United States Search and Rescue Task Force
What Is Weather?
Weather simply refers to the condition of the air on earth at a given place and time – whether it is warm or cold, dry or wet, blowing or calm. The condition of air and how it acts to create weather is influenced primarily by two things – heat (the sun) and water.
Weather on earth starts with the sun. The sun's heat warms our atmosphere and causes two things to happen:
water evaporates into the air
As air rises, its temperature drops. As the moisture in the air cools, it condenses into tiny suspended droplets, forming clouds. The droplets inside the clouds become larger a more moisture is evaporated into the air. Eventually, they are too heavy to remain suspended and fall to earth as precipitation – rain, snow, sleet or hail.
Fronts and Air Masses
Weather maps show the positions of air masses. An air mass is a very large body of air that has similar properties. Air masses move, so they can be tracked over time. They are clues to the future temperature, moisture level and air pressure of the area into which they are moving.
When one air mass meets another, the air in different masses usually does not mix because the properties of the air are different. A boundary, or front, forms between the two air masses. Stormy weather often occurs along fronts.
A cold front forms where a cold air mass moves into a warm air mass. Cold air contracts, becomes more dense, and thus heavier than warm air, so the cold air pushes underneath the warm air. Cumulus clouds and showers or thunderstorms may be scattered along cold fronts. As the front passes, the wind changes direction. Skies begin to clear, and the temperature usually drops.
A warm front forms where a warm air mass moves into a cold air mass. Because the warm air is less dense, it slides up and over the colder air. At first, cirrus clouds might appear. They may be followed by stratus clouds and some precipitation, either rain or snow. When the front passes, the sky clears and the air pressure rises. Temperatures also rise as warm air replaces cold air.
Even though air masses often move, sometimes they don't, resulting in a stationary front. Precipitation and weak winds usually occur in areas with stationary fronts.
High and Low Air Pressure
Certain weather conditions are associated with high and low pressure systems.
High Pressure Areas
High pressure areas, or highs, are shown by “H” symbols. In a high pressure system, air pressure is greater than the surrounding areas. This difference in air pressure results in wind, or moving air. In a high pressure area, air is more dense than in areas of lower pressure. The result is that air will move from the high pressure area to an area of less density, or lower pressure. Winds blow away from high pressure areas toward areas having lower air pressure.
Low Pressure Areas
Low pressure areas, or lows, are shown by “L” symbols. Winds tend to blow into low pressure areas because air, like other gases, moves from areas of higher pressure into areas of lower pressure. As winds blow into a low, the air moves up. This upward flow of air can cause clouds and precipitation to form.
Air pressure is measured by barometers. In general, weather will improve when pressure increases and worsen when pressure decreases.
Precipitation: Water in the Air
Water is one of the few substances that can be found in all three states–solid, liquid and gas–at normal temperatures and pressures. You might not be able to see all the forms of water, but they're there.
All the forms of water that fall from the air to the earth's surface are called precipitation. Whether the precipitation is snow, rain, sleet or hail depends on the temperature of the air that the water falls through. If the air is above freezing, the precipitation will most likely be rain. If the air is below freezing, the precipitation will most likely be snow.
But sometimes, different temperature layers exist within the air through which the water is falling. For example, if snow falling from a high cloud passes through a warmer layer, then it could melt into rain. But if the rain passes through yet another layer that's colder near the ground, then it becomes sleet. Hail is also the result of water passing through different temperature layers.
Water Vapor and Humidity
When people say “It's not the heat–it's the humidity!” they're really referring to the relative humidity, or the amount of water in the air at a certain temperature compared to how much water the air can hold at that temperature.
Air, like all gases, expands at higher temperatures. So air can hold more water vapor at higher temperatures than it can at lower temperatures. The amount of humidity in the air affects how quickly water can evaporate from surfaces such as roads, lakes, even your skin! For example, at an air temperature of 80 degrees, 60 percent humidity might be very comfortable. But at 90 percent humidity, sweat evaporates from your skin very slowly, and you feel hot and sticky.
