No, HDD is not a medical condition. HDD stands for Heating Degree Days. It is a way of using a number to describe the weather conditions.
Some engineers use HDD data to design heating systems to cope with the weather. I use HDD to predict how much fuel will be burned to heat buildings.
The HDD for the day is calculated using 65 degrees Fahrenheit (18C) as a base temperature. The assumption is that if the outdoor temperature is 65 or higher you do not need to heat your building or home. When the outdoor temperature drops below 65, most folks will be turning their furnaces on.
HDD is calculated by comparing the average temperature for the day to 65. As an example, if the average temperature for a particular day is 40 then the HDD for the day is 25 (65 – 40 = 25).
Yesterday, where I live, the high was 36 and the low was 28. The average of those two numbers is 32. That gives a HDD for the day of 33 (65 – 32 = 33). The long-term average HDD for this area is 41. That means that my heating bill for yesterday will be less than average.
Tomorrow is another day, the forecasted high is 18 and the forecasted low is Zero. Tomorrow’s HDD is likely to be 56. (The average of 18 and 0 is 9 and 65 – 9 = 56)
The bigger the HDD number the more heating fuel will be needed. Again the long-term average is 41 so tomorrow will be much colder than average and I can expect to burn much more natural gas than for an average January day.
Forecasting fuel usage is not as simple as I made it sound because there are other factors involved.
Wind chill can increase the heating bill a lot. The more wind there is the more fuel is needed. Wind chill make people feel colder and makes buildings cool off faster.
The amount of sunshine is another factor. Clear sky and sunshine mean less fuel used during the day light hours.
The amount of heat generated inside the building from lights, people and machinery can also change. When the amount of activity in a building changes the need for heating fuel also changes.
I get the weather data I use from two web sites. Weather Underground is a good source for historical data and current HDD numbers for your area.
As an example a high school class mate of mine has a house on the North coast of Spain. When he asked me about using solar heating for his house, I was able to look up the data I needed, on the Weather Underground web site, to give him a good answer. They have very nice weather.
I also use AccuWeather.com for their excellent forecasts. I use their 15-day forecast to predict the HDD for future days.
Other weather descriptive numbers are CDD (Cooling Degree Days) and GDD (Growing Degree Days).
CDD is calculated for average temperatures above 65F as an indication of how much air conditioning is needed to cope with warm weather.
GDD is based on 50F (10C) as an indicator of how well food crops grow. The higher the GDD is the faster crops grow.
Weather forecasters use math to help them predict the weather and engineers use math to design things to cope with the weather and manage energy use. We all get to have fun with numbers.
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