Weather Stress Index
Encyclopedia
The Weather Stress Index, or WSI, is a relative measure of the weather
conditions, often used as a comfort indicator. The index, a number between 0 and 100, represents the percentage of time in the past with temperatures below the current temperature, for a given location, day and time. This makes the index a local measure based in past weather conditions. For example, if for a given location, on the 25th of July at 13:00 UTC
the WSI is 85 for a temperature of 42 Celsius, this means that the temperature was inferior to 42 ºC in 85% of the time in the past, on the same place, on the 25th of July at 13:00 UTC (and superior to 42 ºC in 15% of the time on the same place, day and hour). In other words, the WSI gives the probability of finding a smaller temperature in the local weather history, at a given day and time, than that of the present measurement. Therefore, high values of WSI predict a relative discomfort from excessive heat for local inhabitants.
might refer to a temperature that, in lower latitudes
, could be rated with a WSI of 50 (an average temperature) for the same day and hour.
Weather
Weather is the state of the atmosphere, to the degree that it is hot or cold, wet or dry, calm or stormy, clear or cloudy. Most weather phenomena occur in the troposphere, just below the stratosphere. Weather refers, generally, to day-to-day temperature and precipitation activity, whereas climate...
conditions, often used as a comfort indicator. The index, a number between 0 and 100, represents the percentage of time in the past with temperatures below the current temperature, for a given location, day and time. This makes the index a local measure based in past weather conditions. For example, if for a given location, on the 25th of July at 13:00 UTC
Coordinated Universal Time
Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...
the WSI is 85 for a temperature of 42 Celsius, this means that the temperature was inferior to 42 ºC in 85% of the time in the past, on the same place, on the 25th of July at 13:00 UTC (and superior to 42 ºC in 15% of the time on the same place, day and hour). In other words, the WSI gives the probability of finding a smaller temperature in the local weather history, at a given day and time, than that of the present measurement. Therefore, high values of WSI predict a relative discomfort from excessive heat for local inhabitants.
Using the index
The same WSI for different geographic points might not refer to the same temperature - a WSI of 99.99 for a given location near the North PoleNorth Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...
might refer to a temperature that, in lower latitudes
Latitude
In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...
, could be rated with a WSI of 50 (an average temperature) for the same day and hour.