The Weather Guys

How does weather affect snow-making?

Posted on November 29, 2021 by WeatherGuys Editor

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

Machine-made snow prepares the base at Mt. La Crosse ski hill.
Photo from the La Crosse Tribune Archives

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.

Category: Meteorology, Seasons

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Is our recent cold snap a harbinger of things to come?

Posted on November 23, 2021 by WeatherGuys Editor

Our recent cold snap has brought the November average to just 0.5 degrees above normal through the first 17 days of November after a relatively warm October in Madison, where the average temperature was 5.9 degrees above normal.

NOAA’s outlook for the end of November suggests seasonable temperatures for Wisconsin.

It is unlikely we will see a dramatic enough warmup over the last two weeks of the month to get us even close to as warm as we were in October. In fact, the most recent 10-day forecasts suggest continued seasonable to below-seasonable temperatures in southern Wisconsin through the end of November.

Last year the sequence was nearly exactly reversed as we endured a relatively cold October (temperature 3.8 degrees below normal), while November started really warm, with high temperatures at or above 68 degrees every day from Nov. 3-8.

This year we had a trace (or more) of snow on four consecutive days Nov. 4-7. Such dramatic turns from year-to-year point out how much natural variability there is in the weather at nearly any time of year — though it seems especially capricious in the late fall — perhaps because large departures at this time of year are so obviously experienced as extensions of summer (warm) or early indications of winter (cold).

The fact is that these notable departures are not of much predictive value when it comes to forecasting the coming cold season. Even the brewing La Nina conditions in the central tropical Pacific do not offer much guidance for what we might expect this winter as both warm and cold extremes in our region have been known to accompany such conditions.

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. They also write weekly articles for the Wisconsin State Journal.

Category: Climate, Meteorology, Seasons

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What are punch holes in clouds?

Posted on November 15, 2021 by WeatherGuys Editor

On Nov. 7, numerous “holes” appeared to be punched out of a cloud deck across the Upper Midwest.

Punch holes can occur after a plane flies through the cloud if the cloud droplets are supercooled, with their temperatures below freezing.

A plane flying through a cloud can cause some droplets to freeze. The presence of both liquid water and ice crystal in a cloud yields a unique precipitation-making ability. The water evaporates from the supercooled droplets and flows toward and deposits on the ice crystals. This process is called the Bergeron-Wegener process. It was first proposed by Alfred Wegener in 1911 and explained more extensively by Tor Bergeron.

You can see the ice crystals falling out of the cloud in the accompanying photo, resulting in a cloud-free circle.

There is a difference between freezing a small water droplet and freezing a larger body of water. The freezing temperature of water is 32 degrees at standard pressure. That is the case when water is in dish, ice tray or lake. A 1-millimeter diameter droplet will generally not freeze until the temperature falls below 12.2 degrees. A tiny droplet, but not a large body of water, can be supercooled.

Satellite view of aircraft hole punch clouds over northern Indiana and northeast Illinois.
(Image credit: CIMSS Satellite Blog)

For ice to form, all the water molecules must align in the proper crystal structure. First, a few molecules align, and then the rest quickly follow, turning the liquid into a block of ice. The larger the volume of water, the greater the chances that a few of the molecules will align in the proper manner to form ice when the temperature falls below freezing. In a small volume of water, the chance that some of the molecules will align in the correct structure is reduced, simply because there are fewer molecules.

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.

Category: Meteorology, Phenomena

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What is La Niña and the impact on Wisconsin’s coming winter weather?

Posted on November 8, 2021 by WeatherGuys Editor

La Niña revealed by satellite sensors: Cooler than average sea surface temperatures are depicted by shades of blue along the equator, indicative of La Niña conditions. (Image credit: NOAA Climate.gov)

La Niña refers to a departure from normal in the sea-surface temperature across much of the eastern tropical Pacific Ocean.

The water temperatures off the west coast of South America are typically 60 to 70 degrees. During a La Niña these waters get as much as 7 degrees colder than normal. La Niña conditions recur every few years and last nine to 12 months, though some events have lingered for as many as two years. This cooling results from a strengthening of the winds over the tropical Pacific and its interaction with the underlying ocean waters.

A La Niña event developed in the tropical Pacific in August-September 2020 and ended in May 2021. La Niña conditions have emerged for 2021, the second winter in a row.

Wisconsin winters tend to have more precipitation and near-average temperatures during a typical La Niña.

The National Oceanic and Atmospheric Administration’s (NOAA’s) seasonal outlooks provide the likelihood that temperatures and total precipitation amounts will be above, near or below average.

La Niña are important events to this seasonal outlook, which does not project seasonal snowfall accumulations as snow forecasts are generally not predictable more than a week in advance. NOAA’s 2021 winter outlook (December 2021 through February 2022) predicts wetter-than-average conditions across portions of the northern U.S., primarily in the Pacific Northwest, northern Rockies, Great Lakes, Ohio Valley and western Alaska. Drier-than-average conditions are favored in south-central Alaska, Southern California, the Southwest and the Southeast.

Above-average temperatures are favored across the South and most of the eastern U.S. Below-average temperatures are favored for southeastern Alaska and the Pacific Northwest eastward to the northern Plains. The Upper Mississippi Valley region has equal chances for temperatures at below, near or above average.

Category: Climate, Meteorology, Seasons

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How does frost form?

Posted on November 1, 2021 by WeatherGuys Editor

Trees emit radiation toward the ground insulating small areas which is why frost did not form under the tree in this field. (Photo credit: Tim Wagner)

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.