The Weather Guys

Overall, the chances for snow are relatively slim in October — only one in 10 Octobers has a day with more than 1 inch of snowfall.

Many of us recall that last Oct. 20 we had a period of moderate snow that dusted the ground for a short time in the afternoon.

Because the ground is still usually very warm at this time of year, much of the snow that falls in October does not stick around, and even on the snowiest October day in the past 80 years (Oct. 26, 1997, when 3.8 inches of snow fell at the Madison airport) very little remained on the ground during the next couple of days.

Starting tonight, it appears that we may have snow in the air for a good part of the week, at least on an intermittent basis.

It is unlikely that we will see accumulating snow, but it is not impossible that trick-or-treaters will be dodging snowflakes Thursday night.

Current medium-range forecast models suggest a more substantial threat of accumulating snow coming at the end of the weekend.

Even if that threat turns out to be exaggerated, it does look likely that the end of this month and the beginning of the next will be colder than normal. Thus, it is safe to say that early winter has arrived.

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.

Thus far, October 2019 has been an active weather month over North America.

The month began with a week-long period of moderate to heavy rain over the Great Lakes states associated with an energetic jet stream racing across the northern tier of states.

Near the end of the second week of the month, a major cyclone developed over the High Plains triggering a series of high-impact weather events across a large part of the nation.

The first substantial snows of the season struck Denver and other cities in the Intermountain West, and as the storm moved eastward, it dropped as much as 30 inches of snow in central North Dakota.

Farther to the west, surface high pressure built inland leading to strong Santa Ana winds over central and southern California triggering a number of destructive wildfires.

In Denver, the storm’s passage dropped the temperature from 83 on Oct. 9 to 13 by the end of the next day. That was the fourth largest one-day temperature drop since record keeping began in Denver in 1872.

Late last week a major cyclone struck the northeastern U.S. with heavy rains and hurricane-like winds. Boston had peak wind gusts of 55 mph with some locations in coastal Maine howling up to 80 mph. The storm brought down trees and power lines in a vast number of communities in eastern New England and snarled air travel for several days.

Such high-impact events are characteristic of October — a unique month for the large-scale circulation of the Northern Hemisphere.

During October the decreasing daylight cools the high latitudes and energizes the polar jet stream.

Meanwhile, the last tropical cyclones of the season ensure that the year’s most energetic interaction between the tropics and the mid-latitudes occurs then.

Such interactions can have an enormous impact on the development of weather systems in the middle latitudes at this time, leading to a parade of high-impact events like the one that just visited North America.

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.

Atmospheric conditions can make the actual sunrise and sunset vary slightly from the calculated times. As the Earth moves around the Sun, the length of the day changes.

The length of day at a particular location on Earth is a periodic function of time. This is all caused by the 23.5-degree tilt of the Earth’s axis as it travels around the sun. In the Northern Hemisphere, days are longest at the time of the summer solstice in June, and the shortest days are at the winter solstice in December. At the two equinoxes in March and September, the length of the day is about 12 hours, a mean value for the year.

The length of a day changes far more during the year at higher latitudes than at lower latitudes. At the poles the daytime length varies from 0 to 24 hours, while at the tropics the daytime length varies little.

There is essentially no change in length of day from one day to the next at the time of the solstices. There is more change at the equinoxes. At those times the day-to-day changes can be a few minutes. The day length is changing fastest at the equinoxes.

Consider the percentage change — the change in day length from the previous day divided by the length of day on that day. That can give the impression of how quickly we lose daylight.

The shortening of the length of day in terms of percentage change is at its peak in late October and early November around our latitude.