science

Tornadoes

Tornadoes are most likely to occur when thunderstorms are likely—in spring and early summer, often late in the afternoon when the ground is warm. The Great Plains often have the kind of weather pattern that is likely to create tornadoes: A warm, humid air mass moves north from the Gulf of Mexico into the lower Great Plains. A cold, dry air mass moves south from Canada. When the air masses meet, the cold air moves under the warm air, forcing it to rise. A squall line of thunderstorms is likely to form, with storms traveling from southwest to northeast. A single squall line can produce ten or more tornadoes.

Warm Fronts

warm front, a fast-moving warm air mass overtakes a slowly moving cold air mass. Because cold air is denser than warm air, the warm air moves over the cold air. If the warm air is humid, light rain or snow falls along the front. If the warm air is dry, scattered clouds form. Because warm fronts move slowly, the weather may be rainy or cloudy for several days. After a warm front passes through an area, the weather is likely to be warm and humid.

Occluded Fronts

where a warm air mass is caught between two cooler air masses. The denser cool air masses move underneath the less dense warm air mass and push the warm air upward. The two cooler air masses meet in the middle and may mix. The temperature near the ground becomes cooler. The warm air mass is cut off, or occluded, from the ground. As the warm air cools and its water vapor condenses, the weather may turn cloudy and rain or snow may fall.

Doldrums

Near the equator, the sun heats the surface strongly. Warm air rises steadily, creating an area of low pressure. Cool air moves into the area, but is warmed rapidly and rises before it moves very far. There is very little horizontal motion, so the winds near the equator are very weak. Regions near the equator with little or no wind are called the doldrums.

Stationary Fronts

Sometimes cold and warm air masses meet, but neither one can move the other. The two air masses face each other in a "standoff." In this case, the front is called a stationary front. Where the warm and cool air meet, water vapor in the warm air condenses into rain, snow, fog, or clouds. If a stationary front remains stalled over an area, it may bring many days of clouds and precipitation.

How Hurricanes Form

A typical hurricane that strikes the United States forms in the Atlantic Ocean north of the equator in August, September, or October. A hurricane begins over warm ocean water as a low-pressure area, or tropical disturbance. If the tropical disturbance grows in size and strength, it becomes a tropical storm, which may then become a hurricane

Jet Streams

About 10 kilometers above Earth's surface are bands of high-speed winds called jet streams. These winds are hundreds of kilometers wide but only a few kilometers deep.

anemometer

An instrument used to measure wind speed.

Anticyclones

As its name suggests, an anticyclone is the opposite of a cyclone. Anticyclones are high-pressure centers of dry air. Anticyclones are usually called "highs"—H on a weather map. Winds spiral outward from the center of an anticyclone, moving toward areas of lower pressure. Because of the Coriolis effect, winds in an anticyclone spin clockwise in the Northern Hemisphere. Because air moves out from the center of the anticyclone, cool air moves downward from higher in the troposphere. As the cool air falls, it warms up, so its relative humidity drops. The descending air in an anticyclone generally causes dry, clear weather.

Cold font

As the warm air rises, it expands and cools. Remember that warm air can hold more water vapor than cool air. The rising air soon reaches the dew point, the temperature at which the water vapor in the air condenses into droplets of liquid water or forms tiny ice crystals. Clouds form. If there is a lot of water vapor in the warm air, heavy rain or snow may fall.

Polar Easterlies

Cold air near the poles sinks and flows back toward lower latitudes. The Coriolis effect shifts these polar winds to the west, producing the polar easterlies. The polar easterlies meet the prevailing westerlies at about 60° north and 60° south latitudes, along a region called the polar front. The mixing of warm and cold air along the polar front has a major effect on weather in the United States

Maritime Polar

Cool, humid air masses form over the icy cold North Pacific and North Atlantic oceans. Maritime polar air masses affect the West Coast more than the East Coast. Even in summer, these masses of cool, humid air often bring fog, rain, and cool temperatures to the West Coast.

Continental Tropical

Hot, dry air masses form mostly in summer over dry areas of the Southwest and northern Mexico. Continental tropical air masses cover a smaller area than other air masses. They occasionally move northeast, bringing hot, dry weather to the southern Great Plains.

Prevailing Westerlies

In the mid-latitudes, between 30° and 60° north and south, winds that blow toward the poles are turned toward the east by the Coriolis effect. Because they blow from the west to the east, they are called prevailing westerlies. The prevailing westerlies blow generally from the southwest in north latitudes and from the northwest in south latitudes. The prevailing westerlies play an important part in the weather of the United States.

Continental Polar

Large continental polar air masses form over central and northern Canada and Alaska, as shown in Figure 3. Air masses that form near the Arctic Circle can bring bitterly cold weather with very low humidity. In winter, continental polar air masses bring clear, cold, dry air to much of North America. In summer, the air mass is milder. Storms may occur when continental polar air masses move south and collide with maritime tropical air masses moving north.

Horse Latitudes

Warm air that rises at the equator divides and flows both north and south. latitude is distance from the equator, measured in degrees. At about 30° north and south latitudes, the air stops moving toward the poles and sinks. In each of these regions, another belt of calm air forms. Hundreds of years ago, sailors becalmed in these waters ran out of food and water for their horses and had to throw the horses overboard. Because of this, the latitudes 30° north and south of the equator came to be called the horse latitudes.

Maritime Tropical

Warm, humid air masses form over tropical oceans. Maritime tropical air masses that form over the Gulf of Mexico and the Atlantic Ocean move first into the southeastern United States.

Trade Winds

When the cold air over the horse latitudes sinks, it produces a region of high pressure. This high pressure causes surface winds to blow both toward the equator and away from it. The winds that blow toward the equator are turned west by the Coriolis effect. As a result, winds in the Northern Hemisphere between 30° north latitude and the equator generally blow from the northeast. In the Southern Hemisphere between 30° south latitude and the equator, the winds blow from the southeast. For hundreds of years, sailors relied on these winds to move ships carrying valuable cargoes from Europe to the West Indies and South America. As a result, these steady easterly winds are called the trade winds.

Cyclones

f you look at a weather map, you will see areas marked with an L. The L stands for "low," and indicates an area of relatively low air pressure. A swirling center of low air pressure is called a cyclone.As warm air at the center of a cyclone rises, the air pressure decreases. Cooler air blows toward this low-pressure area from nearby areas where the air pressure is higher. As shown in Figure 6, winds spiral inward toward the center of the system. Recall that, in the Northern Hemisphere, the Coriolis effect deflects winds to the right. Because of this deflection, winds in a cyclone spin counterclockwise in the Northern Hemisphere when viewed from above.Cyclones and decreasing air pressure are associated with clouds, wind, and precipitation.