Meteorology/Everyday Weather

This page refers to the 2010, 2013, and 2016 topic of Meteorology. For sample notesheets for this topic, see Everyday Weather Notes and EpicFailOlympian's Everyday Weather Notes.

General Info

Everday weather is what some would consider the most complex subject for Meteorology, mainly because it covers all bases of Meteorology, even some climate and storm systems. but the major information to learn will include:

• Structure and composition of the modern atmosphere
• Properties of water
• Types of clouds associated with weather
• Heat transport including the energy budget, insolation, albedo, convection and radiation
• Atmospheric circulation including the Coriolis Effect, planetary wind belts, jet stream, local wind patterns (including Chinook winds, mountain and sea breezes) and the three cell model of circulation
• Air Masses
• Fronts
• Surface weather stations
• Satellite imagery,isobars, isotherms,surface weather maps showing isobars, fronts, and radar data; metograms; Doppler imagery
• Weather instrumentation including barometers, thermometers, anemometers, sling psychrometers, rain gauges, radiosondes, rawinsondes, and the Beaufort scale
• Atmospheric phenomena

Everyday Weather focuses on the mechanics of Earth’s atmosphere and how it causes daily weather. This is a large shift from Climate that emphasizes long-term predictable patterns of weather.

The Atmosphere

The sections below illustrate the concepts and importance of the atmosphere in the study of meteorology.

Origins of the Atmosphere

The origin of Earth’s atmosphere is subject to debate. It is fairly certain that the Earth, when it was formed five billion years ago, was too hot to retain the gasses in its primordial atmosphere. Earth’s first atmosphere most likely consisted of helium, hydrogen, ammonia and methane.

What happened over thousands of years is that volcanoes emitted amounts of water vapor, carbon dioxide and nitrogen - the same gases emitted by volcanoes today. This expulsion of gases from Earth’s interior is a process known as outgassing. The water vapor expelled by the hot, volcanic Earth in turn created clouds, which produced rain.

Over time, the rain would accumulate in basins as rivers, lakes and oceans. These basins in turn acted as sinks for accumulated carbon dioxide, which later became locked into deposits of limestone and other sedimentary rocks. Nitrogen, which is not chemically active, accumulated in the atmosphere. Any significant amounts of oxygen probably did not exist in Earth’s early atmosphere.

Only when tiny bacteria living in Earth’s oceans developed the ability to split water molecules apart by using the energy of sunlight could any significant amount of oxygen begin to accumulate in the atmosphere. It was these processes that are believed to have produced the modern atmospheric composition of 78% nitrogen and 21% oxygen.

Organization of the Atmosphere

The atmospheric layers are in order from sea level to space:

Troposphere

The troposphere is where all weather takes place. It is a region of rising and falling pockets of air moving mostly vertically.

The troposphere's height varies between seasons and latitudes, with the equator having the highest troposphere (12-16 km) and the poles the lowest (8km).

Stratosphere

The stratosphere is located above the troposphere, separated from it by the tropopause. You’ve probably flown up into the lower levels of the stratosphere above the clouds on an airplane. Most clouds cannot form in the stratosphere because its temperature inversion inhibits convection. As a result, airflow in the stratosphere is mostly horizontal.

The stratosphere contains the ozone layer, which absorbs the majority of dangerous UV radiation from the sun. It is also the cause of the stratosphere's temperature inversion, because the ozone layer is very warm from absorbing so much radiation.

Mesosphere

Above the stratosphere is the mesosphere. In the mesosphere, temperatures drop with increasing altitude.

In the mesosphere, meteors start to disintegrate due to collisions with gas molecules.

Thermosphere

The outer-most layer. Though the thermosphere has a very high temperature since the molecules move very fast, it has a very low thermal energy because the molecules are spaced so far apart, and so a section of space in the thermosphere would contain very few molecules. (Remember that temperature is the average kinetic energy of each molecule, while thermal energy is total kinetic energy. Neither is to be confused with heat, which refers to the transfer of thermal energy from a hotter object to a cooler one.)

You should know how temperature changes within each layer (see right)

temperature change in each layer

Depending on which reference materials you are utilizing, you may find that more current publishing's state there are 5 layers of Earth's atmosphere, with the Exosphere being the outermost layer.

Atmospheric Circulation

Let’s take a look at the motion of the atmosphere on a global scale and look at how it's associated with everyday weather.

A non-spinning planet with no axial tilt would only experience the influence of unequal heating by the Sun, with the most direct sunlight reaching the tropics and the least amount reaching the polar regions. Under these circumstances, a simple convection system would suffice with extreme heating in the low latitudes causing warm air to rise.

When this rising air mass reaches the top of the tropopause it stops its upward movement and begins to move towards the poles as an upper level wind.

Cooling air at the Polar Regions encourages the air to sink downward and fall towards the surface. At the surface, this cold air then begins to flow towards the equator.

These convection cells transfer heat by the movement air from the equator towards the poles and then cycle air near from the surface to the equator forming the basis of the two cell model.

Air Pressure

Atmospheric pressure is the weight of the overlying column of air. As altitude increases, the amount of overlying air in the atmosphere decreases, and so air pressure decreases, and vice versa.

Around 80% of the mass of Earth’s atmosphere is within the closest 18km to its surface. This is because the air itself is affected by air pressure; the higher air pressures closer to sea level cause the air to be more compressed than at high elevations.

Atmospheric pressure is normally measured in units called millibars (mb). One millibar is equal to 1 gram per centimeter squared (1g/cm2).

At sea level, the average air pressure is 1,013mb. At the top of Mt. Everest the air pressure can get as low as 300mb. Although the concentration remains the same, the amount of oxygen in the atmosphere is decreased at higher elevations because of the lower air pressure, as the pressure of gases such as oxygen is related to density. That means there is only about 1/3 as much oxygen on Mt. Everest as there is at sea level. This why many people who attempt to climb Mt. Everest experience shortness of breath as they climb to higher elevations.

