LOW PRESSURE SYSTEMS • Isobars-lines of equal pressure • Barometric pressure, measured from barometers at various weather stations and various heights by using weather balloons
LOW PRESSURE SYSTEMS • A low is where the pressure is lower than the surrounding area • Move counter-clockwise (cyclonic) in Northern Hemisphere. • Lower level convergence (air coming together) upper level divergence (moving away)
LOW PRESSURE SYSTEMS • As air converges at the surface, it is forced up along with moisture and clouds are formed • This is why low pressure is often associated with bad weather • This surface convergence also leads to frontal systems. Frontal systems are divisions of temperatures, along with wind shifts. • Frontal systems include cold fronts, warm fronts, occluded fronts and stationary fronts.
LOW PRESSURE SYSTEMS • Due to the cyclonic nature of lows, cold air is likely found to the north and west while warm moist air is generally found to the south and east of the low pressure center.
LOW PRESSURE SYSTEMS • Examples of semi-permanent low pressure systems • - Sea breeze/land breeze
LOW PRESSURE SYSTEMS • Semi-permanent systems occur in the Southwest U.S. desert as well as Mexico • Heat from the sun during the day creates low pressure • Often dust devils are created (TURBULENT!)
EXTREME LOW • Hurricanes are extreme low pressure centers • --Need to have very warm waters • -- Winds aloft (upper atmosphere) need to be calm • --Need an existing low or disturbance • --Occurs on water due to absence of friction • --Tight pressure gradient • --Low stacks from surface to 300 mb
HIGH PRESSURE SYSTEM • A high is where the pressure is higher than the surrounding area also called an anticyclone. • Air moves in a clockwise direction in Northern Hemisphere • In a high pressure area, the air is descending. • Convergence aloft (upper air) and divergence at the surface. • As the air descends it warms and generally keeps clouds from forming. • Highs are generally associated with good weather.
HIGH PRESSURE SYSTEMS • Semi permanent high over us is called the Bermuda High (Subtropical High).