Weather Basics


What is weather?

The state of the atmosphere with respect to wind, temperature, cloudiness, moisture, pressure, etc. Weather refers to these conditions at a given point in time (e.g., today's high temperature).


What is the water cycle?  

The water cycle is the continuous movement of water in and around the Earth. The sun drives the entire water cycle and is responsible for its two major components: condensation and evaporation. When the sun heats the surface of water, it evaporates and ends up in the atmosphere as water vapor. It cools and rises, becoming clouds, which eventually condense into water droplets. Depending on the temperature of the atmosphere and other conditions, the water precipitates as rain, sleet, hail or snow.  (How Stuff Works.com) (1)


EarthGuide Diagrams

EarthGuide Diagrams

Why does it rain?

When water becomes warm enough, it evaporates as vapor into the air. When a mass of air quickly cools to its saturation point, the water vapor condenses into clusters of tiny water droplets and frozen water crystals. We call these clusters clouds. Over time, the droplets and crystals that make up a cloud can attract more water to themselves. When water droplets grow heavy enough, gravity pulls them down as raindrops. If the air is cold enough, the ice crystals can remain frozen and grow large enough to fall as snow, sleet, freezing rain or hail. (Discovery Kids)


How do clouds move?  

The wind moves the clouds.  The faster the wind is moving, the faster the clouds move.


What are the different types of clouds?  

There are three main types of clouds - cirrus, cumulus, and stratus.  

Cirrus clouds are wispy, feathery, and composed entirely of ice crystals.  They often are the first sign of an approaching warm front or upper-level jet streak. 

Unlike cirrus, cirrostratus clouds form more of a widespread, veil-like layer (similar to what stratus clouds do in low levels).  When sunlight or moonlight passes through the hexagonal-shaped ice crystals of cirrostratus clouds, the light is dispersed or refracted (similar to light passing through a prism) in such a way that a familiar ring or halo may form. As a warm front approaches, cirrus clouds tend to thicken into cirrostratus, which may, in turn, thicken and lower into altostratus, stratus, and even nimbostratus. (NOAA)

Cirrocumulus clouds are layered clouds permeated with small cumuliform lumpiness. They also may line up in streets or rows of clouds across the sky denoting localized areas of ascent (cloud axes) and descent (cloud-free channels). (NOAA)

The bases of clouds in the middle level of the troposphere, given the prefix "alto-", appear between 6,500 and 20,000 feet. Depending on the altitude, time of year, and vertical temperature structure of the troposphere, these clouds may be composed of liquid water droplets, ice crystals, or a combination of the two, including supercooled droplets (i.e., liquid droplets whose temperatures are below freezing).  (NOAA)

The two main type of mid-level clouds are altostratus and altocumulus.

 Altostratus clouds are "strato" type clouds (see below) that possess a flat and uniform type texture in the mid levels. They frequently indicate the approach of a warm front and may thicken and lower into stratus, then nimbostratus resulting in rain or snow. However, altostratus clouds themselves do not produce significant precipitation at the surface, although sprinkles or occasionally light showers may occur from a thick alto-stratus deck. (NOAA)

Altocumulus clouds exhibit "cumulo" type characteristics (see below) in mid levels, i.e., heap-like clouds with convective elements.  Like cirrocumulus, altocumulus may align in rows or streets of clouds, with cloud axes indicating localized areas of ascending, moist air, and clear zones between rows suggesting locally descending, drier air.Altocumulus clouds with some vertical extent may denote the presence of elevated instability, especially in the morning, which could become boundary-layer based and be released into deep convection during the afternoon or evening.  (NOAA)



Low-level clouds are not given a prefix, although their names are derived from "strato-" or "cumulo-", depending on their characteristics. Low clouds occur below 6500 feet, and normally consist of liquid water droplets or even supercooled droplets, except during cold winter storms when ice crystals (and snow) comprise much of the clouds. (NOAA)

The two main types of low clouds include stratus, which develop horizontally, and cumulus, which develop vertically.