Descending air forms high pressure centers, or divergence.

Polar highs result from the descent of cold air and its movement towards onto the surface. Subtropical highs form as warm air in the 20-30° latitude range in both hemispheres rises and then begins to cool as it falls towards the surface. This air is very dry making surface conditions in these regions very arid. Most of the world’s deserts in both hemispheres are found in this latitude range. High pressure cells move in a clockwise direction in the Northern Hemisphere.

Ascending air forms low-pressure systems or areas of convergence. Tropical lows form as warm air ascends up into the atmosphere. Sub polar lows form as warm air in the 50-60° latitude ranges of both hemispheres rises producing abundant precipitation. Low-pressure cells move in a counterclockwise direction in the Northern Hemisphere.

Air tends to move from areas of high to low pressure. Air is denser in high-pressure cells (anticyclones), which tend to be hot and dry: this is because air tends to heat up when compressed (adiabatic warming), evaporating clouds and leading to drier weather. The evaporation of clouds also decreases surface albedo (reflectivity).

Low-pressure cells tend to bring precipitation because they draw air upwards, and the reduced pressure and temperature at higher altitudes cause water vapor to condense into clouds. Low-pressure areas (or cyclones) are often formed by warm, moist air evaporating over oceans.

Coriolis Effect and the Three Cell Model

If the Earth did not rotate on its axis, there would be a single circulation cell in each hemisphere. The rotation of the earth on its axis from west to east creates the Coriolis Effect, which causes the different air masses created by the unequal heating of the planet’s surface to shift directionlike the picture on the left (click on it to get a better view). Notice the deflection created by the Coriolis Effect and how it changes in each hemisphere and in different regions of each hemisphere.

coriolis deflection

The Coriolis Effect causes wind patterns in the Northern Hemisphere to differ from wind patterns in the Southern Hemisphere. In the Northern Hemisphere, the Coriolis Effect deflects the movement of air to the right. In the Southern Hemisphere, this movement is deflected to the left. This creates the three main wind belts found at the surface of each hemisphere including the easterly trade winds, prevailing westerlies and the polar easterlies.

Three Cell Model

The Coriolis Effect turns the high and low pressure cells of each hemisphere into a series of three different convection cells known as the Hadley Cell, the Ferrel Cell and the Polar Cell. All three are found in both hemispheres.

The three cell model shows the Earth's global winds and the global circulation of air in the form of convection cells, which are separated by pressure belts.

The polar high is found at the poles, which are also the center of the polar cells, and therefore the Polar Easterlies.

The Polar Cell which is the cold high pressure cell around the poles. The winds in this cell blow like you would expect with the Coriolis Effect, creating the Polar Easterlies

In between the Polar Cell and the next cell (Ferrel Cell) you have a subpolar low. Its importance shalt be noted in a minute.

Then you have the Ferrel Cell, which is the mid latitude cell and is a warm cell. The Ferrel Cell also contains the Prevailing Westerlies.

In between the Ferrel Cell and next cell (Hadley) you have a subtropical high and again just remember this in a few minutes.

The Hadley cell is also a warm cell and the winds again blow like you would expect with the Coriolis Effect, creating the Northeast Trade Winds in the Northern Hemisphere and the Southeast Trade Winds in the Southern Hemisphere.

The Real World

As you may know, the real world screws up the concept of the Coriolis Effect

Take this picture for example

As shown, the winds are blowing in the "wrong" direction according to the Coriolis effect, right?

If you read above you will hopefully remember the global cells and the Prevailing Westerlies seem to be blowing "backwards" in is the Ferrel Cell.

This happens because winds blow from high pressure to low pressure. There is a subtropical high at 30 degrees and a sub polar low at 60 degrees therefore the wind blows from 30 degrees to 60 degrees. That wind is still subject to the Coriolis Effect, therefore it still curves, it is just affected differently.

Local Wind Patterns/Global Winds

Planetary Winds

Global scale winds are winds that are created in the different Global circulation Cells.

you have the:

1. Polar Easterlies
2. Prevailing Westerlies
3. Trade Winds

The Polar Easterlies blow from the poles to 60˚ latitude.

The Prevailing Westerlies blow from 60˚ to 30˚ latitude.

The Trade Winds blow from 30˚ latitude to the equator.

Mountain/Valley Winds

During the day, mountains warm, causing the air over them to be warmer than the air over the valley at the same elevation. Warming the air causes it to rise up, creating a valley wind. During the evening, the air cools due to a loss of surface energy to space. The cool dense air moves down slope as a mountain wind.

Chinook Winds

A Chinook wind is a warm dry wind on the leeward side of a mountain. As air descends the leeward side of a mountain (also known as the "Rain Shadow"), it is compressed and adiabatically heated. Warming the air causes the saturation point to increase, causing a decrease in its relative humidity. The new warm and dry wind moves down slope rapidly, and during the spring causes substantial melting of mountain snow. A Chinook is a FOEHN wind that is specific to the Rocky Mountains.

Santa Ana Winds

Santa Ana winds are warm and dry winds. Over plateau regions in the desert region of the United States, high pressure pushes the air off the plateaus, forcing the air into narrow mountain valleys. As the air is forced through the valley it compresses and warms. As the air warms the saturation point rises and its relative humidity drops.

Heat Transport

Earth Energy Budget

The Earth’s Energy Budget is determined by the amount of incoming energy and the amount of outgoing energy. Nearly all of Earth’s incoming energy (99.98%) is from solar radiation. About .013% comes from geothermal energy that is created by the radioactive decay of Earth’s core. About .002% of Earth’s incoming energy comes from the action of tides caused by the interaction of Earth with the Sun and Moon. Waste heat energy from fossil fuel consumption accounts for about .007% of Earth’s Energy Budget. The Earth has an average albedo of about 30% which means that ~30% of incoming solar radiation is radiated back into space before it reaches Earth's surface. After the 30% the atmosphere absorbs 19% and the earths surface absorbs 51%.

