Meteorology

A

Air Masses

An air mass is a body of air that extends over a large area and has nearly uniform temperature and humidity in any horizontal direction. Places where air masses form are called source regions, and they are generally flat with light winds. Ideal source regions are those dominated by large high-pressure areas, such as the arctic plains in winter and the subtropical oceans and desert regions in summer.

Air masses are classified according to their source region. Polar air masses originate over cold regions. Tropical air masses originate over the warm tropics. Continental air masses originate over land, and maritime air masses originate over water. Consequently, a cold, dry air mass that forms over land is called a continental polar air mass and a hot, moist air mass that forms over water is called a maritime tropical air mass.

Generally, the upper-level winds move air masses from one region to another. For example, arctic air masses that form over northern Canada move into the United States when strong upper-level winds, called jet streams, direct these frigid masses of air southward.

B

Fronts

A front is a boundary where air masses with sharply contrasting temperature and humidity meet. Many kinds of storms occur along fronts.

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A cold front marks the boundary where cold air is replacing warm air. On a weather map, cold fronts are drawn as a solid blue line with triangles. The triangles point in the direction of movement. Typically, warmer, more humid air is found in advance of a cold front, while colder, drier air is behind it. Along the front, the warm, humid air often rises and condenses into towering cumulus clouds that may develop into thunderstorms. A narrow band of heavy precipitation, often in the form of rain showers, usually accompanies the front. As a cold front approaches, atmospheric pressure normally drops. As the cold front moves on by, atmospheric pressure rises and the winds shift direction. The passage of the front is often accompanied by the heaviest precipitation and the strongest and gustiest winds. Occasionally, however, a line of thunderstorms may develop, out ahead of a cold front. This line is called a squall line and it produces heavy rain and strong, gusty winds.

A warm front marks the region where warm air is replacing cold air. On a weather map, warm fronts are drawn as a solid red line with half circles. The half circles point in the direction of movement. An average warm front has a more gentle slope than that of a typical cold front. As a warm front advances, warmer air glides up and over the colder, denser surface air. This process, called overrunning, produces widespread cloudiness and precipitation well in advance of the front’s surface position.

Warm fronts are best developed in winter. As a typical warm front approaches, the atmospheric pressure drops and high, wispy cirrus clouds form 12 to 24 hours ahead of the front. These clouds give way to thicker and lower clouds (cirrostratus and altostratus). As the warm front moves closer, cloud level descends and steady rain, snow, sleet, or freezing rain may fall from nimbostratus clouds into the cold air ahead of the front. Just before the front passes, there may be low stratus clouds and fog. As the warm front passes, the air temperature and humidity rise, the atmospheric pressure stops falling, the winds shift, the rain ends, and the fog dissipates. However, these weather changes are less noticeable than those of a typical cold front.

Cold fronts usually move faster than warm fronts. Consequently, when a cold front overtakes a warm front, a new front, called an occluded front, forms. Occluded fronts appear on weather maps as a solid purple line with alternating triangles and half circles, both pointing in the direction toward which the front is moving. Generally, the air behind an occluded front is colder than the air ahead of it. The weather and clouds preceding an occluded front are often similar to that of a warm front.

A stationary front is a cold front or warm front that shows little or no movement. On a weather map, stationary fronts are represented as alternating red and blue lines with half circles pointing toward the colder air and triangles pointing toward the warmer air.

C

Middle Latitude Cyclones

Middle latitude cyclones are huge low-pressure storm systems that consist of a cold front and a warm front, and, usually for part of their lifecycle, an occluded front as well. Middle latitude cyclones usually develop along a slow-moving or stationary front. Such fronts are common at the boundary between the midlatitude cell and the polar cell of the three-cell model. The boundary is a trough of low pressure, with (in the northern hemisphere) warm air to the south and cold air to the north. When a jet stream moves over a stationary front, the front may bend, as a cold front pushes southward and, to its east, a warm front moves northward. The junction of the two fronts is the center of the developing storm and has the lowest atmospheric pressure.

Winds at the ground (in the northern hemisphere) blow counterclockwise and inward around the area of low pressure. As the surface winds converge toward the center of the storm, the air gradually rises, often condensing into clouds. The heat released during condensation supplies some of the energy for the storm’s development (see latent heat). Additional energy is derived as the air masses struggle to obtain equilibrium. Warm air rises along the warm front and cold air sinks behind the cold front. The rising and sinking air transforms potential energy into kinetic energy (energy of motion).

