The Study of Air Pressure • What is it? • Air pressure is the weight of the air in the atmosphere pressing down. • Pressure in general is force applied over an area. • Why is it important? • Air pressure affects winds and precedes weather changes.
How to Measure Air Pressure • Barometer • Tool for measuring air pressure. • Two different types of barometer: • Aneroid – • Uses a thin metal can with a pen attached to a long arm. • As the air pressure around the can changes, the pen moves up or down. • Works in a similar way to the metal thermometer.
How to Measure Air Pressure • Mercury Barometer • A thin column of glass houses mercury. • Mercury is loose in a pan at the bottom of the glass column. • As air pressure changes, the level of the mercury column inside the glass will rise or fall. • At sea level, the column of mercury will be 76 cm tall.
Units of Air Pressure • There are two different units of air pressure used. • The first is given in inches of mercury • Standard pressure is 29.92 inches of mercury • The second is in millibars • Standard pressure is 1013.2 mb
Isobars • Isobars are a way of showing air pressure on a weather map. • They are similar to isotherms in that they connect all the points of equal pressure. • Useful in determining the distribution of air pressures over areas and the changes in pressure over time. The pressure is printed on each isobar. The L indicates a localized low pressure area An H would indicate a localized high pressure area.
Why air pressure changes • If you were to observe a barometer for several day, you would see the pressure change. Why? There are several reasons: • Temperature affects air pressure. • The warmer air gets, the less dense it is and the more space it takes up. • This increase in air temperature results in lower air pressure at the ground. • This is also the principle that allows hot air balloons to work.
Why air pressure changes • The second reason is that the level of water vapor in the air changes from day to day. • The amount of water vapor in the air is known as humidity. • Water vapor is less dense than the different gases that make up air, • So the more water vapor there is in the air, the lower the pressure.
Why air pressure changes • Warm humid air is lighter than cool dry air. • Temperature and humidity are the two biggest factors in air pressure changes. • Meteorologists observe that falling barometer readings will indicate warmer weather and more humid air.
Why air pressure changes • Meteorologists also observe that rising barometer readings will indicate cooler temperatures, and less humid air. • The changes in air pressure can indicate weather patterns. • Falling barometer readings may indicate precipitation, such as rain or snow.
High and Low Pressure • On a weather map, isobars close around each other. Either the pressure is going up or going down as the circles get smaller. • The highest pressure reading is referred to as a pressure high, and the lowest is referred to as a pressure low. • Pressure gradient is the rate of how quickly the pressure changes over an area. Indicated by how close the isobars are to each other. • Pressure gradient = (Pressure 2 – Pressure 1) Distance between
Winds • What is wind? • Wind is the movement of air from an area of high pressure to an area of low pressure. • This is an attempt to maintain equilibrium. • As the air pressure over land decreases in summer since the air is getting warmer, the high pressure air over the ocean will move towards land.
Plotting Winds • Meteorologists can plot the path and strength of wind by looking at an isobar weather map. • The winds will move from the high pressure areas to the low pressure areas across the isobars. • The winds will be stronger in the areas where the isobars are close together.
Plotting Winds • At the Earth’s surface, the winds do not travel in a straight line across the isobars. • They will cross the isobars and turn due to the Coriolis force. • The Coriolis force is an effect felt by all objects moving towards or away from the equator due to the Earth’s rotation.
Plotting Winds • In the northern hemisphere, winds will turn to the right as they cross isobars. • In the southern hemisphere, winds will turn left as they cross the isobars. • Remember, winds travel from high pressure areas to low pressure areas.
Characteristics of Wind • Wind direction is based on the direction it comes from. • A weather vane is a tool that shows direction of wind, it point to the direction the wind is coming from. • Speed of wind varies with the difference in pressure and friction.
Characteristics of Wind • The greater the difference between the pressure levels, the faster the wind. • As the wind rises away from the surface, friction decreases and speed rises. • As altitude reaches about 1000m, the winds run along the isobars, instead of across them. • Wind speed is measured with an anemometer.
Characteristics of Wind • Winds are rarely steady at the surface. • They often blow in sudden bursts called gusts. • Gusts are mainly due to rough surfaces on land. • Winds are usually measured in miles per hour or in knots. • One knot is about 1.15 miles per hour.