Why Isn't the Darkest Month of the Year Also the Coldest?It's something in the water.

Sunday marked the winter solstice—both the official start of winter and the shortest day of the year. But December is rarely as cold as January. Why isn't the month with the least sunlight also the one with the lowest temperatures?

PRINTDISCUSSE-MAILRSSRECOMMEND...SINGLE PAGE

FacebookMySpace Mixx Digg Reddit del.icio.us Furl Ma.gnolia SphereStumbleUponCLOSE

Because water retains heat. Between 70 percent and 75 percent of the Earth's surface is covered in oceans, rivers, and lakes. (There's even more water vaporized in the air or stored in the ground.) During seasons of longer days and more sunlight, these geographical features are able to store up and retain heat over long periods of time, before emitting it as the days get shorter. A body of water is far more effective as a space heater than, say, a big field of rocks: The water holds on to five times as much heat per gram.

Large bodies of water thus serve as giant heat reservoirs, but they're not the only contributing factor to the seasonal lag in average temperatures. Properties of the Earth's atmosphere also contribute to the delayed onset of winter chills. Daily thermometer readings are determined, in part, by the balance between solar heat that's reflected by the planet surface (and bounced back into space) and solar heat that is absorbed into the climate system. This balance shifts as winter proceeds. At the beginning of the season, the temperature drops, which leads to an increased amount of snow covering the globe. This makes the Earth more reflective, with sunlight bouncing off the bright, white surfaces; more reflection exacerbates the cold weather and maintains the low temperatures. Gradually, as the days get longer, the snow melts, and the Earth's atmosphere begins to absorb more heat.

Got a question about today's news? Ask the Explainer.

Explainer thanks Paul Knight of Pennsylvania State University and Tom Moore of the Weather Channel.

PRINTE-MAILRECOMMEND...RSS

FacebookMySpace Mixx Digg Reddit del.icio.us Furl Ma.gnolia SphereStumbleUponCLOSE

SPONSORED CONTENT

• Stopping By Woods
  Tiger Woods' car crash caused $200 worth of damage to a tree. How do you measure that?
  Christopher Beam | Dec. 4, 2009
• Are All NATO Soldiers Created Equal?
  Not if you expect them to fire.
  Brian Palmer | Dec. 3, 2009
• How Important Is the East Anglia Climate Data Set?
  Let's say it's irredeemably corrupt. What would that mean for our understanding of climate change?
  Juliet Lapidos | Dec. 2, 2009
• Search for more Explainer articles
• Subscribe to the Explainer RSS feed
• View our complete Explainer archive