Around 70% of solar energy that is absorbed by the Earth is reradiated as infrared energy. The Earth’s Energy Budget is in equilibrium as the amount of incoming energy is balanced by the same amount of outgoing energy.

Convection

Convection is the transfer of heat from the Earth’s surface into the atmosphere. When a layer of air receives enough heat, it expands and is pushed upward by buoyancy. Then air becomes denser and moves laterally until it begins to sink and then begins to rise again as it warms. Atmospheric convection currents may cause breezes, winds, cyclones and thunderstorms.

Convection Current

Radiation Budget

Radiation budget refers to the balance between incoming radiation from the Sun and the outgoing thermal, or longwave and reflected shortwave energy from Earth. Globally the budget is balanced as the amount of incoming solar radiation is transformed into latent heat, or even kinetic energy. Energy transfers in the oceans along with the atmosphere keep the radiation budget in balance.

But locally the Radiation Budget is unbalanced because tropical regions retain more insolation, while less is retained in higher latitudes.This accounts for differences in the temperature and pressure of air masses that originate in both regions affecting weather throughout the planet

Air masses and Fronts

Air Masses

Large bodies of air that pass slowly over large areas of Earth’s surface and they take on the characteristics of that region such as temperature and humidity. The area from which the air mass derives its characteristics is its source region.

Air mass source regions can be snow covered areas near the poles,arid deserts, or even tropical oceans.

Air masses that form over the ocean are termed maritime air masses.

Those that form over land are called continental air masses.

Any further classification of air masses are normally based on longitude. Tropical air masses are formed in low latitudes. Polar are formed in high latitudes.

Classifications

Air Mass Classification

Air Mass	Description
Continental Tropical (cT)	Formed over land at low latitudes transports warm, dry air
Maritime Tropical (mT)	Formed over sea at low latitudes transports warm, wet air
Continental Polar (cP)	Formed over land at high latitudes transports cold, dry air
Maritime Polar (mP)	Formed over water at high latitudes transports cold, wet air

Air masses

The major air masses that influence the United States are:

Polar air masses that form over Canada and Alaska often affect the weather of the United States as they move south and eastward.

The states along the Gulf of Mexico and the Eastern seaboard often experience the effects of tropical air masses that move northward cuasing humid subtropical and continental climates.

Occasionally, continental arctic (cA) air masses may descend from high latitude regions in the winter months creating bitterly cold weather.

Tropical air masses from the Pacific may affect California and the Southwestern states during the winter months. Although it is influenced by these major air masses, the United States itself is not a favorable source region for fronts because so many weather disturbances disrupt opportunities for the formation of air masses.

Air masses move from their source region due to the Coriolis Effect where they will meet adjacent air masses with different properties. When these two air masses of different origin meet, the boundary between them is termed a front.

Fronts

Frontal boundaries are very narrow, less than 200km wide. Normally one air mass is cooler than the other, giving the warmer air a tendency to flow up and over the cooler air mass. This cooler and denser air acts as a wedge that allows warmer less dense air to rise over it.

Types

There are five types of fronts, each with their own characteristics.

1. Warm Front
2. Cold Front
   

   Cold Front
3. Stationary Front
   

   Warm Front
4. Occluded Front
5. Dry Line

Warm Fronts

A warm front is when warm air moves in and displaces an area of once cooler air. Warm fronts are characterized by an increase in temperature and the appearance of cirrus clouds.

• • shown on a map as semicircles
  • clouds become lower as front nears
  • Slow rate of advance
  • light to moderate precipitation
  • gradual temperature increase

Weather Patterns Associated with a Warm front

Warm Front

	Before Passing	While Passing	After Passing
Winds	South-southeast	Variable	South-southwest
Temperature	Cool-cold, slow warming	Steady rise	Warmer- then steady
Pressure	Usually falling	Levels off	Slight rise-then falling
Clouds	In this order Ci,Cs,As,Ns, St and fog	Stratus	Clearing with scattered Sc; some Cb in summer
Precipitation	Light to moderate rain, drizzle, snow or sleet	Drizzle or none	Usually none; possible light rain
Visibility	Poor	Poor- but improving	Fair in haze
Dewpoint	Steady rise	Steady	Rise, then steady

Cold Fronts

Cold fronts are formed when cooler air replaces an area that was once occupied by warmer air. They are associated with turbulent changes in weather. Temperatures drop as warm air is pushed aside vertically and abruptly. Tall, cumulonimbus clouds take shape and may form thunderstorms.

• • shown on a map as triangles
  • cold air replaces warm air
  • weather is more violent than warm front
  • faster rate of advance
  • precipitation intense
  • clear after front passes

Weather Patterns Associated with a Cold Front

Cold Fronts

	Before Passing	While Passing	After is passes
Winds	South Southwest	Gusty and changing	West Northwest-ish
Temperature	Warm	Sudden drop	Steadily dropping
Pressure	Falling steadily	Minimum-then sharp rise
Clouds	Increasing Ci, Cs and Cb	Cb	Clearing with scattered Sc; some Cb in summer
Precipitation	Short period of showers	Heavy rain, thunder, lightning, hail	Showers, then clearing
Visibility	Fair to poor	Poor followed by improving	Good-possible showers
Dewpoint	High, remains steady	Sharp drop	Lowering

Stationary Fronts

Nearby stationary fronts bring weather patterns that may persist for days.

• • shown on a map as alternating triangles and semicircles facing in opposite directions
  • air flow parallel on both sides
  • doesn't move
  • widespread clouds
  • Precipitation light

Occluded Fronts

Occluded fronts occur when a cold front overtakes a warm front. This often results from the merging of two cold fronts that overwhelm the warm front. The result is a weakening of the storm system that might otherwise occur.

