DESCRIBING WEATHER

Unit Overview
Have you ever wondered why weather conditions are always changing? You can see clouds, wind, rain, and snow that are signs that tell us about the weather. Remember in an earlier unit that weather is the condition of the air around us? There are other things that can be used to describe the weather. In this unit you will learn about temperature, direction, and speed of wind, precipitation, and the barometric pressure in the weather. You will see how some of these qualities affect the weather the most. While you study this unit try to keep in mind the four seasons here on Earth.

VOCABULARY
atmosphere	seasons	expands	limit
contracts	reveals	humidity
absorb	precipitation	steady

The ten vocabulary words above are used throughout unit four. Be prepared to know what the words mean so that you can understand how the words are used in the unit. Below are some definitions you can practice to help you understand the words better.

	DEFINITIONS
atmosphere	is the gases that surround the earth and other planet
absorbed	means to soak up or take up or use up
reveals	to make known or to allow to be seen
steady	means to firmly fix, support, or balanced
expands	to make or become larger or more extensive
controls	means the power to influence people or to influence the course of events
seasons	are each of the four divisions of the year (spring, summer, autumn, and winter) and marked by particular weather patterns and daylight hours
limit	is a point beyond which something does not or may not pass
humidity	is an increased level of moisture in the air
precipitation	is rain, snow, sleet, or hail that falls to or condenses on the ground

The first thing to consider about weather is air temperature. It is air temperature that affects the weather more than anything else. It is the sun that determines the air’s temperature. What happens is the sun sends in energy to the atmosphere. This is where the sun’s energy is absorbed and then turns into heat. Cold air that creates high-pressure areas reveals mild or fair weather. Warm air that creates low pressure does not weigh as much as cold air and it will reveal cloudy or sometimes storming weather.

Now that we have discussed air temperature, think about the way air moves. Moving air is called wind and it has a path that it usually follows. What happens is when there is high pressure in an area; air usually goes to an area of low pressure. Before you look at how these weather qualities can be measured it would be a good time to review weather by using some weather instruments. Air temperature can be checked by a thermometer. Your own body temperature can be checked with a thermometer. A thermometer that is used for checking weather has numbers on it that use a scale. When the mercury in the thermometer’s tube becomes hotter, it expands and rises up the tube. The opposite happens when the mercury cools. As the mercury drops it contracts

Liquid-in-glass thermometer: glass tube filled with liquid (often mercury or alcohol) that expands/contracts with air temperature
	Inside of a thermometer
How a thermometer is used.
One of the most common things about the weather that people measure is how hot or cold it is - the air temperature. (above) You can see how the thermometer is used to measure the air temperature.

The direction of the wind can be found using wind vanes or anemometers. Both of these tools can find the wind direction by which way the wind is coming. Keep in mind that the temperature of the air tells how hot or cold it is outside. As the weather (the condition of the air around us) changes in temperature the wind also changes. Whether the wind is giving a calm day, or a gentle breeze, or a strong wind such as a hurricane, the weather’s temperature and the wind direction and speed help us to understand the seasons here on Earth. Before looking at the seasons, the precipitation and the barometric pressure in weather needs to be better understood. The weather changes from season to season and there are four seasons on Earth. Summer, autumn, winter, and spring are the four seasons. The season of summer will be a good place to start. In the summer the air temperature gets warm and in autumn air temperature gets cooler. In the winter the air temperature gets colder and in spring air temperature turns warmer. It is the air’s pressure that pushes against its surroundings that can explain what barometric pressure is all about when studying weather.

ANEMOMETER
The revolving-cup electric anemometer is used to measure wind speeds. The revolving cups drive an electric generator that operates an electric meter.
Anemometer and direction vane mounted atop an instrument tower.

WHAT IS A WEATHER VANE?
	A weather vane, also called a wind vane, is a movable device attached to an elevated object such as a roof for showing the direction of the wind.

Air pressure can be measured with a barometer. Air pressure changes with the temperature. Warm air will result from this in the air and the air particles move farther apart. As air particles move away from each other, low air pressure results. With cold air the opposite is true. When cold air presents itself, the air particles move closer together. It is good to know that the differences in air pressure in an area that causes the wind to change.

ANEROID BAROMETER
Aneroid barometer is an instrument for measuring the atmospheric pressure. It registers the change in the shape of an evacuated metal cell to measure variations on the atmospheric pressure.

Which direction the wind blows is why a wind or weather vane is used. It is the anemometer that determines the wind (air) speed. As water vapors change in the air three things are affected and they are humidity, clouds, and precipitation. Humidity and clouds will be looked at more closely in a future unit. It is precipitation that is important to check at this time. You learned in the last unit about evaporation and condensation. Now a glance at humidity! A rain gauge is what is used to measure precipitation. Weather people measure precipitation at the same time every day. Precipitation is not just rain, it is snowfall too. However, snow would need to melt before it could be measured accurately. Keep in mind that the amount of water vapor in the air is humidity. When water vapors change so does the weather. Try to recall what was said earlier in this unit about temperature. There is a limit to the amount of water (precipitation) that the air can hold. Because this limit is not steady or constant, it is the temperature of the air that causes the amount of water to change. Think about what the weatherman says on television when he/she talks about the relative humidity during a weather forecast. Two things are important to know when is comes to relative humidity. They are how much water vapor is in the air and the temperature it is outside at the time of the measurement. These are compared to the total amount that could be held at that given temperature. Sounds like a lot of information, doesn’t it? Think of it this way. If the weather person says the relative humidity is 80%, then the air is holding 20% less water than what it usually does. So, the role of water vapor in the air causes different types of precipitation, cloud formation, and humidity.