Wind is caused when air moves from an area of high pressure to one of low pressure. The greater the difference between the areas, the stronger the wind. Since the equator is constantly hot and the poles are cold, there is a general pattern to air circulation on Earth. In many areas the wind usually blows from the same direction. Wind speed is measured with an anemometer; wind direction is indicated with a wind vane.
The heat of the atmosphere is largely influenced by the sun and the degree to which it warms the air, land and oceans. The land and water retain heat and continue to heat the atmosphere after the sun disappears. Air temperature is measured with thermometers.
Meteorologists are scientists who monitor weather conditions. They sample a wide network of weather stations and use satellite images to map out the positions of the large air masses circling the Earth. Since air masses interact in a relatively predictable way, meteorologists are able to predict weather patterns with some degree of accuracy.
As explained above, Fronts are responsible for most changes in weather. They occur when a large mass of cold air meets a large mass of warm air. The front is the zone along which the masses come into contact with each other. Cold Fronts occur when a cold air mass, which has a high density, pushes under a warm air mass. The warm air is pushed upward at a sharp angle, causing moisture to condense rapidly. Heavy precipitation is often the result. Warm Fronts occur when a mass of warm air passes over a mass of cold air at a moderate angle.
Geography – While fronts indicate the character of prevailing weather systems, local weather is greatly influenced by geography.
Every weather prediction is based on the chance that the weather conditions will act together in a certain way. However, forecasts can be wrong. Some general patterns help meteorologists predict weather for a local area.
Most weather conditions across the United States move from west to east. Moving weather conditions are due to wind patterns across the earth. In any given place, a clue to tomorrow's weather may be found by looking at the weather of the area to the west.
However, geographic features in your area such as large bodies of water and mountains can affect local weather. For example, if you live near the Atlantic Ocean, conditions to the east often influence your weather. And whether you live near the east coast or west coast, temperatures near the ocean may be higher than they are even short distances inland. The heat-holding ability of large bodies of water causes warming of the air along the coast. In a city such as Chicago, which is located on Lake Michigan, forecasters often give two sets of information: one for people living within 5-10 miles of the lake, and the second for people in outlying areas. In the spring, for example, it's generally warmer in the outlying areas than it is closer to the lake.
People who live on the eastern side of a mountain range such as the Sierra Nevadas have mostly dry weather. Even when moisture-laden air masses move into the area, their interaction with the mountains causes most of the moisture to fall on the western slopes.
So keep in mind that you can't just look at a weather map and expect to make totally accurate predictions. Lots of factors affect what happens in an area on any given day.
For geography, remember these five points:
- coastal areas have more moderate temperatures than inland areas, and are generally warmer in the winter and cooler in the summer.
- in hilly areas, hot air moves up the slopes during the day and won the slopes at night.
- high altitude areas are usually colder and receive more precipitation than low altitude areas.
- the air above cities is often warmer than the surrounding area. In certain situations, this can create an artificial low pressure system.
- in costal areas, cool air usually blows inland during the day and out to sea at night.
Tools We Use for Weather Prediction
While the National Weather Service uses sophisticated equipment such as Doppler radar and high-altitude balloons to collect data, you can use many of the same tools they use.
Barometers measure the air pressure, which is sometimes referred to as barometric pressure. The pressure of the air on the pool of mercury in the barometer causes the mercury to rise in a tube. We measure the height of mercury in the tube in inches. Therefore, air pressure is often stated in inches of mercury.More common are anaeroid barometers, which don't contain mercury but have a small box inside instead. The air pressure on this box causes it to change shape, moving a needle on a gauge that indicates the air pressure. Normal air pressure readings vary from 28 to 31. Quick changes in air pressure often mean a change in the weather is about to occur. That's why you'll often hear and read about barometric pressure during local weather reports.
