1. LAB 4 due this Friday, September 26 before class
Meteo 003 9/24/14
LAB 4 due this Friday, September before class

2. Terrain Effects on Rain
Windward side of mountains makes air rise/enhances precipitation.
Rain shadow on leeward sides
RAIN SHADOW

3. 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…

4. Visible Imagery
What you see is what you get! Imagine yourself as the satellite in space, looking down.
CLOUDS
CLEAR

5. 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

6. Water Vapor Imagery
Only good for upper troposphere – tells you nothing about the surface!
MOIST
DRY

7. 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

8. 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

9. 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

10. Before Correction

11. After Correction

12. Example of SLP Correction
Allentown, PA
Elevation: 142 meters
Station pressure: 983 millibars
Correction: (10mb/100m) = (x mb/150m)
X = 14.2mb
Corrected pressure: millibars

13. 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

14. 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

15. 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

16. Example State College, PA JULY JANUARY 5605 meters 500 mb 5375 meters
level
5375 meters
Colder Air
Warmer Air
Ground
JULY
JANUARY

17. 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 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