seasonal lag behind astronomic events

[ishmael]

Today is the first day of winter. The shortest day of the year in the northern hemisphere. But it is not, on average, the coldest day. The coldest days come in January and February. Why the lag? I would think that least amount of sun should result in least amount of heat. The same lag happens after the summer solstice - the hottest days follow by a month or more. Obviously something other than the amount of sunlight is causing this. Is it the moderating effect of the oceans? I know the water takes longer to heat and cool than the land does (thus the sea breeze at the shore), but does it lag by a month or more? Is there something else?

[iNow]

Hi Ishmael,

 

It's not so much a factor of "how long the sun is out" nor "how long the day is." The duration of the day is not as important as the angle at which we face the sun. So, in winter, the light we receive from the sun is at more of an "oblique" angle, so basically it's effect is less intense (the radiation from the sun is spread out over a larger area and has to travel farther, therefore is more dissipated on arrival).

 

This is all explained pretty well here. Maybe it will help. Ask more questions if you have them.

 

http://en.wikipedia.org/wiki/Winter

[ishmael]

The oblique angle doesn't change the question. The winter solstice has the most oblique sun angle, but its not the coldest day. The length of days and angle of the suns rays change in tandem.

 

The lag is still the question

[iNow]

Indeed, you're correct. Sorry about that. The answer then is the atmosphere. It's our atmosphere which causes the lag.

 

 

 

http://solar-system-astronomy.suite101.com/article.cfm/first_day_of_winter

Earthly Conditions as a Result of the Solstice

In the winter time, those in the Northern Hemisphere receive fewer hours of sunshine and less direct rays of sunshine, which combine to make it the coldest season. Many people wonder why the "first" day of winter isn't the middle point of winter and why it's not the coldest on the solstice. It is understandable to wonder why, if the December solstice marks the shortest day and the farthest south the sun will get, that it hasn't been colder leading up to the solstice and that it doesn't get warmer more quickly as the sun moves north again.

 

The answer lies in the Earth's atmosphere. It is a cumulative effect. In the summer, it takes the atmosphere a while to warm up from receiving the additional radiation, and therefore it is not hottest at the summer solstice. In the same way, the atmosphere has taken some time to cool down so that it is not the coldest at the winter solstice. The same thing can be noticed in the atmosphere on a daily basis. It is not warmest during the day when the sun is highest but in late afternoon. It is not coldest in the middle of the night but instead just before the dawn.

 

 

Or, using an analogy:

 

http://www.usatoday.com/weather/resources/askjack/wfaqsson.htm

The coldest part of the year lags about a month behind the shortest day, just as the warmest days come a month or so after the longest days. To keep the answer simple, we won't consider warm or cool air coming in from other places, but will assume the air at a particular location stays in place, warming and cooling.

 

We could think of the air in such a place as being like a bank account. If you add money to a bank account, it grows. If you add heat to air it warms up. The Earth is always losing heat, like a bank account that you're always taking some money from. If the amount of heat arriving from the sun is exactly equal to the amount leaving, the temperature stays the same.

 

As days grow longer in spring and early meteorological summer, the balance tips to more heat arriving than leaving. This is like adding money to the account faster than you are withdrawing it. The air grows warmer and warmer. On the longest day, the amount of heat arriving is greatest. But, even after the days begin growing shorter, the amount of heat arriving is more than the amount leaving. It's like continuing to add more money to the bank account than you're taking out, even though you are adding less than you were before.

 

Sometime in the late summer or during the fall, depending on how far north of the equator you are, the heat "account" is in balance. From then on, more heat is leaving than arriving and the days grow colder. Now, you're taking out more than you're adding to the account.

 

In December, when days are shortest, the "withdrawals" from the heat account are greatest. But even after the days start growing longer, more heat is leaving than arriving. The heat account is growing smaller, even though less heat is leaving. Eventually, however, you arrive at a day when the amounts of heat leaving and arriving are in balance. Then, the amount of heat being put into the account becomes greater than the amount being withdrawn. The air begins warming up. Here's a graphic and text that help show

why longest days are not the warmest

.

[SkepticLance]

If you are like me and live near the sea, the lag is even greater. That is because the sea conserves heat as it approaches winter, and takes longer to heat as summer approaches. Since the warmth of the sea affects the warmth of the land that is adjacent, the thermal lag of the sea slows the warming and cooling of the land.

 

For me, today is the longest day. The sea temperature is only 18 C. Within 2 to 3 months it will reach 21 C. When we hit our shortest day, the sea temperature will be only 17 C, and will chill off to about 14 C over the following 2 to 3 months before it begins to warm again.

 

This effect of slow warming and cooling of the sea, and its effect on the nearby land, makes my home more equable than inland regions. We get much reduced temperature extremes in the local atmosphere.

[npts2020]

I would take issue with the notion that the atmosphere or ocean are the *primary* cause of the lag. In order to have net warming you have to recieve a certain amount of sunlight each day. That means that after the winter solstice (I will use Canada as my reference for seasons) the Earth is still cooling, until it reaches a point where there is net warming each day. Read carefully iNow's second reference.