Why is January the coldest month when the shortest day is in December?
Why are the hardest frosts in January?
The shortest day of the year falls around 21 December. That is when the Sun shines for the shortest time of the whole year. And yet the hardest frosts usually come in January. How can that be?
Less sunshine in winter
In winter the Sun stays low in the sky, shines at a slant and only for a few hours a day. So the ground gets much less heat than in summer.
Heat comes in and goes out
During the day the ground gets heat from the Sun, and it loses heat all the time, day and night: every warm thing gives heat off, and the warmer it is, the more. When more heat goes out than comes in, the ground cools down.
The ground is like a big pot
Soil and above all water hold a lot of heat and give it up slowly, like a pot of soup that stays hot long after you turn off the stove. So the ground cools down and warms up only gradually. The sea holds even more heat, so by the sea the coldest time often comes only in February.
After the shortest day
After 21 December the ground gets a little more heat from the Sun every day. But it cools slowly, so for several more weeks it still loses more than it gets. That is why it keeps getting colder, and the coldest days come only around the middle of January.
Summer works the same way
The longest day is around 21 June, but the hottest days usually come in July and early August. For a while after the longest day, the ground still gets more heat than it loses.
Try it yourself
The same happens within a single day. In summer the Sun is highest around one o’clock in the afternoon, but the warmest time is usually around three or four, and the coldest just after dawn. Look at the hourly temperature graph in a weather app.
So why are the hardest frosts in January?
Whether it gets colder depends not on how much heat the Sun is giving right now, but on whether more heat goes out or comes in. The ground cools slowly, so after the shortest day it still loses more than it gets for weeks. That is why the hardest frosts come only in January.