Introduction to Weather Terms—Cloudy or Clear?
The forecast itself becomes more interesting and more understandable if we look into the meanings of the terms that forecasters use. Some parts of the weather forecast seem easy enough to understand—for example, the predicted high and low temperatures. But weather terms such as "high-pressure system," "occluded front," or "temperature-humidity index" sound more complicated. The meanings of these terms become clear, however, once you know something about the major factors that produce the weather and how these factors influence one another.
The Varities of Precipitation
Precipitation refers to rain, snow, sleet, and hail—types of moisture that fall from the atmosphere. Moisture enters the atmosphere through the evaporation of water from lakes, rivers, soil, and even green plants on Earth's surface. This evaporated water is called water vapor.
At a given temperature and pressure, the air can hold only a certain amount of moisture, and colder air can hold less than can warmer air. The dew point is the temperature of the air at which water vapor begins to condense (turn to liquid). The vapor may condense on Earth's surface as dew or frost or in the air around tiny particles of dust or pollution.
High in the air, the tiny water droplets form the clouds; near ground, they create fog. If the temperature of the air is below freezing, the water vapor that forms clouds turns into microscopic ice crystals.
Rain falls when the microscopic water droplets in clouds collide, merge, and finally become too heavy for the air to support them. If the air temperature is above about 4 °C (39 °F), ice crystals in clouds melt as they fall to Earth, also creating rain. At slightly lower temperatures, the result is sleet. Below about 3 °C (37 °F), the moisture falls as snow.
Hailstones are frozen raindrops or ice pellets that are swept up by powerful updrafts (upward movements of air) in storm clouds and coated with a layer of icy water that freezes. The pellets drop and then get carried back up again to freezing heights by the updraft. As this process repeats, the hailstones grow until they find their way into a downdraft that carries them to the ground. If the hailstones are large enough, they will not completely melt as they fall to Earth, even on a warm summer day.
In weather forecasts, forecasters often refer to the chance of precipitation as a percentage. A 30 percent chance of rain, for example, means that it rained in the past on 30 days out of every 100 with similar weather conditions.
Tracking Humidity
Humidity is a measure of the water vapor in the air. Absolute humidity is the actual amount of water vapor in a given volume of air. Weather forecasts more often refer to relative humidity—a ratio between the actual moisture and the maximum amount of moisture that air at that temperature could hold. If the temperature outside drops to the dew point, relative humidity becomes 100 per cent and fog is likely to form.
Relative humidity affects our comfort. In hot weather, high relative humidity slows the evaporation of water from the skin and makes us feel uncomfortably hot and sticky. In cold weather, high relative humidity conducts heat away from the body and makes the air feel cold and raw.
The temperature-humidity index (THI) sometimes mentioned in forecasts was once called the discomfort index. This scale estimates the degree of discomfort caused by hot, moist weather. The higher the reading, the more discomfort people feel. Most people feel comfortable with a THI below 75.
Barometric Pressure
Air pressure is the weight of the atmosphere pressing down on the surface of Earth. Meteorologists call it barometric pressure because the instrument used to measure air pressure is called a barometer. The barometer was invented in 1643 by Evangelista Torricelli, an Italian physicist, who demonstrated that the pressure of the atmosphere at sea level could push a column of mercury 76 centimeters (30 inches) up a glass tube. Hence, standard barometric pressure is about 30 inches of mercury at sea level.
In the United States, weather forecasts give barometric pressure in inches. Meteorologists who use the metric system express barometric pressure in units called bars and millibars. A millibar is 1/1000 of a bar, and standard barometric pressure averages 1,013 millibars at sea level.
Zones of high atmospheric pressure, also called highs, contain dense air. In the Northern Hemisphere, many highs form in polar regions because the colder a mass of air gets, the denser it becomes, and this increases its pressure. A high also occurs where an air mass cools in relation to the surrounding air. As the cooling air becomes denser, it sinks. This further compresses the air, warming it somewhat and increasing its ability to hold moisture. As a result, the water drops that make up clouds evaporate, and the sky over a high-pressure zone is usually clear.
Low-pressure zones, or lows, contain air that is less dense. Lows form where the air is warm and rises. The warm air cools as it rises, and the water vapor it holds begins to condense. As a result, the sky over a low is generally cloudy.
Changes in barometric pressure help forecast weather conditions. A high with rising barometric pressure typically signals fine, clear weather ahead, whereas a low with falling barometric pressure often indicates bad weather ahead—usually heavy, prolonged rain.
Weather Fronts
Fronts, where most abrupt changes in the weather occur, are the boundaries between huge masses of air, each with its own temperature and moisture content. The movements of these air masses influence local weather. For example, an air mass from the equator can produce balmy weather, while an air mass from northern Canada can bring a cold spell. Air masses of different temperatures meet and do battle along fronts. Fronts were discovered toward the end of World War I (1914–1918) and are so named because the clashing of air masses reminded meteorologists of the clashing of armies along a battlefront.
Strong fronts, which occur where there is a large difference in temperature or moisture between air masses, bring high winds and stormy weather. Weak fronts, where the two air masses are close in temperature, often pass unnoticed, except by meteorologists. In North America, highs, lows, and fronts generally move from west to east and follow curving paths. A front may be warm or cold, depending upon which air mass—the warmer or the colder—is pushing the air ahead of it.
