1. 4.1 Water in the Atmosphere
Chapter 4: Weather
4.1 Water in the Atmosphere

2. Review of the Water Cycle
Evaporation turns liquid water into water vapor.
Condensation turns water vapor back into liquid water droplets, which form clouds.
Precipitation falls from the clouds, and then goes back into the ground and bodies of water.

3. Review of the Water Cycle

4. Humidity Humidity: measure of the amount of water vapor in the air.
Warm air can hold more water vapor.
Relative humidity: percentage of water vapor that is actually in the air compared to the maximum amount that could be held

5. Humidity
Relative humidity: percentage of water vapor that is actually in the air compared to the maximum amount that could be held.
Measured with a psychrometer.

6. Chapter 4: Weather
4.2 Clouds

7. Condensation
Clouds form when water vapor in the air condenses to form liquid water or ice crystals.
Water condenses at cooler temperatures: it begins at the dew point.
Particles must be present in the atmosphere, such as dust or salt, for condensation to occur.

8. Condensation Liquid water that condenses on the surface is called dew.
Ice deposited on a surface that is below freezing is called frost.

9. Types of Clouds Cirrus clouds: Wispy and feathery
High altitudes and low temperatures
Made of ice
Fair weather

10. Types of Clouds Altocumulus/altostratus clouds: Medium altitudes
Indicate precipitation

11. Types of Clouds Cumulus clouds: Puffy, cotton-like clouds
Short clouds indicate fair weather
Stacked cumulonimbus clouds produce storms

12. Types of Clouds Cirrocumulus clouds: Small, cotton ball clouds
Indicate a storm is on the way

13. Types of Clouds Stratus clouds: Cover all or most of the sky
Dull and gray
If precipitation falls from them, then they are nimbostratus clouds.

14. Types of Clouds
Fog:
Clouds that form near the ground

15. Chapter 4: Weather
4.3 Precipitation

16. Precipitation
Precipitation: any form of water that falls from clouds and reaches Earth’s surface.
Rain
Sleet
Freezing rain
Snow
Hail

17. Rain Most common type Smaller drops make drizzle
Smallest drops make mist
Measured using a rain gauge: open ended tube that collects rain.

18. Sleet Formed when raindrops freeze on their way to Earth’s surface.
Ice particles smaller than 5 mm in diameter.

19. Freezing Rain
Raindrops fall as liquid water, but freeze when they touch the cold surface.

20. Snow
Formed when water vapor in a cloud changes right into ice crystals.
Measured by using a measuring stick or by melted collected snow to see the water’s depth.

21. Hail Pellet of ice larger than 5 mm in diameter.
Contains layers of ice.
Forms inside cumulonimbus clouds during storms.

22. Floods Flood: overflowing of water in a dry area
many occur when the volume (amount) of water in a river increases so much that the river overflows.

23. Floods Can uproot trees, remove boulders, wash away buildings, etc.
Controlled by:
Dams: barrier across a river to redirect its flow
Levees: embankment built along a river to prevent flooding on the land.

24. Droughts Drought: long period of scarce rainfall or dry weather.
Reduces groundwater amounts
Results in a water shortage

25. Droughts Usually caused by dry weather systems.
Can cause crop failure and famine.
Water conservation and preservation help prepare for these.

26. Chapter 4: Weather
4.4 Air Masses

27. Air Masses
Air mass: huge body of air in the lower atmosphere that has similar temperature, humidity, and air pressure.

28. Air Masses There are four main types of air masses:
Tropical: warm masses that form in the tropics; low air pressure
Polar: cold masses that form near the poles; high air pressure
Maritime: form over oceans; very humid
Continental: form over land; less humid

29. Air Masses
Maritime polar: cool, humid air form in the N. Atlantic; pushed to sea by westerly winds.
Continental polar: form over Canada/Alaska; bitterly cold and low humidity.

