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?
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
Air masses • Air masses have different temperatures • They also have different amounts of moisture • The source region is the area over which a particular air mass forms • It usually gives the air mass its characteristics: temperature and humidity!
Air mass modification When 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.
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 intertropical convergence zone
Did you figure it out? • Warm, moist air in the topics rises • Cold air can hold less moisture than warm air • As the moist air rises, it condenses and forms clouds! More on this later
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 _____ if you are north 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’sand 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 Hon 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 more or less along the jet stream and bring us our weather changes
Fronts and Air Masses • An air mass is a large body of air whose temperature and moisture are fairly similar at a given altitude • Fronts are boundaries separating different air masses • 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
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 diagram • 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 • Often found close to the low pressure center
Ready for a little quiz?You’ll need a sheet of paper • Write your answers as we go • 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
7. It causes air and water to be deflected to the right north of the equator
9. Warm air holds ( more or less ) moisture than cold air
10. • If there is a bigH 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 2. Cold 9. More 3. Cold 10. Fair 4. Maritime polar (mP) 5. Convection 6. Rise 7. Coriolis