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-- Calculate wind chill. If temperature is low but it's windy, the temperature
-- feels lower than the actual measured temperature. Wind chill formula from
-- <a href="http://www.nws.noaa.gov/om/windchill/">http://www.nws.noaa.gov/om/windchill/</a>.
-- @param temperature Temperature in Fahrenheit, must be 50 or less
-- @param wind_speed Wind speed in miles per hour
-- @return Wind chill of the given conditions, or nil if invalid input received
function calc_wind_chill(temperature, wind_speed)
	local apparent = nil
	if (temperature ~= nil and wind_speed ~= nil and temperature <= 50) then
		if (wind_speed > 3) then
			local v = math.pow(wind_speed, 0.16)
			apparent = 35.74 + 0.6215 * temperature - 35.75 * v + 0.4275 * temperature * v
		elseif (wind_speed >= 0) then
			apparent = temperature
		end
	end

	return apparent
end

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function fahrenheit_to_celsius(fahrenheit, digits) 
    return round((5/9) * (fahrenheit - 32), digits or 2)
end

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function km_to_miles(km, digits) 
    return round((km * 0.62137119223733), digits or 2)
end

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function round(value, digits)
	if not value or not digits then
		return nil
	end
		local precision = 10^digits
        return (value >= 0) and
		  (math.floor(value * precision + 0.5) / precision) or
		  (math.ceil(value * precision - 0.5) / precision)
end