Stratus clouds are uniform and flat, producing a gray layer of cloud cover which may be precipitation-free or may cause periods of light precipitation or drizzle.  Low stratus decks are common in winter in the Ohio Valley, especially behind a storm system when cold, dismal, gray weather can linger for several hours or even a day or two. (NOAA)

Stratocumulus clouds are hybrids of layered stratus and cellular cumulus, i.e., individual cloud elements, characteristic of cumulo type clouds, clumped together in a continuous distribution, characteristic of strato type clouds. Stratocumulus also can be thought of as a layer of cloud clumps with thick and thin areas. These clouds appear frequently in the atmosphere, either ahead of or behind a frontal system. (NOAA)

Nimbostratus clouds are generally thick, dense stratus or stratocumulus clouds producing steady rain or snow . 

In contrast to layered, horizontal stratus, cumulus clouds are more cellular (individual) in nature, have flat bottoms and rounded tops, and grow vertically. In fact, their name depends on the degree of vertical development. For instance, scattered cumulus clouds showing little vertical growth on an otherwise sunny day used to be termed "cumulus humilis" or "fair weather cumulus," although normally they simply are referred to just as cumulus or flat cumulus. (NOAA)

cumulus cloud that exhibits significant vertical development (but is not yet a thunderstorm) is called cumulus congestus or towering cumulus. If enough atmospheric instability, moisture, and lift are present, then strong updrafts can develop in the cumulus cloud leading to a mature, deep cumulonimbus cloud, i.e., a thunderstorm producing heavy rain. In addition, cloud electrification occurs within cumulonimbus clouds due to many collisions between charged water droplet, graupel (ice-water mix), and ice crystal particles, resulting in lightning and thunder. (NOAA)

How does a cloud form?

Clouds form from water in the sky. The water may evaporate from the ground or move from other areas. Water vapor is always in the sky in some amount but is invisible. Clouds form when an area of air becomes cooler until the water vapor there condenses to liquid form. This is referred to as the dew point  At the dew point, the air is said to be "saturated" with water vapor. The air where the cloud forms must be cool enough for the water vapor to condense. The water will condense around things like dust, ice or sea salt - all known as condensation nuclei. The temperature, wind and other conditions where a cloud forms determine what type of cloud it will be.  (NASA) 



What are the different types of precipitation?

The four types of precipitation are rain, sleet, snow, and hail.  Depending on the temperature of the air, precipitation may fall as a liquid or as a solid.



How does wind form?  

Differences in atmospheric pressure generate winds. At the Equator, the sun warms the water and land more than it does the rest of the globe. Warm equatorial air rises higher into the atmosphere and migrates toward the poles. This is a low-pressure system. At the same time, cooler, denser air moves over Earth’s surface toward the Equator to replace the heated air. This is a high-pressure system. Winds generally blow from high-pressure areas to low-pressure areas. (National Geographic Education) 




What are the types of fronts?

Warm, cold, stationary, and occluded are the types of fronts. Fronts form when large masses of warm and cold air meet. Weather often changes as the front passes through, depending on the type of front.

A front is called a cold front if the cold air mass is replacing the warm air mass. The air behind a cold front is colder and typically drier than the air ahead of it, which is generally warm and moist. There is typically a shift in wind direction as the front passes, along with a change in pressure tendency (pressure falls prior to the front arriving and rises after it passes). Cold fronts have a steep slope, which causes air to be forced upward along its leading edge. This is why there is sometimes a band of showers and/or thunderstorms that line up along the leading edge of the cold front. Cold fronts are represented on a weather map by a solid blue line with triangles pointing in the direction of its movement.  (NOAA)



A warm front occurs when a cold air mass is receding (i.e. a warm air mass is replacing a cold air mass). The air behind a warm front is warm and moist, while the air ahead of a warm front is cooler and less moist. Similar to the cold front, there will a shift in wind direction as the front passes and a change in pressure tendency. Warm fronts have a more gentle slope than cold fronts, which often leads to a gradual rise of air. This gradual rise of air favors the development of widespread, continuous precipitation, which often occurs along and ahead of the front. Warm fronts are represented on a weather map by a solid red line with semi-circles pointing in the direction of its movement.  (NOAA)

