Ch. 2 Weather

1. Ch. 2 Weather Section 1 What is Weather? Section 2 Weather Patterns Section 3 Weather Forecasting

2. Section 1 What is Weather? • Refers to the state of the atmosphere at a specific time and place

3. Weather Factors • Describes conditions such as air pressure, wind, temperature, and humidity, or moisture in the air. • The Sun provides almost all of Earth’s energy. • Drives the Water Cycle • Causes wind to occur

4. Temperature Review • Temperature is a measure of air molecule movement • High temp. = Warm = air molecules move rapidly • Low temp. = Cold = air molecules move slowly

5. Wind • Movement of air in a specific direction = from regions of higher pressure to regions of lower pressure • Cool air is denser so higher pressure • Warm air is less dense so lower pressure

7. Direction can be determined using a: • Wind Vane - has an arrow that points in the direction from which wind is blowing • Wind Sock - has one open end that catches the wind, causing sock to point in the direction toward which wind is blowing

8. Wind speed can be measured using an: • Anemometer - has rotating cups that spin faster when the wind is strong

9. Humidity • The amount of water vapor present in the air • Warm air can hold more water vapor than cool air. • Water vapor condenses and sticks together when cooled • Expressed as g/m3

10. Relative Humidity • A measure of the amount of water vapor present in air compared to the amount needed for saturation at a specific temp. • Expressed as a %

11. Saturation • When relative humidity is 100% • Or, in other words, when air is holding the maximum amount of water vapor it can hold at a certain temp.

12. Relative Humidity & Saturation • Air at 25°C is saturated when it contains 22 g of water vapor per cubic meter of air. • The relative humidity is 100 percent.

13. Relative Humidity & Saturation • If air at 25°C contains 11 g of water vapor per cubic meter, the relative humidity is 50 percent.

14. Dew Point • The temperature at which air is saturated and dew forms • The dew point changes with the amount of vapor present in the air. • When temperature drops, less water vapor can be present in the air. • The vapor will condense into a liquid or ice crystals

15. Because land cools so quickly, air in contact with the cold ground can reach its dew point, causing dew to form on the ground. • If the temp. is near 0 ˚C, frost will form. • Condensation forms on the outside of a glass because the cold glass brings the air next to it down to its dew pt.

16. Forming Clouds • Clouds form when warm, moist air is forced upward, expands, and cools to its dew point • Water vapor condenses around tiny particles called condensation nuclei. • Water droplets are so small that they stay suspended. Billions of these droplets form a cloud.

17. Ways in which air lifts and cools: • Rays from the Sun heat the ground and the air next to it making it buoyant. The warm air rises and cools. • As moist air moves over mountains, it is forced upward and cools. • When cool air meets warm, moist air along frontal boundaries the warm air rises and cools.

19. Classifying Clouds • Classified mainly by shape and height • Shapes: • Stratus - form layers, or smooth even sheets in the sky • Fog - stratus clouds that form near the ground • Cumulus - puffy, white clouds, often with flat bases • Cirrus - thin, white feathery clouds made of ice crystals

20. Height: (prefixes) • Cirro - high clouds • Alto - middle-elevation clouds • Strato - low clouds

21. Rain- or Snow-Producing Clouds • Have the word nimbus or nimbo attached to them • Sunlight usually cannot penetrate through • Ex: cumulonimbus, nimbostratus

22. Precipitation • Water falling from clouds • 4 main types: • Rain • Snow • Sleet • Hail • Form is determined by air temperature

23. Rain • When the air is warm, water vapor forms raindrops that fall as rain • Size depends on: • Strength of updraft • Stronger = larger drops • Rate of evaporation • If the air is dry, size can be reduced by evaporation before reaching the ground

24. Snow • Forms when the air temp. is so cold that water vapor changes directly to a solid

25. Sleet • Forms when raindrops pass through a layer of freezing air near Earth’s surface

26. Hail • Forms in cumulonimbus clouds when water vapor freezes in layers around a small nucleus of ice • The stronger the updraft, the larger the hailstone • Most hailstones are < 2.5 cm, but some can be as large as a softball • Causes the most damage of all forms of precipitation

27. Question 1 The state of the atmosphere at a specific time and place is __________. A. climate B. meteorology C. season D. weather

28. Question 2 The amount of water vapor present in the air is __________. A. dew B. condensation C. fog D. humidity

29. Question 3 What is the dew point?

30. Section 2 Weather Patterns

31. Air Mass • A large body of air that has properties similar to the part of Earth’s surface over which it developed • Can cover thousands of square km • Daily changes in Wx are due to the movement of air masses.

32. 6 air masses of 4 diff. types affect the US: • Maritime Polar (mP) - NW/NE • cool moist • Continental Polar (cP) - N • cold dry • Maritime Tropical (mT) - SW/SE • warm moist • Continental Tropical (cT) - S • hot dry

34. Highs and Lows • Air pressure is measured using a barometer. • Remember: Winds blow from high to low pressure areas.

35. Highs • Winds blow away from a center of high pressure • Earth’s rotation causes the wind to swirl in a CW dir. • Called anticyclones • Sinking motion in highs makes it difficult for air to rise and clouds to form • High pressure usually signals good Wx.

36. Lows • Winds blow toward a center of low pressure • Earth’s rotation causes the wind to swirl in a CCW dir. • Called cyclones • Air rises in a low causing clouds to form. • Lows are usually associated w/ cloudy and/or rainy Wx.

37. Front • Boundary b/w two air masses of diff. density, moisture, or temp. • Cloudiness, ppt, and storms sometimes occur at frontal boundaries as air is lifted • 4 types: • Cold, Warm, Occluded, Stationary • Need to know symbols!

38. Cold Front • Occurs when colder air advances toward warm air • Cold air wedges under warm air like a plow with a steep front causing air to lift and cool rapidly • Generally produces heavy ppt that starts and stops abruptly • Thunderstorms common; tornadoes may form

40. Warm Front • Occurs when lighter, warmer air advances over heavier, colder air • Usually forms a boundary with a gentle slope causing air to lift and cool gradually • Generally produces steady ppt that lasts for hours or days

42. Occluded Front • Occurs when a cold air mass moves toward cool air w/ warm air b/w the two • Colder air forces warm air upward closing it off from the surface

44. Stationary Front • Occur when a boundary b/w air masses stops advancing • May remain in place for many days producing light wind and ppt

46. Severe Weather • Poses danger to people, structures, or animals • Includes: • Thunderstorms • Tornadoes • Hurricanes • Blizzards

47. Thunderstorms • Form in warm, moist air masses and along fronts whenever air lifts and cools rapidly forming cumulonimbus clouds • Falling raindrops cool the surrounding air. Cool, dense air sinks forming downdrafts.

48. Extremely powerful storms can produce large hailstones. • Damage: • Flash flooding • Strong winds (>89 km/h = severe thunderstorm) • Hail • Strong downdrafts • Lightning • Tornadoes

49. Lightning • Occurs when rapid movement of air w/in a storm cloud causes diff. parts of the cloud to become oppositely charged • Lightning is the flow of current b/w regions of op. elec. charge.

50. Can occur w/in a cloud, b/w clouds, or b/w a cloud and the ground • Can reach temp. of about 30,000 ˚C