Source code for ladybug_comfort.hi

# coding=utf-8
"""Utility functions for calculating Heat Index (HI)."""
from __future__ import division

import math


def heat_index(ta, rh):
    """Calculate heat index (HI) from air temperature and relative humidity.

    Heat index is derived from original work carried out by Robert G. Steadman [1],
    which defined heat index through large tables of empirical data.
    The formula here approximates the heat index to within +/- 0.7C and is
    the result of a multivariate fit [2].
    Heat index was adopted by the US's National Weather Service (NWS) in 1979.

    Note:
        [1] Steadman, R. G. (July 1979). "The Assessment of Sultriness. Part I: A
        Temperature-Humidity Index Based on Human Physiology and Clothing Science".
        Journal of Applied Meteorology. 18 (7): 861–873.

        [2] Lans P. Rothfusz. "The Heat Index 'Equation' (or, More Than You Ever
        Wanted to Know About Heat Index)", Scientific Services Division
        (NWS Southern Region Headquarters), 1 July 1990.
        https://www.weather.gov/media/ffc/ta_htindx.PDF

    Args:
        ta: Air temperature [C]
        rh: Relative humidity [%]

    Returns:
        hi -- Heat index [C]
    """
    tf = ta * 9. / 5. + 32.  # convert to fahrenheit

    if tf < 80:
        hif = 0.5 * (tf + 61.0 + ((tf - 68.0) * 1.2) + (rh * 0.094))
    else:
        hif = -42.379 + 2.04901523 * tf + \
            10.14333127 * rh - \
            0.22475541 * tf * rh - \
            6.83783e-3 * tf ** 2 - \
            5.481717e-2 * rh ** 2 + \
            1.22874e-3 * tf ** 2 * rh + \
            8.5282e-4 * tf * rh ** 2 - \
            1.99e-6 * tf ** 2 * rh ** 2
        if tf >= 80 and tf <= 112 and rh < 13:
            adjust = ((13. - rh) / 4.) * math.sqrt((17. - abs(tf - 95.)) / 17.)
            hif = hif - adjust
        elif tf >= 80 and tf <= 87 and rh > 85:
            adjust = ((rh - 85) / 10) * ((87 - tf) / 5)
            hif = hif + adjust

    hi = (hif - 32.) * 5. / 9.  # convert to celsius

    return hi


def heat_index_warning_category(hi):
    """Get the category of warning associated with a given heat index (HI).

    Categories are used by the US National Weather Service (NWS) and National
    Oceanic and Atmospheric Administration (NOAA) to issue the following warnings:

    * 0 = No Warning. Satisfactory temperature. Can continue with activity.
    * 1 = Caution: Fatigue is possible with prolonged exposure and activity.
      Continuing activity could result in heat cramps.
    * 2 = Extreme caution: Heat cramps and heat exhaustion are possible.
      Continuing activity could result in heat stroke.
    * 3 = Danger: Heat cramps and heat exhaustion are likely.
      Heat stroke is probable with continued activity.
    * 4 = Extreme danger: Heat stroke is imminent.

    Args:
        hi: Heat index [C]

    Returns:
        category -- An integer indicating the level of warning associated with the
        heat index. Values are one of the following:

        -   0 = No Warning
        -   1 = Caution
        -   2 = Extreme Caution
        -   3 = Danger
        -   4 = Extreme Danger
    """
    if hi < 26.6:
        category = 0
    elif hi < 32.2:
        category = 1
    elif hi < 40.5:
        category = 2
    elif hi < 54.4:
        category = 3
    else:
        category = 4

    return category