Heating Season in Connecticut
Hartford typically opens its heating season around September 24 and closes it around May 23. Here is the full window, the rest of Connecticut, how cold the nights get, and any active National Weather Service cold product.
Typical heating season at Hartford: September 24 to May 23Connecticut's heating season
1991-2020, HartfordThe Connecticut River valley shows up in the heating numbers exactly as it does in the summer ones. Hartford needs 5,883 heating degree days a year to Bridgeport's 5,140, and takes 46.9 nights at or below 20 F to Bridgeport's 26.1, because Long Island Sound will not let the shoreline get properly cold. Norfolk up in the Litchfield Hills needs 7,311, a Vermont number inside Connecticut.
Heating across Connecticut, station by station
A single state figure would hide the thing that decides a winter: elevation, latitude and distance from water move these numbers further inside Connecticut than three decades of warming does. Each row is one NOAA reference station. The degree days and the night counts are quoted straight from NOAA's published 1991 to 2020 normals; the opening and closing dates are our own derivation from the same agency's daily observations, because NOAA publishes no date shaped heating variable.
The spread here runs from 5,140 heating degree days a year at Bridgeport to 7,311 at Norfolk, a difference of 2,171 inside one state.
| Station | Heating degree days | Season opens | Season closes | Nights 32 F or below | Nights 20 F or below |
|---|---|---|---|---|---|
| Hartford | 5,883 | September 24 | May 23 | 126 | 47 |
| Bridgeport | 5,140 | October 4 | May 23 | 92 | 26 |
| Norfolk | 7,311 | September 12 | June 8 | 152 | 70 |
Longest heating load
heating degree days a year at Norfolk, the most of this page's Connecticut stations.
Quoted NOAA normalColdest nights
nights a year at Norfolk whose low reaches 20 F or below.
Quoted NOAA normalColdest on record
at Hartford on January 22, 1961, in a station record running 1942 to 2026.
Our derivation
The nights set the load, the days only ease it
A heating degree day is built from the daily mean, which means the overnight low is doing half the arithmetic. Two places can share an afternoon high and part company completely after dark, and the one whose nights fall further pays for it in every hour before sunrise, when there is no sun on the walls and nothing to slow the loss.
In Connecticut, Norfolk averages 70 nights a year at or below 20 F, and the coldest daily maximum on record at any station here is -9 F at Norfolk: a day the air never rose past it. Those counts are NOAA's published normals, and NOAA's thresholds include the named value, so a night that bottomed out at exactly 20 F is counted as one.
Nights at or below 32 F
a year at Hartford. NOAA's published 1991 to 2020 normal.
Quoted NOAA normalNights at or below 0 F
a year at Hartford. Below zero the gap between inside and outside is more than 70 degrees, and every hour of it is heating load.
Quoted NOAA normalDays that never thaw
days a year at Hartford whose HIGH stays at or below 32 F. An ice day is the one where the building gets no relief at all, because even the afternoon is below freezing.
Quoted NOAA normal
What the National Weather Service has out in Connecticut
Everything below is read live from api.weather.gov when this page loads and is shown exactly as the issuing forecast office wrote it, with the office and the times it covers.
Checking api.weather.gov for active cold products.
Product text, issuing office, area and times are passed through exactly as the National Weather Service issued them. The NWS retired Wind Chill Warning, Wind Chill Watch, Wind Chill Advisory, Hard Freeze Warning and Hard Freeze Watch on 1 October 2024. Read . Source: api.weather.gov
Loading the National Weather Service gridpoint forecast.
What Connecticut asks for, month by month
Heating degree days are the measure utilities and mechanical engineers plan against, and NOAA publishes them directly, so every figure in this section is NOAA's published normal. At Hartford the coldest month is January, carrying 1,176 degree days on its own, and the months that carry real heating demand run January through December. December, January and February together account for 54 percent of the year's total, which is the number that tells you whether this is a concentrated winter or a long shallow one.
U.S. Energy Information Administration, Degree days“The base temperature we use is 65°F. If the mean temperature is above 65°F, we subtract the base temperature (65°F) from the mean, and the result is Cooling Degree Days (CDD). If the mean temperature is below 65°F, we subtract the base temperature from the mean, and the result is Heating Degree Days (HDD).”
U.S. Energy Information Administration, Degree days“A cooling degree day indicates a hot day and measures how much air conditioning we need to keep a building cool. A heating degree day indicates a cold day and measures how much heating we need to keep a building warm.”
NOAA's 1991 to 2020 normals publish, without any arithmetic of ours: annual and monthly heating degree days, the mean number of nights a year at or below 32 F, 20 F and 0 F, and the mean number of days whose high never rises above 32 F. NOAA publishes no date shaped heating variable, so the opening and closing dates on this page are this site's own derivation, labelled as such.
One caveat. Adding up daily heating degree days from the raw observations does not reproduce NOAA's published normal exactly: NCEI derives its degree-day normals from smoothed daily temperature normals with an adjustment for how days scatter around the mean, and it estimates days a station missed, while this site sums the days that reported and drops incomplete years. The two land about a percent apart in either direction, which is why the degree day figures here are always NOAA's.