Warm and Cold Fronts
A warm front occurs where a relatively warm air mass advances on a colder air mass. The warm air, which is lighter, pushes up and over the edge of the cold air. As the warm air rises, it cools and thereby loses some of its ability to hold moisture. If the air is fairly humid, some of this moisture may turn into rain or snow. If the air is dry, clouds may form but precipitation will be slight. The gentle rain or drizzle that warm fronts often bring usually lasts several days. After the front passes, the temperature rises and the sky clears.
A cold front forms where a relatively cool air mass overtakes a warmer air mass. The cold air, being denser, slides under the edge of the warmer air, lifting it rapidly. Tall clouds form as the warm air quickly cools on its steep ascent. The more moisture in the air, the larger the clouds are.
Cold fronts bring bad weather, but they move faster than warm fronts, and the bad weather usually ends soon. If a cold front moves very fast, it may slide under a warm air mass, lift it off the ground, and collide with another cold air mass on the far side of the warm air. In this case, an occluded front forms. The word occlude comes from a Latin word meaning to close off, and the two cold air masses essentially “close off” the warm air mass. Occluded fronts usually bring less severe weather than warm or cold fronts and tend to stay in one place for a longer time.
Sometimes air masses meet and do not move, forming a stationary front. This kind of front usually brings unsettled weather that lasts a while.
A thermal inversion is a weather condition that occurs when a mass of warm air forms over cooler air near the ground. The lighter warm air sits like a lid atop the cooler air, blocking the normal air circulation. In large cities such as Los Angeles, a thick haze of automobile exhaust, industrial chemicals, and other pollutants may build up during the thermal inversion. The warm air above is very stable and prevents the pollutants from rising and scattering and results in a type of air pollution often called smog.
Winds and Wind Speed
Wind is simply air in motion over Earth's surface. The sun's uneven heating of Earth's surface sets air in motion. As warm air rises, cool air rushes in to replace it.
The uneven heating of land and water surfaces along coastlines creates local winds. During the day, the land warms faster than the water and heats the air above it. As this warm air rises, cooler air over the water rushes in to replace it. At night, the land cools faster than the water, and cooler air blows from land to replace warm air rising over the water.
But the strongest winds occur along fronts. The greater the differences in temperature and pressure along the front, the stronger the winds are.
Winds also spiral into low-pressure zones, like water into a whirlpool. For this reason, meteorologists call low-pressure zones cyclones, a term that comes from the Greek word for circle. A high-pressure zone is called an anticyclone because wind flows out of it in the direction opposite that of a cyclone. North of the equator, the winds move counterclockwise around a cyclone and clockwise around an anticyclone due to Earth's rotation. South of the equator, cyclone winds move clockwise; anticyclone winds, counterclockwise.
A jet stream is a band of strong winds at a high altitude. Jet streams follow meandering paths from west to east at speeds up to 400 kilometers per hour (kph) (250 miles per hour [mph]), changing their course frequently. Over the Northern Hemisphere, cold air masses predominate to the north of a jet stream and warm air masses to its south.
Wind chill provides an estimate of the effect of wind speed on air temperature. Wind accelerates the loss of heat from the body, even on a hot day. The faster the wind blows, the more heat the body loses and the colder the temperature feels. On a day with a temperature of 1.7 °C (35 °F), for example, a wind of 8 kph (5 mph) has the effect of lowering the temperature to 0 °C (32 °F). A wind that is blowing twice as fast appears to lower the wind-chill temperature to -5.6 °C (22 °F).
Wind shear is a rapid change in the speed or direction of wind. Wind shear can be dangerous for airplane travel, especially if the change is a downburst, a gust of wind that blows straight to the ground. Downbursts may develop when precipitation falls through dry air, suddenly cooling the air and making it more dense. The air may then plummet to the ground.
Storms
A hurricane or typhoon is a violent, swirling storm that develops in a low-pressure area over tropical ocean regions. Hurricanes grow in size and strength as they travel, feeding on heat from the warm water. They peter out when they pass over land or cold water, which robs them of their source of energy. As hurricanes move onto land, their heavy rains often cause floods.
A tornado is a small, intense funnel of wind that extends downward from the dark clouds that form during thunderstorms. Wind speeds in a tornado can exceed 320 kph (200 mph). Strong updrafts of wind inside the funnel are powerful enough to lift automobiles and mobile homes into the air. In the United States, tornadoes occur most frequently in the Midwest in spring.
Winter storms and thunderstorms often form when air that is moist and relatively warm is set in motion, perhaps by an advancing cold front. Water vapor in the air quickly condenses as the warm air rises, forming massive, towering clouds that produce heavy rains, blizzards, or ice storms.
In thunderstorms, the motion of the air also causes electric charges to build up inside the cloud, producing lightning. Flashes of lightning heat the surrounding air, causing the air to expand violently and create the sound waves known as thunder.
Even when such severe weather is not threatening, it's still a good idea to understand the meaning of common weather terms. And knowing about the actions of fronts, highs, and lows can help you understand why the forecaster predicts that the weather will be fair or foul.