Water in the Air Air Masses and Fronts Severe Weather Forecasting Weather

1. UNDERSTANDING WEATHER Water in the Air Air Masses and Fronts Severe Weather Forecasting Weather

2. VOCABULARY weather air mass water cycle front Humidity thunderstorm relative humidity lightning Condensation thunder dew point tornado cloud hurricane Precipitation

3. VOCABULARY thermometer barometer windsock wind vane anemometer isobars

4. Water in the Air • Although the Earth’s atmosphere contains only about 0.001 percent of the total volume of water on the planet (about 1.46 x 109 km3), it is an essential link between land masses and bodies of water on Earth. • The rate at which water evaporates into Earth’s atmosphere is about 5.1 x 1017 L per year. • About 78% of all precipitation falls over Earth’s oceans. Of the 22% that falls on land, about 65% returns to the air by evaporation. • Rainbows are evidence that air contains water. Water in the air affects weather and weather is the condition of the atmosphere at a particular time and place.

5. The Water Cycle The water cycle is the continuous movement of water from water sources, such as lakes and oceans, into the air, onto and over land, into the ground.

7. Physical Science Connection At the Earth’s surface and in the atmosphere, energy from the sun moves water thru the water cycle. Sunlight provides the energy for evaporation, transpiration, condensation and precipitation.

8. Life Science Connection When air is humid, hair becomes frizzy. Hair is made of a protein called keratin. Each hair has a scaly outer cuticle, which allow moisture to enter the inner part of the hair fiber. Humidity can cause hair length to change as much as 2.5%.

9. Condensation Condensation occurs when water vapor cools and changes back into liquid droplets. This is how clouds form. It is the process by which a gas, such as water vapor, becomes a liquid. Before condensation can occur the air must be saturated.

10. Evaporation Evaporation occurs when liquid water changed into a water vapor, which is a gas.

12. Transpiration Transpiration is the process by which plants release water vapor into the air through their leaves.

13. Precipitation Precipitation occurs when rain, snow, sleet, or hail falls from the clouds onto the Earth’s surface.

14. 4 Types of Precipitation • Rain ( measured by a rain gauge) • Snow • Sleet • Hail

15. Runoff Runoff is water, usually from precipitation, that flows across land and collects in rivers, streams, and eventually the ocean.

16. Humidity Humidity is the amount of water vapor or moisture in the air. It is the moisture in the air that makes for bad hair days! Air’s ability to hold water vapor depends on the temperature. As temperature increases, the air’s ability to hold water also increases. Page 423, figure 3 shows the relationship between air temperature and air’s ability to hold water.

17. Relative Humidity Relative humidity is the amount of moisture the air contains compared with the maximum amount it can hold. When air holds all the water it can at a given temperature, the air is said to be saturated. Saturated air has a relative humidity of 100%. A psychrometer measures relative humidity.

18. Math Concept Relative humidity is the amount of moisture the air is holding compared with the amount it can hold at a particular temperature. The relative humidity of air that is holding all the water it can at a given temperature is 100%, meaning it is saturated. You can calculate it with the following equation: (present) g/m3 x 100 = relative humidity (saturated) g/m3

19. How does Humidity relate to the water cycle? Evaporation occurs when liquid water changes into water vapor and returns to the air. Humidity is the amount of water vapor in the air.

20. Dew Point Dew point is the temperature to which air must cool to be completely saturated. It must also have a surface to condense on. Have you ever noticed the dew on the grass?

21. Clouds A cloud is a collection of millions of tiny water droplets or ice crystals. They form as warm air rises and cools becoming saturated. At saturation the water vapor becomes liquid.

22. Cumulus Clouds

23. Cumulus Clouds Cumulus clouds form when warm air rises. They tend to have flat bottoms are white and puffy

24. Cumulus Clouds Larger cumulus clouds form thunderstorms. A cumulus cloud that produces thunderstorms is called a cumulonimbus cloud. When –nimbus or nimbo- is used as part of a clouds name, it means that it might rain.

