import math


def windchillF(T_F, V_mph):
    """Calculate wind chill.
    http://www.nws.noaa.gov/om/cold/wind_chill.shtml
    
    T_F: Temperature in Fahrenheit
    
    V_mph: Wind speed in mph
    
    Returns Wind Chill in Fahrenheit
    """

    if T_F is None or V_mph is None:
        return None

    # only valid for temperatures below 50F and wind speeds over 3.0 mph
    if T_F >= 50.0 or V_mph <= 3.0:
        return T_F

    WcF = 35.74 + 0.6215 * T_F + (-35.75 + 0.4275 * T_F) * math.pow(V_mph, 0.16)
    return WcF