Why does fog form over the lake?

Posted on November 18, 2019 by WeatherGuys Editor
Steam fog on a Wisconsin lake in October.
Photo credit: Tom Achtor

The recent cold weather was accompanied by interesting fog over the open water of our lakes.

Fog is essentially a ground-hugging cloud, composed of tiny liquid water droplets.

This particular fog, called a steam fog, forms when cold air drifts across relatively warm water. The lake water evaporates into the air above the lake surface. The lake must be unfrozen.

The air is cooled and moistened, causing the dew point to increase. As the dew point approaches the air temperature, condensation occurs, forming fog droplets.

The condensation further warms the air. The warmed air rises and mixes with the cold air above it, reaching saturation and causing more fog to form.

When there is a large difference in temperature between the air above the lake and the water at the surface of the lake, there will also be considerable turbulence in the air over the lake.

The combination of steam fog production, turbulence over the lake, and strong winds can create one of nature’s most awesome spectacles — steam devils.

The name was first used in 1971 after observations over Lake Michigan.

Steam devils are swirling columns of steam fog (analogous to dust devils) that look like a whirlwind of steam fog on a cold day. Steam devils can rise up to 1,500 feet above the lake.

One of the many features of living near large lakes is that they are often shrouded in a fog in the fall and early winter.

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.

Category: Meteorology, Phenomena

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What is a polar vortex

Posted on November 11, 2019 by WeatherGuys Editor

From ClimateReanalyzer.org, provided by the Climate Change Institute at the University of Maine.

The term “polar vortex” was popularized by media outlets in 2014, when the country experienced a brutal January cold snap.

The polar vortex is a band of strong winds, high up in the atmosphere that spins counter-clockwise around the North Pole.

During the summer months, the vortex is weak, and its southern edge sits at high latitude. As the sun sets on the North Pole in late September, the vortex gains strength and gradually edges southward.

At the southern edge of the vortex is the so-called polar jet stream, which separates warm air to its south from the ever expanding and increasingly colder air to its north.

The nature of the polar vortex changes throughout the winter — sometimes it is strongly west-to-east, and other times it is characterized by high amplitude waves, which can rapidly transport warm air toward the North Pole in some locations and frigid air southward in others.

For instance, last week during our cold spell over the central United States, it was balmy in Anchorage, Alaska, which enjoyed highs in the low 40s.

Our recent cold spell has not prompted media to use the term and, as it turns out, that is actually appropriate.

A characteristic aspect of the true wintertime polar vortex is the existence of strong polar night jet — a band of strong west-to-east winds in the lower stratosphere (between 10-15 miles high).

The polar night jet is just beginning to establish itself at this time of year. If pieces of that vortex break off later in the winter and head southward, it is appropriate to refer to the ensuing cold air outbreaks as being related to changes in the polar vortex. But that is not the case in early November.

Category: Climate, Seasons

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Did Madison break any snowfall records last month?

Posted on November 4, 2019 by WeatherGuys Editor

As we begin Winter Weather Awareness week in Wisconsin (Nov. 4-8), too many of us probably feel like grizzled veterans already.

What an end to the month of October!

It was a chilly month overall, with the average temperature ending up 1.5 degrees F below normal. This chill was especially notable in the last 10 days of the month, which averaged 6.7 degrees F below normal and were attended by a record setting 8.1 inches of snow in nearly back-to-back, but separate, events on Oct. 28-29 and Oct. 30-31.

The 8.1 inches of snow set the all-time monthly record for Madison, shattering the old record of 5.2 inches set more than a century ago in 1917.

The single storm total of 5.1 inches from the second of the two snowfalls also broke the previous record of 3.8 inches for a single storm set on Oct. 26-27, 1997.

Shoveling snow on Halloween in Madison revealed Autumn leaves below.
Image credit: M. Mooney

Finally, the 4 inches of snowfall on Halloween became the earliest 4 inches or greater snowfall in Madison’s history, displacing the prior record set on Nov. 3, 1951.

The fact that nearly 70 years have elapsed since such an early 4 inches of snowfall suggests that we are at the very limit of how early such an accumulation can occur in Madison.

Given that our average first inch of snow falls here on Nov. 27, we are nearly a full month ahead of schedule.

Finally, with 1 inch of snow on Oct. 28, this year contained Madison’s second-shortest interval between 1 inch or greater snowfalls on the bookends of summer (April 27 to Oct. 28).

It is tempting to wonder if this week’s early snow portends anything about the coming winter. Our strong suspicion is that the answer is no — but don’t let that dim your enthusiasm for what was a truly astounding, record-setting week.

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.
Category: Meteorology, Seasons

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What are the chances for a snowy Halloween?

Posted on October 28, 2019 by WeatherGuys Editor

The last half of October has been a bit cooler than normal, and there is a real chance we will see our first accumulating snow of the season this week.

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.
Category: Meteorology, Seasons

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Why has October been so stormy?

Posted on October 22, 2019 by WeatherGuys Editor

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.
Category: Meteorology, Seasons

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