Hewitt//Suchocki/Hewitt Conceptual Physical Science Fourth Edition

1. Hewitt//Suchocki/HewittConceptual Physical Science Fourth Edition Chapter 25: DRIVING FORCES OF WEATHER

2. This lecture will help you understand: • Atmospheric Moisture • Weather Variables • Cloud Development • Air Masses, Fronts, and Storms • Violent Weather • Weather Forecasting

3. Weather Four factors influence the weather: • Atmospheric moisture • Temperature • Air pressure • Arrangement of land and water features

4. Atmospheric Moisture Humidity is the mass of water vapor a given volume of air contains. Relative humidity is the ratio:

5. Atmospheric Moisture Air that contains as much water vapor as it possibly can is saturated. • Warm air holds more water vapor than cold air. • As air cools, it holds less and less water vapor. Saturation can occur when air temperature drops, causing water vapor to condense. Saturation and condensation are more likely in cold air—slower-moving molecules.

6. Atmospheric Moisture

7. Atmospheric Moisture Evaporation rate depends on temperature. Condensation rate does not. It depends only on humidity. When the evaporation rate is greater than the condensation rate, the relative humidity is always less than 100%.

8. Atmospheric Moisture Dew point is the temperature at which saturation occurs. Condensation occurs when the dew point is reached. • Water vapor condenses high in the atmosphere and forms clouds. • Water vapor condenses close to the ground surface to form dew, frost, and/or fog.

9. Weather Variables Air pressure: the movement of air molecules into each other. The faster the air molecules move, the greater their kinetic energy and the greater the air pressure. Warm air exerts more air pressure on its surroundings than cooler air.

10. Weather Variables The denser the air, the more molecular collisions and the higher the air pressure. Air pressure, density, and temperature are interrelated. Adiabatic processes occur when air is expanded or compressed without heat exchange.

11. Weather Variables Ideal gas law: Pressure = density  temperature

12. Weather Variables With adiabatic expansion, the temperature of a dry (unsaturated) air parcel decreases by about 10°C for each kilometer rise. This rate of cooling of dry air is called the dry adiabatic lapse rate.

13. Weather Variables Chinooks—warm, dry winds—occur when cold air moving down a mountain slope is compressed as it moves to lower elevations and becomes warmer.

14. Weather Variables If rising air stays warmer than the surrounding air, it will continue to rise instead of returning to its starting position. This is unstable air. Eventually, the air parcel will expand and cool sufficiently to match the surrounding air. When the temperatures match, the air parcel rises no further, but it does not sink back to its starting position. So, unstable air rises, but stable air does not.

15. Weather Variables When upper regions of the atmosphere are warmer than lower regions, which is opposite of what normally occurs, we have a temperature inversion.

16. Weather Variables CHECK YOUR NEIGHBOR What does the image below demonstrate? • Condensation • Air pressure • Temperature inversion • All of the choices

17. Weather Variables CHECK YOUR NEIGHBOR What does the image below demonstrate? • Condensation • Air pressure • Temperature inversion • All of the choices

18. Cloud Development

19. Cloud Development Clouds develop when condensation rate exceeds evaporation rate above the lifting condensation level. A rising air parcel cools at the dry adiabatic lapse rate until it reaches saturation. After saturation, the moist adiabatic lapse rate controls how thick the cloud will become.

20. Cloud Development Height of the cloud base and how thick the cloud becomes depends on environmental lapse rate, dry adiabatic lapse rate, and moist adiabatic lapse rate.

21. Cloud Development CHECK YOUR NEIGHBOR Which of the following clouds appears at highest altitude? • Stratus • Nimbostratus • Altocumulus • Cirrus

22. Cloud Development CHECK YOUR ANSWER Which of the following clouds appears at highest altitude? • Stratus • Nimbostratus • Altocumulus • Cirrus

23. Precipitation Formation Each step toward precipitation is part of the collision-coalescence process. • Formation of dust • Updrafts • Growth of stationary drops of water • Falling of raindrops Vertical development in the cloud is necessary so that enough droplet collisions occur.

