Corel Clipart of Storm Cloud  
Peralta Colleges, Physical Geography
Rita D. Haberlin, Instructor


  1. The changing weather of the mid-latitudes is caused by different air masses. An air mass is a large body of air extending hundreds of miles horizontally with uniform temperature and moisture conditions. The properties of an air mass are picked up from the land and ocean surfaces over which it has rested. These source regions are usually areas of high pressure causing the air mass to move outward to regions of lower pressure.

  3. Air masses are classified according to their source region and latitude. Tropical (T) air masses originate in the tropics. Polar (P) air masses originate in high latitudes, and Arctic (A) and Antarctic air masses over the North and South Pole. They are described as continental (c) if they form on a continent and maritime (m) if they form over the ocean. Thus, a mT air mass is warm and moist and a cP air mass is dry and cold.

  5. A front is a zone of contact between two different air masses. There are four types of fronts. A stationary front occurs when cold and warm air masses meet but neither is advancing. Cold fronts occur where cold air is advancing and gaining ground over warm air. Warm fronts occur where warm air is advancing. Occluded fronts occur as mid-latitude storms gradually dissolve.

  1. Mid-latitude cyclonic storms are low pressure centers that develop along the polar front where warm and cold air masses meet. According to wave cyclone theory, they progress through a series of stages. They travel from west to east at about 20-40 miles per hour. They may be up to 600-1000 miles in width.

  3. In a mature cyclone, warm fronts form when warm air masses are forced to rise over a cold air mass along a low angle slope. Stratiform clouds and steady rain or drizzle occur along a wide area in advance of the warm front.

  5. Cold front weather is more violent. Cold air is pushed beneath the rising warm air producing short, sharp showers and cumuliform clouds. Winds change quickly after the passage of the cold front. Cold, clear weather and anticyclones follow.

  7. As the cyclone moves east, the cold front overtakes the warm front forcing the warm air up. This produces an occluded front that may be cloudy and rainy.

  9. Violent storms such as hurricanes, tornadoes, and thunderstorms are similar because they are all low pressure centers and depend upon the latent heat of condensation for energy.

  11. Tropical cyclones are also known as cyclones (Indian Ocean), hurricanes (Atlantic Ocean), or typhoons (Pacific Ocean). Hurricanes develop over warm ocean water (80°F) in latitudes 7°-15° N and S of the equator in late summer or early fall. They do not develop at the equator, as the coriolis effect does not work at the equator. Hurricanes (100-300 miles wide) move westward in the Trade Winds Belt. They later come under the influence of the Westerlies and move eastward. Hurricanes are well-developed low pressure systems with strong convectional air movement that forms a wall of clouds and spiraling winds. In the center of the hurricane is an eye that is noted for its calm, clear conditions.
  12. Damage from hurricanes comes from high winds (75-125 miles per hour), storm wave surges, and flash floods. Hurricanes tend to weaken over land where the water supply is less. They also weaken over cold waters where there is less evaporation taking place.

  14. Tornadoes are the most violent of storms and often form in advance of a cold front. They occur frequently in spring and early summer in the Great Plains and the Middle Western United States. They are smaller in size and shorter in duration than hurricanes. Tornadoes have very high winds (250-500 miles per hour), cumulonimbus clouds, heavy rains, or hail. They may have a diameter of a quarter of a mile and travel from SW to NE.

  16. Destruction from tornadoes is the result of high wind velocity, explosion resulting from pressure differences in buildings, and the strong updraft of air within the funnel.

  18. Storms are an important mechanism for transferring heat around the earth. In addition to the exchanges of warm and cold air that occur during storms, latent heat, and ocean currents contribute to global heat transfer between the low and high latitudes.



  1. Define air mass and source region.
  2. Describe how air masses are classified and the symbols used.
  3. Describe the four kind of fronts and the symbols used to show them on weather maps.
  4. Describe a mid-latitude cyclone in terms of size, direction, and speed of travel.
  5. Describe the weather associated with warm fronts.
  6. Describe the weather associated with cold fronts.
  7. Describe an occluded front.
  8. Name two features that violent storms have in common.
  9. Describe the hurricane in terms of location of occurrence, direction of travel, size, and weather.
  10. Explain the danger from hurricanes. How do they die out?
  11. Describe tornadoes. When and where do they occur? Describe their size, duration, and path of travel.
  12. Explain the damage from tornadoes.
  13. What role do storms play in the global environment?



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This Geography Site Is Maintained By Patricia A. Kulda  Last Update July 14, 2010