Why is weather so changeable?

Masses of air that are either warm or cold, wet or dry, contain air at either high or low-pressure areas, constantly moving across the land and the sea. Weather changes as one air mass replace or meets another.

The most dramatic weather changes occur along a front, the boundary between two air masses with different temperatures and densities. A warm front is a boundary between an advancing warm air mass and the cooler one it is replacing. Because warm air is less dense than cool air, and an advancing warm front will rise over a mass of cool air.

As the warm front raises, its moisture starts to condense, and it produces many layers of clouds at different altitudes. High, wispy clouds are the first signs of an advancing warm front. Gradually the cloud thickens, descends to a lower altitude, and often releases its moisture as rainfall. A moist warm front can bring days of cloudy skies and drizzle.

A cold front is the leading edge of an advancing mass of cold air. Since cold air is denser than warm air, an advancing cold front stays close to the ground and wedges underneath less dense warmer air. An approaching cold front rapidly moving towering clouds called thunderheads. As the overlying masses of warm air are pushed upward, it cools, and its water vapor condenses and falls to the Earth's surface as a precipitate. As a cold front passes through, we often experience high surface winds and thunderstorms. We usually experience cooler temperatures and a clear Sky after the front passes through.

Weather is also affected by changes in atmospheric pressure. An Air mass with high pressure contains cool, dense air that descends towards the Earth's surface and becomes warmer. Fairweather follows as long as the high-pressure air mass remains over an area.

In contrast, a low-pressure air mass, called a low, produces cloudy, sometimes Stormy weather. This happens because less dense; warm air spirals inward toward the center of a low-pressure air mass. Because of its low pressure and low density, the center of the low rises, and its warm air expands and cools. This causes moisture to condense (because cool air cannot hold as much moisture as warm air). Precipitation often follows.

In addition to normal weather changes, we sometimes experience weather extremes. Examples are violent Storms called tornadoes (which form overland) and tropical cyclones (which form over warm ocean waters and sometimes pass over coastal land). Tropical cyclones are called hurricanes in the Atlantic and typhoons in the Pacific.

Twice a day, metrologists use devices such as weather balloons, aircraft, radar, and satellites to obtain data on variables such as atmospheric pressures, precipitation, temperatures, wind speed, and locations of air masses and fronts. These data are then fed into computer models to draw separate weather maps for the seven troposphere levels from the ground to 19 km up used in forecasting. Computer models use the map data to forecast the weather in each box of 7 layered grids for the next 12 hours. Other computer models project the weather for the next several days by calculating the probabilities that air masses, winds, and other factors will move and change in certain ways.