Our summary of information published by NOAA NCEI 1991-2020 U.S. Climate Normals
Cold weather, in the agencies' own words
Cold safety guidance is the National Weather Service's. Here it is on the cold itself, quoted directly, and then the products it issues about it.
National Weather Service, Cold Weather Safety“Extremely cold air comes every winter into at least part of the country and affects millions of people across the United States. The arctic air can be dangerous. Combined with brisk winds, dangerously cold wind chill values can result. People exposed to extreme cold are susceptible to frostbite and can succumb to hypothermia in a matter of minutes.”
National Weather Service, Cold Weather Safety“Hypothermia occurs when the body loses heat faster than it can produce it.”
National Weather Service, Cold Weather Safety“A freeze occurs when the temperature drops below 32°F. Freezes and their effects are significant during the growing season. Frost develops on clear, calm nights and can occur when the air temperature is in the mid-30s.”
On 1 October 2024 the National Weather Service retired five product names in its hazard simplification work. Wind Chill Warning became Extreme Cold Warning, Wind Chill Watch became Extreme Cold Watch, Wind Chill Advisory became Cold Weather Advisory, Hard Freeze Warning was folded into Freeze Warning, and Hard Freeze Watch became Freeze Watch. The agency's own announcement is at weather.gov. It matters here because code still asking for a retired name gets an empty answer, which looks identical to an all clear. The panel above asks only for the current five.
Carbon monoxide: the agencies, in their own words
Heating season is carbon monoxide season. The guidance below is the EPA's, verbatim, and the full guidance lives with the EPA and with the Consumer Product Safety Commission.
“Carbon monoxide is an odorless, colorless and toxic gas. Because it is impossible to see, taste or smell the toxic fumes, CO can kill you before you are aware it is in your home. The effects of CO exposure can vary greatly from person to person depending on age, overall health and the concentration and length of exposure.” (US EPA, Carbon Monoxide's Impact on Indoor Air Quality)
On what exposure does, again the EPA: “At low concentrations, fatigue in healthy people and chest pain in people with heart disease. At higher concentrations, impaired vision and coordination; headaches; dizziness; confusion; nausea. Can cause flu-like symptoms that clear up after leaving home. Fatal at very high concentrations.”
And on generators, which is where a cold snap and a power cut meet: “Never use a portable generator inside homes, garages, crawlspaces, sheds or similar areas. Deadly levels of carbon monoxide can quickly build up in these areas and can linger for hours, even after the generator has shut off.”
Full guidance: EPA Carbon Monoxide and indoor air quality and CPSC Carbon Monoxide Information Center. The EPA and the CPSC are the authorities on carbon monoxide, detectors and heating equipment, and their guidance is the guidance.
How the Connecticut figures are calculated
Two registers, kept apart. Quoted: the heating degree days and the counts of cold nights and ice days are NOAA NCEI's published 1991 to 2020 U.S. Climate Normals, printed here as NOAA publishes them. Derived: the opening and closing dates, the season span and the record extremes are ours, computed from NOAA NCEI GHCN-Daily station observations, because the normals product publishes no date shaped heating variable at all.
The rules are the same at every station. A heating day is a day that accrues at least one heating degree day under NOAA's own whole-degree convention. A season runs July to June, labelled by its starting year, because every US heating season crosses New Year. The season opens on the first day of its first run of 7 or more consecutive heating days and closes on the last day of its last such run; 7 days is this site's threshold, stated on every page, because one cold morning in August does not open a heating season and a week that never averages above 65 F does. A season only counts if at least 85 percent of its days carry a clean pair of readings, and incomplete seasons are excluded rather than filled in. Where a percentile lands on a season that never sustained a heating run at all, no date is printed.
The cross check: NOAA publishes counts of nights at or below 32 F, 20 F and 0 F, and this site computes its own from the daily records with the same inclusive rule. At Hartford the derived count is 126 against NOAA's 126 freezing nights, and across all 150 stations in this dataset 145 agree within two days a year. The same counts also agree within 1.2 days everywhere against the frozen pipes vertical, which computes its 20 F counts independently over a different season window. One detail matters if you try to reproduce this: a US station reports whole degrees Fahrenheit and NOAA's day count normals include the threshold itself, so a night recorded at exactly 20 F counts here as it does for NOAA, and requesting the archive in metric would quietly lose it.
The page's shape follows those numbers. Where a state's primary station needs 6,000 heating degree days a year or more, the dates carry little information and the page leads with length and cold instead. Where it needs under 2,000, leading with dates would oversell the season. Connecticut is on the defined shape, at 5,883 degree days.
Live conditions come from api.weather.gov, read in your browser at page load. Sources: NOAA NCEI 1991-2020 U.S. Climate Normals, NOAA NCEI GHCN-Daily, EIA Degree days. All are US Government works in the public domain.
Connecticut heating season FAQ
The rest of Connecticut's winter
Heating season in nearby states
Heating degree days and cold night counts are quoted from NOAA NCEI's 1991-2020 U.S. Climate Normals. Opening and closing dates, season spans and record extremes are our derivation from NOAA NCEI GHCN-Daily observations. Live products come from the National Weather Service and are shown verbatim. National picture: heating season in every state