Why Do We Have Weather?

An Introduction What is weather? Weather Patterns Weather Forecasts

WHY DO WE HAVE THE WEATHER ?  Just think about it!  Why is there wind? Why does it blow from one direction one day and another the next?  Why is it rainy one day and dry the next?  How come it’s cold in the winter?  How can we have hail in the summer?  What causes snow and freezing rain?

Let’s take a look at the weather picture and why we have weather!

What is Weather?  State of the atmosphere at a specific time and place  Includes such conditions as air pressure, wind, temperature, and moisture in the air  Temperature is a measure of air molecule movement –Sun’s energy causes air molecules to move rapidly; temperatures are high and it feels warm –When less of the Sun’s energy reaches air molecules, they move less rapidly and it feels cold

Wind  Air moving in a specific direction  As the sun heats air, it expands, becomes less dense, rises, and has low atmospheric pressure  Cooler air is denser and sinks, causing high atmospheric pressure  Air moves from high pressure areas to low pressure areas, causing wind

Humidity  The amount of water vapor in the air  Warmer air can hold more water vapor, tending to make it more humid  Relative humidity – the amount of water vapor in the air compared to what it can hold at a specific temperature  When air cools, it can’t hold as much water vapor, so the water vapor condenses to liquid or forms ice crystals  Dew point – the temperature at which air is saturated and condensation forms

If we were to pick one term to help explain why we have weather, what do you think would be a good word? You might pick heat or sun….but another good choice would be Convection

Convection is the transfer of heat, usually in gases or liquids.

After the atmosphere is warmed by radiation and conduction, the heat is transferred throughout the atmosphere by convection.  Since warmed air has more space between the molecules, it’s less dense and rises  Cooled air is more dense and tends to sink  In general, air near the equator tends to rise and air near the poles tends to sink

Take a look at this!

Notice the band of clouds around the equator ? This is the ITCZ or inter tropical convergence zone

Why do you think there is this band of clouds near the equator?

Did you figure it out?  Warm, moist air in the tropics rises  Cold air can hold less moisture than warm air  As the moist air rises, it condenses and forms clouds!

Clouds  Clouds form as warm air is forced upward and cools. Then water vapor condenses in tiny droplets that remain suspended in the air  Shape and height of clouds vary with temperature, pressure, and water vapor in atmosphere

Clouds  Shape –Stratus-smooth, even sheets or layers at low altitudes –Cumulus-puffy, white clouds, often with flat bases –Cirrus-high, thin, white feathery clouds made of ice crystals  Height –Cirro – high clouds –Alto – middle-elevation clouds –Strato – low clouds –Nimbus clouds are dark and so full of water that sunlight can’t penetrate them  Precipitation – water falling from clouds –When water droplets in clouds combine and grow large enough, precipitation falls to Earth –Air temperature determines whether the droplets form rain, snow, sleep, or hail

Cirrus

Cumulus

Stratus

Consequences of Rotation: the Coriolis effect

The Weather Highways  The rotation of the earth creates the Coriolis effect.  The Coriolis effect causes the air and water to be deflected to the right north of the equator.  This creates global weather highways

The Westerlies  Because of our latitude, most of our weather comes from the west  Looking at the weather map, what type of weather might we expect?  What type of weather might we expect in a few days?

Let’s break for a short review 1.Transfer of heat in liquids or gases_____ 2. _____ air is dense and tends to sink. 3. Band of clouds found around the equator______ 4. Cold air holds _____ moisture than warm air 5. The Coriolis effect causes the air and water to be deflected to the _____ of the equator

How did you do? 1. CONVECTION 2. COLD 3. ITCZ 4. LESS 5. RIGHT

Now What?  Ok, so we know that the weather moves around on these highways and that warm air rises and cold air sinks.  But why is it sunny one day, and rainy the next?

Let’s take another look at the weather map  Notice that there are H ’s and L ’s on the map  There are also blue lines with spikes and red lines with half circles  Let’s take a closer look!