• • active cold front overtakes a warm front
  • weather is complex
  • precipitation associated with warm air

There are two types of occluded fronts, known as cold occlusion and warm occlusion. In cold occlusion, the approaching cold air mass creating the cold front is colder than the retreating cold/cool air mass in front of the warm front. The opposite is true for warm occlusions, where the cold/cool air mass creating the cold front is warmer than the retreating cold air mass in front of the warm air mass. Also to note, cold occlusions are much more common than warm occlusions.

Weather Patterns Associated with an Occluded Front

Occluded Front

	Before Passing	While Passing	After is passes
Winds	Southeast-south	Variable	West-northwest
Temperature (Cold Occlusion)	Cold-cool	Dropping	Colder
Temperature (Warm Occlusion)	Cold	Rising	Milder
Pressure	Usually falling	Low point	Usually rising
Clouds	In order Ci, Cs, As, Ns	Ns, sometimes Tcu and Cb	Light to moderate precipitation-clearing
Precipitation	Light, moderate or heavy	Continuous light, moderate or heavy	Light to moderate, then clearing
Visibility	Poor	Poor	Improving
Dewpoint	Steady	Slight drop	Slight drop, may rise if warm occluded

Dry Lines

A dry line is a line that separates a moist air mass from a dry air mass. It can also be referred to as a dew point front because the dew point temperature changes drastically across the dry line. The most common place to find a dry line is just east of the Rocky mountains separating the dry air that comes over the mountains from the moist air coming off of the Gulf of Mexico and the Atlantic ocean. Dry lines are extremely rare east of the Mississippi River.

Front Symbols

Clouds

types	symbol for a stationary front	100pxsymbol for a cold front	symbol for a warm front
types	symbol for a occluded front	Dry Line symbol

Precipitation and Clouds

Water in the Atmosphere

The atmosphere of our planet is laden with water. In temperate and tropical regions, water exists mainly in liquid form. At the poles and higher latitudes, much of Earth’s water exists as ice locked away in alpine or continental glaciers. The physical composition of Earth’s atmosphere consists primarily of water vapor.

Humidity refers to the amount of water vapor that is in the air. This water vapor exists in a gaseous state. The process in which water changes from a liquid into a gaseous state is evaporation. Each water molecule that becomes water vapor also takes with it a parcel of heat energy from the surface it evaporates from cooling the surface (evaporative cooling). Evaporative cooling explains why you may feel a chill after swimming as water evaporates off the surface of your skin, taking with it heat from your body.

During the spring when the amount of daylight is increasing and the declination of the hemisphere is tilting towards the Sun, the intensity of solar radiation increases causing ice crystals in the upper troposphere to melt and fall as rain. As the water is exposed to increased solar radiation it evaporates and returns to the atmosphere in a gaseous state-as water vapor. The humidity of the atmosphere increases as spring changes to summer.

Water vapor is measured using an instrument known as a hygrometer. Measurements of humidity are often expressed as a percentage, which is termed relative humidity. The complete saturation of the air (100% relative humidity) occurs when the amount of water vapor in the air equals the amount of water vapor that the air can hold.

Precipitation

Precipitation is any form of water that falls to the surface of Earth. The most common forms precipitation occurs in are:

• rain
• snow
• sleet
• hail

Liquid Precipitation

Mist consists of droplets less than .05mm in diameter. Drizzle is anything larger than.05mm but less than .5mm larger than 0.5mm across is rain. Most raindrops are not larger than 5mm across because of air drag effects that would tear larger droplets into smaller droplets as they descended through the air.

Frozen Precipitation

Snowflakes fall as ice crystals and have diameters of between 1mm and 2cm. They form through deposition of water vapor onto particles such as dust, and the water vapor is directly converted into ice.

Graupel is around 5mm and falls as soft and mushy ice.

Sleet is similar to graupel, but it is smaller, forming when raindrops freeze as they fall.

Hail is larger than 5mm and is formed from rounded clumps of hard, layered ice. It is usually associated with thunderstorms. Each individual hailstone is repeatedly cycled through the interior of a thunderstorm by strong updrafts, allowing layers of ice to accumulate before it finally becomes too massive and falls to the ground.

Another variation is rime, a deposit of ice that freezes onto a surface that has a temperature below 0°C. Rime forms from water droplets that are supercooled: below the freezing point, but unable to freeze into ice until coming into contact with something (similar to the concept of supersaturation). Rime droplets are very small, and can often be observed on snowflakes.

Graupel essentially consists of snowflakes that have accumulated large masses of rime, to the extent that the original snowflake's structure is completely obscured.

Virga

Virga-precipitation that falls from clouds but evaporates while falling and never reaches the surface of the Earth.

At high altitudes, precipitation falls mainly as ice crystals before they melt and evaporate before reaching the ground because of compressional heating that occurs as a result of increasing air pressure closer to the ground (remember- air that is compressed becomes warmer).

Streams of falling precipitation that never reach the ground make the clouds appear to have commas attached to them as aloft winds push the bottom ends of the virga into angles. Virgas can be hazardous to pilots because the pockets of extremely cold air descending from the upper atmosphere can create microbursts.

Clouds

Formation of Clouds

Clouds are nothing more than small droplets of water and ice crystals that clump together within the atmosphere. They may produce precipitation in the form of liquid water and/or ice crystals that fall to the Earth’s surface. Rising air is an important process in the formation of clouds. As air rises, it expands causing it to lose heat energy and voila the temperature of the air decreases. The water vapor molecules that are in the air also increase the humidity of the air until it is saturated (100% relative humidity).

Excess water vapor condenses changing from a gas into a liquid on large aerosol particles in the atmosphere if the relative humidity is not in excess of 100%. When the atmosphere cools, it will reach the point at which the air is saturated with water vapor and can precipitate.This is the dew point. The dew point is defined as the temperature to which a particle of air would need to be cooled in order to reach this point of saturation. The air’s capacity to hold water vapor is temperature dependent. Warmer air tends to hold more moisture, while cooler air holds less.

