What do these graphed items mean?

Current Outside Dewpoint     Outside Dewpoint History


Dew point temperature is defined as the temperature to which the air would have to cool (at constant pressure and constant water vapor content) in order to reach saturation. A state of saturation exists when the air is holding the maximum amount of water vapor possible at the existing temperature and pressure.


Current Wind Chill     Wind Chill History


The software calculates wind chill based on the temperature and wind speed readings for the archive period (that is, the temperature at the end of the archive interval and the average wind speed during the interval). WeatherLink uses the Osczevski (1995) equation to calculate wind chill. This is the method adopted by the US National Weather Service in September of 2001.


Current Heat Index   Heat Index History


The Heat Index, also known as the Temperature/humidity index (THI) uses the temperature and the relative humidity to determine how hot the air actually "feels." When humidity is high (i.e. the air is saturated with water vapor) the apparent temperature will be higher than the air temperature because perspiration cannot readily evaporate into the surrounding air. Technically, Heat Index is calculable only when air temperature is above 68 °F (20 °C) because it is a measure of heat stress, which is not significant at lower temperatures. Davis Instruments smooths the curve relating air temperature to Heat Index between 68 °F (20 °C) and 57 °F (13.9 °C). Below 57 °F (13.9 °C), the Heat Index is equal to outside temperature. Conversely, 135 °F (52 °C) is the highest Heat index for which calculation factors are available. Note that at very low humidity values, the air temperature can be as high as 140 °F. In terms of storage to the data logger and database, Heat Index is a special case. It is not stored in archive memory or in the database, rather it is calculated as necessary (for example, when plotting or displaying database information). When Heat Index data is needed, the software calculates an average for each archive interval based on the temperature and humidity readings for the archive period.


Current THW Index


The THW Index uses humidity, temperature and wind to calculate an apparent temperature that incorporates the cooling effects of wind on our perception of temperature.