Heating Degree Day (HDD), also known as Degree Days, is a metric used to measure the severity of winter weather and the need for energy to heat buildings. HDD is determined by subtracting the daily average temperature from the number 65 degrees Fahrenheit (18.33C), which is the baseline temperature at which buildings turn on heating systems.

A day is considered a Heating Degree Day if the day's average temperature falls below 65 degrees Fahrenheit, which is when buildings turn on their heating systems to ensure indoor temperatures are an average of 70 degrees Fahrenheit (21.11C). Negative values are usually rounded off, but HDD is typically set to zero if it has a negative value.

HDD is used in the calculation of weather futures contracts, which are a popular financial risk management tool for industries such as construction and agriculture that rely heavily on weather conditions for the success of their operations. Weather futures contracts indemnify consumers against losses suffered due to severe weather conditions by providing a hedge against risks posed by inclement weather.

In addition, HDD can be used as a measure of air conditioning needs for people living in warm climates. Just as HDD measures the need for heating in colder climates, Cooling Degree Days (CDD) measure the need for air conditioning in hot climates. These calculations are often used by energy companies and utility providers to determine the energy demand in specific geographical regions, and to ensure they have the necessary resources to meet summer demands.

Overall, Heating Degree Day is an important factor to consider when assessing indoor comfort levels and energy demand. HDD is widely used by a variety of industries as a way to ensure appropriate heating needs are met, and also to protect against losses posed by weather-related disruptions. By understanding HDD, businesses, contractors and utility providers will be better equipped to accurately predict energy demand and provide reliable heating and cooling services.