The dew point and relative humidity can be measured using a psychrometer, a weather measurement tool consisting of two identical thermometers mounted side by side. One of the thermometers- the dry bulb measures air temperature. The other thermometer- the wet bulb- has a damp wick wrapped around it allowing it to measure any decrease in temperature. This indicates the maximum amount of cooling that can result from evaporation.

To use a psychrometer, it needs to be exposed to a flow of air by slinging it around on a handle. The humidity of the air is directly proportional to the amount of moisture evaporating off the wet bulb. If the two thermometers have identical readings, than no evaporation has taken place and the air is saturated with water vapor. The more significant the measured difference between the two thermometers, the drier the air and the lower the level of humidity.

Cloud formation is closely related to the cooling of humid air masses. As water vapor expands- it cools in temperature. Likewise, when air is compressed- it heats up. This change in temperature caused by the expansion or contraction of gases is known as adiabatic temperature change. This is a cooling or warming of the air caused by expansion or contraction and not by the increase or irradiation of heat.

The effects of adiabatic temperature change in Earth’s atmosphere can be dramatic. Air sinking down from higher latitudes is warmed by an increase in atmospheric pressure as it contracts. Likewise, warm air that climbs in altitude is under less pressure and cools as it expands. When this air is enriched in water vapor and cooled down to its dew point, condensation and cloud formation can take place.

Cloud ID

For the event you need to be able to identify clouds and weather that come with each cloud. Cloud prefixes tell you where the clouds are located.

"cirr-", like cirrus clouds, can be located at high levels

"alto-", like altostratus, can be found at middle levels

Cloud types are classifies by height of the ground these are three of the classifications

• Upper Clouds
  • 9000 meters
  • Cirus clouds
• Intermediate Clouds
  • 3000-7000 meters
  • Altocumulus
• Lower Clouds
  • 2000 meters
  • Nimbostratus
• High Fogs
  • Under 1000 meters

This is a list identifying which clouds go in which layer. These are from "The cloud book: how to understand the skies" by Richard Hamblyn

1. Low Clouds
   1. Stratocumulus
   2. Stratus
   3. Cumulus
2. Medium Clouds
   1. Altocumulus
   2. Altostratus
   3. Nimbostratus
3. High clouds
   1. Cirrus
   2. Cirrocumulus
   3. Cirrostratus
4. Multi-level
   1. Cumulonimbus

Clouds

Cloud Picture	Name	Description
	Stratus Cloud	Low clouds Below 2,000 m Gray, fog-like clouds Flat
	Stratocumulus	low-level clouds that develop below 2,000 meters that generally appear as low, lumpy layers of clouds. usually fair-weather clouds sometimes accompanied by low-intensity precipitation such as drizzle. vary in color from dark to light gray with breaks of clear sky in between them
	Nimbostratus	dark, low level (below 2,000 meters) clouds that are usually accompanied by light to moderately falling levels of precipitation. thick enough to block out the Sun entirely(gloomy day clouds) In the winter months they may also contain ice particles and snow.
	Altostratus	gray/blue gray cloud covers entire sky over large area
	Altocumulos	ppear as parallel bands or perhaps as rounded masses at medium altitudes (2,000-6,000m) white, fluffy appearance (commonly nicknamed ‘sheep-back’ clouds because of their wooly appearance) usually form by the convection of air in an unstable layer aloft because of the gradual lifting of air in advance of an approaching cold front
	Cirrostratus	sheet-like, high level clouds composed of ice crystals. Cirrostratus clouds can cover the entire sky and be up to several thousand feet thick relatively transparent as the Sun and Moon can easily be seen through them. often form when a broad layer of air is lifted by a large-scale convergence as shown in the diagram below. Cause of halos
	Cirrocumulus	round white puffs
	Cirrus	Cirrus clouds are found at high altitudes (greater than 6,00 meters) and appear as thin and often wispy sheets with a feathery appearance. Cirrus clouds are usually composed of ice crystals that originate from the freezing of super cooled water droplets. Cirrus clouds are usually fair weather clouds and point in the direction of air movement at their elevation.
	Cumulus	looks like a piece of floating cotton life span of around 40 minutes
	Cumulonimbus	thunderstorm cloud forms by Cumulus cloud growth convective updrafts sometimes at speeds of over 50 knots
	Lenticular Cloud	A lens shaped cloud
	Mammatus Clouds	baggy clouds seen after thunderstorms can mean tornadoes
	Contrails	Jet produced clouds trail of water vapor
	Nacreous Clouds	soft pearly looking

Great Cloud page http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/home.rxml

Great Cloud book is "The cloud book: how to understand the skies" by Richard Hamblyn

Clouds symbols

Optical Phenomena

There are many types of weather phenomena but here are the major ones.

Phenomena

Picture	Name	Description
	Mirage	An optical phenomenon that occurs when light is refracted to produce an image of an object or the sky where it is not.
	Halo	Like rainbows, haloes are formed around the Sun due to moisture being refracted from the Sun’s rays in the upper atmosphere.
	Sundog	An atmospheric phenomenon that creates bright spots of light in the sky, often on a luminous ring or halo on either side of the sun.
	Virga	An observable streak or shaft of precipitation that falls from a cloud but evaporates or sublimes before reaching the ground.
	Rainbow	An atmospheric phenomenon that is caused by reflection of light in water droplets in the Earth's atmosphere, resulting in a spectrum of light appearing in the sky.
	Crepuscular Ray	Rays of sunlight that stream through gaps in clouds (particularly stratocumulus) or between other objects who's columns of sunlit air are separated by darker cloud-shadowed regions.
	Aurora Borealis	A natural light display in the sky particularly in the high latitude regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere.
	Green Flash	An optical phenomenon that occurs shortly after sunset or before sunrise, when a green spot is visible, usually for no more than a second or two, above the sun, or it may resemble a green ray shooting up from the sunset point.
	Belt of Venus	Occurs during dusty evenings when a band of pinkish or brownish sky will appear between the sky and the horizon.
	St Elmo’s Fire	Luminous plasma that appears like fire on objects, such as the masts of ships or lightning rods.

