CHAPTER 11

WEATHER PATTERNS

I. Air Masses

A. Weather patterns are the result of the movement of large bodies of air called air masses. An air mass is an immense body of air, usually 1600 km or more across, and perhaps several kilometers thick, which is characterized by a homogeneity of temperature and moisture at any given altitude.

B. It may take several days for an air mass to transverse an area, the region under its influence will probably experience fairly constant weather, a situation called air-mass weather.

C. Source regions are the areas where an air mass acquires its characteristic properties of temperature and moisture.

• Air masses are classified according to heir source region.
• Polar air masses originate in high latitudes.
• Tropical air masses originate in low latitudes.
• The designation polar or tropical gives an indication of the temperature characteristics of the air mass. Polar indicates cold and tropical indicates warm.
• Air masses are also classified according to the nature of the surface in the source region. Continental designates land and maritime indicates water. The continental and maritime suggests the moisture characteristics of the air mass. Continental air is usually and maritime air is humid.
• Continental tropical and maritime tropical, and continental polar and maritime polar are the four types of air masses.

II. Fronts

A. Fronts are boundaries that separate air masses of different densities, one warmer and often higher in moisture content that the other. Above the ground the frontal surface slopes at a low angle so that warmer air overlies cooler air. In the ideal case, the air masses on both sides of the front move in the same direction and at the same speed.

B. Warm fronts develop when the surface position of a front moves so that warm air occupies territory formerly covered by cooler air. On a weather map the front is denoted by a line with semicircles extending into the cooler air

C. Cold fronts develop when cold air is actively advancing into a region occupied by warmer air. As with warm fronts, friction tends to slow the surface position of a cold front more so than its position aloft. On the average cold fronts are twice as steep as warm fronts. Also, cold fronts advance much faster than warm fronts.

D. The differences in the rate of movement and steepness of slope largely account for the more violent nature of cold front weather.

E. Stationary fronts develop when the surface position of a front does not move and its is stationary. On the weather map, stationary fronts are shown with triangular point on one side of the front and semicircles on the other. If some overrunning occurs along a stationary front the result is extended period of widespread cloudiness, light rain, or light snow.

F. Occluded fronts occurs when an active cold front overtakes a warm front. As the advancing cold air wedges the warm front upward, a new front emerges between the advancing cold air and air over which the warm front sliding. The precipitation associated with this type of front is due warm air to be forced aloft.

III. The Middle Latitude Cyclone

A. The fronts described above influence the weather in the mid-latitudes. They are usually associated with a low pressure system called a middle-latitude or wave cyclone.

B. Life cycle of a wave cyclone

o       Wave cyclones form along fronts where they change in a somewhat predictable way. This life cycle can last for a few hours or for several days.

o       The life cycle is as follows:

§         A front develops.

§         Cyclonic circulation is developed.

§         An occluded front is fully developed.

§         The cyclone dissipates.

C. Cyclone formation occurs whenever the flow aloft is relatively straight, that is, from west to east, very little cyclonic activity occurs at the surface. However, when the upper air begins to meander widely in a north-to-south direction, high-amplitude waves consisting of alternating troughs and ridges are produced, and surface cyclonic activity intensifies.

D. Divergence aloft initiates upward air movement, reduced surface pressure, and cyclonic flow.

E. Convergence along the jet stream results in general subsidence of the air column, increased surface pressure, and anticyclonic surface winds.

IV. Thunderstorms

A. Stages of a thunderstorm.

• During the cumulus stage, strong updrafts act to build the thunderstorm.
• The mature stage is marked by heavy precipitation and cool down drafts in part of the storm.
• The dissipating stage, the warm updrafts disappear completely, precipitation becomes light, and the cloud begins to evaporate.
• The life span of a cumulonimbus cell within a thunderstorm complex is only about an hour, but as the storm moves, fresh supplies of warm, water laden air generate new cells to replace those that are dissipating.

B. Thunder and lightening are obvious features of a thunderstorm.

• Lightning is similar to electrical shock you have experienced on a very dry day upon touching a metal object. The upper portion of the cloud acquires a positive charge, while the lower portion of the cloud maintains an overall negative charge while containing small positively charged pockets. These charges build up to millions of volts before a lightning stroke acts to discharge the cloud by striking the earth, or possibly another cloud.
• The electrical discharge of lightning rapidly heats the air, which, inturn, causes it to expand explosively, and create the thunder we hear.

V. Tornadoes

A. Tornadoes are local storms of short duration that are ranked high among nature's most destructive forces. Tornadoes are violent windstorms that take the form of a rotating column of air that extends downward form the cumulonimbus cloud. Pressures within are estimated to be as much as 10% lower than immediately outside the storm.

B. Drawn by the much lower pressure in the center of the storm, air near the ground rushes into the tornado from all directions.

C. As the air streams inward it is spiraled upward around the core until it merges with the airflow of the parent thunderstorm deep in the cumulonimbus tower.

D. Most often tornadoes are spawned along the cold front of a middle-latitude cyclone.

E. General characteristics of a tornado.

• The average diameter is between 150-600 meters.
• It travels across the landscape at about 45 km/hr.
• It cuts a path of about 10 km long.

F. The Fujita intensity scale or F-scale is used to measure the strength of tornado winds and destruction created by such winds.

G. Tornado warning is used to alert people to the possibility of tornadoes. A tornado watch is used to alert people of the existence of favorable conditions for a tornado.

VI. Hurricanes

A. Hurricanes are whirling tropical cyclones that may have wind speeds reaching 300 km/hr.

B. Hurricanes form in all tropical waters (except those in the South Atlantic and eastern South Pacific) between the latitudes of 5 and 20 degrees. In the Pacific hurricanes are known as typhoons and in the Indian Ocean, cyclones.

C. A steep pressure gradient generates the rapid, inward spiraling winds of a hurricane. As the inward rush nears the core of the storm, it turns upward and ascend in a ring of cumulonimbus towers. This doughnut shaped wall of intense convective activity surrounding the center of storm is called the eye wall. It is here that the greatest wind speeds and heaviest rainfall occur. Surrounding the eye wall are curved bands of clouds that trail away in a spiral fashion. Near the top of the hurricane the airflow is outward, carrying the rising air away from the storm center, thereby providing room for more inward flow at the surface.

D. At the very center of the storm is the eye of the hurricane. In the eye the rain and wind subside. The air within the eye gradually descends and heats by compression, making it the warmest part of the storm.

E. A hurricane can be described as a heat engine that is fueled by the latent heat liberated when huge quantities of water vapor condense.

F. When a cyclone's strongest winds do not exceed 61 km/hr, it is called a tropical depression. When winds are between 61 and 119 km/hr it is called a tropical storm.

G. hurricanes diminish in intensity whenever they move over ocean waters that cannot supply warm moist tropical air, move onto land; or reach a location where the large scale flow aloft is unfavorable.