Chapter 2 Section 3

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. 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 Local Winds

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… • Moist and cold • Continental Polar… • Dry and Cold • Maritime Tropical… • Moist and warm • Continental Tropical… • Dry and warm

23. Air Masses and Their Origin

25. North American Air Masses 1 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