Weather Technology

Instruments

Instrument	Uses
Actinometer	measures solar radiation
Barometer	measures air pressure
Barograph	makes a continuous recording of atmospheric pressure
Ceilometer	measures the height of a cloud base
Evaporimeter	measures evaporation of water
Hygrometer	measures moisture content of gases/gas
Psychrometer	measures relative humidity
Radiosonde	measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes are usually placed in weather balloons.
Rawinsondes	a radiosonde that is designed to only measure wind speed and direction.
Rain gauge	measures rainfall
Thermometer	measures temperature
Wind vane	shows wind direction

Forecasting

There are many ways to forecast, but the simplest way is to take today's weather and say that tomorrow is going to be the same. This can be called the Persistence method. This method is useful in some places (e.g., Florida during the summer) but can also be very unreliable in other places (e.g., Chicago during the spring).

The next method can be called the trends method it relies on mathematics to forecast. This method involves being able to get a accurate measurement of the speed at which the weather system is moving and putting the numbers into(S=d/t) and determining the time at which the system will be at your position. Here is an example of the trend model:

trend model

The Climatology Method is another easy way of forecasting. This method involves averaging weather statistics accumulated over many years to make a forecast.

The Analog Method is a more complicated method of producing a forecast. It involves examining today's forecast scenario and remembering a day in the past when the weather scenario looked very similar. Than you would predict that the weather in this forecast will behave similar to the day in the past.

Radar

Before we get into any satellite imagery or the like you need to know Radar.

Radar is a very important part of today's meteorology. It gives us a chance to have early warning of approaching storms and rain. So lets start with the basics.

Radar Basics

Radar is a simple concept, you have a beam you send out and it reflects back to an antennae that collects the beam and sends it via wire to a receiver. the return signals are arranged to produce a radar image.

Radar transmitting

Radar Recieving

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So when the beam of energy (electromagnetic) hits it target it is reflected in all directions, some going up, some going down, some going back toward its origin point. The most important part is the returning energy. This returning energy is much weaker when it is returning than when it left. The size of the reflecting particle determines the strength of the return signal. This means that the larger the particle, the stronger the signal, and the smaller the particle, the weaker the signal. Also the more particles there are, the stronger the return signal because the returning beams combine to form a stronger signal

The amount/size of the reflectivity is proportional to the number and size of the drops encountered by the electromagnetic pulse. Because of this high reflectivity normally means heavy precipitation while low reflectivity means lighter precipitation.

Since radar uses electromagnetic pulses or electromagnetic waves, wavelength will affect the signal and pulses. The two wavelengths are short and long wavelengths. Short wavelengths are good at measuring small particles like dust and cloud droplets. However the short wavelengths are also easily absorbed by the the materials it reflects off of (this is called attenuation). This feature makes it hard to measure distant targets.

Long wavelengths have the advantage that absorption by the particles is reduced. This feature makes this wavelength very useful for looking at severe storms and other large scale/distant weather. Long wave radar is used in most Doppler radars like the National Weather Service's WSR-88D (NEXRAD) Doppler radars.

How To read weather maps/satellite imagery

A good place to start is a basic weather map, the kind that you will most likely see at a competition:

1. Start by identifying the different pressure zones on the map. Above it is already done but if it wasn't marked look for sections that have a circle with a very high or low pressure.
2. Look for fronts. This can be done by looking at the station ball symbols. (those are the yellow circles with the tails the tails indicate wind direction) look and find a sudden change in wind direction/pressure that will normally indicate a front. I marked where I believe a front to be.

There are many lines on this map. Each one means something:

• The blue lines are isobars which mean that they are lines of constant pressure.
• The yellow circles with "tails" are called station ball symbols of station models; they are explained in the next section.

Something else you might see on a weather map is a isotherm, which is a line of constant temperature.

Station Models

This image is a station model. It can tell you many different things, like wind speed, wind direction, temperature, dew point, current weather, cloud cover, and pressure, given that you know how to read and interpret it. Some symbols have more information than others on them, but here is a basic overview:

• The 48 is the current temperature
• The 45 is the dew point
• The "whatever" that is in between the two numbers is the current weather. On this one it is a light rain.

This is what tells you information about the wind. The direction the stick faces shows the wind direction, and how many lines on the end of it show the wind speed. A half line signifies five knots, a full line ten knots, and a bold line 50 knots.

This indicates how much cloud cover there is. There are nine choices:

Cloud Cover

Picture	Meaning	Picture	Meaning	Picture	Meaning	Picture	Meaning	Picture	Meaning	Picture	Meaning
	Clear		1/8		1/4		3/8		1/2		Missing
	5/8		3/4		7/8		overcast		Obscured

Symbols

this is from here:http://www.sover.net/~redcamp/wxplegend.gif

another symbol sheet in case you want two examples

Rawinsondes/Radiosondes

Rawinsondes and Radiosondes are interconnected they both are part of the weather equiptment luanched in weather balloons.

A radiosonde or rawinsonde will normally record and "observe"

• Pressure
• Altitude
• Location (coordinates)
• Temperature
• Wind speed and direction

METAR

This is a way that meteorologists convey what is happening at a point on Earth. It is a very abbreviated language. Here is an example of METAR:

KCLL 312253Z 14007G15KT 10SM CLR 31/15 A2990 RMK AO2 SLP120 T03110150

all of this means something, you just have to know what the abbreviations mean.

