Why Do We Have Why Do We Have Weather?Weather?

WHY DO WE HAVE THE WHY DO WE HAVE THE WEATHER ?WEATHER ?

Just think about it!Just think about it! Why is there wind? Why does it blow Why is there wind? Why does it blow

from one direction one day and from one direction one day and another the next?another the next?

Why is it rainy one day and dry the Why is it rainy one day and dry the next?next?

How come it’s cold in the winter?How come it’s cold in the winter? How can we have hail in the summer?How can we have hail in the summer? What causes snow and freezing rain?What causes snow and freezing rain?

If we were to pick one term to help If we were to pick one term to help explain why we have weather, what explain why we have weather, what do you think would be a good word?do you think would be a good word?

You might pick heat or sun….but You might pick heat or sun….but another good choice would beanother good choice would be

ConvectionConvection

Convection is the transfer of Convection is the transfer of heat,heat,

usually in gases or liquids.usually in gases or liquids.

After the atmosphere is warmed by radiation After the atmosphere is warmed by radiation and conduction, the heat is transferred and conduction, the heat is transferred

throughout the atmosphere by convection.throughout the atmosphere by convection. Since warmed air has Since warmed air has

more space between more space between the molecules, it’s less the molecules, it’s less dense and risesdense and rises

Cooled air is more Cooled air is more dense and tends to sinkdense and tends to sink

In general, air near the In general, air near the equator tends to rise equator tends to rise and air near the poles and air near the poles tends to sinktends to sink

Take a look at this!Take a look at this!

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

equator?equator?

Did you figure it out?Did you figure it out?

Warm, moist air in the topics risesWarm, moist air in the topics rises Cold air can hold less moisture than Cold air can hold less moisture than

warm airwarm air As the moist air rises, it condenses and As the moist air rises, it condenses and

forms clouds!forms clouds!

More on this laterMore on this later

Hadley CellsHadley Cells

Caused by convection Caused by convection currents.currents.– Warm air near the Warm air near the

equator risesequator rises– Cold air near the poles Cold air near the poles

sinkssinks

Consequences of Rotation: the Coriolis Consequences of Rotation: the Coriolis EffectEffect

The Weather HighwaysThe Weather Highways

The rotation of the The rotation of the earth creates the earth creates the Coriolis effect.Coriolis effect.

The Coriolis effect The Coriolis effect causes the air and causes the air and water to be water to be deflected to the deflected to the West.West.

This creates global This creates global weather highways.weather highways.

The Prevailing WesterliesThe Prevailing Westerlies

Because of our latitude, Because of our latitude, most of our weather most of our weather comes from the westcomes from the west

Looking at the weather Looking at the weather map, what type of map, what type of weather might we weather might we expect?expect?

Combining Hadley Cells, the Coriolis Effect, and Prevailing Westerlies

gives you... Tradewinds:

Where do you think

they got their name?

Who were these winds

particularly important to?

Let’s break for a short reviewLet’s break for a short review

1.1. Transfer of heat in liquids or Transfer of heat in liquids or gases_____gases_____

2.2. _____ air is dense and tends to sink._____ air is dense and tends to sink.3.3. Cold air holds _____ moisture than Cold air holds _____ moisture than

warm airwarm air4.4. The Coriolis effect causes the air and The Coriolis effect causes the air and

water to be deflected to the _____ of water to be deflected to the _____ of the equatorthe equator

How did you do?How did you do?

1.1. CONVECTIONCONVECTION

2. 2. COLDCOLD

3. 3. LESSLESS

4. 4. RIGHTRIGHT

Now What?Now What?

Ok, so we know that the weather Ok, so we know that the weather moves around on these highways and moves around on these highways and that warm air rises and cold air sinks.that warm air rises and cold air sinks.

But why is it sunny one day, and rainy But why is it sunny one day, and rainy the next?the next?

Let’s take another look at the Let’s take another look at the weather mapweather map

Notice that there Notice that there are are HH’s and ’s and LL’s on ’s on the mapthe map

There are also blue There are also blue lines with spikes lines with spikes and red lines with and red lines with half circleshalf circles

Let’s take a closer Let’s take a closer look!look!

