Weather Basics

What is weather?

Weather describes the conditions of the outside atmosphere.  For example, is it cold or warm outside? Cloudy or sunny?  Dry or raining?  Windy or not windy? 

What is the water cycle?  

The water cycle is the movement of water in and around the Earth.  The Sun is the power source.  The main steps of the water cycle are evaporation, condensation, and precipitation. Evaporation occurs when the Sun heats the water of Earth.  The heat causes the water to change into water vapor (gas state of water).  The water vapor travels into the atmosphere, becoming liquid water droplets due to the cold air there.  There, it combines with other water droplets and dust to form clouds.  This is the process of condensation.  Depending on the temperature of the atmosphere and other conditions, the water falls back down to Earth as rain, sleet, hail, or snow.  The water falling back down to Earth is also called precipitation.

• Water Cycle Information - http://ga.water.usgs.gov/edu/watercycle-kids-text.html

• Water Cycle Information - http://www.epa.gov/safewater/kids/flash/flash_watercycle.html

• Interactive Water Cycle Diagram -  http://earthguide.ucsd.edu/earthguide/diagrams/watercycle/

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) (2)

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 usually thin, wispy clouds that are made of ice.  These clouds are usually found high up in the sky.  

Cumulus clouds are usually low level clouds that are puffy, much like cotton balls.  These clouds are often referred to as “fair weather” clouds.  However, these clouds can grow into storm clouds, called cumulonimbus clouds.

Stratus clouds appear as smooth sheets, and can contain light rain or drizzle.

• Cloud Information and Games - http://eo.ucar.edu/webweather/cloud3.html

• Types of Clouds - http://www.teachersdomain.org/asset/ess05_int_cloudtype/

How does a cloud form?

All air contains water, but near the ground it is usually in the form of an invisible gas called water vapor. When warm air rises, it expands and cools.  Cool air can't hold as much water vapor as warm air, so some of the vapor condenses onto tiny pieces of dust that are floating in the air and forms a tiny droplet around each dust particle. When billions of these droplets come together they become a visible cloud. (Weather WizKids)

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.

• Types of Precipitation - http://www.eo.ucar.edu/basics/wx_2_b.html

• Types of Precipitation - http://www.srh.noaa.gov/jetstream/global/preciptypes.htm

• Interactive - http://profhorn.meteor.wisc.edu/wxwise/precip/precip.html  (Java Applet)

How does wind form?  

As the sun warms the Earth's surface, the atmosphere warms too. Some parts of the Earth receive direct rays from the sun all year and are always warm. Other places receive indirect rays, so the climate is colder. Warm air, which weighs less than cold air, rises. Then cool air moves in and replaces the rising warm air. This movement of air is what makes the wind blow. (Weather Wizkids) (3)

• Wind information - http://www.weatherwizkids.com/weather-wind.htm

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.

• Types of Fronts - http://www.phschool.com/atschool/phsciexp/

active_art/weather_fronts/

What is dew?  How did it get there?

On a clear day, water evaporates from the warm ground into the atmosphere. When night falls, the ground radiates the day's warmth into the skies. The ground becomes much cooler, causing the water vapour to condense. This condensed vapour is dew.  On a cloudy night, the clouds send the heat back to the ground so the ground never gets cold enough for the dew to be formed.  When the nights get colder, this dew gets transformed into frost.  (Pitara). 

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)(4)

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)(4)

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)(4)

Severe Weather

What causes thunder and lightning?

Thunder is caused by lightning. When a lightning bolt travels from the cloud to the ground it actually opens up a little hole in the air, called a channel. Once then light is gone the air collapses back in and creates a sound wave that we hear as thunder. The reason we see lightning before we hear thunder is because light travels faster than sound!  (Weather WizKids)(6)  

Lightning is an electric current. Within a thundercloud way up in the sky, many small bits of ice (frozen raindrops) bump into each other as they move around in the air. All of those collisions create an electric charge. After a while, the whole cloud fills up with electrical charges. The positive charges or protons form at the top of the cloud and the negative charges or electrons form at the bottom of the cloud. Since opposites attract, that causes a positive charge to build up on the ground beneath the cloud. The grounds electrical charge concentrates around anything that sticks up, such as mountains, people, or single trees. The charge coming up from these points eventually connects with a charge reaching down from the clouds and - zap - lightning strikes!  (Weather WizKids)(7)

• Lightning Experiment - http://www.planet-science.com/categories/under-11s/our-world/2011/05/what-is-lightning.aspx

• More thunder and lightning information!  http://scijinks.nasa.gov/lightning 

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. 

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. 

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)

(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.

Time Lapse Seasons Simulator -

• http://www.climate.gov/teaching/resources/time-lapse-season-demonstrator

Seasons Interactive Simulator -

• http://www.sepuplhs.org/students/iaes/simulations/SEPUP_Seasons_Interactive.swf

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.

• Drought Data Website - http://droughtmonitor.unl.edu/

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.

• http://spaceplace.nasa.gov/blue-sky/