METAR

Abreviation	Explanation	What is says
KCLL	this tells you where the METAR station is located
312253Z	this tells you the date and time in zulu*	this says that it is the 31st of the month and is 22:53 zulu
14007G15KT	wind direction and speed	this says that the wind is from 140 degrees at 7kt with gusts to 15 kts
10SM	visibility	this says there are 10 statute miles of visibility
CLR	clear skies
31/15	temperature and Dew point in Celsius
A2990	inches of mercury(29.90)	to get millibars multiple by 33.863
RMK	Remarks	this is where the station will post claritive data
A02	type of automated station	AO1; automated station without a precipitation descriminator. AO2; automated station with precipitation discriminator
SLP120	sea level pressure
T03110150	acually temperature	this says that the temperature is actually 31.1 and the dew point is 15

Zulu is a time measurement based off of a 24-hour clock

here is a more in depth METAR guide: http://www.met.tamu.edu/class/metar/quick-metar.html

Stuve Diagrams

A Stuve diagram is the compilation of data gathered in Weather Balloon flights.

As you can see the image above is extremely cluttered and hard to understand so lets break it down.

This image shows the Stuve diagram at is most basic level. The left Y-axis shows air pressure and elevation (in meters).

The X-axis shows temperature in Kelvin and well as Celsius.

Now that we have the X and Y axes in order, lets talk about the yellow horizontal and vertical lines that look similar to graph paper except are not constant. The vertical lines represent isotherms, or lines of constant temperature and the horizontal lines represent isobars, or lines of constant pressure.

Since the axis and lines are down, direct your view to the "barbs" along the right side, which show wind speed and direction. They are on the same scale as the Station Model symbols.

Now that everything is clear, lets get to what I bet you have been staring at since you first started; The red line. The red line shows the relationship between air temperature and altitude.

As you can see more information has been added; the dashed green lines and the dashed black line.

The dashed green line shows represents the saturation mixing ratio which is the amount of water vapor needed in a parcel of air (the amount needed to make a cloud).

The black line shows how the dew point temperature changes with altitude.

Since it makes it more interesting, lets add yet more elements to this chart. These are the yellow line and also the solid diagonal lines.

The yellow line show the temperature of a parcel of air as it is moved through the atmosphere. The solid diagonal lines are called dry adiabats and show the rate at which unsaturated of dry air will cool down as it rises up through the atmosphere, at a rate of about 10˚C/km.

If an air parcel is initially unsaturated, it will cool off at the dry adiabatic lapse rate as it rises (note that the yellow line is parallel to the solid diagonal lines).

So let us assume the parcel of air starts off at an altitude of 500m with a temperature of 22˚C. If it gets lifted up, it will cool off at 10ºC for every km it rises. This is shown by the yellow line. At an altitude of 2000m it will have cooled to a temperature of about 7˚C. At this point it has cooled down enough that it is now saturated.

Why does this happen you ask? Remember that the saturation mixing ratio for any temperature and pressure can be found using the dashed green lines on the graph.Well, at 800 mb and 7˚C (where the yellow line ends) the dashed green line which would go through this point would have a value of 8 g/kg. This is the saturation mixing ratio for this point. But this was also the actual mixing ratio for this air parcel. So now the air has cooled down enough that the actual mixing ratio is the same as the saturation mixing ratio. The altitude at which this happens is the lifting condensation level (LCL). This is the point at which moisture contained in a rising parcel of air can begin to condense. this is shown in the list of data at the right-hand side of the figure. Look under “PARCEL”, then find “LCL:800”. This indicates that the lifted air parcel would reach its lifting condensation level at 800 mb.

Finally, the last part of a Stuve diagram are the solid green lines which are referred to as saturated adiabats. These show the rate at which saturated air cools as it rises.

The lines are somewhat curved because the saturated adiabatic lapse rate fluctuates between 2˚C/km to nearly 10˚/km (the dry adiabatic lapse rate), depending moisture content of the parcel of air

Images above come from this site

Meteograms

A meteogram is a mix of station ball symbols and metar File:Meteogram.gif

The graph along the top of the Meteogram shows temperature and Dew point. temperature is the top line(green) and dew point is the bottom line (blue)

The abreviation EXTT (extreme temperature) shows the maximum and minimum temperatures which are plotted below the chart at specific reporting times. 12Z and 18Z are generally low temperatures and 0Z and 6Z are high temperatures.

The WX shows the current weather data at the time of data collection

SNWDP would show snow-depth if there where any.

PREC would tell you precipitation in inches

VIS tells you visibility in miles

WGST would tell you wind gusts

WIND (winds and cloud cover) - This is same sympols wind and cloud cover symbols used in the station models

Cloud chart will give cloud layer information. The vertical axis is height of cloud base in feet. The layers are plotted as horizontal lines.

• Clear skies are plotted as a 'C'.
• Scattered cloud layers (1/8th to 3/8th coverage) are plotted as a single short dash.
• Broken cloud layers (4/8th to 7/8th coverage) are plotted as two short dashes. *Overcast layers are plotted as a single long dash.

The actual cloud ceiling is displayed below the chart in 100s of feet if the ceiling is below 10,000 feet.

Pressure chart - This chart plots sea level pressure (or altimeter setting if pressure not reported) in millibars.

