feat(windchill): Add windchill module

* Add wind chill index and wind chill effect functions

* Add windchill temp calculation

* add tests for windchill module

* minor docstring changes in windchill_index functions

* rename module from windchill to wc

ladybug_comfort/wc.py

@@ -0,0 +1,164 @@
+# coding=utf-8
+"""Utility functions for calculating Wind Chill Index (WCI) and Wind Chill
+Temperature (WCT)"""
+
+import math
+
+
+def windchill_index(ta, ws):
+    """Calculate the Wind Chill Index (WCI) from air temperature and wind
+    speed.
+
+    Wind Chill Index is derived from original work carried out by Gregorczuk[1].
+    It qualifies thermal sensations of a person in wintertime. It is especially
+    useful at low and very low air temperature and at high wind speed.
+
+    Note:
+        [1] https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/196900031
+        09_1969003109.pdf,  equation 55, page 6-113
+
+    Args:
+        ta: Air temperature [C]
+        ws: Wind speed [m/s]
+
+    Returns:
+        wci: Wind Chill Index [W/m2]
+    """
+    wci = (10 * math.sqrt(ws) + 10.45 - ws) * (33 - ta) * 1.163
+
+    return wci
+
+
+def windchill_index_effect_category(wci):
+    """Get the category of effect associated with a given wind chill index
+    (WCI).
+
+    Each number (from -4 to 3) represents a certain WCI thermal sensation
+    category. With categories being the following:
+        -4 = Extreme frost
+        -3 = Frosty
+        -2 = Cold
+        -1 = Cool
+         0 = Comfortable
+         1 = Warm
+         2 = Hot
+         3 = Extremely hot
+
+    Args:
+        wci: Wind Chill Index [W/m2]
+
+    Returns:
+        category: An integer indicating the level of effect associated with
+            the wind chill index. Values are one of the following:
+                -4 = Extreme frost
+                -3 = Frosty
+                -2 = Cold
+                -1 = Cool
+                 0 = Comfortable
+                 1 = Warm
+                 2 = Hot
+                 3 = Extremely hot
+    """
+    if wci >= 2326:
+        category = -4
+    elif wci >= 1628.2:
+        category = -3
+    elif wci >= 930.4:
+        category = -2
+    elif wci >= 581.5:
+        category = -1
+    elif wci >= 232.6:
+        category = 0
+    elif wci >= 116.3:
+        category = 1
+    elif wci >= 58.3:
+        category = 2
+    else:
+        category = 3
+
+    return category
+
+
+def windchill_temp(ta, ws):
+    """Calculate the Wind Chill Temperature (WCT) from air temperature and wind
+    speed.
+
+    It is the perceived decrease in air temperature felt by the body on
+    exposed skin due to the flow of air. It's used by both National
+    Weather Service (NSW) in US and Canadian Meteorologist service.
+    The formula is derived from Envirnoment Canada [1].
+
+    Note:
+        [1]  "Environment Canada - Weather and Meteorology - Canada's Wind
+        Chill Index". Ec.gc.ca. Retrieved 2013-08-09.
+
+    Args:
+        ta: Air temperature [C]
+        ws: Wind speed [m/s]
+
+    Returns:
+        twc: Wind Chill Temperature [C]
+    """
+    ws_km_h = ws * 3.6   # convert wind speed from m/s to km/h
+    twc = 13.12 + 0.6215 * ta - 11.37 * (ws_km_h ** 0.16) + 0.3965 * ta * \
+        (ws_km_h ** 0.16)
+
+    return twc
+
+
+def windchill_temp_effect_category(twc):
+    """Get the category of effect associated with a given wind chill
+    temperature (WCT).
+
+    Each number (from -6 to 0) represents a certain WCT thermal sensation
+    category. With categories being the following:
+         0 = No discomfort. No risk of frostbite formost people
+        -1 = Slight increase in discomfort. Low risk of frostbite for most people
+        -2 = Risk of hypothermia if outside for long periods without adequate
+             protection. Low risk of frostbite for most people
+        -3 = Risk of hypothermia if outside for long periods without adequate
+             clothing or shelter from wind and cold. Increasing risk of frostbite
+             for most people in 10 to 30 minutes of exposure
+        -4 = Risk of hypothermia if outside for long periods without adequate
+             clothing or shelter from wind and cold. High risk of frostbite
+             for most people in 5 to 10 minutes of exposure
+        -5 = Serious risk of hypothermia if outside for long periods without
+             adequate clothing or shelter from wind and cold. High risk of
+             frostbite for most people in 2 to 5 minutes of exposure
+        -6 = Danger! Outdoor conditions are hazardous. High risk of frostbite for
+             most people in 2 minutes of exposure or less
+
+    Args:
+        twc: Wind Chill Temperature [C]
+
+    Returns:
+        category: An integer indicating the level of warning associated with the
+            heat index. Values are one of the following:
+         0 = No discomfort
+        -1 = Slight increase in discomfort
+        -2 = Risk of hypothermia if outside for long periods without adequate
+             protection
+        -3 = Risk of hypothermia if outside for long periods without adequate
+             clothing or shelter from wind and cold.
+        -4 = Risk of hypothermia if outside for long periods without adequate
+             clothing or shelter from wind and cold.
+        -5 = Serious risk of hypothermia if outside for long periods without
+             adequate clothing or shelter from wind and cold.
+        -6 = Danger! Outdoor conditions are hazardous.
+    """
+    if twc >= 0:
+        category = 0
+    elif twc >= -9:
+        category = -1
+    elif twc >= -27:
+        category = -2
+    elif twc >= -39:
+        category = -3
+    elif twc >= -47:
+        category = -4
+    elif twc >= -54:
+        category = -5
+    else:
+        category = -6
+
+    return category

tests/wc_test.py

@@ -0,0 +1,42 @@
+# coding utf-8
+import pytest
+
+from ladybug_comfort.wc import windchill_index
+from ladybug_comfort.wc import windchill_index_effect_category
+from ladybug_comfort.wc import windchill_temp
+from ladybug_comfort.wc import windchill_temp_effect_category
+
+
+def test_windchill_index():
+    """Test the windchill_index function"""
+    assert windchill_index(32, 85) == pytest.approx(20.5216, rel=1e-3)
+    assert windchill_index(20, 50) == pytest.approx(471.1182, rel=1e-3)
+
+
+def test_windchill_index_effect_category():
+    """Test the windchill_index_effect_category function"""
+    assert windchill_index_effect_category(2350) == -4
+    assert windchill_index_effect_category(1700) == -3
+    assert windchill_index_effect_category(1000) == -2
+    assert windchill_index_effect_category(600) == -1
+    assert windchill_index_effect_category(400) == 0
+    assert windchill_index_effect_category(150) == 1
+    assert windchill_index_effect_category(90) == 2
+    assert windchill_index_effect_category(30) == 3
+
+
+def test_windchill_temp():
+    """Test the windchill_temp function"""
+    assert windchill_temp(32, 85) == pytest.approx(36.3012, rel=1e-3)
+    assert windchill_temp(20, 50) == pytest.approx(17.6540, rel=1e-3)
+
+
+def test_windchill_temp_effect_category():
+    """Test the windchill_index_effect_category function"""
+    assert windchill_temp_effect_category(10) == 0
+    assert windchill_temp_effect_category(-5) == -1
+    assert windchill_temp_effect_category(-15) == -2
+    assert windchill_temp_effect_category(-31) == -3
+    assert windchill_temp_effect_category(-45) == -4
+    assert windchill_temp_effect_category(-50) == -5
+    assert windchill_temp_effect_category(-60) == -6