Why Do We Have Weather?

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?

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

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

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!

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

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

Hadley Cells Caused by convection currents. Warm air near the equator rises Cold air near the poles sinks

Consequences of Rotation: the Coriolis Effect

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 West. This creates global weather highways.

The Prevailing 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?

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 review Transfer of heat in liquids or gases_____ _____ air is dense and tends to sink. Cold air holds _____ moisture than warm air The Coriolis effect causes the air and water to be deflected to the _____ of the equator

How did you do? 1. CONVECTION 2. COLD 3. LESS 4. 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’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!

High Pressure Areas When cooler air sinks and is warmed, the air can hold more moisture This usually means sunny and clear 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 Tornadoes and hurricanes can produce very low pressure readings.

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

This map shows the air mass source regions and there paths

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?

Cold Fronts This is the symbol for a cold front Cold air pushes under a warm air mass. Warm air rises quickly=narrow bands of violent storms form

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

This is the weather map symbol for an 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 a stationary front Warm or cold front stops moving. Light wind and precipitation may occur across the front boundary

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

5. That important weather word that refers to the transfer of heat

6. In general, air near the equator tends to_____ ( rise or fall )

7. It causes air and water to be deflected to the right north of the equator

8. Which of the weather highways usually controls our weather

Warm air holds ( more or less ) moisture than cold air 9. Warm air holds ( more or less ) moisture than cold air

10. If there is a big H on the weather map where you live, would you expect fair or stormy weather

Let’s check the answers! 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