Weather and Climate

1. Weather and Climate

2. Weather is a description of the changeable aspects of the atmosphere, the temperature, rainfall, pressure, and so forth, at a particular time. These changes usually affect your daily life one way or another, but some of them seem more inconvenient than others.

3. Clouds and Precipitation

4. Introduction • Hydrological Cycle – 4 main parts • Evaporation of water from the oceans • Transport of water vapor in the atmosphere • Condensation and precipitation • Return of water to ocean by rivers and streams

5. The main events of the hydrologic cycle are: (1) The evaporation of water from the ocean, (2) the transport of water vapor through the atmosphere, (3) condensation and precipitation of water on the land, and (4) return of water to the ocean by rivers and streams.

6. Cloud-forming Processes • Upward Air Movement • Convection resulting from differences in temperature • Barriers such as mountain ranges which provide lift to air masses • Meeting of moving air masses with different densities.

7. Adiabatic Cooling • Decrease in temperature of an expanding gas • Adiabatic Heating • Heating of a gas as it undergoes compression • Dry Adiabatic Lapse Rate • Rate of cooling in the absence of condensation • About 10 OC for each increase or decrease of 1 km (5.5OF/1,000 ft)

8. As a parcel of dry air is moved upward, it expands and cools according to the dry adiabatic lapse rate. This graph compares the temperature of a rising and adiabatically cooling parcel of dry air with the average temperatures of the surrounding atmosphere when the temperature at the surface is 30OC (86O F).

9. Atmospheric Stability • Stability • When the atmospheric lapse rate is less that the dry adiabatic lapse rate • Instability • When the atmospheric lapse rate is greater than the dry adiabatic lapse rate.

10. If a parcel of air is moved up or down, it will cool or warm according to the dry adiabatic lapse rate. In a state of atmospheric stability, the parcel of air will always be cooler, and therefore more dense, than the surrounding air at any altitude. It will, therefore, return to the original level when the upward force is removed.

11. In a state of atmospheric instability, a parcel of air will always be warmer, and therefore less dense, than the surrounding air at any altitude. The parcel will, therefore, continue on in the direction pushed when the upward force is removed.

12. Wet Adiabatic Lapse Rate • The lapse rate for a parcel of air at a slow rate and releasing the temperature equal to its latent heat of vaporization • Supersaturated • When the air contains more water vapor than is the normal amount of water vapor

13. When the dew point temperature is reached in a rising parcel of air, the latent heat of vaporization is released as water vapor condenses. This release of heat warms the air, decreasing the density and accelerating the ascent. The new lapse rate from the release of latent heat is called the wet adiabatic lapse rate.

14. Origin of Precipitation • Processes of precipitation formation • Coalescence • When water droplets merge with millions of other water droplets. • Growth of ice crystals • Ice crystals can capture other water molecules and grow to enormous sizes.

15. Supercooled • If water remains in the liquid state after the temperature is below the freezing point. • Ice-forming nuclei • Solid particles on which the ice forms

16. Precipitation is water in the liquid or solid form that returns to the surface of the earth. The precipitation you see here is liquid, and each raindrop is made from billions of the tiny droplets that make up the clouds. The tiny droplets of clouds become precipitation by merging to form larger droplets or by the growth of ice crystals that melt while falling.

17. Weather Producers

18. Introduction • There is a shift in weather during the changes of the seasons that is related to: • Movement of air masses • Leading fronts of air masses • high and low pressure areas

19. Air Masses • Polar air mass • An air mass that moves from a cold region • Tropical Air Mass • An air mass that moves from a warm region • Continental Air Mass • Moves in from a land mass • Maritime Air Mass • Moves in from over an ocean

20. Major types of air masses • Continental Polar • Cold • Dry • Maritime Polar • Cold • Moist

21. Continental Tropical • Warm • Dry • Maritime tropical • Warm • Moist

22. The general movement of the four main types of air masses that influence the weather over the contiguous United States. The tropical air masses visit most often in the summer, and the polar air masses visit most often during the winter. During other times, the polar and tropical air masses battle back and forth over the land.

23. This satellite photograph shows the result of a polar air mass moving southeast over the southern United States. Clouds form over the warmer waters of the Gulf of Mexico and the Atlantic Ocean, showing the state of atmospheric instability from the temperature differences.