A stationary front is a front that is not moving. Although the frontal boundary does not move, the air masses may move parallel to the boundary. Stationary fronts can also produce significant weather and are often tied to flooding events. Stationary fronts are represented on a weather map by alternating red and blue lines, with blue triangles and red semi-circles facing opposite directions.  (NOAA)

Generally, cold fronts move faster than warm fronts. Sometimes in a storm system the cold front will "catch up" to the warm front. An occluded front forms as the cold air behind the cold front meets the cold air ahead of the warm front. Which ever air mass is the coldest undercuts the other. The boundary between the two cold air masses is called an occluded front. Occluded fronts are represented on weather maps by a solid purple line with alternating triangles and semi-circles, pointing in the direction of its movement. (NOAA)


What is dew?  How did it get there?

Because of daylight heating, it is generally warmer during the day than it is at night.  During the day when it's warm, water evaporates into the air and the amount of water the air can hold depends on how warm the temperature is.  At night as the temperature cools down, the moisture that evaporated during the day is still in the air, but the air is no longer warm enough to be able to hold as much moisture as it could during the warmer day.  As a result, the water begins to condense on the cooling ground.  In other words it comes out of the air forming dew.  When the temperature warms up again in the morning the air can contain the water again and the water evaporates back into the air.


What are the different forms of heat transfer?

The different forms of heat transfer are radiation, conduction, and convection.

Radiation - This type of heat transfer can be observed on sunny days. You face will feel warm when you are standing in the sun. The sunlight is absorbed by your face and warms your face, without warming the air around you. The energy from the sun that is absorbed by your face is called radiant energy or radiation. Radiation is the transfer of this heat energy by electromagnetic waves. (NWS/NOAA)

Conduction - Conduction is the transfer of heat from one molecule to another within a substance. Imagine you are holding a metal pin between your fingers and you place this pin in a flame. The pin absorbs the energy from the flame and the molecules inside the pin begin to move faster (warmer temperature). These faster moving molecules cause adjoining molecules to move faster and will eventually cause the molecules in your fingers to move faster. The heat is now being transferred from the pin to your finger and your finger will heat up. This is an example of heat transfer through conduction. When heat is transferred through conduction, it flows from warmer to colder regions and will transfer more rapidly with greater temperature differences. (NWS/NOAA)

Convection - Convection is the transfer of heat through the movement of a fluid, such as water or air. This type of heat transfer can occur in liquids and gases because they move freely, making it possible to set up warm or cold currents. Convection occurs naturally in the atmosphere on a warm, sunny day. As the earth's surface absorbs sunlight, certain portions of the surface absorb more than other portions. The earth's surface and the air near the surface heats unevenly. The warmest air expands, becomes less dense than the surrounding cooler air, becomes buoyant and rises. These rising "bubbles" of warm air, called thermals, act to transfer heat up into the atmosphere. Cooler, heavier air then flows toward the surface to replace the warm air that just rose. When the cooler air reaches the surface, it is warmed and it too eventually rises as a thermal. (NWS/NOAA)


Severe Weather

What causes thunder and lightning?  

NASA - Scijinks

NASA - Scijinks

Thunder is caused by lightning, which is essentially an electrical current flowing between or within clouds, or between a cloud and the ground. The air surrounding the electron stream is heated to as hot as 50,000 degrees Farhenheit, which is three times hotter than the surface of the sun.   

As the superheated air cools it produces a resonating tube of partial vacuum surrounding the lightning's path. The nearby air rapidly expands and contracts. 

