MET 1010 Introduction to Weather Dr. Arturo Rodriguez Miami Dade College North Campus Air Pressure & Winds Chapter 6 Air Pressure & Winds Air Pressure & Winds Atmospheric Pressure Air moves in response to horizontal differences in pressure. Air pressure is simply the mass of air above a given level. Atmospheric pressure always decreases with increasing height. Air pressure, air density (the mass of air in a given volume), and air temperature are all interrelated. Air Pressure & Winds Measuring Air Pressure Air pressure can be defined as the force exerted by the air molecules over a given area. Instruments that detect and measure pressure changes are called barometers. In meteorology, the bar is a unit of pressure that describes a force over a given area. Air Pressure & Winds By definition, a bar is a force of 100,000 newtons acting on a surface area of 1 square meter. A newton is the amount of force required to move an object with a mass of kilogram so that it increases its speed at a rate of 1 meter per second each second. Air Pressure & Winds Because surface pressure changes are normally small, the unit of pressure most commonly found on surface weather maps is the millibar (mb), where one millibar is one-thousandth of a bar. The unit of pressure designed by the International System (SI) of measurement is the pascal (Pa), where 1 pascal is the force of 1 newton acting on a surface of 1 square meter. A more common unit is the hectopascal (hPa), which equals 1 millibar. Air Pressure & Winds A common pressure unit used on TV and radio weather broadcasts is inches of mercury. At sea level, standard atmospheric pressure is 1013.25 mb=1013.25hPa=29.92in.Hg Standard atmospheric pressure at sea level is the pressure exerted by a column of mercury 29.92 in. (760 mm) high, having a density of 1.36 X 104 kg/m³, and subject to an acceleration of gravity of 9.80 m/sec². Air Pressure & Winds Fig. 6.3 compares pressure readings in millibars and inches of mercury. Fig. 6-3, p.145 Air Pressure & Winds Barometers Because we measure atmospheric pressure with an instrument called barometer, atmospheric pressure is also referred to as barometric pressure. Evangelista Torricelli invented the mercury barometer in 1643. Fig. 6-4, p.146 Air Pressure & Winds The most common type of home barometer is the aneroid barometer. The altimeter and barograph are two types of aneroid barometers. Altimeters measure pressure, but are calibrated to indicate altitude. Barographs are recording aneroid barometers. Fig. 6-5, p.146 Aneroid Barometer Uses a small, flexible metal box called an aneroid cell. The aneroid cell is made from an alloy of beryllium and copper. The capsule is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the movements of the capsule are amplified and displayed on the face of the aneroid barometer. Aneroid Barometer More Aneroid Barometers Fig. 6-6, p.147 Air Pressure & Winds Pressure Readings Fig. 6.7 gives the station pressure measured at four locations only a few hundred kilometers apart. To properly measure horizontal changes in pressure, barometer readings must be corrected for altitude. Altitude corrections are made so that a barometer reading taken at one elevation can be compared with a barometer reading taken at another. Air Pressure & Winds Station pressure observations are normally adjusted to a level of mean sea level – the level representing the average surface of the ocean. The adjusted reading is called sea-levelpressure. The size of the correction depends primarily on how high the station is above sea level. Air Pressure & Winds Near the earth’s surface, atmospheric pressure decreases on the average by about 10mb for every 100 m increase in elevation (about 1 in. of Hg for each 1000 ft rise). This decrease in atmospheric pressure with height (10mb/100m) occurs when the air temperature decreases at the standard lapse rate of 6.5 C/1000 m. Fig. 6-7, p.147 Air Pressure & Winds Notice in Fig. 6.7 the isobars, lines connecting points of equal pressure. Drawn as solid dark lines at intervals of 4 mb, with 1000 mb being the base value. With its isobars, the bottom chart (Fig. 6.7c) is now called a sea-level-pressure chart, or simply a surface map. When weather data are plotted on the map, it becomes a surface weather map. Air Pressure & Winds Surface and Upper-Air Charts Fig. 6.8a is a simplified surface map that shows areas of high and low pressure and arrows that indicate wind direction – the direction from which the wind is blowing. The large blue H’s on the map indicate the centers of high pressure, which are also called anticyclones. Air Pressure & Winds The large red L’s represent centers of low pressure, also known as depressions, mid-latitude cyclones , or extra-tropical cyclones because they form in the middle latitudes, outside the tropics. The solid dark lines are isobars with units in millibars. Fig. 6-8, p.148 Air Pressure & Winds Notice that the surface winds tend to blow across the isobars toward regions of low pressure. In the Northern Hemisphere the winds blow counterclockwise and inward toward the center of the lows and clockwise and outward form the center of the highs. Air Pressure & Winds Fig. 6.8b shows an upper-air chart for the same day as the surface map in Fig. 6.8a. The upper-air-map is a constant pressure chart because it is constructed to show height variations along a constant pressure (isobaric) surface, which is why these maps are also known as isobaric maps. This particular isobaric map shows height variations at a constant level of 500 mb (which is about 5600 m or 18,000 ft above sea level). Hence, this map is called a 500millibar map. Air Pressure & Winds The solid dark lines on the map are contour lines – lines that connect points of equal elevation above sea level. Although contour lines are height lines, they illustrate pressure much like isobars do. Consequently, contour lines of low height represent a region of lower pressure, and contour lines of high height represent a region of higher pressure. See Focus Section on page 149. Air Pressure & Winds Notice on the 500-mb map (Fig. 6.8b) that the contour lines typically decrease in value from south to north. The reason for this fact is illustrated by the dashed red lines, which are isotherms – lines of equal temperature. Observe that colder air is generally to the north and warmer air to the south, and recall that cold air aloft is associated with low pressure, warm air aloft with high pressure. Air Pressure & Winds The contour lines are not straight, however, they bend and turn, indicating ridges (elongated highs) where the air is warmer and indicating depressions or troughs (elongated lows) where the air is colder. The arrows on the 500-mb map show the wind direction. Unlike the surface winds that cross the isobars, the winds in the 500mb chart tend to flow parallel to the contour lines in a wavy west-to-east direction. Surface and upper-air charts are valuable tools for the meteorologist. Air Pressure & Winds Surface maps describe where the centers of high and low pressure are found, as well as the winds and weather associated with these systems. Upper-air charts are extremely important in forecasting the weather. The upper-level winds not only determine the movement of surface pressure systems, but they determine whether these surface systems will intensify or weaken.