WHAT IS A RAIN GAUGE?
A rain gauge is a type of instrument used by meteorologists and hydrologists to gather and measure the amount of liquid or solid (snow, sleet, and hail) precipitation over a set period of time.

Over the course of the hot summer we have all experienced a humid day.  A device called a hygrometer informs us on the humidity level each day.

WHAT IS A HYGROMETER?
This is a picture of an indoor hygrometer.	This is a picture of an outdoor hygrometer. You can read outdoor humidity from indoors.

What is Humidity?
Humidity is the amount of moisture in the air. It can be measured in various ways, but the most usual is to describe it as 'relative' humidity. This is expressed as a percentage. A relative humidity of 100% means the moisture content of the air is the maximum possible at any particular temperature. The hotter the air, the more moisture it can hold.

Why is it Important?
When relative humidity is low, evaporation is rapid. Soil dries out, wet clothes dry quickly and perspiration evaporates from the skin. When relative humidity is high, clothes dry slowly and body sweat cannot evaporate easily. We feel sticky and hot and because our perspiration cannot evaporate quickly, we feel uncomfortable. We can stand dry heat much better than damp heat which makes us feel listless and takes our energy away.

Supercomputers

NOAA ACTIVATES NEWEST CLIMATE AND WEATHER SUPERCOMPUTERS
	The National Oceanic and Atmospheric Administration’s newest weather and climate supercomputers, increasing the computational might used for the nation's climate and weather forecasts by 320 percent. The new IBM machines process 14 trillion calculations per second at maximum performance and ingest more than 240 million global observations daily. Photo credit “NOAA.”

Weather Maps

WEATHER MAPS
	Weather maps are maps that show predictions of coming weather or report on weather that is actually happening. Weather is how hot or cold or how wet or dry it is in a location over a short period of time.

Radar

RADAR
“Is it going to rain or should I pack the sunscreen?” One of the first things many of us do before leaving the house each morning is check the weather forecast. Although we often complain about the accuracy (or seeming lack thereof) of the weatherman’s predictions, the truth is that weather forecasts are usually very reliable, at least in the short term. This is due, in large part, to the use of weather radar. The idea that radar could be used to detect precipitation (rain, snow, etc.) was first suggested in 1941. A year later, testing of early radar systems showed that rain did show up on radar screens. In fact, when the weather showed up on these early radar screens it created big problems. Engineers found that storms appeared as large areas of moving “clutter” on their screens and obscured the targets (such as ships or airplanes) that they were seeking. The military were quick to exploit the discovery that radar could be used to detect weather. The military trained dozens of meteorologists in radar technology and by 1943 surface-based and airborne weather radar observations were being established worldwide by the United States armed forces.
	(left) A typical weather radar image, similar to one we might see on the local news. This one is of weather over and around Japan. Courtesy: The Weather Channel
(right) Weather radar can give us advance warning of potentially dangerous weather, such as tornadoes. Courtesy: National Severe Storms Laboratory.
One of the radars in the NEXRAD network. Courtesy: National Oceanic and Atmospheric Administration.
In the United States, the federal government operates a network of weather radar stations called NEXRAD, which completely covers the entire country. The data generated by NEXRAD are disseminated to governmental and commercial users for use in the preparation of storm and flood forecasts and meteorological studies. Information gleaned from NEXRAD is used to create time-lapse radar displays of storms, a regular feature of TV weather reports that show weather systems in motion.

Weather Satellites

TIROS I THE FIRST METEOROLOGICAL SATELLITE
During the 1950s weather scientists gave serious thought to capturing images of meteorological phenomena from satellites launched into space by powerful rockets. Dr. Harry Wexler, chief scientist of the Weather Bureau during that period, strongly advocated meteorological satellites. In March 1958, the President's Science Advisory Committee stated "The satellite that will turn its attention downward holds great promise for meteorology and the eventual improvement of weather forecasting." Two months later, Project TIROS (Television and Infra-Red Observation Satellite) was born when the U.S. Committee on Meteorological Satellites began its initial planning sessions.
	The first image taken from TIROS I on April 1, 1960. When stations on Earth received data from TIROS, technicians recorded it on 35 millimeter film for making prints and large projections of the image. Forecasters dreams of watching weather from space had finally come true.
TIROS I, the world's first meteorological satellite blasts off aboard a Thor-Able rocket on April 1, 1960, from Cape Canaveral, Florida. The day marked the first time it was possible to observe the Earth's weather on a regular basis from space.

A barometer, wind vane, anemometer, rain gauge, supercomputers, weather maps, weather satellites, hygrometer and radar are all instruments used to measure the parts of the weather. Learning about the seasons and how they change will be easier when you understand the parts that make up the weather.

Activity One:
On a sunny day, take a clear, plastic drinking cup and turn it upside and place it outside on the grass in a safe place. Watch what happens! Water molecules from the grass and soil will evaporate and form water vapor in the glass.

Activity Two:

Take a piece of paper and get it wet. Hold the wet paper and blow on it or take it outside and let the wind blow on it. What do you think will happen? The paper will dry more quickly than a wet paper that has been placed in an area where the air is still and quiet. Wind will speed up evaporation. Also, using a fan in this activity will show quick results.

The following list of information is being given to you if you want to learn more about the weather. A funny book to read about weather is called Cloudy with a Chance of Meatballs by Judi Barrett.

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Below are additional educational resources and activities for this unit.

Unit 4 Science 140