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Windchill
A microclimate that shields against wind during a short freeze can protect tender plants: Windchill is a measure of the cooling effect of wind. Wind increases the rate at which a body loses heat through evaporation. The following calculator and charts give you an idea of how much wind can speed up the reduction in temperature or how much a protective wall or even a blanket can help in the short run.

Windchill-Factor
Usually windchill is expressed as an equivalent temperature with no wind. At this temperature the rate of cooling is the same. However, Wind and temperature combine to produce a COOLING POTENTIAL, not a temperature. One important value is -- at about 1500 kcal/m2, exposed flesh will freeze. At higher wind speeds and lower temperatures than those that combine to create 1500, flesh will simply freeze more quickly.

    
WINDCHILL CALCULATOR
Wind Speed:
Air Temperature: 1 º
Equivalent Temp: 2
Factor: kcal/m2/h 3 Watts/m2/s 4
Increase in Heat Loss due to wind:
Comment:
Note: 5
1 Air Temperature as measured by a dry thermometer.
2 Temp at which the cooling effect would be the same without wind.
3 Wind chill in kilocalories lost per square meter of exposed flesh in one hour.
4 Wind chill in Watts lost per square meter in one second.
5 Wind chill formulas are not valid at all for wind speeds outside the range from 4 mi/h to 25 m/s. 
______________________________________________________________________________________
        | ACTUAL THERMOMETER READING (DEG. F)
        |
WIND    | 50      40      30      20      10       0     -10     -20     -30     -40
SPEED   |
IN      |
MPH     |
        |_____________________________________________________________________________
        | EQUIVALENT TEMPERATURE (DEG. F)
        |
Calm    | 50      40      30      20      10       0     -10     -20  |  -30     -40
        |                                                     ________|
 5      | 48      37      27      16       6      -5     -15  |  -26     -36     -47
        |                                             ________|
10      | 40      28      16       4      -9     -21  |  -33     -46     -58     -70
        |                                     ________|                       ________
15      | 36      22       9      -5     -18  |  -36     -45     -58     -72  |  -85
        |                             ________|                       ________|
20      | 32      18       4     -10  |  -25     -39     -53     -67  |  -82     -96
        |                             |                       ________|
25      | 30      16       0     -15  |  -29     -44     -59  |  -74     -88     -104
        |                             |                       |
30      | 28      13      -2     -18  |  -33     -48     -63  |  -79     -94     -109
        |                             |                       |
35      | 27      11      -4     -20  |  -35     -49     -67  |  -82     -98     -113
        |                             |                       |
40      | 26      10      -6     -21  |  -37     -53     -69  |  -85    -100     -116
______________________________________|_______________________|________________________
OVER    |        LITTLE DANGER        |    INCREASING DANGER  |      GREAT DANGER
40 MPH  |                             |                       |
(little |                             |
added   |(for properly clothed person)|    (Danger from freezing of exposed flesh)
effect) |                             |
________|_____________________________|________________________________________________
Source: Rhapidophyllum, July 1996.