What it means
A hot day often raises demand for air conditioning, and cooling degree days convert daily average temperature into an index of warmth above a reference point. The standard US example uses 65 degrees Fahrenheit, about 18 degrees Celsius.
The US National Weather Service defines the daily mean as the high plus the low divided by two for its degree-day explanation, and when the mean is above the base, subtract the base to obtain cooling degree days; below it, the daily CDD is zero. If the mean is 78 degrees Fahrenheit, the daily value is 13 CDDs on a 65-degree base.
A sequence of warm days adds up: ten days at 13 CDDs each contribute 130 CDDs to the period total. The base represents a modelling convention, not the precise thermostat threshold for every building, so insulation, humidity, occupancy and equipment efficiency can cause two buildings to use different power with the same CDD count.
CDD differs from heating degree day, which measures how far the daily mean falls below the base. A day can contribute to one side of that simple convention, but not both at the same base under the daily-mean formula.
Utilities can compare CDDs with historical electricity demand to plan generation and purchases, though the relationship is empirical and can change as customers install air conditioning or improve efficiency. Weather derivatives can settle on an observed temperature index rather than measured losses.
CME Group lists index-based weather products for specific cities and periods, and the contract defines its station, observation method, base and payout rule. A company may buy a contract that pays when CDDs exceed an agreed level, offsetting some extra cooling costs, but if its own costs rise for other reasons or the selected city's weather differs, the hedge can fall short.
A numerical index can also be used for speculation. A payoff based on temperature is not proof that the buyer suffered a loss, since weather contracts can gain or lose value independently of a business's operations.
Investopedia describes both daily high-low means and more frequent temperature readings as calculation approaches, which need not produce identical results for the same day, so follow the exact method in the data series or contract. A calendar month with the same total CDD as another may have a different pattern of extreme days, and peak power costs can depend heavily on when heat occurs, so a simple monthly sum can miss timing risk.
Locations matter too, because a temperature observation at an airport may not match a building's microclimate or the utility's entire service area. The practical check is to record the base, units, weather station, averaging convention and contract multiplier, since without those details identical-looking CDD numbers may describe different exposures.
In practice
Real-world examples.
Example
A 75-degree Fahrenheit daily mean gives 10 CDDs using a 65-degree base.
Example
A utility compares a hot month's CDD total with its electricity demand to plan supply.
Example
A weather contract references a specific city's CDD index, which may differ from the insured business's local weather.
Formula
Calculation
Daily CDD = max(0, (daily high + daily low)/2 - base temperature). At a high of 85 degrees Fahrenheit, low of 65 and base of 65, mean temperature is 75 and CDD is 10. Monthly CDD = sum of daily CDDs. A weather contract's payout might depend on the excess over a strike multiplied by an agreed dollar amount, but actual terms vary.
As an illustration, suppose a 30-day month averages 10 CDDs per day, so the monthly total is 30 x 10 = 300 CDDs. If a contract has a strike of 280 CDDs and pays $20 per CDD above the strike, the payout is (300 - 280) x $20 = $400. If the month totals only 270 CDDs, the payout is zero.Case study
Seen in the real world.
Fictional example: A hotel operator in a warm region budgets summer electricity costs. The team compares its past bills with local cooling degree days and observes that hotter months often cost more. But higher occupancy and tariff changes also affect the bill. The operator evaluates a weather contract based on a nearby station.
It checks the base temperature, settlement period and dollar multiplier, then models both a hot local month and a mismatch between the station and hotel. It treats CDD as a planning input, not a perfect reimbursement for power costs. In the review, the finance team notes that a $400 payout on a month with a much larger power bill would barely dent the overspend. They decide to size any contract against the bill they actually expect to vary with temperature, not against the whole electricity budget.
Watch out
Common mistakes.
- Equating one CDD with a fixed amount of electricity use for every building.
- Using the wrong base temperature or observation station when comparing contracts.
- Assuming a weather derivative necessarily pays exactly when operating costs rise.
Questions
People also ask.
What does a CDD of 10 mean?
Under a 65-degree Fahrenheit daily-mean convention, the day's mean was 10 degrees above that base.
Is this actual energy consumption?
No. It is a temperature-based proxy; building design and behaviour also affect energy use.
Why do traders care?
Some weather contracts settle on degree-day indices used to manage or take exposure to temperature risk.
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