Why Do We Have Why Do We Have Weather? Weather?
Mar 27, 2015
Why Do We Have Why Do We Have Weather?Weather?
An IntroductionAn Introduction
Take good notes! There will be Take good notes! There will be a quiz at the end of this a quiz at the end of this
presentation.presentation.
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?
Let’s take a look at the weather Let’s take a look at the weather picture and why we have picture and why we have
weather!weather!
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!
Notice the band of clouds Notice the band of clouds around the equator ?around the equator ?
This is the This is the ITCZITCZ or inter or inter tropical convergence zonetropical convergence zone
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
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 deflected water to be deflected to the right north of the to the right north of the equator.equator.
This creates global This creates global weather highwaysweather highways
The WesterliesThe 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?
What type of weather What type of weather might we expect in a might we expect in a few days?few days?
Let’s break for a short reviewLet’s break for a short review
1.Transfer of heat in liquids or gases_____1.Transfer of heat in liquids or gases_____2. _____ air is dense and tends to sink.2. _____ air is dense and tends to sink.3. Band of clouds found around the 3. Band of clouds found around the
equator______equator______4. Cold air holds _____ moisture than 4. Cold air holds _____ moisture than
warm airwarm air5. The Coriolis effect causes the air and 5. The Coriolis effect causes the air and
water to be deflected to the _____ of the water to be deflected to the _____ of the equatorequator
How did you do?How did you do?
1.1. CONVECTIONCONVECTION
2. 2. COLDCOLD
3. 3. ITCZITCZ
4. 4. LESSLESS
5. 5. 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 skies sunny 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 When warm air rises
and is cooled, the air and is cooled, the air can not hold as much can not hold as much moisturemoisture
Often, these areas are Often, these areas are associated with associated with precipitation and precipitation and stormy weatherstormy weather
Winds tend to move Winds tend to move counter clockwise counter clockwise around the lowaround the low
So, if you see a big So, if you see a big HH on the on the weather map over the area you weather map over the area you
live, you can expect fair live, you can expect fair weatherweather
When you see a big When you see a big LL in your in your area, there will probably be area, there will probably be
stormy weatherstormy weather
These highs and lows move These highs and lows move or less along the jet stream or less along the jet stream and bring us our weather and bring us our weather
changeschanges
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?
Warm FrontsWarm Fronts
A warm front is A warm front is warm air displacing warm air displacing cool air diagramcool air diagram
Shallow leading Shallow leading edge warm air edge warm air must “overrun” must “overrun” cold aircold air
These are usually These are usually slow movingslow moving
Cold FrontsCold Fronts
Cold air advances Cold air advances into region of warm into region of warm airair
Intensity of Intensity of precipitation greater, precipitation greater, but short livedbut short lived
Clearing conditions Clearing conditions after front passesafter front passes
Usually approaches Usually approaches from W or NWfrom W or NW
Stationary FrontsStationary Fronts
Surface Surface positions of the positions of the front do not front do not movemove
Often a region Often a region of cloudsof clouds
Occluded FrontsOccluded Fronts
Cold front Cold front overtakes warm overtakes warm frontfront
Often found close Often found close to the low to the low pressure centerpressure center
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
tend to_____ ( rise or fall )tend 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