High Pressure AreasHigh Pressure Areas When cooler air When cooler air

sinks and is sinks and is warmed, the air can warmed, the air can hold more moisturehold more moisture

This usually means This usually means sunny and clear sunny and clear skies skies

Winds tend to move Winds tend to move clockwise around a clockwise around a highhigh

Low Pressure AreasLow Pressure Areas When warm air rises and is When warm air rises and is

cooled, the air can not hold cooled, the air can not hold as much moistureas much moisture

Often, these areas are Often, these areas are associated with associated with precipitation and stormy precipitation and stormy weatherweather

Tornadoes and hurricanes can Tornadoes and hurricanes can produce very low pressure produce very low pressure readings.readings.

Fronts and Air MassesFronts and Air Masses

An air mass is a large body of air An air mass is a large body of air whose temperature and moisture are whose temperature and moisture are

fairly similar at a given altitudefairly similar at a given altitude Fronts are boundaries separating Fronts are boundaries separating

different air massesdifferent air masses

There are four different air masses There are four different air masses that affect the United Statesthat affect the United States

The Air MassesThe Air Masses

cPcP( continental polar) : cold, dry stable( continental polar) : cold, dry stable cTcT( continental tropical) : hot, dry, ( continental tropical) : hot, dry,

stable air aloft, unstable at the surfacestable air aloft, unstable at the surface mPmP( maritime polar) : cool, moist, ( maritime polar) : cool, moist,

unstableunstable mTmT( maritime tropical) : warm, moist, ( maritime tropical) : warm, moist,

unstableunstable

This map shows the air mass This map shows the air mass source regions and there pathssource regions and there paths

Ok, now we see the difference Ok, now we see the difference in the air massesin the air masses

Let’s look at the different fronts and their impact on Let’s look at the different fronts and their impact on weatherweather

Can you see the four different types of fronts on the Can you see the four different types of fronts on the map?map?

Cold Fronts

Cold air pushes under a warm air mass. Warm air rises quickly=narrow bands of violent storms form

This is the symbol for a cold front

Warm Front

Warm air slides over departing cold air- large bands of precipitation form

This is the symbol on a map for a warm front

Occluded Front

2 air masses merge and force warm air between them to rise quickly. Strong winds and heavy precipitation will occur

This is the weather map symbol for an occluded front

Stationary Front

Warm or cold front stops moving. Light wind and precipitation may occur across the front boundary

This is the weather map symbol for a stationary front

Ready for a little quiz?Ready for a little quiz?You’ll need a sheet of paperYou’ll need a sheet of paper

Write your answers as we Write your answers as we gogo

Here we go!Here we go!

1.1. Winds in a low Winds in a low

pressure pressure system move system move _____ around _____ around the lowthe low LL

2.2. What type of What type of

front can be front can be found close to found close to point point D D ??

3.3. Which of these Which of these

fronts would fronts would you expect to you expect to have greater have greater precipitation, precipitation, but be short but be short lived as the lived as the front passes?front passes?

4.4. Give the name Give the name

of the air mass of the air mass that would that would have the have the following following characteristics:characteristics:

cool, moist, cool, moist, unstableunstable

5.5.That important weather word That important weather word that refers to the transfer of that refers to the transfer of

heatheat

6.6.In general, air near the equator In general, air near the equator

tends to_____ ( rise or fall )tends to_____ ( rise or fall )

7.7.

It causes air and water to be It causes air and water to be deflected to the right north deflected to the right north

of the equatorof the equator

8.8.Which of the weather highways Which of the weather highways

usuallyusuallycontrols our weather controls our weather

9.9.

Warm air holds ( more or less ) Warm air holds ( more or less ) moisture than cold airmoisture than cold air

10.10. If there is a bigIf there is a big

HH on the on the weather map weather map

where you live, where you live, would you would you

expect fair or expect fair or stormy weatherstormy weather

HH

How did you do?How did you do?

Let’s check the answers!Let’s check the answers!

AnswersAnswers

1. 1. Counterclockwise Counterclockwise 8. 8. WesterliesWesterlies

2. 2. Cold Cold 9. 9. MoreMore

3. 3. Cold Cold 10. 10. FairFair

4. 4. Maritime polar (mP)Maritime polar (mP)

5. 5. ConvectionConvection

6. 6. RiseRise

7. 7. CoriolisCoriolis