Degree day

BBC 2005 rigueur climatique

Degree day is a measure used in meteorology, agriculture, and building energy performance to estimate the demand for energy to heat or cool buildings. It is a reflection of the demand for energy based on the extent to which the daily average temperature deviates from a base temperature—typically 65°F (18°C) in the United States. Degree days are used in calculations relating to the effect of outside air temperature on building energy consumption.

Definition

A degree day is computed as the difference between the mean outdoor temperature of a single day and a base temperature. Degree days can be categorized into two types: Heating Degree Days (HDD) and Cooling Degree Days (CDD).

• Heating Degree Days (HDD) are calculated when the daily average temperature is below the base temperature. They are a measure of the heating needs of a building. The higher the HDD, the colder the weather, and thus, the higher the demand for heating.
• Cooling Degree Days (CDD) are calculated when the daily average temperature is above the base temperature. They indicate the cooling needs of a building. The higher the CDD, the warmer the weather, and therefore, the higher the demand for cooling.

Calculation

The calculation of a degree day is straightforward. For HDD, the formula is:

\[ HDD = \max(0, T_{base} - T_{avg}) \]

Where \(T_{base}\) is the base temperature and \(T_{avg}\) is the average temperature of the day. For CDD, the formula is:

\[ CDD = \max(0, T_{avg} - T_{base}) \]

Applications

Degree days have a wide range of applications, including:

• Energy Management: They are used by utility companies to forecast energy demand and by building managers to optimize energy use.
• Agriculture: Degree days help in predicting plant growth stages, pest emergence, and crop harvesting times.
• Climate Change Studies: Long-term trends in degree days can indicate changes in climate patterns.

Limitations

While degree days are a useful tool, they have limitations. They do not account for solar radiation, wind speed, or humidity, all of which can affect building energy needs. Additionally, the choice of base temperature can significantly impact the calculation of degree days, making comparisons between different regions or studies challenging.

See Also

• Meteorology
• Building Energy Performance
• Energy Management
• Climate Change

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