The Weather Guys

The winter solstice (In Latin, sol, “sun,” and stice, “come to a stop”) is the day of the year with the fewest hours of daylight in the Northern Hemisphere.

In 2021, this occurs for the Northern Hemisphere on Dec. 21 at 9:59 a.m.

As Earth orbits the sun, its axis of rotation is tilted at an angle of 23.5 degrees from its orbital plane. Because Earth’s axis of spin always points in the same direction — toward the North Star — the orientation of Earth’s axis to the sun is always changing as Earth orbits around the sun.

As this orientation changes throughout the year, so does the distribution of sunlight on Earth’s surface at any given latitude. This links the amount of solar energy reaching a location to the time of year and causes some months of the year to always be warmer than others — in other words, the seasons.

On the Northern Hemisphere’s winter solstice, the northern spin axis is pointed away from the sun and latitudes north of the Arctic Circle (66.5 degrees North) have 24 hours of darkness. Meanwhile, the Southern Hemisphere is having short nights and long days.

The time of day that the sun reaches its highest point in its journey across the sky is called solar noon. In early December, true solar noon comes nearly 10 minutes earlier by the clock than it does at the solstice. With true noon coming later on the solstice, so will the sunrise and sunset times.

This discrepancy between clock time and solar time causes the Northern Hemisphere’s earliest sunset to precede the December solstice. The precise date of the earliest sunset depends on your latitude. The latest sunrise doesn’t come on the solstice either. From mid-northern latitudes, the latest sunrise comes in early January.

After the winter solstice, the days get longer, very slowly at first, but at ever larger daily intervals as the March equinox approaches, marking springtime.

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. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Ski resorts often use snow machines to make snow and lay down a good base for the coming season.

To make snow for ski trails requires temperatures near or below freezing. The humidity also plays a role in snow-making; a lower humidity is better. A low temperature and a low humidity is the best atmospheric condition for snow-makers, as it yields the driest snow.

Wet bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at a constant pressure. The wet bulb temperature is directly related to both relative humidity and outside air temperature. Web bulb temperatures of less than 20 degrees are ideal for snow-making with snow guns. Those temperatures can produce a dry snow.

Snow can also be made with wet bulb temperatures between 27 and 20 degrees, but the snow will be on the wet side. It is possible to make wet snow with temperatures slightly above freezing but that requires relative humidities less than about 40% (wet bulb temperatures are near 27 degrees).

Snow-making produces snow by mixing air and water. The nozzle of a snow gun produces tiny water droplets that freeze as they travel through the air. Compressed air or fans are used to loft the small droplets over the targeted section of the trail. The water droplets must freeze before hitting the ground, which would produce an ice glaze.

The machine-made snow is more like frozen water droplets than snow crystals. This machine-made snow has a greater density than natural snow but is good for laying a snow base for the season.

The wind also affects snow-making. The wind can help direct the flow of the snow and can help the hang time of a merging snowflake.

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 and authors of Ask the Weather Guys in the Wisconsin State Journal.