Cardinal Parameter Information
CALCULATED PARAMETERS
Wind Chill
Wind chill is based on the amount of heat that is lost from the body due to exposure to cold temperatures and wind. As wind speeds increase, heat is drawn from the body, leading to a decrease in skin temperature, and by extension, a decrease in internal body temperature. In effect, high wind speed coupled with existing cold air temperature can make the environment feel colder. Wind chill data are available from stations with instantaneous measurements of temperature and wind speed.
For more detailed information regarding cold weather safety — including the equation used to calculate wind chill — please visit the following page: https://www.weather.gov/safety/cold-faqs
Heat Index
Heat index was devised as a way to determine how warm the outside temperature feels when relative humidity is factored in. When body temperature increases, perspiration begins to take place. If conditions are cool enough and dry enough, this perspiration can evaporate off of the body, effectively cooling its temperature. However, if this perspiration cannot evaporate — such as in an environment with high air temperature and high humidity — body temperature can’t be regulated. In effect, the human body feels warmer in humid conditions. Heat index data are available from stations with instantaneous measurements of temperature and relative humidity.
For more detailed information regarding the equation used to calculate heat index, please visit the following page: https://www.wpc.ncep.noaa.gov/html/heatindex_equation.shtml
Station Pressure and Sea Level Pressure
Station pressure is the air pressure that is observed at a specific elevation, and is the true barometric pressure of a location. It is ultimately a measurement of the pressure exerted by the atmosphere at given location, as a result of gravity acting upon the “column” of air that lies directly above that location. Locations with higher elevations above sea level experience lower pressure, given air’s mass decreases with increases in elevation. While many stations directly measure station pressure, this parameter can also be calculated for stations with instantaneous measurements of temperature and sea level pressure, with the station’s elevation factored in.
Most often used by meteorologists to track weather systems at the surface, sea level pressure is a reduced pressure that uses observed conditions to remove the effects of elevation from pressure readings. As a result, it estimates the pressure that would exist at sea level at a point directly below the station using a temperature profile based on temperatures that actually exist at the station. While many stations directly measure sea level pressure, this parameter can also be calculated for stations with instantaneous measurements of temperature and station pressure, with the station’s elevation factored in.
Reference Crop Evapotranspiration (Using Penman-Monteith Method)
The Food and Agriculture Organization of the United Nations (FAO) derived a standard Penman-Monteith method to determine the rate of moisture transport away from a surface (i.e., evaporation rate from water surfaces and transpiration rate from plant surfaces). This method requires measurements of net radiation, ground heat flux, temperature, humidity, and wind speed.
Solar radiation measurements are not readily available at all stations, so the needed solar radiation measurement is replaced with the Hargreaves radiation estimate. Hargreaves’ equation states that the incoming solar radiation is proportional to the extraterrestrial radiation, the square root of the difference between daily maximum and minimum temperatures, and a coefficient for the proximity to water. Extraterrestrial radiation is the idealized amount of radiation from the sun if it were striking a flat plane perpendicular to the sun’s rays at the top of the atmosphere, over a certain latitude, and during a certain time of year and day. The coefficient for the proximity to water ranges from 0.16 for interior locations to 0.19 for coastal locations. Due to North Carolina’s relative location, a value of 0.18 is used for the entire state.
For more detailed information on the FAO56 Penman Monteith method, please visit: https://www.fao.org/3/x0490e/x0490e06.htm
Wet Bulb Globe Temperature
The Wet Bulb Globe Temperature, or WBGT, is a measure of heat stress as felt by the human body. It generally gives a more representative result than Heat Index since WBGT includes a measure of the temperature in direct sunlight and accounts for evaporation via the natural wet bulb temperature.
Prior to November 1, 2022, all WBGT data in Cardinal used the Stull methodology for calculating natural wet bulb temperature using only air temperature and relative humidity. After an evaluation of historical data across North Carolina, and in order to be consistent with forecasts from the National Weather Service, we switched to using the Boyer methodology — which includes air temperature, relative humidity, wind speed, and solar radiation — on November 1, 2022.
Note that any WBGT data requested and retrieved prior to this date may not match the currently available data using the updated methodology. If this applies to you, we suggest you re-request this data.
DATA NOTICES
AWOS Precipitation
Use this data with caution! Observations from AWOS stations are transmitted every 20 minutes, resulting in 3 observations of accumulated rainfall that resets to zero at the top of the hour (:00 minutes). The last observation within a given hour holds the hourly accumulated precipitation, however this particular observation can be taken anywhere from 40 to 59 minutes after the hour before the precipitation counter resets. As a result, any rain falling after the last observation for that hour, but before the 59 minute mark, will be missed.