AOSC Lesson 11 Fig Centrifugal Force The Mechanism for Geostrophic Flow GEOSTROPHIC FLOW EVENTUALLY A BALANCE IS REACHED BETWEEN THE CORIOLIS FORCE AND THE PRESSURE GRADIENT FORCE IF THERE IS NO FRICTION THEN THIS OCCURS WHEN THE PARCEL OF AIR IS PARALLEL TO THE ISOBARS. AT THIS POINT THERE IS NO NET FORCE ON THE AIR PARCEL - NO ACCELERATION - IT NOW MOVES WITH CONSTANT VELOCITY. UNDER THESE IDEALIZED CONDITIONS THE AIRFLOW IS SAID TO BE IN GEOSTROPHIC BALANCE. WIND GENERATED IS CALLED THE GEOSTROPHIC WIND NORMALLY ONLY APPLIES TO WINDS ALOFT. SURFACE WINDS ARE SUBJECT TO FRICTION. Fig. 6-15, p. 172 GRADIENT WINDS WINDS AROUND CENTERS OF HIGH OR LOW PRESSURE FOLLOW CURVED PATHS IN ORDER TO STAY PARALLEL WITH THE ISOBARS. THESE WINDS ARE CALLED GRADIENT WINDS. LOW PRESSURE CENTERS ARE CALLED CYCLONES - ROTATION IS COUNTER- CLOCKWISE - SAME AS THE EARTH CENTERS OF HIGH PRESSURE ARE CALLED ANTI-CYCLONES. IN SOUTHERN HEMISPHERE THE DIRECTIONS OF FLOW ARE REVERSED Fig. 6-12, p. 168 Fig. 6-18, p. 175 Fig. 6-24, p. 181 SURFACE WINDS FRICTION AFFECTS WINDS ONLY CLOSE TO THE EARTH'S SURFACE. NOW WE MUST BALANCE THREE FORCES - CORIOLIS, PRESSURE GRADIENT AND FRICTION. NET EFFECT IS TO INDUCE A NET INFLOW AROUND A CYCLONE, AN EFFECT KNOWN AS CONVERGENCE. AROUND AN ANTICYCLONE WE GET A NET OUTFLOW, DIVERGENCE Aneroid Barometer On an aneroid barometer fair corresponds to high surface pressure. Air is subsiding and is subject to adiabatic heating. This lowers the relative hunidity. Hence possibility of clouds forming is low. Rain corresponds to low pressure. Air is rising and is subject to adiabatic cooling. The raises the relative humidity. Possibility of clouds is high. Effect of temperature on pressure Initially the pressure above each city is the same. But if we heat the air above one of them the column of air will expand. If we cool the air above the other it will contract. If we now look at the pressure at the top pf the cool air it will be lower than that in the heated column. This is because pressure is defined as the weight of air above a given altitude. The weight of air above the cold column is less than that at the same altitude in the not column This will cause air to move from the hot column to the cold column. Hence the pressure will increase in the cold column at the ground, and the pressure will decrease in the hot column. Fig. 6-22, p. 179 Sea (Land) Breeze Effect SEA BREEZES ARE THE RESULT OF DIFFEERENTIAL HEATING OF THE OCEAN AND THE LAND DURING THE DAY THE LAND HEATS UP QUICKLY WHILE THE OCEAN HEATS UP SLOWLY HIGH TEMPERATURE OVER THE LAND, LOWER TEMPERATURE OVER THE OCEAN AT THE SURFACE - HIGH PRESSURE OVER THE OCEAN, LOW PRESSURE OVER THE LAND - CAUSES WIND AT THE SURFACE TO FLOW FROM THE OCEAN TO THE LAND (SEA BREEZE) AT NIGHT THE LAND COOLS RAPIDLY TO A TEMPERAURE BELOW THAT OF THE OCEAN. WIND REVERSES - FLOWS FROM THE LAND TO THE OCEAN AT THE SURFACE (LAND BREEZE) Fig. 7-6, p. 175 Stepped Art Fig. 6.26