What is the difference between an average and a normal?

The arithmetic average of a meteorological element over 30 years is defined as the climatological normal. So, the key difference between the two is that a normal is strictly for 30 years, whereas an average can be computed over any time span. Official normals are calculated by the National Climatic Data Center for temperature and precipitation elements and are updated through the end of each decade ending in zero (for example, 1971-2000). These official normals can be slightly different from a strict average because they also take into account changes in the station over the thirty-year period. These changes can include changes in the instrumentation, the location, the observing practices, the observation time, or in the surrounding environment.

How about between an average temperature and a mean temperature?

Typically when climatologists talk about the mean temperature, they are referring to the average of the maximum and minimum temperatures. Referring to just to an "average" temperature can be a bit to vague - it isn't necessarily clear whether you are referring to the average high, the average low or the average mean for a specific time period.

What is the Wind Chill?

The Wind Chill Temperature Index, sometimes also known as the equivalent temperature, is used to describe how cold people and animals feel when they experience heat loss caused by the combined effects of low temperature and wind.

When the wind blows across exposed skin, it removes the insulating layer of warm air that lies adjacent to the skin. This in turn drives down the skin temperature and eventually the internal body temperature. The faster the wind blows, the faster the heat is carried away, the greater the heat loss and the colder it feels.

A new Wind Chill Temperature Index took effect on November 1, 2001, replacing the original wind chill index that was derived in 1945. The original Wind Chill Index was developed by two Antarctic explorers and was based on research involving the time it took water in a plastic container to freeze. The new Wind Chill Temperature Index includes the latest advances in science, technology and computer modeling. It takes into account a calculated wind speed at average face height based on readings from winds measured at the national standard height of 33 feet. It is based on the exposure of a human face to cold versus a plastic container, incorporates modern heat transfer theory, lowers the calm wind threshold from 4 miles per hour to 3 miles per hour, and has a consistent standard for skin tissue resistance. The new Wind Chill Temperature Index currently assumes no impact from the sun, but it may yet be revised again for solar radiation impacts under various sky conditions (clear, partly sunny, cloudy).

Lastly, while exposure to low wind chills can be life threatening to humans and animals, the only effect that wind chill has on an inanimate objects, such as vehicles, is that it shortens the time it takes the object to cool to the actual air temperature. The object cannot be cooled below the actual temperature.

What is the Heat Index?

The Heat Index, also known as the apparent temperature, is an accurate measure of how hot it really feels when relative humidity (RH) is added to the actual air temperature. The National Weather Service derived the Heat Index in an effort to alert the public to the dangers of exposure to extended periods of heat, especially when high humidity acts along with the high temperatures to reduce the body's ability to cool itself.

One important fact to realize about the Heat Index is that it is computed for temperature readings taken in the shade and for a wind speed of approximately 6 miles per hour. Exposure to full sun can add up to 15 F to the Heat Index value!

To find the Heat Index from the table, find the air temperature along the left side of the table and the relative humidity along the top. Where the two intersect is the Heat Index.

Courtesy of Midwestern Regional Climate Center