High Pressure Areas  When cooler air sinks and is warmed, the air can hold more moisture  This usually means sunny skies  Winds tend to move clockwise around a high

Low Pressure Areas  When warm air rises and is cooled, the air can not hold as much moisture  Often, these areas are associated with precipitation and stormy weather  Winds tend to move counter clockwise around the low

So, if you see a big H on the weather map over the area you live, you can expect fair weather

When you see a big L in your area, there will probably be stormy weather

These highs and lows move or less along the jet stream and bring us our weather changes

Fronts and Air Masses  Because air and moisture move in the atmosphere, weather is constantly changing  Air pressure – measured by barometer  An air mass is a large body of air whose temperature and moisture are fairly similar at a given altitude; properties like the part of Earth’s surface over which it formed  Fronts are boundaries separating different air masses – clouds, precipitation, and storms occur at frontal boundaries  There are four different air masses that affect the United States

The Air Masses  cP( continental polar) : cold, dry stable  cT( continental tropical) : hot, dry, stable air aloft, unstable at the surface  mP( maritime polar) : cool, moist, unstable  mT( maritime tropical) : warm, moist, unstable

This map shows the air mass source regions and there paths

Ok, now we see the difference in the air masses  Let’s look at the different fronts and their impact on weather  Can you see the four different types of fronts on the map?

Warm Fronts  A warm front is warm air displacing cool air  Shallow leading edge warm air must “overrun” cold air  These are usually slow moving

Cold Fronts  Cold air advances into region of warm air  Intensity of precipitation greater, but short lived  Clearing conditions after front passes  Usually approaches from W or NW

Stationary Fronts  Surface positions of the front do not move  Often a region of clouds

Occluded Fronts  Cold front overtakes warm front  Involves three air masses of different temperatures  Often found close to the low pressure center close to the low pressure center

Ready for a little quiz?  Here we go!

1.  Winds in a low pressure system move _____ around the low

2.  What type of front can be found close to point D ?

3.  Which of these fronts would you expect to have greater precipitation, but be short lived as the front passes?

4.  Give the name of the air mass that would have the following characteristics:  cool, moist, unstable

5. That important weather word that refers to the transfer of heat

6. In general, air near the equator tend to_____ ( rise or fall )

7. It causes air and water to be deflected to the right north of the equator

8. Which of the weather highways usually controls our weather

9. Warm air holds ( more or less ) moisture than cold air

10.  If there is a big H on the weather map where you live, would you expect fair or stormy weather

How did you do? Let’s check the answers!

Answers 1. Counterclockwise 8. Westerlies 1. Counterclockwise 8. Westerlies 2. Cold 9. More 2. Cold 9. More 3. Cold 10. Fair 3. Cold 10. Fair 4. Maritime polar (mP) 4. Maritime polar (mP) 5. Convection 5. Convection 6. Rise 6. Rise 7. Coriolis 7. Coriolis

Severe Weather  Thunderstorms occur inside warm, moist air masses and at fronts –Warm, moist air is forced rapidly upward, wehre it cools and condenses –Strong updrafts of warm air and sinking, rain- cooled air cause strong winds

Lightning  Movement of air inside a storm cloud causes parts of the cloud to become oppositely charged  Current flows between the regions of opposite electrical charge, forming a lightning bolt  Thunder – lightning superheats the air, causing it to expand rapidly and then contract, forming sound waves

Tornado  Violent, whirling wind that moves in a narrow path over land

Hurricane  Large, swirling, low-pressure system that forms over tropical oceans

Blizzard  A winter storm with strong winds, cold temperatures, and low visibility, that lasts more than three hours  A winter storm with strong winds, cold temperatures, and low visibility, that lasts more than three hours

Severe Weather Safety  A National Weather Service WATCH means conditions are favorable for severe weather to develop  A warning means that severe weather conditions already exist  Meteorologists study and predict weather  National Weather Service makes weather maps

Reading a weather map  ISOBAR= connects areas of equal pressure BAR comes from BARometric pressure

Reading a weather map...  Isotherm: Connects areas of equal temperature; therm means temperature

Weather Station (not on the TV) Weather conditions at specific location Current Conditions!!