What Are HDD and CDD in Natural Gas Trading?

Degree days measure how far the average daily temperature is above or below a base temperature.

In the United States, the standard base temperature is usually 65°F.

A day with an average temperature close to 65°F produces few or no degree days. Colder conditions produce HDD, while hotter conditions produce CDD.

Degree days are not a direct measurement of natural gas consumption. They are a standardized weather metric used to estimate how temperature may affect heating and cooling demand.

Heating Degree Days, or HDD, measure how cold a day is relative to 65°F.

HDD = max(65°F - daily average temperature, 0)

For example, if the average daily temperature is 40°F:

If the average temperature is 70°F, the day produces zero HDD because heating demand is not expected based on the standard degree-day calculation.

Higher HDD generally indicates stronger space-heating demand and potentially higher natural gas consumption. However, the demand impact also depends on where the cold occurs, how long it lasts and how current conditions compare with normal.

Cooling Degree Days, or CDD, measure how hot a day is relative to the same 65°F base temperature.

CDD = max(daily average temperature - 65°F, 0)

For example, if the average daily temperature is 80°F:

If the average temperature is 60°F, the day produces zero CDD.

Higher CDD generally indicates stronger air-conditioning demand. This can increase natural gas consumption in the power sector as electricity generators respond to higher cooling load.

The relationship is indirect. CDD affects electricity demand first, while the resulting natural gas demand depends on the regional generation mix, renewable output and other power-market conditions.

Total Degree Days, or TDD, combine heating and cooling degree days into a single metric.

TDD = gwHDD + pwCDD

TDD provides a broad view of total temperature-driven energy demand.

During winter, HDD normally accounts for most of the total. During summer, CDD becomes the dominant component. During shoulder seasons, both values may remain relatively low.

TDD is useful for comparing the overall demand intensity of different forecast periods, but HDD and CDD should still be reviewed separately because heating and cooling affect natural gas demand through different channels.

A simple national average treats every location equally.

That approach can be misleading for natural gas analysis because the same temperature change can have very different demand effects in different parts of the country.

Cold weather in a major gas-heating region may matter more than the same temperature change in an area with limited heating demand. Similarly, extreme heat affecting a major population center may create more cooling demand than hotter conditions in a lightly populated area.

NatgasApp uses:

• Gas-weighted HDD (gwHDD), giving greater importance to locations where cold weather has a larger effect on natural gas heating demand.
• Population-weighted CDD (pwCDD), giving greater importance to locations where hot weather affects more people and creates greater cooling load.

NatgasApp aggregates HDD and CDD across 181 representative locations in the contiguous United States.

This produces a more demand-relevant national view than a simple unweighted temperature average.

A forecast value becomes more useful when it is compared with the expected seasonal level.

A forecast of 150 HDD may represent unusually cold conditions in one period but near-normal conditions in another. The absolute number alone does not provide enough context.

In general:

• Above-normal HDD indicates colder-than-normal conditions and stronger-than-normal heating demand.
• Below-normal HDD indicates milder conditions and weaker heating demand.
• Above-normal CDD indicates hotter-than-normal conditions and stronger cooling demand.
• Below-normal CDD indicates cooler conditions and weaker cooling demand.

NatgasApp derives normals independently from historical observations using the same location set used for forecast aggregation.

This allows the forecast and the normal to be compared on a consistent basis.

Daily degree days show when heating or cooling demand is expected to occur.

Cumulative degree days show the total demand estimate across a selected forecast period.

Both views are useful.

A cumulative forecast may show that a model has added 10 HDD, but the daily view shows where those HDD were added. A near-term change may carry more practical importance than the same adjustment near the end of the forecast period.

Daily charts help identify:

• The timing of cold and heat
• The duration of a weather event
• Changes concentrated on specific dates
• Transitions between heating and cooling demand

Cumulative charts make it easier to compare the total demand effect across models and forecast runs.

Natural gas traders do not look only at the absolute HDD or CDD value. The direction of change between model runs can be equally important.

For example:

The latest run has added forecast heating demand.

However, one run should not be evaluated in isolation. The change becomes more meaningful when it persists across several updates, appears in both GEFS and ECMWF EPS, or is accompanied by reduced ensemble uncertainty.

A large adjustment in the distant forecast period may also be less reliable than a smaller change occurring in the near term.

Degree days are useful demand indicators, but they are not complete trading signals.

Natural gas prices are also affected by:

• Production
• Storage inventories
• LNG feedgas demand
• LNG terminal outages and maintenance
• Adverse shipping conditions, including hurricanes and other maritime disruptions
• Pipeline constraints
• Renewable generation
• Coal-to-gas switching
• Market positioning
• Existing weather expectations

A colder forecast is not automatically bullish if the change was already expected or if other fundamentals offset the additional demand.

HDD and CDD should be treated as part of a wider natural gas market analysis.