Understanding weather

1. Understanding weather Chapter 7 Notes

2. Water in the Air • Weather is the condition of the atmosphere at a certain time and place. • The condition is affected by the amount of water in the air.

3. The Water Cycle • Water is constantly being recycled through the water cycle.

4. Humidity • As water evaporates from lakes oceans and plants it becomes water vapor. • The amount of water vapor in the air is called humidity. As the temperature increases, the amount of water vapor the air can hold increases.

5. Relative Humidity • Relative Humidity is the amount of water vapor in the air compared with the maximum amount of water vapor that the air can hold at a certain temperature

6. Factors that affect Relative Humidity Amount of Water Vapor Temperature The relative humidity decreases as the temperature rises and increases as the temperature drops • The more water vapor in the air the higher the relative humidity. • The water vapor drops the relative humidity drops.

7. Measuring Relative Humidity • A psychrometer is an instrument that is used to measure relative humidity. • It consists of two thermometers: one wet bulb thermometer and one dry bulb thermometer. • The difference in the temperature readings between the two thermometers indicates the amount of humidity.

8. Measuring Relative Humidity

9. Dew Point • Dew point: the temperature at which a gas condenses into a liquid. • Air is saturated at dew point

10. Condensation • Condensation: the process by which a gas, such as water vapor, becomes a liquid. • Condensation occurs when saturated air (air full of humidity) cools.

11. Clouds • A cloud is a collection of millions of tiny water droplets or ice crystals. • Clouds form as warm air rises and cools ( condensation)

12. Clouds

13. Cumulus Clouds • Puffy, white clouds that tend to have flat bottoms • Form when warm air rises • Indicate fair weather • When they get larger they produce thunderstorms – cumulonimbus clouds (AKA Thunderheads) • Clouds with nimbus or nimbo will produce precipitation

14. Cumulus Clouds Cumulonimbus Cloud 

15. Stratus Clouds • Stratus Clouds form in layers • Cover large areas and often block out the sun • Caused by a gentle lifting of a large body of air. • Nimbostratus clouds are dark stratus clouds that produce continuous rain. • Fog is a stratus cloud found near the ground.

16. Stratus Clouds

17. Cirrus Clouds • Cirrus Clouds are thin feathery white clouds found at high altitudes • Cirrus clouds form when the wind is strong • They can indicate a change in weather.

18. Cirrus Clouds

20. Clouds and Altitude • Clouds are classified by the altitude that they form

21. Cloud Classification • Low clouds – have no prefix • Ex. Cumulus, Stratus • Middle clouds – Have the prefix alto • Ex. Altocumulus, Altostratus • High clouds – Have the prefix Cirro • Ex. Cirrostratus, Cirrus

22. Precipitation • Precipitation is water in solid or liquid form that falls from the air to the Earth • There are four major forms • Rain • Snow • Sleet • Hail

23. Rain • The most common form of precipitation. • Rain is produced when the water droplets in a cloud become a certain size.

24. Sleet and Snow • Sleet forms when rain falls through a layer of freezing air. • Snow forms when temperatures are so cold that water vapor changes directly to a solid.

25. Hail • Hail are balls or lumps of ice that fall from clouds. • Hail forms in cumulonimbus cloudsdue to updrafts freezing the water droplets.

26. Air Masses and Fronts • Changes in weather are caused by the movement and interaction of air masses. • An air mass is a large body of air where temperature and moisture content are similar throughout .

27. Air Masses • Air Masses are characterized by their moisture content and temperature which is determined by the area over which the air mass forms(Source Region)

28. Types of air masses • Maritime (m) Forms over water; wet • Continental ( c ) Forms over land; dry • Polar ( P) Forms over polar regions; cold • Tropical (T) develops over the tropics: warm

29. Cold Air Masses • Cold air masses are responsible for bringing extremely cold winters.

30. Warm Air Masses • Warm air masses bring warm air weather systems into the United States. • This brings milder temperatures • This can also bring severe weather during the summer months.

31. Fronts • Front: the boundary between air masses of different densities and usually different temperatures • Four kinds of fronts • Cold front • Warm front • Occluded front • Stationary front

32. Cold Fronts • A cold front forms where cold air moves under warm air • Move quickly and bring thunderstorms, heavy rain, or snow • Cooler weather follows a cold front

33. Warm Fronts • A warm front forms where warm air moves over colder denser air • Warm air replaces cold air • Bring drizzly rain • Followed by clear and warm weather

34. Occluded Front • An occluded frontforms when a warm air mass is caught between two colder air masses • This produces cold temperatures and large amounts of rain and snow.

35. Stationary Fronts • Stationary fronts form when a cold air mass meets a warm air mass. • Brings many days of cloudy wet weather.

36. Locate the 4 types of fronts on this weather map.

37. Cold Fronts

38. Warm Front

39. Stationary Front

40. Occluded Front

41. Be a Weather Forecaster You are planning to travel to Alabama in 2 days. The high temperature there for today is 68º F. Use the map to help you predict whether the temperature in Alabama will increase, decrease, or stay the same. Explain why you think so. Weather Fronts 41

42. Be a Weather Forecaster There is a cold front approaching. The temperatures will probably be cooler behind the front. 42

43. Air Pressure and Weather • Areas that have LOWER pressure than the surrounding areas do are called Cyclones. • These areas the air masses come together and RISE

44. Air Pressure and Weather • Areas that have HIGH pressure are called anticyclones. • Anticyclones are areas where the air moves apart and SINKS

45. Air Pressure and Weather • By keeping track of the low pressure (cyclones) and High Pressure (anticyclones), meteorologists can predict the weather. • Low pressure cause cloudy possibly stormy weather • High pressure brings dry clear weather

46. Severe Weather • Thunderstorms • Tornadoes • Hurricanes • Severe weather safety

47. Thunderstorms • Thunderstorms are small intense weather systems that produce strong winds, heavy rain, lightning, and thunder. • Two atmospheric conditions are needed to produce a thunderstorm: • Warm and moist air near the Earth’s surface • Unstable atmosphere

48. Thunderstorms Cumulonimbus Clouds

49. Lightning • Lightning is an electric discharge that occurs between a positively charged area and a negatively charged area. • Can occur • Between two clouds • Between Earth and a cloud • Between two parts of the same cloud

50. Lightning • When lightning strikes, energy is released. • Energy is transferred to the air. • Thunder is the sound that results from the rapid expansion of air along the lightning strikes.