<p>Primarily because of <strong>inertia</strong>.  This phenomenon is called <a href="https://en.wikipedia.org/wiki/Seasonal_lag" rel="nofollow noreferrer">seasonal lag</a>.</p>
<p>It is true that the December solstice is the moment that the northern hemisphere gets the lowest total amount of insolation.  Conversely, the June solstice corresponds to the moment of maximum insolation.  The oceans, and to a lesser degree the land, absorb a lot of heat.  Due to the large heat capacity of water and the large amounts of water, the seasonal lag can be considerable.  In general, mid-latitude areas near large bodies of water have the largest seasonal lag, to the extent that the warmest month of the year in most of western Europe is August.</p>
<p>It is illustrated very well by this <em>Warmest day of the Year</em> map that NOAA NCDC (now NCEI) produced for the contiguous United States:</p>
<p><a href="https://i.sstatic.net/b4A5P.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/b4A5P.jpg" alt="NCDC/NCEI map of warmest day of the year" /></a></p>
<p>As expected, areas with a humid climate, such as southern Texas and nearby areas, tend to have the warmest day of the year in August.  The west coast is even more extreme, with some places having the warmest day in September.  Conversely, desert areas in the southwest (southern Arizona, New Mexico, western Texas) have the warmest day of the year in June.  (For the Big Bend area in Texas, it's even in the <em>first</em> half of June, which has different reasons; I'm no expert on the climate of the region, but according to <a href="https://earthscience.stackexchange.com/questions/7233/why-does-winter-get-colder-after-the-solstice/7234?noredirect=1#comment59315_7234">a comment by David Hammen</a>, it's <em>due to the North American Monsoon. July and August are much cloudier (and occasionally, much more rainy) than is June in the Big Bend area</em>).
There is a corresponding map for coldest day of the year:</p>
<p><a href="https://i.sstatic.net/46JC8.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/46JC8.jpg" alt="Coldest day of the year" /></a><br />
(Source: <a href="https://www.ncdc.noaa.gov/news/when-to-expect-coldest-day-of-year" rel="nofollow noreferrer">NCEI</a>)</p>
<p>The detailed structure is somewhat different, which has to due with local climate and circulation, but the overall trend related to humidity and proximity to ocean shows up in both.  We can also see the mountains, which I would guess to be a snow albedo feedback effect (ever notice how nights get colder when the ground is snow-covered?).  At NCEI you will also find <a href="https://www.ncdc.noaa.gov/news/when-to-expect-coldest-day-of-year" rel="nofollow noreferrer">similar maps for the rest of the USA</a> as well as <a href="https://www.ncei.noaa.gov/news/when-expect-warmest-day-year" rel="nofollow noreferrer">updated information focussing on the 1991-2020 timeframe</a>.</p>

CC BY-SA 4.0
2023-07-04T16:57:06.937Z
I see so many charts like these nowadays. No label for the axis. No unit. No explanation. After looking at it for 10 minutes, I finally understand: for Chart 1, if the location is marked "purple" or "violet", that means its warmest month is at the end of August or in September...
Stefanie Gauss
CC BY-SA 4.0
2023-07-04T16:58:40.027Z
Then I don't understand why Boston or the East Coast, when it can store a lot of energy in the ocean too, why is it not behaving like the West Coast?
Stefanie Gauss
CC BY-SA 4.0
2023-07-04T20:06:52.050Z
@StefanieGauss The label with the unit is right there below the map, incidating the date of the coldest day of the year.  I don't know what more there is to explain.  The east coast has less seasonal leg than the west coast because prevailing winds in this latitude are from the west, so the west coast typically has a wind from the ocean whereas Boston typically has a wind from the land.
gerrit
CC BY-SA 4.0
2023-07-05T08:49:12.273Z
An updated (1991-2020, as opposed to 1981-2010) version of the map for the warmest day of the year is at [When to Expect the Warmest Day of the Year](https://www.ncei.noaa.gov/news/when-expect-warmest-day-year).
David Hammen
CC BY-SA 4.0
2023-07-05T08:53:04.210Z
Regarding "For the Big Bend area in Texas, it's even in the first half of June, which must have entirely different reasons (probably related to dynamics) because a negative seasonal lag doesn't make much sense." That's due to the North American Monsoon. July and August are much cloudier (and occasionally, much more rainy) than is June in the Big Bend area. The warmest day of the year in Terlingua, Texas is June 11 -- well before the solstice.
David Hammen
CC BY-SA 4.0
2016-01-12T18:51:19.730Z
2023-07-06T06:17:03.617Z
2023-07-06T06:17:03.617Z
gerrit
<p>Intuitively, it makes perfect sense to think that the coldest day of the year would be the day that gets the least sunshine, the winter solstice.  In the northern hemisphere, this occurs a few days before Christmas.  But as anyone who lives in places that gets snow can tell you, the most bitter parts of winter are in January and February, not December.</p>

<p>Why does it get so much colder when sunlight (and the warmth it should be bringing with it) is increasing?</p>

CC BY-SA 3.0
2016-01-12T20:51:24.043Z
This question is related to: http://earthscience.stackexchange.com/questions/4505/why-is-march-colder-than-september-in-northern-hemisphere/4512#4512 & http://earthscience.stackexchange.com/questions/3193/how-can-the-average-coldest-day-of-the-year-be-before-the-winter-solstice & http://earthscience.stackexchange.com/questions/2133/july-august-hottest-months-northern-hemisphere-because-of-continued-warming-o/2135#2135
Fred
<climate><seasons>
Why does winter get colder after the solstice?
Mason Wheeler
# Why does winter get colder after the solstice?

