Weather is the state of the atmosphere (mass of air surrounding Earth) at a particular place and point in time. Rain showers, gusty winds, thunderstorms, cloudy skies, droughts (prolonged period of dry weather), snowstorms, and sunshine are all examples of weather conditions.
Weather scientists, called meteorologists, use measurable factors like atmospheric pressure (pressure caused by weight of the air), temperature, moisture, clouds, and wind speed to describe the weather. Meteorologists make predictions of future weather based on observations of present regional weather patterns and past trends.
Weather prediction, or forecasting, is an important part of meteorology (weather science). Advance warning of such weather phenomena as extreme hot and cold temperatures, heavy rainfall, drought, and severe storms can protect people’s property and save lives.
The weather patterns that a region experiences over tens, hundreds, or thousands of years are called climate. For example, the northeastern United States experiences a wide range of weather during an average year.
Below-freezing temperatures and heavy snowfall are typical weather conditions in winter, while warm temperatures and afternoon thunderstorms are common in the summer.
Communities of plants and animals (ecosystems) adapt over thousands of years to survive the weather extremes of their particular climate. In New England, plants lie inactive, mammals grow shaggy coats, and birds fly south during the cold dark winter.
In the spring, trees pull sap from their roots and grow leaves, animals bear young, and seeds germinate in time to take advantage of mild temperatures and long, sunny days in the summer. Climate change happens over hundreds and thousands of years, but weather varies from day to day, hour to hour, and sometimes from minute to minute.
Weather conditions: pressure, temperature, and moisture
The atmosphere presses down on Earth’s surface. (There is no atmosphere in outer space. Without their pressurized space suits, astronauts’ bodies would explode.)
The weight of the column of air molecules above a surface is called atmospheric pressure. The average weight of the atmosphere on one square inch of ground at sea level is 14.7 pounds. People do not feel this pressure because their senses are adjusted to it and the human body is designed to withstand it.
Meteorologists use an instrument called a barometer to measure pressure, and atmospheric pressure is also called barometric pressure. Evangelista Torricelli (1608–1647), an Italian physicist, invented the barometer in 1643. His instrument, “Torricelli’s tube,” was a glass tube full of dense, liquid mercury with its end in an open dish of mercury.
His barometer works the same way that mercury barometers work in modern day. Air pressing down on the mercury in the dish pushes some of the mercury upwards into the glass tube. As air pressure increases, the mercury in forced into the tube and the column of mercury rises.
When air pressure decreases, the mercury flows back into the dish and the column of falls. Barometric pressure is often measured in inches of mercury. When a weather forecaster says the mercury is falling, it means that air pressure is falling, and bad weather may be approaching.
Atmospheric pressure differs from one place on Earth to another due to temperature, moisture, and topography (physical surface features). Pressure decreases with elevation. There are many fewer air molecules above a square foot (kilometer) on the summit of Mt. Everest than above a square foot of Waikiki Beach.
Air currents, better known as winds, blow from areas of high pressure to areas of low pressure. Rapidly changing patterns of winds, precipitation (any form of water falling), clouds, and storms develop around moving high and low pressure centers in Earth’s atmosphere.
Temperature affects air pressure and moisture in the atmosphere. Warmer air expands and rises, so pressure falls beneath rising columns of warm air. Warm air also holds more moisture, in the form of water vapor, than cool air.
Rising warm air in low pressure zones often carry water vapor high into the atmosphere. When the warm air begins to cool, the moisture condenses into droplets or freezes into ice crystals and clouds form. Precipitation and storms are common in low-pressure centers.
As air cools it contracts, causing air pressure to rise under the sinking air. Because cool air holds less moisture, and because sinking air masses are usually already dry, high pressure areas usually are low in humidity (air moisture).
High and low pressure systems
Major east and west-blowing winds blow high and low-pressure weather systems around Earth. High-pressure systems, also called anticyclones, consist of winds spiraling out from a high-pressure center under sinking, dry air.
Low-pressure systems, or cyclones, have low-pressure centers and winds that spiral toward their centers. High-pressure systems are called anticyclones. A cyclone has a column of warm air rising from its center. In anticyclones, the air sinks toward the center and warms as it descends.
In the northern hemisphere (half of the Earth), anticyclones spin clockwise and cyclones spin counterclockwise, and the reverse is true in the southern hemisphere. Because air travels from high to low pressure areas, high-pressure anticyclones often follow low-pressure cyclones.
In North America, the jet stream (high-speed winds that race around the planet at about five miles above the Earth) blows cyclones and anticyclones from west to east. In general, cyclones bring intense weather in the form of rain, snow, clouds, and storms.
Dry, clear, calm weather usually accompanies the passage of anticyclones. (The parched residents in deserts of the American Southwest might look forward to the clouds and rain storms a cyclone brings. A southward dip in the Jet Stream causes a near-permanent zone of high pressure over Arizona, New Mexico, and Southern California, and moisturebearing weather systems tend to bypass the region.)
Trade winds (persistent tropical winds that blow generally toward the west) blow low-pressure systems that develop in the tropical Atlantic Ocean west toward the Caribbean Sea and east coast of the United States. These tropical cyclones feed on warm ocean waters and can develop into massive storm systems called tropical storms and hurricanes.
Air masses and fronts
An air mass is a large body of air that has similar temperatures and moisture content throughout. Several air masses contribute to weather patterns in North America: cold, dry air over northern Canada; hot, dry air in the American Southwest; cool, moist air moving east over the Pacific Northwest; and warm, moist air traveling north from the Gulf of Mexico.
The boundaries between air masses are called fronts. A cold front occurs where a cold air mass is moving in to replace warm air. Clouds, precipitation, and storms are common at cold fronts. The incoming cold, dense mass lifts the warm, moist air and creates unstable conditions where moisture rapidly condenses and winds organize clouds into storms.
Once a cold front has passed, temperatures and humidity drop and a highpressure system moves in. A warm front precedes an incoming warm air mass. Warm fronts bring moisture and higher temperatures. Stationary fronts separate unmoving air masses.
A typical cyclone in the American Mid-West is a rotating pinwheel of three air masses and three fronts moving east toward the Atlantic Ocean. Cold, dry air flows south from Canada behind cool, moist air flowing from the Pacific Northwest. Warm air from the Gulf of Mexico moves north and contributes moisture to the system. Thunderstorms and blizzards develop along cold fronts.