def calculate_wind_chill(temperature, wind_speed):
"""
Uses the air temperature and wind speed to calculate the wind chill, the purpose of which is to represent a
"felt-air temperature" close to what a human actually feels given the temperature and wind speed. This index does
not take into account the humidity or solar radiation, and so is not the most accurate measure of a true
"feels-like" temperature. For that, see `calculate_thw_index` and `calculate_thsw_index`. The algorithm used and
its constants are sourced from the chart at http://www.srh.noaa.gov/ssd/html/windchil.htm, and the function is
tested against the same chart. In this algorithm:
T is the temperature in degrees Fahrenheit
WS is the wind speed in miles per hour
This function returns `None` if the input temperature is above 40F.
:param temperature: The temperature in degrees Fahrenheit
:type temperature: int | long | decimal.Decimal
:param wind_speed: The wind speed in miles per hour
:type wind_speed: int | long | decimal.Decimal
:return: The wind chill temperature in degrees Fahrenheit to one decimal place, or `None` if the temperature is
higher than 40F
:rtype: decimal.Decimal
"""
if temperature > WIND_CHILL_THRESHOLD:
return None
T = temperature
WS = _as_decimal(wind_speed)
if WS == ZERO: # No wind results in no chill, so skip it
return T
V = WS ** WC_V_EXP
wind_chill = (
WC_C1 + (WC_C2 * T) - (WC_C3 * V) + (WC_C4 * T * V)
).quantize(ONE_TENTH, rounding=decimal.ROUND_FLOOR)
return T if wind_chill > T else wind_chill
# noinspection PyPep8Naming
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