24. Air mass weather • When the weather in an area is under the influence of an air mass.

25. Weather Fronts • Front • A boundary between two different air masses • Cold Front • When a cold air mass moves into a warmer area, displacing the warm air mass • Provides lift to adiabatically cool the warm air, resulting in towering cumulus and thunderclouds.

26. (A)A cold air mass is similar to a huge, flattened bubble of cold air that moves across the land. The front is the boundary between two air masses, a narrow transition zone of mixing. (B) A front is represented by a line on a weather map, which shows the location of the front at ground level.

27. An idealized cold front, showing the types of clouds that might occur when an unstable cold air mass moves through unstable warm air. Stable air would result in more stratus clouds rather than cumulus clouds.

28. Warm Front • When a warm air mass moves into an area, displacing the cold air mass • A gently sloping front as the Warm air moves over top of the cooler air. • Stationary Front • When the edge of a front ceases to advance

29. An idealized warm front, showing a warm air mass overriding and pushing cold air in front of it. Notice that the overriding warm air produces a predictable sequence of clouds far in advance of the moving front.

30. Waves and Cyclones • Occluded Front • One that has been lifted completely off the ground • Has s low pressure center and cyclonic activity • Cyclone • a low pressure area with winds moving into the low pressure area and being forced upward. • Friction and the Coriolis effect cause the air to move to the right of the direction of movement. • Anticyclone • A high pressure center

31. The development of a low-pressure center, or cyclonic storm, along a stationary front as seen from above. (A) A stationary front with cold air on the north side and warm air on the south side.

32. (B) A wave develops, producing a warm front moving northward on the right side and a cold front moving southward on the left side.

33. (C) The cold front lifts the warm front off the surface at the apex, forming a low-pressure center.

34. (D) When the warm front is completely lifted off the surface, an occluded front is formed.

35. (E) The cyclonic storm is now a fully developed low-pressure center.

36. Cyclonic storms usually follow principal storm tracks across the continental United States in a generally easterly direction. This makes it possible to predict where the low-pressure storm might move next.

37. Air sinks over a high-pressure center that moves away from the center on the surface, veering to the right in the Northern Hemisphere to create a clockwise circulation pattern. Air moves toward a low-pressure center on the surface, rising over the center. As air moves toward the low-pressure center on the surface, it veers to the left in the Northern Hemisphere to create a counterclockwise circulation pattern.

38. Major Storms • Thunderstorms • Conditions • Uplift of air • Cumulus clouds • Frontal Thunderstorms • Move with the front that produced them

39. Stages • Cumulus • Air is lifted and cools adiabatically to the dew point and a cumulus cloud develops. • Heat of vaporization is released and accelerates the uplift. • Mature • When the moisture that is produced reaches the surface of the Earth • This series of updrafts, downdrafts, and precipitation release electrical charges which become the lightening associated with the storm • Final • All of the updrafts are cut off and only downdrafts exist. • Lightening • Thunder • hail

40. Three stages in the life of a thunderstorm cell. (A) The cumulus stage begins as warm, moist air is lifted in an unstable atmosphere. All the air movement is upward in this stage.

41. (B) The mature stage begins when precipitation reaches the ground. This stage has updrafts and downdrafts side by side, which create violent turbulence.

42. (C) The final stage begins when all the updrafts have been cut off, and only downdrafts exist. This cuts off the supply of moisture, and the rain decreases as the thunderstorm dissipates. The anvil-shaped top is a characteristic sign of this stage.

43. Different parts of a thunderstorm cloud develop centers of electric charge. Lightning is a giant electric spark that discharges the accumulated charges.

44. These hailstones fell from a thunderstorm in Iowa, damaging automobiles, structures, and crops.

45. Tornados • Most violent, focused storm on Earth • Speeds in excess of 300 km/hr (200 mi/hr) • Hurricanes • A hurricane is a violent storm that is produced over the warm tropical ocean near the equator. • Tropical Cyclone • Typhoon

46. A tornado might be small, but it is the most violent storm that occurs on the Earth. This tornado, moving across an open road, eventually struck Dallas, Texas.

47. This is a satellite photo of hurricane John, showing the eye and counterclockwise motion.

48. Cross section of a hurricane.

49. Weather Forecasting

50. Weather predictions are based on information about air masses, fronts, and associated pressure systems in an area. • This information is used to produce a model of behavior for weather using a computer. • Many models are used and then summarized when the different models agree fairly closely to a model of the weather.