24. Precipitation Formation Raindrops shrink as they fall, because the evaporation rate exceeds the condensation rate once they leave the cloud. If enough evaporation occurs, raindrops may disappear before they reach the ground—this is called virga.

25. Air Masses, Fronts, and Storms Air masses fall into six categories:

26. Air Masses, Fronts, and Storms

27. Air Masses, Fronts, and Storms Atmospheric lifting: lifting of air. Three types: • Convectional lifting—cumulus clouds • Orographic lifting—rain shadow • Frontal lifting—cirrus clouds changing to cumulonimbus clouds

28. Air Masses, Fronts, and Storms Convectional lifting:

29. Air Masses, Fronts, and Storms Orographic lifting:

30. Air Masses, Fronts, and Storms Frontal lifting: Cold Front

31. Air Masses, Fronts, and Storms Frontal Lifting: Warm Front

32. Air Masses, Fronts, and Storms Frontal Lifting Occluded Front

33. Air Masses, Fronts, and Storms: Cyclones A cyclone is an area of low pressure around which winds flow. Due to the Coriolis force, winds in a cyclone move: • Counterclockwise in the Northern Hemisphere • Clockwise in the Southern Hemisphere Air converges in the center (lowest pressure) and is forced to rise upward.

34. Air Masses, Fronts, and Storms CHECK YOUR NEIGHBOR What is the name for a front that occurs when a cold front and warm front merge? • Convection • Occluded • Stationary • Turbulent

35. Air Masses, Fronts, and Storms CHECK YOUR ANSWER What is the name for a front that occurs when a cold front and warm front merge? • Convection • Occluded • Stationary • Turbulent

36. Violent Weather Storms are defined as violent and rapid changes in the weather. Three major types of severe storms: • Thunderstorms • Tornadoes • Hurricanes

37. Violent Weather Thunderstorms begin with humid air rising, cooling, and condensing into a single cumulus cloud. When fed by unstable, moist air, a cumulus cloud grows into a thundercloud. Thunderstorms contain immense amounts of energy.

38. Violent Weather All thunderstorms include thunder and lightning. The electrical energy flowing from cloud to ground is lightning. As lightning heats up the air, the air expands and we hear thunder.

39. Violent Weather Tornado: a funnel-shaped column of air rotating around a low-pressure core that reaches from a cumulonimbus cloud to the ground. A funnel cloud is similar to a tornado, but it does not touch the ground. Tornadoes are dangerous because of their suction and also the battering of their high winds.

40. Violent Weather Hurricanes are the greatest storms on Earth—energy comes from latent heat released from condensing water vapor. • Rising warm air creates low pressure near the surface, drawing in more moist air. • Winds rotate around a central low-pressure area—the eye of the storm. • There is a continuous supply of energy from tropical waters— a hurricane weakens as fuel is cut off (land fall or cooler water).

41. Violent Weather Strong vertical wind shear can cause warm air to tilt inward and spiral. It can develop into a tropical depression. If the storm intensifies, it progresses into a tropical storm, with increased wind speeds. Hurricane—winds up to 300 km/hour.

42. Violent Weather CHECK YOUR NEIGHBOR What is the eye of a hurricane? • Area of high pressure • Area of highest level of precipitation • Area of low pressure • Area where upper-level air descends

43. Violent Weather CHECK YOUR ANSWER What is the eye of a hurricane? • Area of high pressure • Area of highest level of precipitation • Area of low pressure • Area where upper-level air descends

44. Weather Forecasting Weather forecasting involves collecting data from all over the world. Computers can plot and analyze data and predict weather, although the many variables make accuracy difficult.

45. Weather Forecasting Weather maps identify points of equal pressure with isobars—lines. Lows and highs are marked.