This causes the column to vibrate like a tubular drum head and produces a tremendous crack. As the vibrations gradually die out, the sound echoes and reverberates, generating the rumbling we call thunder. We can hear the thundering booms 10 miles or more distant from the lightning that caused it. (Scientific American)

Lightning is an electric current. To make this electric current, first you need a cloud.  When the ground is hot, it heats the air above it. This warm air rises. As the air rises, water vapour cools and forms a cloud. When air continues to rise, the cloud gets bigger and bigger. In the tops of the clouds, temperature is below freezing and the water vapour turns into ice.  Now, the cloud becomes a thundercloud. Lots of small bits of ice bump into each other as they move around. All these collisions cause a build-up of electrical charge.  Eventually, the whole cloud fills up with electrical charges. Lighter, positively charged particles form at the top of the cloud. Heavier, negatively charged particles sink to the bottom of the cloud.  When the positive and negative charges grow large enough, a giant spark - lightning - occurs between the two charges within the cloud. This is like a static electricity sparks you see, but much bigger.Most lightning happens inside a cloud, but sometimes it happens between the cloud and the ground.  A build up of positive charge builds up on the ground beneath the cloud, attracted to the negative charge in the bottom of the cloud. The ground's positive charge concentrates around anything that sticks up - trees, lightning conductors, even people! The positive charge from the ground connects with the negative charge from the clouds and a spark of lightning strikes.  (Planet Science.com)



How are tornadoes classified?

Currently, tornadoes are classified using the Enhanced Fujita Scale.  The scale classifies tornadoes based on  the type and severity of damage the tornado produces.   Because of their short lives and violent winds, tornadoes are difficult to study when they are on the ground.  Because of this, scientists have to study the damage left behind.



How often do tornadoes strike?  

Georgia’s average number of tornadoes in a year is 6.


Can a tornado destroy a whole city?

The largest width recorded for a tornado was 2.6 miles.  While this would definitely damage or even destroy a small town, it would not destroy a large metropolitan city. 

How are hurricanes classified?

Unlike tornadoes, exact, immediate data about the hurricane can be gathered and monitored. The scale used to classify hurricanes is referred to as the Saffir-Simpson Scale.  This scale focuses on the intensity, especially wind speed.  The information also alerts residents on the possibility of a strong storm surge.  



Which is the most dangerous - tornadoes or hurricanes?  Why?

Both weather phenomena are dangerous.  

Tornadoes occur very suddenly.  Deaths usually occur because people had no idea that the tornado has touched down and is heading their way.  Advances in technology have increased the amount of time to proceed to safety.  The average time for people to take shelter, also known as lead time, is 13 to 14 minutes.  This is almost three times as long as the lead time in the 1980’s, which was about 5 minutes.

In 2011, there was a tornado outbreak, with a total of 358 tornadoes in the span of 4 days (April 25-28).  The number of fatalities exceeded 550.  At least four EF5’s were identified from the tornado outbreak.  The outbreak caused approximately $11 billion dollars.

Hurricanes, unlike tornadoes, take days to weeks to form and move.  This makes them easier to track.  However, hurricanes can be massive, several miles across.  The danger from the hurricane is not only in its wind, but also in the storm surge.  Storm surges are often the greatest threat from a hurricane. It is primarily caused by extremely high winds as those winds push the ocean water on shore. As a result a huge wave is formed.  (CEMA)(8)

In 2005, Hurricane Katrina moved into the Gulf of Mexico.  From August 23 to August 31, Katrina grew into a Category 5 hurricane with 170 mph winds, and then made landfall as a Category 3, with 125 mph.  The death toll was over 1300, mainly due to the storm surge.  Damage exceeded $81 billion dollars. 

GOES time-lapse animation of Hurricane Katrina Mississippi on Aug. 29, 2005.

What is the difference between a severe weather watch and a severe weather warning?

A severe weather watch, or any other watch, indicates the atmospheric conditions are present for the severe weather to occur.  People in the area should be aware of the conditions, and plan accordingly.

A severe weather warning, or any other warning, indicates the the severe weather is occurring at the present time.  People in the area should immediately take shelter and listen for updates.


Weather Instruments and Forecasting





What are all the lines on a weather map?

There are several types of lines found on a weather map.

An isotherm is a line on a map connecting points having the same temperature at a given time or on average over a given period.