25. Stratus Clouds Stratus clouds form in layers and cover large areas. These clouds are formed by a gentle lifting of warm air into the atmosphere.

26. Cirrus Clouds Cirrus clouds are thin, feathery white clouds found at high altitudes and form when the wind is strong. They may indicate approaching bad weather if they thicken and lower in altitude.

27. Cirrus Clouds

30. Cloud Types Based on Form and Altitude • High Clouds- made of ice crystals and the prefix cirro- is used to describe high clouds. • Middle Clouds- can be made of both water droplets and ice crystals. The prefix alto- is used to describe middle clouds. • Low Clouds- made of water droplets. The prefix strato- is used to describe low clouds.

31. Air Masses and Fronts An air mass is a large body of air that has similar temperature and moisture throughout. It gets it moisture and temperature characteristics from the area in which it forms. See page 430, figure 14.

32. Warm Air Masses Four warm air masses influence the weather in the United States. As shown in figure 14.

34. Cold Air Mass Most of the cold winter weather in the United States is influenced by the three polar air masses. The cP is dry and the 2 mP’s are moist.

35. Cold and Warm Fronts

36. Occluded and Stationary Fronts

37. Quiz • If a cP air mass moves over Ohio in the summer, what would the weather be like? • Why does the cT air mass that forms over northern Mexico bring clear, dry, hot weather? • Explain how a cold front develops. • What kind of weather is associated with a stationary front?

38. Answers • Cool and Dry • It forms over the desert, which is hot and contains little moisture. • A cold air mass moves under a warm air mass, forcing the warmer air upwards. • Will be cloudy and rainy as long as the front lies over an area. After the front passes, the weather will usually be clear.

39. Severe Weather Severe Weatheris weather that can cause property damage and even death. Examples of severe weather include thunderstorms, tornadoes and hurricanes.

40. Thunderstorms Thunderstorms are small, intense weather systems that produce strong winds, heavy rain and lightning. They occur when warm, moist air rises rapidly in an unstable atmosphere. A typical thunderstorm produces approximately 470 million liters of water and enough electricity to power the “ENTIRE” United States for 20 minutes.

41. Weird Science Trees sometimes explode when struck by lightning.Why? Lightning causes the sap in the tree to vaporize (turn from a liquid to a gas).The steam expands rapidly as it is heated, causing the tree to explode.

42. Lightning and Thunder Lightningis a large electrical discharge that occurs between two oppositely charged surfaces. When lightning strikes, energy is released. The energy is transferred to the air and causes the air to expand rapidly and send out sound waves. Thunder is the sound that results from the rapid expansion of air along a lightning strike.

44. Math & More To find the distance of a thunderstorm in kilometers, count the seconds between a lightning flash and the thunder and divide the number by 3. If you see a lightning flash and then hear the thunder 21 seconds later, how far away is the storm? 9 kms? 7 kms?

45. Tornadoes Tornadoes are produced in only 1% of all thunderstorms. A tornado is a small, rotating column of air that has high wind speeds and low central pressure and touches the ground. A tornado starts out as a small funnel cloud and hangs in the air. It is called a tornado when it makes contact with the Earth’s surface.

47. Tornadoes contd. About 75% of tornadoes occur in the United States. The average tornado has wind speeds of 120-180 km/hr, but rarer, more violent tornadoes have spinning winds up to 500 km/hr.

48. Hurricanes Hurricanes are large, rotating tropical weather systems with wind speeds of at least 119 km/hr. They are the most powerful storms on Earth!They have different names depending on where they form.Western Pacific- Typhoons. Indian Ocean- Cyclones.

49. Hurricanes generally form in the area between 50 and 200 north and south latitude over warm, tropical oceans. At higher latitudes, the water is too cold for hurricanes to form. They vary in size from 160 km to 1500 km in diameter, and they can travel thousands of miles.

50. Hurricane Formation A hurricane begins as a group of thunderstorms moving over tropical ocean waters. Winds traveling in two different directions collide, causing the storm to rotate over the area of low pressure. Because of the Coriolis effect, the storm turns counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.