1. 1 Chapter 12-Meteorology

2. 2 I. Causes of Weather A. Meteorology is the study of atmospheric phenomena. 1. Clouds, raindrops, snowflakes, fog, dust and rainbows are all types of atmospheric meteorology.

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5. 5 B. Weather and climate 1. Weather is the current state of the atmosphere. 2. Weather is referred mainly to short-term variations in the atmosphere. 3. These variations could occur over minutes, hours, days, weeks, or months.

6. 6 4. Long term variations in weather for a particular area make u the climate of that area. 5. Climate is usually averaged over the course of 30 years or more.

7. 7 Weather

8. 8 Climate

9. 9 C. A question of balance 1. The sun ’ s rays are spread out all over the earth. 2. There are some areas that have high amounts of the sun ’ s rays and some areas that have low amounts of the sun ’ s rays.

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11. 11 D. Air masses 1. Air mass is a large body of air that takes on the characteristics of the area over which it forms. 2. Air masses form over land or water. 3. Air masses are classified according to their regions.

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13. 13 4. All five main types of air masses can be found in North America because of the continent ’ s proximity to the source regions associated with each air mass. 5. Air masses do not stay in one place indefinitely. They move.

14. 14 Air mass

15. 15 II.Weather systems A. Global wind systems. 1. Trade winds occur at 30 degrees north and south latitude. 2. There, air sinks, warms and moves toward the equator in a westerly direction.

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17. 17 3. The second wind system, the prevailing westerly, flows between 30 degrees and 60 degrees north and south latitude in a circulation pattern opposite that of the trade winds.

18. 18 4. The polar easterlies lies between 60 degrees latitude and the poles. 5. Similar to the trade winds, the polar easterlies flow from the northeast to the southwest in the northern hemisphere.

19. 19 B. Jet Streams 1. Narrow bands of fast, high-altitude, westerly winds called jet streams flow at speeds up to 185 km/h. 2. Disturbance form along jet streams and give rise to large-scale weather systems that transport surface cold air toward the tropics and surface warm air toward the poles.

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21. 21 3. A front is the narrow region separating the two air masses of different densities.

22. 22 4. Cold fronts – dense air displaces warm air and forces the warm air up along a step front.

23. 23 5. Warm fronts- Advancing warm air displaces cold air.

24. 24 6. Stationary front- two air masses meet and neither advanced into the other ’ s territory.

25. 25 7. Occluded fronts- Cold air mass moves so rapidly that it overtakes a warm front.

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27. 27 C. Pressure systems 1. High-pressure systems, air sinks, so that when it reaches Earth ’ s surface, it spreads away form the center. 2. Low-pressure systems, air rises.

28. 28 HIGH PRESSURE

29. 29 LOW PRESSURE

30. 30 III. Gathering weather data A. Surface data 1. One of the most common weather instruments is a thermometer, a device used to measure temperature.

31. 31 2. Barometers measure air pressure. 3. In a mercury barometer, changes in air pressure are indicated by changes in the height of a column of mercury.

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33. 33 4. An anemometer is used to measure wind speed.

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35. 35 5. Hygrometer measures relative humidity.

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37. 37 6. Ceilometers measure the height of cloud layers and estimates the amount of sky covered by clouds.

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39. 39 7. The instrument of choice gathering upper – level data is a balloon- born package of sensors called a radiosonde.

40. 40 B. Weather radar 1. The Doppler effect is the change in wave frequency that occurs in energy, such as sound or light as the energy moves toward or away from an observer.

41. 41 IV. Weather analysis A. Surface Analysis 1. Station models provide information for individual sits. 2. To plot data nationwide or globally, meteorologists use isopleths, which are lines that connect points of equal or constant values.

42. 42 3. Digital forecasting is the main method used by modern meteorologists. 4. It is highly dependent on the density of the data available, basically, the more data, the more accurate the forecast.

43. 43 5. Another type of forecast, an analog forecast, involves comparing current weather patterns to patterns that took place in the past. 6. Long term forecasts involving months and seasons are based largely on patterns or cycles.

44. 44 The end