2 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?
4 You might pick heat or sun….but another good choice would be 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 beConvection
5 Convection is the transfer of heat, usually in gases or liquids.
6 Air masses Air masses have different temperatures They also have different amounts of moistureThe source region is the area over which a particular air mass formsIt usually gives the air mass its characteristics: temperature and humidity!
7 Air mass modificationWhen an air mass travels over land or water that has characteristics different from those of its source region, the air mass can acquire some of the characteristics of that land or water and undergo modification.
8 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 risesCooled air is more dense and tends to sinkIn general, air near the equator tends to rise and air near the poles tends to sink
9 Take a look at this! Notice the band of clouds around the equator? This is the ITCZ or intertropical convergence zone
10 Why do you think there is this band of clouds near the equator?
11 Did you figure it out? Warm, moist air in the topics rises Cold air can hold less moisture than warm airAs the moist air rises, it condenses and forms clouds!More on this later
13 The Weather HighwaysThe 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
14 The WesterliesBecause of our latitude, most of our weather comes from the westLooking at the weather map, what type of weather might we expect?What type of weather might we expect in a few days?
15 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 _____ if you are north of the equator
16 How did you do?1. CONVECTION2. COLD3. ITCZ4. LESS5. RIGHT
17 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?
18 Let’s take another look at the weather map Notice that there are H’s and L’s on the mapThere are also blue lines with spikes and red lines with half circlesLet’s take a closer look!
19 High Pressure AreasWhen cooler air sinks and is warmed, the air can hold more moistureThis usually means sunny skiesWinds tend to move clockwise around a high
20 Low Pressure AreasWhen warm air rises and is cooled, the air can not hold as much moistureOften, these areas are associated with precipitation and stormy weatherWinds tend to move counter clockwise around the low
21 So, if you see a big H on the weather map over the area you live, you can expect fair weather
22 When you see a big L in your area, there will probably be stormy weather
23 These highs and lows move more or less along the jet stream and bring us our weather changes
24 Fronts and Air MassesAn air mass is a large body of air whose temperature and moisture are fairly similar at a given altitudeFronts are boundaries separating different air massesThere are four different air masses that affect the United States
25 The Air Masses cP( continental polar) : cold, dry stable cT( continental tropical) : hot, dry, stable air aloft, unstable at the surfacemP( maritime polar) : cool, moist, unstablemT( maritime tropical) : warm, moist, unstable
26 This map shows the air mass source regions and there paths
27 Ok, now we see the difference in the air masses Let’s look at the different fronts and their impact on weatherCan you see the four different types of fronts on the map?
28 Warm Fronts A warm front is warm air displacing cool air diagram Shallow leading edge warm air must “overrun” cold airThese are usually slow moving
29 Cold Fronts Cold air advances into region of warm air Intensity of precipitation greater, but short livedClearing conditions after front passesUsually approaches from W or NW
30 Stationary FrontsSurface positions of the front do not moveOften a region of clouds
31 Occluded Fronts Cold front overtakes warm front Often found close to the low pressure center
32 Ready for a little quiz? You’ll need a sheet of paper Write your answers as we goHere we go!
33 1.Winds in a low pressure system move _____ around the low
34 2.What type of front can be found close to point D ?
35 3.Which of these fronts would you expect to have greater precipitation, but be short lived as the front passes?
36 4.Give the name of the air mass that would have the following characteristics:cool, moist, unstable
37 5. That important weather word that refers to the transfer of heat
38 6. In general, air near the equator tend to_____ ( rise or fall )
39 7.It causes air and water to be deflected to the right north of the equator
40 8. Which of the weather highways usually controls our weather
41 Warm air holds ( more or less ) moisture than cold air 9.Warm air holds ( more or less ) moisture than cold air
42 10.If there is a big H on the weather map where you live, would you expect fair or stormy weather
43 Let’s check the answers! How did you do?Let’s check the answers!