Transcript rain shadow
Meteo 003 9/24/14 LAB 4 due this Friday, September 26 before class Terrain Effects on Rain • Windward side of mountains makes air rise/enhances precipitation. • Rain shadow on leeward sides RAIN SHADOW Wind Direction • In California – prevailing winds are westerly, so the central valley is in a rain shadow, as well as the CA/NV border. • In Hawaii, prevailing winds are northeasterly, so… where would the rain shadow be? Remember what a NE wind means… Visible Imagery • What you see is what you get! Imagine yourself as the satellite in space, looking down. CLOUDS CLEAR Infrared Imagery • All about temperature! – Higher clouds are cooler, lower clouds warmer. • Cold usually = bright in meteo-nation. Ground usually = dark LOW CLOUDS THE GROUND HIGHER CLOUD TOPS Water Vapor Imagery • Only good for upper troposphere – tells you nothing about the surface! MOIST DRY Radar Shortfalls • Precipitation from low clouds far away from radar site can go unnoticed, the beam can overshoot due to the earth’s curvature and beam angle • It will look as if there is nothing there A note about winds • The pressure gradient force will always point towards lower pressure, regardless of which hemisphere you are in (perpendicular to isobars). • What other two “forces” must we consider? H L Sea Level Correction • If you plotted actual station pressures, mountains would always have the lowest pressure, and coastal locations would always have the highest. • We need to correct this. • Simplified way to do it: – For every 100m of elevation, you add 10mb of pressure to correct to sea-level Before Correction After Correction Example of SLP Correction • Allentown, PA – Elevation: 142 meters – Station pressure: 983 millibars – Correction: (10mb/100m) = (x mb/150m) – X = 14.2mb – Corrected pressure: 997.2 millibars Highs and Lows • For the feature to be analyzed, you need at least one closed isobar contour • The example shows 5mb intervals, U.S. standard is 4mb, but any interval may be used L H Wind Reminder • Remember: The PGF is directed from high to low pressure. With a moderate (medium) amount of friction and the Coriolis effect, the wind crosses isobars at (approximately) a 30 degree angle, toward lower pressure. Lower Pressure Wind Direction Isobars Higher Pressure Pressure Levels • Pressure surface elevation is proportional to the average temperature of the column of air beneath it. • Higher average temperature means higher pressure surfaces. • So, the elevation of the 500mb pressure surface is higher at a warm, tropical location than a cold, polar location • In the mid latitudes (like here in State College) the 500mb surface is higher in summer than in winter Example • State College, PA 5605 meters 500 mb level 5375 meters Warmer Air Ground JULY Colder Air JANUARY Lab 4 – Due Friday at start of class 4.14 a, b (draw an arrow showing wind direction) 4.15 c (use dewpoint to get vapor pressure- just substitute dewpoint for temperature in the table) 5.4 a, b, c 5.10 a, b 5.18 (Fig. 5.27 is helpful) 6.2 a, b (draw AT LEAST 8 wind arrows on each) 6.7 a, b 6.9 a, c, d (you may use arrows instead of wind barbs if that’s easier for you- but make sure they point in the correct direction!) 7.9 a, b