The storm’s development and movement depend upon the winds aloft. Strong winds above the storm quickly sweep the rising air downwind. If the winds aloft remove the air above the storm more quickly than the surface air converges, the surface pressure drops and the storm system intensifies. Conversely, if the converging surface air is greater than the removal of air aloft, the surface pressure rises and the storm system weakens. Because the winds above the surface storm typically blow from the southwest (in the northern hemisphere), the center of the surface low normally moves northeastward.

As the storm system moves northeastward, the faster-moving cold front catches up to the slower-moving warm front. Eventually the cold front overtakes the warm front and the storm system becomes occluded. With cold surface air on both sides of the occluded front, warm air is no longer rising and the cold air is no longer sinking. The storm is now without its primary source of energy (the conversion of potential energy into kinetic energy during the forceful lifting of warm air) and the storm system dies out and dissipates.

D

Tropical Cyclones

Tropical cyclones, also known as hurricanes and typhoons, are storms with sustained winds in excess of 120 km/h (74 mph). They form over warm, tropical waters between 5° and 20° latitude, where the winds are light and the humidity is high.

Tropical cyclones form over water when a mass of thunderstorms becomes organized and spirals in toward the storm’s center, or eye. The storm’s highest winds, strongest thunderstorms, and heaviest rain, occur just outside the eye in the region called the eye wall. In the eye itself, winds are usually light and skies are partly cloudy.

Tropical cyclones tend to form along a weak area of low pressure, called a disturbance or wave, in the intertropical convergence zone (ITCZ). As the disturbance becomes more organized, it first becomes a tropical depression, then a tropical storm, and finally a tropical cyclone. For the tropical cyclone to intensify, the outflow of air above the storm must exceed the inflow of air at the bottom. Tropical cyclones derive their energy from the transfer of heat from the warm water and from the latent heat given up to the system during condensation. Tropical cyclones dissipate when they are cut off from their energy sources either by moving over cold water or a large land mass. Although tropical cyclones often take erratic paths, the prevailing easterly winds in the tropics tend to steer tropical cyclones westward or northwestward until they leave the tropics, then the prevailing westerlies tend to sweep them northeastward.

IX

Weather Prediction

Weather forecasting entails predicting how the present state of the atmosphere will change. Present weather conditions are obtained by ground observations, observations from ships and aircraft, radiosondes, Doppler radar, and satellites. This information is sent to meteorological centers where the data are collected, analyzed, and made into a variety of charts, maps, and graphs. These charts, maps, and graphs are then sent electronically to forecast offices where local and regional weather forecasts are made. In addition, these offices prepare weather advisories and warnings of impending severe weather.

Modern high-speed computers transfer the many thousands of observations onto surface and upper-air maps. Computers draw the lines on the maps with help from meteorologists, who correct for any errors. A final map is called an analysis. Computers not only draw the maps but predict how the maps will look sometime in the future. The forecasting of weather by computer is known as numerical weather prediction.

To predict the weather by numerical means, meteorologists have developed atmospheric models that approximate the atmosphere by using mathematical equations to describe how atmospheric temperature, pressure, and moisture will change over time. The equations are programmed into a computer and data on the present atmospheric conditions are fed into the computer. The computer solves the equations to determine how the different atmospheric variables will change over the next few minutes. The computer repeats this procedure again and again using the output from one cycle as the input for the next cycle. For some desired time in the future (12, 24, 36, 48, 72 or 120 hours), the computer prints its calculated information. It then analyzes the data, drawing the lines for the projected position of the various pressure systems. The final computer-drawn forecast chart is called a prognostic chart, or prog.

A forecaster uses the progs as a guide to predicting the weather. There are many atmospheric models that represent the atmosphere, with each one interpreting the atmosphere in a slightly different way. The forecaster learns the idiosyncrasies of each model and places more emphasis on the ones that do the best job of predicting a particular aspect of the weather.

Weather forecasts made for 12 and 24 hours are typically quite accurate. Forecasts made for two and three days are usually good. Beyond about five days, forecast accuracy falls off rapidly.

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