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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. - PowerPoint PPT Presentation
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Meteo 0039/24/14
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.
CLOUDSCLEAR
Infrared Imagery• All about temperature! – Higher clouds are
cooler, lower clouds warmer. • Cold usually = bright in meteo-nation. Ground
usually = dark
HIGHER CLOUD TOPS
LOW CLOUDS THE GROUND
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
LH
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
Higher Pressure
Wind Direction
Isobars
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
Ground JULY JANUARY
500 mb level
5605 meters
5375 meters
Warmer Air Colder Air
Lab 4 – Due Friday at start of class4.14 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, b6.9 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