When people say that it is going to rain, or that it is cold outside, what is the “it” that they are talking about? The “it” refers to the atmosphere – the blanket of air that surrounds the Earth. The atmosphere is mainly made up of nitrogen (78%) and oxygen (21%) gas. It is an important renewable resource. The atmosphere is also called the source of weather. In fact, the weather really is what is happening in the atmosphere at a given time in a particular place.

The mixed gases in the atmosphere, known as air, are usually invisible to us. However, air is a form of matter which means it has mass and takes up space. Because we are surrounded by invisible air, we are not always aware of it. I f you put air into a balloon and squeeze it, you sill see that air takes up space inside the balloon. W hen air moves around us, we can feel it as w ind.

M ost of the time, we are not aware of the air, but air is always pushing down on us. We live at the bottom of an ocean of air. The weight of air high in the atmosphere compresses, or squeezes together, air closer to the Earth’s surface, causing a higher air pressure. The weight of the air causes air pressure – the force applied by the weight of the air pushing down. Air pressure is not always the same. It changes based on the amount of moisture in the air, how cold or warm the air is, and other factors. Meteorologists use an instrument called a barometer to measure air pressure. Barometers generally fall into two main groups: When air pressure changes, the weather usually changes as well. So, studying air pressure helps meteorologists predict weather.

Some barometers use mercury, a silver liquid. A long glass tube sits in mercury open to the air. Air pressure is often measured in inches of mercury. A second type of barometer is round in shape. It uses a metal spring, like the spring of a spring scale. Air pressure squeezes the spring. A needle points to the amount of air pressure. Air pressure can sometimes be called barometric pressure. Air pressure can be measured in millibars, kg per cm², or pounds per square inch.

A barometer is a very effective tool to help predict what type of weather is ahead. When there is an increase in air pressure, which causes the barometer to rise, we can expect mild weather –clear skies, sun, little wind, and dry weather ahead. H - High pressure climate/air-pressure-and-wind.htm When there is a decrease in air pressure, which causes the barometer to fall, the opposite is probably ahead-clouds, less sun, more wind, and possibly precipitation of some kind. L - Low pressure

S ome air masses have high pressure and others have low pressure. Differences in air pressure are caused by the uneven heating of Earth and the atmosphere. As air gains heat energy, its particles move faster and spread out. This makes an air mass that has low pressure. The pressure differences between different air masses makes wind blow from areas of high pressure to areas of lower pressure. If an air mass cools, its particles slow down and get closer together, making an air mass that has high pressure. Different amounts of pressure in air masses (huge bodies of air) are what cause wind. The greater the difference in pressure, the faster the wind blows.

Meteorologists measure wind speed with an anemometer. The simplest anemometer consists of 4 cups, each mounted on one end of four arms. The flow of the air turns the cups. Measuring the cups’ motion over a period of time can tell the average wind speed in km per hour. cience/weather-and-climate/weather- instruments.htm Wind is important because it moves weather from place to place. When is wind helpful to us? When is wind harmful to us?

Knowing the direction of the wind is an important part of predicting the weather because wind brings us our weather. A wind vane spins and points in the direction from which the wind is coming. (So, if the arrow is pointing to the east, it means the wind is coming from the east. cience/weather-and-climate/weather- instruments.htm To use a wind vane, you must know where north, south, east and west are. A weather vane or wind sock measures the direction of the wind.

Warm Front: A warm front indicates a warmer air mass is moving in the direction toward which the semicircles are pointing. The warm front is drawn at the leading edge of the warm air mass.. When a warm air mass catches up to a cold air mass. The warm air slides up over the cold air. Clouds form and precipitation may fall. The sky clears, then the temperature becomes warmer. A border that forms when 2 air masses meet (they don’t mix).

Cold Front: A cold front indicates a colder air mass is moving in the direction toward which the triangles are pointing. The cold front is drawn at the leading edge of the cold air mass. es2002/es2002page01.cfm?chapter_no=visualization When a cold air mass catches up to a warm air mass. When the warm air is pushed up, it forms clouds. Rain or thunderstorms may occur. After the rain, the temperature is cooler.

 T o describe the weather, meteorologists use special instruments to measure conditions. Meteorologists use a thermometer to measure the amount of heat (thermal) energy in matter, or how hot or cold something is. They measure temperatures in either in degrees Celsius (°C) or Fahrenheit ( ° F). Temperature is the key to all air movement. Most thermometers are closed glass tubes containing liquids such as alcohol or mercury. When air around the tube heats the liquid, the liquid expands and moves up the tube.

Meteorologists use a rain gauge to measure the amount of rain that has fallen in inches or centimeters. The rain gauge measures the height of the column of water it captures. The amount of rainfall is important because it affects how much water is available to farmers and others.

A map is a diagram representing a place. It shows where things are located. A map’s key explains the symbols used on a map. A direction indicator or compass shows directions (N, S, E, W). A weather map is a special kind of map. It shows weather patterns. The location of clouds, precipitation, wind speed and direction, temperatures and air pressure can all be shown on a weather map. Weather maps can be used to predict future weather.

W eather data can also be recorded in a table or a graph. Below is a bar graph showing the average monthly rain fall in Houston, Texas.

Barometer measures this Warm Front Cold Front ThunderstormRain Partly Cloudy SunnyCloudySnow

The atmosphere is the blanket of air that surrounds the Earth. The atmosphere is mainly made up of nitrogen and oxygen gas. The atmosphere is the main source of weather. When the air moves around us, we feel this as wind. We measure the weight of the air around us as air pressure. A barometer is used to measure air pressure.

To describe the weather, scientists use special instruments to measure conditions: Temperature- Measures how hot or cold the air is in Celsius (°C) or Fahrenheit (°F). Precipitation- Measured with a rain gauge. The amount of rain that has fallen in inches or centimeters. Wind- Wind speed with an anemometer. Wind direction is measured with a weather vane. A ir Pressure- Measured with a barometer

Weather maps- Specialized map that shows weather patterns. It indicates the conditions present in the atmosphere. A key on map is used to explain what each map symbol represents. Symbols on a map are used to record information about the weather. A weather map often shows: Temperature Wind Direction Air Pressure Wind Speed Precipitation Cloud Conditions