The Weather Guys

Frost on Berries, Frautschi Point, Madison Wisconsin. Credit: Fred Best

Frost on objects is just water vapor in the air that has deposited itself as ice onto a surface. Frost forms on objects close to the ground, such as blades of grass.

At night, a blade of grass loses energy by emitting radiation (a non-lethal kind) while it gains energy by absorbing the energy emitted from surrounding objects. Under clear nighttime skies, objects near the ground emit more radiation than they receive from the sky, and so a blade of grass cools as its energy losses are greater than its energy gains. If the temperature of a grass blade gets cold enough and there is sufficient water vapor in the environment, frost will form on the grass.

Overnight cooling of the air near the ground causes morning frost on grass and car windshields. Frost will form on a surface only where the temperature is at or below freezing. The observed air temperature may be higher than 32 degrees, since those air temperature observations are taken at about 4 feet above the ground, where it can be warmer than the ground.

You may notice that frost forms in an open field but not under a tree. Trees emit more radiation toward the ground than does the clear sky. Energy losses at the ground under the tree are therefore less than those of the grass in the open field. The grass in the open field cools faster and reaches the frost point before the grass blades under the tree.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

GOES-16 True Color image from September 21, 2017. Credit: Kaba Bah, CIMSS/SSEC
(click to enlarge)

Even the least observant person has probably recognized by now that the days are growing shorter.

In fact, on Friday at 3:02 p.m., we will reach the autumnal equinox and the night will be as long as the day for the first time since late March.

At the North Pole, the situation is more dire, as Saturday morning will be the first of 182 straight days during which the Sun will not rise there! As each day goes by, the portion of the polar regions that gets no sunlight continues to expand until late December. Thus, larger and larger areas of the high latitudes have continual night over the next 3 months.

This circumstance drives the production of increasingly extensive cold air masses near the surface which gradually begin to spread southward as the season progresses.

In North America, the cold air production during this period can be enhanced if early season snow falls over wide areas of northwestern Canada because air over a snow-covered surface can get colder overnight than air over a bare surface.

So, how cold the fall and early winter will be in Madison can be strongly connected not only to when the snow begins to fall to our far northwest but also how much actually falls.

Keep an eye on northwest Canada!

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.