Understanding Air Pressure Earth Science 19.1 Understanding Air Pressure Understanding Air Pressure

Earth Science 19.1 Understanding Air Pressure Of the various elements of weather and climate, changes in air pressure are the least noticeable. When you listen to a weather report, you listen to the temperature, the precipitation, and humidity. Although you might not perceive day-to-day or hour-to-hour changes in air pressure, these changes are very important in producing changes in our weather.

Earth Science 19.1 Understanding Air Pressure Variations in air pressure from place to place can generate winds that have hurricane force like the ones at right. Winds, in turn, bring changes in temperature and humidity. Air pressure is one of the most basic of weather elements and is major factor in predicting weather. Air pressure is closely tied to the other elements of weather in a cause-and-effect relationship.

Air Pressure Defined Air pressure is simply the pressure exerted by the weight of the air above. Average air pressure at sea level is about 1 kilogram per square centimeter. This pressure is roughly the same pressure that is produced by a column of water 10 meters in height.

Air Pressure Defined You can calculate that the air pressure exerted on a 50 cm by 100 cm school desk exceeds 5000 kilograms, which is about the mass of a 50 passenger school buses. Why then doesn’t the desk collapse under the weight of the air above it. Air pressure is exerted in all directions; down, up and sideways. The air pressure pushing down on an object balances against the air pressure pushing up on the same object.

Air Pressure Defined Imagine a tall aquarium has the same dimensions as the desktop we just mentioned in the previous example. When the aquarium is filled to a height of 10 meters, the water pressure at the bottom equals 1 atmosphere, or 1 kilogram per square meter. Now imagine what will happen if you place this aquarium on top of the desk. The desk collapses downward because the pressure downward is greater than the pressure in all directions.

Air Pressure Defined If you could place the desk inside the aquarium however, the desk would not collapse but sink to the bottom because the water pressure is exerted in all directions on it, not just downward. The desk, like your body, is built to withstand the pressure of 1 atmosphere.

Measuring Air Pressure When meteorologists measure atmospheric pressure, they use a unit of measure called millibars. Standard air pressure is 1013.2 millibars. You might have heard the phrase “inches of mercury” which is used by weathermen to describe atmospheric pressure. This expression dates to 1643 when an Italian scientist named Torricelli invented the mercury barometer.

Measuring Air Pressure A barometer is a device used for measuring air pressure. Torricelli correctly described the atmosphere as a vast ocean of air that exerts pressure on us and all objects around us. To measure this force, he filled a glass tube, closed at one end, with mercury. He then put the tube, upside down, into a dish of mercury as shown at right. The mercury flowed out of the tube until the weight of the air pressure exerted on the surface of the mercury was equal with the weight of the mercury in the column.

Measuring Air Pressure As a result, when air pressure increases, the mercury in the tube rises. When air pressure decreases, the mercury in the tube goes down. With some modern improvements, the mercury barometer is still the standard instrument used today for measuring air pressure.

Measuring Air Pressure The need for smaller and more portable instruments for measuring air pressure led to the invention of the aneroid barometer. The aneroid barometer uses a metal chamber with some air removed. This partially emptied chamber is very sensitive to variations in air pressure. This chamber changes shape and compresses as the air pressure is increases, and it expands as the air pressure decreases. 2 examples of aneroid barometers

Measuring Air Pressure One advantage of an aneroid barometer is that it can be easily attached to a recording device. This type of device provides a continuous record of pressure changes over time.

Factors Affecting Wind As important as vertical motion is, far more air moves horizontally, a phenomena called wind. What causes wind however? Wind is the result of horizontal differences in air pressure. Air flows from areas of high pressure to areas of lower pressure. You may have experienced this flow of air when you open a vacuum-packed can of coffee or tennis balls. The noise you hear is the air rushing from the higher pressure outside the can to the lower pressure inside the can.