Beaufort Scale

Beaufort Scale

Force	Wind Speed(knots)	Classification	Appearance	Picture
0	>1	Calm	Calm, smoke rises vertically
1	1-3	Light Air	Smoke drift indicates wind direction,wind vanes still	File:Beau1.gif
2	4-6	Light Breeze	Wind felt on face, leaves rustle,wind vanes begin to move
3	7-10	Gentle Breeze	Leaves and small twigs constantly moving, light flags extended
4	11-16	Moderate Breeze	Dust, leaves, and loose paper lifted, small tree branches move
5	17-21	Fresh Breeze	Small trees begin to sway
6	22-27	Strong Breeze	Larger tree branches moving, whistling in wires
7	28-33	Near Gale	Whole trees moving, resistance felt walking against wind
8	34-40	Gale	Whole trees in motion, resistance felt walking against wind
9	41-47	Strong Gale	Slight structural damage occurs, slate blows off roofs
10	48-55	Storm	Seldom experienced on land, trees broken or uprooted, "considerable structural damage"
11	56-63	Violent Storm
12	64+	HURRICANE	Look up the great hurricane of 1900(see also Katrina)

Reference

Terms To Know

Some basic terms

Term	Definition
Absolute Humidity	Mass of water vapor in a volume of air
Absolutely Stable Atmosphere	Atmospheric condition that occurs when the environmental lapse rate is less than moist adiabatic rate
Absolutely Unstable Atmosphere	Atmospheric condition that occurs when the environmental lapse rate is greater than the dry adiabatic rate
Adiabatic Process	Possess that takes place without transfer of heat between systems, compression results in heating, expansion results in cooling
Air Mass	A large body of air that has similar characteristics throughout it
Air Pressure	Pressure exerted by a mass of air at a given point
Albedo	Percent of radiation being reflected by substance
Black Body	Hypothetical object that absorbs all radiation that strikes it
Celsius Scale	Temperature scale where zero=freezing and 100=boiling
Cloud Burst	Sudden heavy rainfall
Cut Off Low	Cold upper-level low that has dissipated out of basic westerly flows
Daily Range of Temperatures	Difference between maximum and minimum temperature
Derecho	Strong damaging straight line winds associated with a thunderstorm
Doppler Lidar	A radar that uses light beams to determine velocity of objects by using Doppler shift
Doppler Shift	Change in frequency of waves that occurs when emitter or observer is moving toward or away from each other
Downburst	Severe downdraft that can be experienced beneath a severe thunderstorm
Dry Adiabatic Lapse Rate	Rate of change in temperature in a rising or descending unsaturated air parcel. Rate is about 10˚C/km
Eddy	A small volume of air that acts differently than surrounding air
Flash Flood	Flood that starts with little or no advanced warning
Geostrophic Wind	A theoretical horizontal wind blowing in a straight path parallel to isobars or contours at a constant speed
Heat Capacity	The ratio of heat absorbed by a system to the corresponding temperature change
Isobar	Line connecting areas of equal pressure
Isotherm	Line connecting areas of constant temperature
Lake Breeze	Wind blowing off the surface of a lake
Land Breeze	Coastal breeze blows from land to sea
Lapse Rate	Rate at which an atmospheric variable decreases with height
Lee-side Low	Storm system that form on downwind side of a mountain chain
Local Winds	Winds that normally blow over a small area due to regional effects
Low Level Jet Streams	Jet streams that form near earths surface (below 2km) at speeds less than 60kts
Macroburst	Strong downdraft greater than 4km wide that can occur beneath a severe thunderstorm
Macroclimate	Climate of a large area say a country
Nuclear Winter	The dark, cold, and miserable conditions that could be brought on be a nuclear war
Parcel of Air	Small body of air a few meters wide that is used to explain behavior of air
Pressure	Force per unit of area
Pressure Gradient	Rate of change of pressure per unit of horizontal distance
Prevailing Wind	Wind direction most frequently observed during a period of time
Radiational Cooling	Process by which the earth's surface releases infrared energy
Ridge	An elongated area of high atmospheric pressure
Rotors	Turbulent eddies that form downwind of a mountain chain creating hazardous eddies
Saturated Adiabatic Lapse Rate	Rate of change in temperature in a rising or descending unsaturated air parcel. Rate is about 2˚C/km to 10˚C/km
Sea Breeze	Coastal local wind that blows from sea to land
Sea Level Pressure	Atmospheric pressure at mean sea level
Sensible Heat	Heat that can be felt and measured with a thermometer
Sensible Temperature	Sensation of temperature that the human body feels in contrast to actual temperature
Snow Squall	Intermittent heavy shower of snow that reduces visibility
Specific Heat	Ratio of energy absorbed to the change in temperature
Station Pressure	Actual air pressure at the observing station
Streamline	Line that shows wind flow patterns
Tcu	Abbreviation for towering cumulus cloud
Teleconnections	Linkage between weather changes occurring in widely different parts of the world
Temperature Inversion	Increase of air temperature with height
Thermal	Small rising parcel of air caused by unequal heating of earth's surface
Thermal Circulation	Air flow resulting from heating and cooling of air
Trace of Precipitation	Amount of precipitation less than .025 cm
Urban Heat Island	Increased air temperature over urban regions compared to the surrounding countryside
Weather	State of the atmosphere at a certain point in time
Zonal Wind Flow	Wind that has a predominant west to east component

Here is an explanation of degree days.

From a meteorological standpoint, degree days are defined with respect to 65 F as a reference point. The National Weather Service calculates it as follows: Heating Days / Cooling Days The daily high and low temperature, both measured to the nearest whole degree Fahrenheit, are averaged. If the average is not an integer, it is rounded up. Subtract this average from 65. If it is a positive quantity, it is the number of COOLING degree days for that day. If it is negative, its absolute value is the number of HEATING degree days.

As an example, yesterday in Omaha, Nebraska the high was 55 and the low 28. Averaging those two numbers:

Average = (High + Low)/2 = (55 + 28)/2 = 83/2 = 41.5. This needs to be rounded up to 42 (it is always rounded up).

Degree Days = Average - 65 = 42 - 65 = -23. It's negative so yesterday there were 23 HEATING degree days in Omaha.

Degree days are reported daily in climate summaries issued by the National Weather Service. Monthly and yearly sums of the daily degree days are also reported. Counts for heating and cooling degree days are kept separate.

Note: the "year" for heating degree days is typically that sum from the period July 1-June 30, while cooling runs from January 1-December 31.

Degree days can be calculated with other reference values than 65, but the National Weather Service standard is 65 degrees.

Links

This is a great meteorology site and some of the images on this page have come from here

Also, see these Everyday Weather Meteorology Notes

EpicFailOlympian's Everyday Weather Notes