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of the Midwest | Resources & Links | FAQ
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| 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. 
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| 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.
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| Do you keep records
of the Heat Index? If not, how do I calculate it?
Because the Heat Index is a calculated value based off of air temperature and humidity, it is not archived with historic weather data. To calculate a specific value for a previous date, you will need to know the air temperature and humidity. This data is not typically available for cooperative observing stations, but is generally available for airports. The equation to calculate the Heat Index is as follows:
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| What is the Wind Chill?
To find the Wind Chill Temperature Index from the table below, find the air temperature along the top of the table and the wind speed along the left side. Where the two intersect is the Wind Chill Temperature. Click to see wind chill temperature table 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. 
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| Do you keep records
of Wind Chill? If not, how do I calculate it?
Because the Wind Chill Temperature Index is a calculated value based off of air temperature and wind speed, it is not archived with historic weather data. To calculate a specific Wind Chill Temperature for a previous date, you will need to know the air temperature and wind speed. This data is not typically available for cooperative observing stations, but is generally available for airports. The equation to calculate the Wind Chill Temperature Index is as follows:
Where:
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| What are Degree Days
(Heating, Cooling, & Growing)? Heating Degree Days Heating Degree Days (HDD) are used as an index to estimate the amount of energy required for heating during the cool season. When the daily mean temperature falls below 65 degrees Fahrenheit, most buildings require heat to maintain a comfortable interior temperature. By monitoring heating fuel usage and heating degree day accumulation over a period of time, a building's energy consumption per HDD can be calculated and this value used, for example, in fuel consumption monitoring, energy efficiency evaluation or future fuel supply estimates. The daily mean temperature is found by adding together the high and low temperature for the day and dividing by two. When the mean temperature is above 65 F, the HDD total is zero. If the mean temperature is below 65 F, the HDD amount is the difference between 65 F and the mean temperature. For example, if the high temperature for the day was 68 F and the low 52 F, the mean temperature for the day would be 68 + 52 = 120 / 2 = 60 F. The Heating Degree Day total would then be 65 - 60 = 5 HDD. In equation form: Where:
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| 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.
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| 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). For additional information on the new Wind Chill
Temperature Index, please see http://www.nws.noaa.gov/om/windchill. 
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| What are Degree Days
(Heating, Cooling, & Growing)? Growing Degree Days To calculate GDDs, you must first find the mean temperature for the day. The mean temperature is found by adding together the high and low temperature for the day and dividing by two. If the mean temperature is at or below TBASE, then the Growing Degree Day value is zero. If the mean temperature is above TBASE, then the Growing Degree Day amount equals the mean temperature minus TBASE. For example, if the mean temperature was 75°F, then the GDD amount equals 10 for a TBASE of 65°F. You can think of Growing Degree Days as similar to Cooling Degree Days, only the base temperature can be something besides 65°F. In equation form:
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| What are Degree Days
(Heating, Cooling, & Growing)? Cooling Degree Days Cooling Degree Days (CDD) are used as an index to estimate the amount of energy required for cooling during the warm season. When the temperature rises above 65°F, many buildings use air conditioning to maintain a comfortable indoor temperature. By monitoring air conditioner energy usage and cooling degree day accumulation over a period of time, a building's energy consumption per CDD can be calculated and this value used, for example, in energy use monitoring, energy efficiency evaluation or future energy usage estimates. The daily mean temperature is found by adding together the high and low temperature for the day and dividing by two. When the mean temperature is below 65°F, the CDD total is zero. If the mean temperature is above 65°F, the CDD amount is the difference between the mean temperature and 65. For example, if the high temperature for the day was 92°F and the low was 68°F, the mean temperature for the day would be 92 + 68 = 160 / 2 = 80°F. The Cooling Degree Days would then be 80 - 65 = 15 CDD.
In equation form: Where:
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| What are Degree Days
(Heating, Cooling, & Growing)? Growing Degree Days Growing Degree Days (GDD) are used to estimate the growth and development of plants and insects during the growing season. The basic concept is that development will only occur if the temperature exceeds some minimum development threshold, or base temperature (TBASE). The base temperatures are determined experimentally and are different for each organism.
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| Modified Growing Degree
Days
Modified Growing Degree Days are similar to Growing Degree Days with several temperature adjustments. If the high temperature is above 86°F, it is reset to 86°F. If the low is below 50°F, it is reset to 50°F. Once the high / low temperatures have been modified (if needed), the average temperature for the day is computed and compared with a base temperature, which is usually 50°F. Modified Growing Degree Days are typically used to monitor the development of corn, the assumption being that development is limited once the temperature exceeds 86°F or falls below 50°F. For example, if the high for the day was 92°F and the low 68°F, the average for use in the modified GDD calculation would be 86 + 68 = 154 / 2 = 77.
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