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