1. Why Do We Have Weather?

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?

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

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 be
Convection

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

6. After the atmosphere is warmed by radiation and conduction, the heat is transferred throughout the atmosphere by convection.
In general, air near the equator tends to rise and air near the poles tends to sink

7. Take a look at this!

8. Notice the band of clouds around the equator ?
This is the Inter-tropical convergence zone

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

10. 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!

11. 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?

12. 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?

13. 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!

14. 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

15. 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

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

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

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

19. 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

20. Warm Fronts A warm front is warm air displacing cool air diagram
These are usually slow moving

21. Cold Fronts Cold air advances into region of warm air
Clearing conditions after front passes
Usually approaches from W or NW

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

23. Occluded Fronts Cold front overtakes warm front
Often found close to the low pressure center