Meteorologists use anemometers to measure wind speed, but you can estimate wind speed just by looking around. Watch how smoke rises in chimneys, how leaves move in trees, and how flags wave in the wind. Sailors and other people sometimes rate their observations of wind speed according to the Beaufort scale.If you don't have an anemometer to measure wind speeds, you can get a good idea of how fast the wind is blowing just by looking at objects around you. In 1805, the British Admiral Sir Francis Beaufort devised an observation scale for measuring winds at sea. The Beaufort Scale measures winds by observing their effects on sailing ships and waves. Beaufort's scale was later adapted for use on land and is still used today by many weather stations.
Look at the diagrams below. Each represents one level on the Beaufort scale.
Compare the two pictures below of winds at different speeds on water with the Beaufort scale.Wind Vane
While anemometers measure how fast the wind is blowing, wind vanes tell you from which direction the wind is blowing. And knowing where the wind is coming from might give you clues to the temperature and the amount of water in the air moving into an area. For example, winds from the south are often warmer and carry more moisture than winds from the north.
Psychrometers, or wet bulb thermometers, measure relative humidity. A psychrometer uses two thermometers, one bulb of which is covered with a wet cloth. As the cloth dries, the cooling effect of evaporation lowers the temperature on that thermometer. Then the temperatures on the two thermometers are compared on a special chart to find the relative humidity. Often, the relative humidity is the weather condition that makes people the most uncomfortable.
Thermometers measure the air temperature via the expansion or contraction of a liquid or a metal as the air temperature changes. Some thermometers contain red-colored alcohol, others contain mercury, while still others have a bimetal coil attached to a gauge.
In the United States, temperature is measured using two scales, both of which are based on the state of water at sea level. Most people use the Fahrenheit scale, on which water freezes at 32 degrees and boils at 212 degrees. Scientists and other people who regularly use metric measurements measure temperature on the Celsius scale, on which water freezes at 0 degrees and boils at 100 degrees.
Rain gauges are very simple instruments used to measure the amount of liquid precipitation. Any open container with a flat bottom and straight sides will work just by adding a scale of inches to it. Other kinds of precipitation are usually recorded by collecting the precipitation in a similar instrument, then letting the precipitation melt to find out the liquid equivalent. But at home, you might just want to use a ruler or yardstick to measure a deep snowfall! Then you can use the average conversion of 10 inches of snow equals 1 inch of rain to find out how much water fell.
Look up in the sky. What kind of clouds do you see? Clouds are made up of liquid water or ice crystals, and the water droplets or particles are not large or heavy enough to fall to the ground. Fog is also made up of liquid water and can be called a “cloud on the ground.”
In general, different kinds of clouds indicate different kinds of weather.
Low Clouds – Under 10,000 Feet
Cumulus clouds –
Cumulus is Latin for heap. Cumulus clouds are usually associated with fair weather, but can produce precipitation if they are very tall. When large and bunched, they can cause heavy showers, especially in warm weather.
Stratus clouds –
Stratus is the Latin word for layer or blanket. Stratus clouds form a low layer that can cover the entire sky like a blanket. Rain and drizzle often come from stratus clouds. If they lift quickly in the morning, they often indicate a fine day ahead.
Nimbostratus Clouds –
Nimbostratus clouds are dark sheets of clouds which blot out the sun and are often followed by lengthy precipitation within a few hours.
Stratocumulus Clouds –
Stratocumulus clouds are low, rolling mass of thin, lumpy gray to white clouds. They may produce light precipitation but usually dissipate by the end of the day.
Middle Clouds – 10,000 to 20,000 Feet
Altocumulus Clouds –
Altocumulus clouds are larger than cirrocumulus clouds and, they are patterned white to gray clouds that are often rippled or appear in waves. Considered fair weather clouds, they often follow storms.
Altostratus Clouds –
Altostratus clouds are formless gray to bluish clouds that form a thin veil over the sun and moon. If the clouds gradually darken and blot out the sun or moon, precipitation will follow.
High Clouds – Over 20,000 Feet
Cirrus clouds –
In Latin, the word cirrus means curl. Cirrus clouds are very high in the atmosphere where the air is very cold. These clouds of ice crystals are usually associated with fair weather, but may sometimes indicate that storms are on their way.
