How Does Temperature Affect Air Pressure and Wind? The earth is surrounded by a layer of air that presses down on the earth. This pressing down is called air pressure. Most of the time you don ‘t even notice air pressure. You may have noticed a change in air pressure when you rode in an elevator, in an airplane, or up a hill in a car. The sudden changes in air pressure can make your ears pop. Think about the air temperature as you go higher. Of course, it gets cooler. Near the surface of the earth, the air is warmer. The surface heats the air above it. As you move higher and higher above the earth’s surface, the air gets colder. Not only does the temperature change as you go higher above the surface of the earth. The air pressure changes as well. The higher you are the less air is above you. Less air presses down on you. The air pressure is lower. Near the surface of the earth , the air molecules are close together. Higher up the air molecules are spread out more. The molecules keep getting farther and farther apart. What causes changes in air pressure? Temperature change can bring about air pressure change. Sunlight can heat the surface of the earth. As the air becomes warmer, its molecules move farther apart and the air becomes lighter. The warm air rises. As warm air rises, the warm air pushes down on the surface of the earth with less pressure. This causes a low pressure area. When the warm air rises cooler air moves in to take its place. Cooler air sinks because it is heavier than warm air. The cold air pushes down on the surface of the earth with more pressure than warm air. This is called a high-pressure area where the colder air sinks. Wind is caused by air moving from an area of high pressure to an area of low pressure. People need to know the air pressure in different places so they can predict the weather. A tool called a barometer is used to measure air pressure. One kind of barometer is made of a hollow tube that is closed on one end and open on the other. This tube is filled with mercury. The mercury moves down the tube a little and air pressure pushes down on the mercury in the dish and holds the mercury at a certain level. As the air pressure gets higher, it will press down more. The added pressure in the dish makes the mercury go higher. When the air pressure is lower, it presses less on the mercury on the dish. Another kind of barometer has a box inside it. The box has a needle hooked to it. As the higher pressure pushes on the sides of the box, the needle points to a higher number on the front of the barometer. A tool called a wind vane shows the direction the wind is coming from. The letters on the vane show which way is north, south, east, and west. An arrow turns to point into the wind. A wind sock can also show wind direction. An anemometer measures the speed of the wind. As the wind blows faster, it spins faster. A dial on the anemometer shows how fast it is blowing.