1. Chapter 2 Section 3
Winds

2. What are winds?
A wind is the horizontal movement of air from an area of high pressure to an area of lower pressure
Winds are caused by differences in air pressure

3. Convection Currents
Form when an area of Earth’s surface is heated by the Sun’s rays
Air over the heated surface expands and becomes less dense. As the air becomes less dense, its air pressure decreases

4. Convection Currents
Cool dense air with a higher pressure flows underneath the warm, less dense air= warm air rising

5. How do we measure wind speed?
Winds are described by their direction and speed
Anemometers are used to measure wind speed

6. Wind Direction
The name of a wind tells you where the wind is coming from
For example: A west wind comes from the west and blows toward the east
Wind direction is determined with a wind vane
The wind swings the wind vane so that one end points into the wind

7. Wind Direction Represented on a Map
Wind direction is shown by a line leading into a circle
The example below shows that the wind blew in from the east

8. Wind Speed Represented on a Map
The wind speed is shown by the “feathers” on the wind direction line
One “feather” =10 knots
Half of a “feather”=5 knots
A circle within a circle= calm, no wind
A flag=50 knots
(1 knot=1.15mph)
(feathers are also sometimes called “barbs”)

9. Weather Symbol Examples
Example #1: What is the wind speed and direction?
Wind coming from the east at a speed of 85 knots

10. Pressure and Wind
Isobars are lines joining places on a weather map that have the same air pressure
Meteorologists use isobars on weather maps to observe atmospheric pressure changes over an area and to make predictions about wind speed
Remember-Wind is a result of air pressure differences and wind blows from high to low pressure
The closer the isobars are= the faster the wind speed
The further apart the isobars are= the slower the wind speed

11. Wind Chill Factor Wind blowing over your skin removes body heat
The stronger the wind, the more friction is created from the air molecules hitting your body
This creates heat energy which is absorbed by the water near the surface of your skin, causing it to evaporate
This evaporation removes heat from your body and makes you feel colder

12. Local Winds Local winds are winds that blow over short distances
Local winds are caused by the unequal heating of Earth’s surface within a small area
Unequal heating often occurs along the shore of a large body of water
It takes more energy to warm up a body of water than it does to warm up an equal area of land (water has a high specific heat)
During the day the land warms up faster than the water (unequal heating)
At night, the land cools faster than the water (unequal heating)

13. Local Winds
Land Breeze –Local wind that blows from the land to a body of water during the night
Sea Breeze -Local wind that blows from an ocean or lake to the land during the day

14. Global Winds
Global winds are winds that blow steadily from specific directions over long distances
Global winds are created by the unequal heating of Earth’s surface
Warm air rises at the Equator and cold air sinks at the poles

16. Jet Streams
About 10 km above Earth’s surface are bands of high-speed winds called jet streams
Meteorologists use the location of some of the jet streams as an aid in weather forecasting
Jet streams generally blow west to east
Jet streams form at the boundaries of adjacent air masses with significant differences in temperature (polar region vs. Equator)
Jet streams have a meandering shape

17. Jet Stream Example

19. Chapter 3 Section 1 Weather

20. Air Masses
A huge body of air that has similar temperature (warm or cold), humidity (dry or wet), and air pressure (high or low) at any given height is called an air mass
Four major types of air masses influence the weather in North America:
Maritime tropical
Continental tropical
Maritime polar
Continental polar

21. Types of Air Masses Continental: land (dry) Mar: sea (wet)
Tropical: warm/hot
Polar: cold
The area that the air mass forms over will determine its characteristics
For example:
If the source region is an ocean, the air mass will have a lot of moisture
If the source region is land, the air mass will be drier

22. Putting it together Maritime Polar… Continental Polar…
Moist and cold
Continental Polar…
Dry and Cold
Maritime Tropical…
Moist and warm
Continental Tropical…
Dry and warm

23. Air Masses and Their Origin
Maritime Tropical
Gulf of Mexico and Atlantic and Pacific Ocean
Continental Tropical
Southwest and Northern Mexico
Maritime Polar
North Pacific and North Atlantic
Continental Polar
Central and Northern Canada and Alaska

25. North American Air Masses1 6 2 4 5 3

26. How Air Masses Move
Prevailing Westerlies- push air masses west to east
Jet stream

27. Fronts The boundary where 2 unlike air masses meet is called a front
4 types of fronts
Cold
Warm
Stationary
Occluded
Storms and changeable weather often develop along fronts

28. Symbols for Fronts

29. Front Symbols and Direction
Front symbols point in the direction of movement
Example: In the picture below, the warm, cold, and occluded fronts are moving north and the stationary front is not moving in any direction

30. Front Symbols and Direction
Draw a cold front moving east:
Draw a warm front moving south:
Draw an occluded front moving northeast:

31. Cold Fronts
A cold front is when a fast-moving cold air mass takes over a warm air mass
Possible weather: Clouds with possible thunderstorms with heavy rains or snow; cooler weather on the way

32. Warm Fronts
A warm front is when a warm air mass overtakes a slow-moving cold air mass
Possible weather: Humid, light rain or snow for several days; warmer weather on the way

33. Stationary Fronts
A stationary front is when cold and warm air masses meet, but neither can move the other
Possible weather: The air masses remain stalled over an area and may bring many days of clouds and precipitation

34. Occluded Front
An occluded front is when a warm air mass is caught between two cooler air masses
Possible weather: Cloudy and rain or snow

35. Front Symbols on a Weather Map

36. Low Pressure and Weather
If you look at a weather map, you will see areas marked with a letter L
and the L stands for “low pressure”
Low pressure systems lead to cloudy conditions that often bring precipitation
Here’s how it works:
Areas of low pressure are caused by massive amounts of air rising from the ground into the atmosphere
As the air molecules rise, they take their mass with them
Air pressure is a force
Force = mass x acceleration
Less mass of air molecules left at ground level means low air pressure
As this air rises it takes whatever water vapor it is holding along with it
As altitude increases in the troposphere, the temperature decreases
Eventually the air mass reaches a temperature at or below the dew point and the water vapor condenses
Relative humidity rises because colder air cannot hold as much water vapor
This condensation forms clouds
If there is enough water vapor in the air, it might also bring precipitation

37. High Pressure and Weather
If you look at a weather map, you will see areas marked with a letter H
and the H stands for “high pressure”
High pressure systems lead to clear and calm weather
Here’s how it works:
Areas of high pressure are caused by massive amounts of air sinking from the upper troposphere down towards the ground
As the air molecules sink, they take their mass with them
Air pressure is a force
Force = mass x acceleration
More mass of air molecules coming to ground level means high air pressure
As this air sinks it takes whatever water vapor it is holding along with it
As altitude decreases in the troposphere, the temperature increases
The higher the temperature is, the farther from the dew point you get
Warmer air is less dense so it has more space to hold water vapor so relative humidity drops
This means that condensation does not occur so no clouds form
Because there are no clouds, there will be no precipitation

38. Weather Map Example

39. Weather Map Example