Cirrostratus Clouds –
Cirrostratus clouds are milky, white-veined clouds that produce a halo around the sun or moon. Often called ‘bed-sheet' clouds, if they are replaced by cirrostratus clouds it usually means that precipitation will follow.
Cirrocumulus Clouds –
Cirrocumulus clouds appear in layers that look like rippled sand or fish scales. Nicknamed ‘mackerel sky,' they are considered an omen of good weather.
Contrail Clouds –
Contrail clouds are thin, high altitude clouds that are formed when moisture released from jet engines turns into ice crystals.
Towering Clouds – Up To 60,000 Feet
Swelling Cumulus Clouds –
Swelling cumulus clouds are flat-bottomed and have growing, cauliflower-like towers. They often form in midday and precede cumulonimbus clouds.
Cumulonimbus Clouds –
Cumulonimbus clouds are towering storm clouds that bring rain, sleet, hail, thunder, lightning and tornadoes. The top of the cloud is often anvil-shaped.
Thunderstorms and Lightning – Thunderstorms occur when large air masses rise quickly into the atmosphere, forming huge cumulonimbus clouds. Severe air currents inside the clouds cause water droplets and ice crystals to crash into one another continually, and the friction between these particles creates static electricity in the cloud.
Over time, opposite charges build between the top and bottom of the cloud, and the bottom of the cloud and the earth. When these opposing charges become intense, a gigantic spark occurs (lightning) which jumps the gap between the cloud and the earth. The thunder accompanying lightning is the noise produced by the discharge.
Hurricanes – Hurricanes are high speed windstorms accompanied by torrential rains. They begin over the ocean where air rising from warm seas creates a severe low pressure zone. This zone draws air to it with such force the winds rotate around the core (the eye) at up to 185 mph.
The ‘eye' averages about 20 miles in diameter while the hurricane may be up to a few hundred miles in diameter. After the onset of a hurricane at sea, the storm moves slowly toward one of the poles and loses force as it moves into cooler areas or over land. Hurricanes are usually predicted by meteorologists well in advance of their occurrence.
Tornadoes – One of the most destructive natural phenomena, tornadoes are swirling winds that spin at phenomenal speeds of up to 400 mph. Like hurricanes, they are caused by severe low pressure zones that attract winds with force. They have relatively small bases, a fact which greatly magnifies their destructive power. Tornadoes are difficult to predict and often strike without warning. Meteorologists expect to be able to predict a tornado in the near future.
Waterspout – Waterspouts are tornadoes over water which draw up large columns of water. If a waterspout moves from sea to shore, expect fish and other sea creatures to rain down!
Dust Devil – Dust Devils' are small tornadoes over land that draw up dust and light debris. They are most common in desert areas.
Sundog – Sundogs are mock suns that appear to the left or right of the sun when it is low on the horizon. They are caused by light refracted through ice crystals present in cirrus or cirrostratus clouds.
Rainbows – Rainbows are caused when sunlight penetrates raindrops and is broken into bands of red, orange, yellow, green, blue, indigo and violet. You can simulate a rainbow easily by placing your back to the sun and looking through the mist of a lawn sprinkler.
Aurora Borealis – Aurora Borealis is caused when atomic particles from space react with atoms high in the atmosphere. Visible from Canada and the northern United States, auroras illuminate the night sky with pulsing curtains of colored light.
Do any of these sound familiar to you? Here are their meanings…
“Red sky at night, sailor's delight. Red sky in morning, sailor take warning.” (Red sunsets are usually followed by dry nights. A red morning sky means rain is on the way.)
“A sun-shiny shower, won't last half an hour.” (Showers that happen while the sun shines are brief.)
“Mackerel sky and mare's tails make tall ships carry low sails.” (Certain clouds are often followed by high winds.)
“Coming storms cause shooting corns.” (Aches and pains are aggravated when a storm approaches.)
Wind Chill Chart