Factors Affecting Wind The unequal heating of Earth’s surface generates pressure differences. Solar energy is therefore the ultimate source of energy for the creation of wind. If Earth did not rotate and their were no friction what-so-ever between moving air and earth’s surface, air would flow in a straight line from areas of high pressure to areas of low pressure. But both factors, Earth’s rotation and friction, do exist and the flow of air is therefore not so straightforward. Three factors combine to control wind: pressure differences, Coriolis effect, and friction.

Pressure Differences Wind is created by differences in pressure: the greater these differences are, the greater the wind speed is. Over Earth’s surface, variations in air pressure are determined from barometric readings taken at hundreds of weather stations. These pressure data readings are shown on a weather map using isobars. Isobars are lines on a map that connect places of equal air pressure.

Pressure Differences The spacing of isobars indicates the amount of pressure change over a given distance. These pressure changes are expressed as the pressure gradient. A steep pressure gradient, like a steep hill, causes great acceleration of a parcel of air. A less steep gradient causes a slower acceleration. Closely spaced bars indicate a steep pressure gradient and high winds. Widely spaced bars indicate a weak pressure gradient and light winds.

Pressure Differences The pressure gradient is the driving force of wind. The pressure gradient has both magnitude (strength) and direction. It’s magnitude is reflected in the spacing of the isobars. The closer the spacing, the stronger the winds. The direction of force is always from areas of high pressure to areas of low pressure and at right angles to the isobars. Friction affects wind speed and direction while Coriolis effect affects wind direction only.

Coriolis Effect The weather map at right shows typical air movements associated with high and low pressure systems. Air moves out of regions of higher pressure and into the regions of lower pressure. However, wind does not cross the isobars at right angles as one would expect. This change in direction results from the rotation of the earth and is called the Coriolis effect.

Coriolis Effect The Coriolis effect describes how Earth’s rotation affects moving objects. All free-moving objects or fluids, including the wind, are deflected to the right of their path of motion in the Northern Hemisphere. In the Southern hemisphere, they are deflected to the left.

Coriolis Effect Imagine the path of a rocket launched from the North Pole toward a target located at the equator. The true path of the rocket is straight, however, in the time it would take for the rocket to fly from the North pole to the equator, the Earth would have rotated underneath the rocket by 15 degrees. The rocket arrives at a spot to the right of where it was intended because the Earth moved under it while it flew. The counterclockwise rotation of the Northern hemisphere causes this path to deflect.

Coriolis Effect This apparent shift in direction is attributed to the Coriolis effect. This deflection Is always directed at right angles to the direction of airflow Affects only wind direction and not wind speed Is affected by wind speed; the greater the wind speed the greater the deflection Is strongest at the poles and weakens toward the equator, becoming nonexistant at the equator

Friction The effect of friction on wind is important only within a few kilometers of Earth’s surface. Friction acts to slow air movement, which changes wind direction. To illustrate friction’s effect on wind direction, first think about a situation in which friction does not play a role in wind’s direction. When air is above the friction layer, the pressure gradient moves across the isobars. As soon as air starts to move, the Coriolis effect acts at right angles to motion. The faster the wind speed, the greater the deflection.

Friction The pressure gradient (PGF) and Coriolis effect (CF) balance in high-altitude air, and wind generally flows parallel to isobars. The most prominent features of airflow high above the friction layer are the jet streams. Jet streams are fast-moving rivers of air near the tropopause that travel between 120 and 240 kilometers per hour in a west-to-east direction. One such jet stream is situated over the polar front, which is the zone separating cool polar air from warm subtropical air. Jet streams were first encountered by high-flying bomber pilots during World war II.

Friction For air close to Earth’s surface , the roughness of the terrain determines the angle of airflow across the isobars. Over the smooth ocean surface, friction is low, and the angle of airflow is small. Over rugged terrain, where friction is higher, winds move more slowly and cross the isobars at greater angles. Friction causes winds to flow across the isobars at angles as great as 45 degrees. Slower wind speeds caused by friction decrease the Coriolis effect.