You might think that December 21st, the shortest day of the year in the United States (US), would also be the coldest day of the year. After all, the Northern hemisphere is at its furthest point from the sun on that day, receiving less sunlight than any day before and after.
The coldest day actually depends on where you live in the U.S. Some states experience their coldest temperatures at a date close to or on December 21st while others experience it toward the end of January.
Why is this? What makes some states have their coldest day of the year far later than other states? For answers on these and more, read the content below!
Your State’s Coldest Day
Our infographic uses three decade’s worth of data on the average normal climate temperatures and precipitation for each state, presenting:
- The coldest day of the year for each state, as you will see on the map.
approximatetemperature range you can expect for each state, spanning from 0F (far below freezing) to 60F, as seen on the key.
The temperature ranges are associated with a gradation of blue—the lighter shades displaying higher state temperatures and the darker shades displaying lower state temperatures.
You will notice that there are various patterns across the graphic:
- From the West coast to the East coast, the coldest day lands further from December 21st, being as late as January 24th (with Utah as the obvious exception).
- States West to East of the US receive similar, warmer temperatures, whereas the northern and middle states are colder.
- The West coast is warmer than the East coast on average. This is due to the differences between the Pacific Ocean (West) and the Atlantic (East).
- Hawaii is an outlier in the data, having its coldest day of the year in early February.
- Only Arizona, Louisiana, and Hawaii reach 40F-60F temperatures.
- The northernmost tip of the East coast receives colder temperatures (0F-20F) than the rest of the East coast (20F-40F). Massachusetts and Rhode Island are the exceptions to this trend.
To see more, such as the weather of specific days throughout the last three decades, visit the National Oceanic and Atmospheric Association (NOAA) website.
The West coast is naturally warmer than the East coast, and the South is naturally warmer than the North. Why?
The US is surrounded by multiple bodies of water that carry hot and cold air from one place to another. The largest contributors to this process are of course the Pacific and Atlantic oceans.
West and East - And the Impact of the Oceans
Larger bodies of water act as heat sinks, storing solar heat in their surface layers while creating additional heat as their waters constantly evaporate.
The Pacific Ocean is particularly good at this. It carries heat North from the equator to the Arctic Ocean/North Pole, moving along the West coast of the US in the process. This constant supply of heat creates warmer weather, high moisture, and heavy rainfall rather than snow among the West coast. This is why they experience mild winters.
The southern end of the East coast is mainly affected by the Gulf of Mexico. The Gulf Stream carries humid, hot air, which it pushes inland. This is what allows states such as Georgia, South Carolina, and Florida to have warmer winters than their northeastern counterparts.
On the other hand, the Atlantic carries frigid air from the North to the equator, producing colder winters along the East coast of the US.
The Gulf and the Atlantic are connected, allowing the heat from the Gulf Stream to be taken upward along the East coast. The problem, however, is that the Gulf loses its influence the farther the Atlantic flows North. For this reason, the winter season gets harsher in the mid-East coast and northeast, until the temperature drops as low as 0F on average.
North and South
The sun warms up the earth unevenly. The equator receives more heat than anywhere else on the globe. Because of this, the US’s southern states are warmer than the northern states.
The North deals with an additional influence: a polar jet stream.
Jet streams are narrow, strong wind currents that blow West to East but often drift to the North and South. These phenomena are the most pronounced in the winter, stretching from the bottom of Canada to the northern and central areas of the US, and sometimes the eastern coast.
There are four primary jet streams around the world—two polar jet streams and two subtropical. The one that the US experiences in the North brings in cold air from the Arctic, disproportionately cooling the northern and central states in comparison to their southern neighbors.
Your state and/or immediate area may also have topographical reasons that affect your state’s coldest day:
- The Great Lakes. The five Great Lakes create what’s called a “lake effect” around the surrounding states. This means that the states experience colder, snowier winters because of the warm air over the water mixing with the low temperatures and the cold air coming from Canada. This phenomenon is intensified in cities near the Great Lakes, as cities act as heat sinks also, generating more moisture in the air that eventually turns into snow.
- Elevation. Heat rises and cold air sinks. Depending on the predominant topography of your city or the surrounding area, the altitude will impact the weather. For example, the Cascade Mountains are found in the Pacific Northwest. As the moist, hot air from the Pacific works its way over the west side of the mountains, it cools off, forming clouds and precipitation. On the east side, the cooled/cold air warms up again and dries out. So, those who live on the west side of the mountains will undergo a wetter season than those who reside on the opposite side.
- El Niño-Southern Oscillation (ENSO) cycle. While El Niño and La Niña aren’t topographical per se, they are native to the Pacific Ocean and affect our winters (January-March). El Niño and La Niña occur every 3 to 7 years. They are natural large-scale temperature fluctuations between the Pacific Ocean and the atmosphere that alter typical weather patterns. El Niño warms and La Niña cools large portions of the Pacific, heavily influencing how much rain or snow will fall in a given area. During an El Niño season, the Pacific Northwest has a warmer winter than usual, and the South (from California to the Carolinas) has a wetter season than average. La Niña does the reverse.