(Asked by: username_0 on 2016-01-12)

Intuitively, it makes perfect sense to think that the coldest day of the year would be the day that gets the least sunshine, the winter solstice. In the northern hemisphere, this occurs a few days before Christmas. But as anyone who lives in places that gets snow can tell you, the most bitter parts of winter are in January and February, not December.

Why does it get so much colder when sunlight (and the warmth it should be bringing with it) is increasing?

**Tags:** climate, seasons

> **Comments:**
> **username_1**: This question is related to: http://earthscience.stackexchange.com/questions/4505/why-is-march-colder-than-september-in-northern-hemisphere/4512#4512 & http://earthscience.stackexchange.com/questions/3193/how-can-the-average-coldest-day-of-the-year-be-before-the-winter-solstice & http://earthscience.stackexchange.com/questions/2133/july-august-hottest-months-northern-hemisphere-because-of-continued-warming-o/2135#2135  

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## 1 answers

### Answer by username_2 on 2016-01-12. Score: 13 | Accepted answer

Primarily because of **inertia**. This phenomenon is called [seasonal lag](https://en.wikipedia.org/wiki/Seasonal_lag).

It is true that the December solstice is the moment that the northern hemisphere gets the lowest total amount of insolation. Conversely, the June solstice corresponds to the moment of maximum insolation. The oceans, and to a lesser degree the land, absorb a lot of heat. Due to the large heat capacity of water and the large amounts of water, the seasonal lag can be considerable. In general, mid-latitude areas near large bodies of water have the largest seasonal lag, to the extent that the warmest month of the year in most of western Europe is August.

It is illustrated very well by this *Warmest day of the Year* map that NOAA NCDC (now NCEI) produced for the contiguous United States:

[![NCDC/NCEI map of warmest day of the year](https://i.sstatic.net/b4A5P.jpg)](https://i.sstatic.net/b4A5P.jpg)

As expected, areas with a humid climate, such as southern Texas and nearby areas, tend to have the warmest day of the year in August. The west coast is even more extreme, with some places having the warmest day in September. Conversely, desert areas in the southwest (southern Arizona, New Mexico, western Texas) have the warmest day of the year in June. (For the Big Bend area in Texas, it's even in the *first* half of June, which has different reasons; I'm no expert on the climate of the region, but according to [a comment by username_4 username_4](https://earthscience.stackexchange.com/questions/7233/why-does-winter-get-colder-after-the-solstice/7234?noredirect=1#comment59315_7234), it's *due to the North American Monsoon. July and August are much cloudier (and occasionally, much more rainy) than is June in the Big Bend area*).
There is a corresponding map for coldest day of the year:

[![Coldest day of the year](https://i.sstatic.net/46JC8.jpg)](https://i.sstatic.net/46JC8.jpg)  
(Source: [NCEI](https://www.ncdc.noaa.gov/news/when-to-expect-coldest-day-of-year))

The detailed structure is somewhat different, which has to due with local climate and circulation, but the overall trend related to humidity and proximity to ocean shows up in both. We can also see the mountains, which I would guess to be a snow albedo feedback effect (ever notice how nights get colder when the ground is snow-covered?). At NCEI you will also find [similar maps for the rest of the USA](https://www.ncdc.noaa.gov/news/when-to-expect-coldest-day-of-year) as well as [updated information focussing on the 1991-2020 timeframe](https://www.ncei.noaa.gov/news/when-expect-warmest-day-year).

> **Comments:**
> **username_3**: I see so many charts like these nowadays. No label for the axis. No unit. No explanation. After looking at it for 10 minutes, I finally understand: for Chart 1, if the location is marked "purple" or "violet", that means its warmest month is at the end of August or in September...  
> **username_3**: Then I don't understand why Boston or the East Coast, when it can store a lot of energy in the ocean too, why is it not behaving like the West Coast?  
> **username_2**: @username_3 The label with the unit is right there below the map, incidating the date of the coldest day of the year.  I don't know what more there is to explain.  The east coast has less seasonal leg than the west coast because prevailing winds in this latitude are from the west, so the west coast typically has a wind from the ocean whereas Boston typically has a wind from the land.  
> **username_4**: An updated (1991-2020, as opposed to 1981-2010) version of the map for the warmest day of the year is at [When to Expect the Warmest Day of the Year](https://www.ncei.noaa.gov/news/when-expect-warmest-day-year).  
> **username_4**: Regarding "For the Big Bend area in Texas, it's even in the first half of June, which must have entirely different reasons (probably related to dynamics) because a negative seasonal lag doesn't make much sense." That's due to the North American Monsoon. July and August are much cloudier (and occasionally, much more rainy) than is June in the Big Bend area. The warmest day of the year in Terlingua, Texas is June 11 -- well before the solstice.  

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