Air Pressure and Winds-I GEOL 1350: Introduction To Meteorology 1 2 Pressure gradient force is in balance with gravity Hydrostatic relations Means no vertical motion initially 3 How does atmospheric pressure change with altitude ? Pressure always decreases with increasing height. In the low atmosphere, pressure decreases by about 10 mb (hPa) for every 100 m increase in elevation 4 Pressure can change as we go up or down in the atmosphere. Can pressure change as we move from one location to another assuming the two locations are at the same elevation ? 5 Two columns of air– same temperature same distribution of mass 500 mb level 5500 m MSL 1000 mb 1000 mb 6 Cool the left column; warm the right column The heated column The cooled expands column contracts 500 mb original 500 mb level 500 mb 1000 mb 1000 mb 7 The level of the 500 mb surface changes; the surface pressure remains unchanged The level corresponding to 500 mb is displaced downward The 500 mb surface is displaced upward in the in the cooler column warmer column new 500 mb level in warm air original 500 mb level new 500 mb level in cold air The surface pressure remains the same since both columns still contain the same mass of air. 1000 mb 1000 mb 8 A pressure difference in the horizontal direction develops above the surface The 500 mb surface is The 500 mb surface is displaced downward in displaced upward in the the cooler column warmer column new 500 mb level in warm air original 500 mb level Low High new 500 mb level in cold air The surface pressure remains the same since both columns 1000 mb 1000 mb still contain the same mass of air. 9 Air moves from high to low pressure in middle of column, causing surface pressure to change. Difference in pressure in horizontal direction induces horizontal winds 500 mb 450 mb Low High 550mb original 500 mb level 500 mb Surface pressure rises Air above moving away as air moving into the from the column – 10 column from above 1003 mb 997 mb surface pressure drops Air moves from high to low pressure at the surface Difference in pressure in horizontal direction induces horizontal winds 500 mb original 500 mb level Low High 500 mb High Low 1003 mb 997 mb 11 Two columns of air– same temperature same distribution of mass Heat more 1000 mb 1000 mb 1003 mb 997 mb 12 How does the atmosphere converts heating into motions ? • Starting with a uniform atmosphere at rest, we introduced differential heating • The differential heating caused different rates of expansion in the air columns • The differing rates of expansion resulted in pressure differences along a horizontal surface. • The pressure differences then induce flow. Heating or cooling of a column of air can establish horizontal variations in pressure that cause the air to move horizontally 13 14 Station Pressure and Sea Level Pressure • Station pressure – the pressure reading at a particular location and elevation • Sea-level pressure - the pressure adjusted to altitude 0 m, which is the mean sea level. Pressure drops approximately 10 mb for 100 m increase in height 15 Surface Maps 16 Surface High and Low Pressure and Winds Near surface in the Northern Hemisphere, winds blow counter clockwise around and towards a low pressure center Clockwise around and outwards from a high pressure center 17 Why? 18 19 Upper-level map and winds aloft At upper levels, winds blow parallel to the height contours in a wavy west20 to east direction Pressure gradient force – horizontal difference in atmospheric pressure causes air to move 500 mb original 500 mb level Low P High P 500 mb High Low 1003 mb 997 mb 21 Higher water level in A creates high fluid pressure at the bottom and a net force towards the low fluid pressure at the bottom of Tank B, causing water to move from A to22 B 23 The pressure gradient force . the force that causes wind to blow . directed from higher towards lower pressure at right angle to the isobars . directly related to the pressure gradient – the change of pressure over a given distance (ΔP/d) . steep pressure gradients correspond to stronger pressure gradient force and vice versa 24 Computing PGF for each direction [Stull, Meteorology for Scientists and Engineers] 25 26 SUMMARY 1. Atmospheric pressure is the pressure exerted by the mass of air above a region. 2. A change in surface air pressure can be brought about by changing the mass above the surface. 3. Difference in horizontal air pressure produces a horizontal pressure gradient force (PGF). 4. The pressure gradient force (PGF) is always directed from higher pressure toward lower pressure and it is the PGF that causes the air to move and the wind to blow. 5. Steep pressure gradients indicate strong pressure gradient forces and high wind; gentle pressure gradients indicate weak pressure gradient forces and light winds. 27 .