30. Air Masses
Maritime tropical: warm, humid masses that form over the Pacific; bring hot, humid weather with storms OR heavy rain/snow.
Continental tropical: hot, dry air masses that form in the Southwest of the US.

31. Air Masses Prevailing westerlies push air from west to east in the US.
Jet streams are bands of high-speed winds that blow from west to east.
A front is a boundary where air masses meet; storms can develop at fronts.

32. Cold Fronts
Cold air is dense and sinks; warm air is less dense and rises.
A cold front is when cold air slides underneath warm air.
Clouds, heavy rain, and snow are possible, as well as thunderstorms.

33. Cold Fronts

34. Warm Fronts A fast-moving warm air mass moves over a cold mass.
Weather can be rainy or cloudy for several days.

35. Occluded Fronts Warm air mass is caught between two cold masses.
The warm mass rises, and the cold masses mix.
Temperatures get much cooler; clouds and precipitation may occur.

36. Occluded Fronts

37. Stationary Fronts Warm and cold masses meet, but do not move.
Water vapor condenses into rain, snow, fog, or clouds.

38. Cyclones A cyclone is a swirling center of low air pressure.
Cyclones and low pressure systems are associated with clouds, wind, and precipitation.

39. Anticyclones An anticyclone is a high-pressured center of dry air.
Causes dry, clear weather.

40. Chapter 4: Weather
4.5 Storms

41. Storms Storm: violent disturbance in the atmosphere.
Sudden change of air pressure.

42. Winter Storms Most precipitation is formed as snow all year round.
In the winter, it is cold enough (0 C or 32 F) for the snow to remain snow.

43. Thunderstorms
Thunderstorm: small storm often accompanied by precipitation, thunder, and lightning.
Form in cumulonimbus clouds.
Hot, humid afternoons/evenings

44. Thunderstorms
Positive and negative charges build up, releasing in lightning – sudden spark or discharge of energy between clouds and the ground.

45. Hurricanes
Hurricane: tropical cyclone with winds of 119 km/h or higher.
Atlantic Ocean, Pacific Ocean (typhoons), and Indian Ocean (cyclones).
Begins over warm water as a low-pressure area.

46. Hurricanes Gets energy from warm air over water
As warm air rises, more air is pulled into the system with high winds and heavy rains.
Strongest around the center, which is called the eye.

47. Hurricanes Can last a week or more.
Over land, it begins to weaken without warm air to gain energy from.
Storm surge: a dome of water that sweeps across the coast

48. Tornadoes
Tornado: rapidly whirling, funnel-shaped cloud that reaches down from a t-storm.
Over water – waterspout
Touch the ground for 15 minutes or less; winds can approach 500 km/h.

49. Tornadoes
Mostly form in cumulonimbus clouds, but can form in any severe weather.
Can occur anywhere, but are most common in the Great Plains – called Tornado Alley.

50. 4.6 Predicting the Weather
Chapter 4: Weather
4.6 Predicting the Weather

51. Predicting the Weather
The first step is to collect data-
Making observations: watching the clouds can tell a great deal about the future weather.
Gathering data: using balloons, satellites, radar, and surface instruments.
Interpreting information: maps, charts, computers, and technology help prepare forecasts.

52. Using Technology
Automated weather stations: gather info for temperature, air pressure, relative humidity, rainfall, and wind speed/direction.

53. Using Technology
Weather Balloons: carry instruments into troposphere to measure temperature, air pressure, and humidity.
Weather Satellites: orbit Earth in the exosphere; capture images of clouds, storms, and snow cover.

54. Using Technology
Computer forecasts: computers analyze data quickly to make fast predictions.

55. Weather Service Maps
Curved lines connect places with similar conditions of temp/air pressure.
Isobars: lines joining places on the map that have the same air pressure.
Isotherms: lines joining places that have the same temperatures.

56. Weather Service Maps

57. Weather Forecasts
The “butterfly effect” – a small change in the weather today can lead to a big change in the weather next week.
This makes it extremely difficult to predict weather far in the future.