An isobar is a line on a map connecting points having the same atmospheric pressure at a given time or on average over a given period.





Why is there a H or a L on a weather map?  What does it mean?

The letters are the symbols for the different types of pressure systems found in an area.  H designates a high pressure area, whereas the L represents a low pressure area.

What does an anemometer do?

An anemometer is an instrument that measures wind speed.

What does a thermometer do?

A thermometer is an instrument that measures temperature.

What does a barometer do?

A barometer is an instrument for determining the pressure of the atmosphere.


What does a psychrometer do?

A psychrometer is used to measure humidity. It has two bulbs in order to function properly - one is wet and the other is dry.  The temperature differences between the two are noted to determine current humidity conditions.


What does a wind vane do?

A wind vane identifies the direction the wind is moving.


What does a meteorologist do?

Meteorologists compare temperature readings, winds, atmospheric pressure, precipitation patterns, and other variables to form an accurate picture of our climate. From past readings, meteorologists are able to draw conclusions and make predictions about how our climate will translate into local weather every day. They can also develop computer models that predict how climate and weather may vary in the future as a result of human activity. Meteorologists also carry out basic research to help us understand the way the atmosphere works, ranging from why hurricanes and tornadoes form when and where they do, to why the ozone hole formed over the Antarctic in the spring. They use satellites, aircraft, ships, and balloons to take the data needed to help understand, document, and predict weather and climate. (NASA.gov) (9)

Markina Brown -  CBS Atlanta

Markina Brown -  CBS Atlanta

(Interested in becoming a meteorologist?  Check out the "Interested in a Career?" link on the Menu Board.)  


Climate and Seasons

What is climate?

Climate refers to the "average" weather conditions for an area over a long period of time (e.g., the average high temperature for today's date).


Why are there seasons?

Seasons occur on the Earth because of the tilt of the Earth's axis, which is tilted approximately 23.5 degrees. Because the Earth's spin is not exactly aligned with the Earth's orbit around the sun, the Sun sometimes shines directly over the northern hemisphere, and other times shines directly over the southern hemisphere. For instance, the sun reaches its most northerly position (the tropic of cancer) on June 21. This day is called the "summer solstice." If you were standing on the tropic of cancer on this day, the sun would be directly overhead at noon. June 21 is the first day of summer in the northern hemisphere, and the first day of winter in the southern hemisphere. Because the sun is shining more directly in the north than in the south, the weather is warmer in the north in June. In December, the opposite happens. The sun is over the tropic of Capricorn, and is shining directly at the southern hemisphere. With less direct sunlight in the northern hemisphere, the temperature drops, bringing fall and winter.  So December is summer in Australia and Brazil, but winter in America, Europe, and Asia.


Interactive Seasons Simulator -  

Time Lapse Seasons Simulator -

Ignite Video


What provides the energy for weather?

The Sun is the source of the energy in weather.


What is a drought?

A drought is a prolonged period of abnormally low precipitation which leads to a shortage of water.


How long has Georgia had a drought?  

The most recent “severe” drought period in Georgia began in September 2010 following a summer of only 70% of normal rainfall over 2 months.  However, in late spring 2013, the “severe” drought designation was lifted for the majority of Georgia after a state average of almost 10 inches of rainfall. 


Meteorological Oddities

What does it mean when someone says that it is “muggy”?  

“Muggy” usually refers to an unpleasantly warm and humid atmosphere.  The word derived from the Scandinavian word “mugga” mist, which means “to drizzle”.


My grandparents’ joints ache when a storm is coming.  Why?  

When storms are rolling in, barometric pressure decreases.  This drop affects sensory nerves in joints, commonly hips, knees, elbows, shoulders and hands.  The nerves react with an aching sensation as the barometric pressure fluctuates.  



Why is the sky blue?

Despite popular misconceptions, the sky is not blue due to the reflection from the oceans.  The blue color of the sky is actually due to light traveling through the sky.  Much like a prism, the molecules of the sky bend or stop some colors of the light spectrum.  However, the “blue” light travels through, making the sky blue.