What causes April showers?

Posted on April 24, 2023 by WeatherGuys Editor
The vertical distribution of temperature will often determine the type of precipitation (rain vs. snow vs. sleet vs. freezing rain) that occurs at the surface during the development of “wintry mix.” More often than not, the temperature does not decrease with height but increases, many times by several degrees, before decreasing. This increase, then decrease is called an inversion. During wintry mix precipitation, an inversion can be critical in determining the type or types of weather. (Image credit: weather.gov)

The weather this past week in Madison has been very interesting.

On at least three occasions we experienced a mix of precipitation types — with snow, hail, graupel and rain in various combinations.

Early in the week the precipitation was delivered by the passage of a mid-latitude cyclone. These disturbances are organized on very large scales and have a characteristic distribution of clouds and precipitation associated with them. The early week precipitation was most closely tied to the passage of the cold frontal portion of the larger cyclone.

By the nature of their large-scale structure, such storms usually have a region of quite cold air at about 3 miles above the surface trailing to their west and northwest. When this air moves over a given location, the temperature difference between the surface and 3 miles above the surface naturally increases. This increased temperature difference reduces the resistance air parcels have to move in the vertical direction. Since this usually substantial resistance is lessened in such cases, upward vertical motions (which produce cloud and precipitation) are more easily generated.

The widespread, but intermittent, snow and graupel over southern Wisconsin on Saturday was largely a consequence of this set of physical circumstances. The fact that the showers abruptly ceased after sunset is another characteristic of this kind of precipitation generation mechanism in action.

Since April, as we all know, is still capable of delivering pretty cold temperatures, it turns out that a large fraction of this month’s fabled April showers are produced in exactly this manner.

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, Seasons

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How could we be in a fire danger this early in spring after all the winter snow?

Posted on April 17, 2023 by WeatherGuys Editor
A Red Flag Warning was issued by the National Weather Service in conjunction with the Wisconsin Department of Natural Resources on Friday, April 14, 2023. A Red Flag Warning is issued when a variety of weather factors come together to create especially dangerous wildland fire conditions.

This 2022-23 winter — December through February — was exceptional in that it was Wisconsin’s wettest meteorological winter on record, and those records go back to 1895.

The state also experienced significant snowfall in March. When the snow slowly melts into the soil, it provides needed water for plant growth.

Fires require fuel to burn, air to supply oxygen, and a heat source to get the fuel to its ignition temperature. Regardless of how much snow fell during winter, if we have a few days of hot, dry and windy weather in early spring, vegetation will dry out, providing fuel to burn if ignited.

A wet winter also can become a wildfire problem later in the summer. The snowfall provides moisture for plant production. A summer drought can kill plants, leaving behind a fuel source for a fire.

This past week, Wisconsin and other northern states were under a red flag warning. The National Weather Service issues a red flag warning when warm temperatures, very low humidity and strong winds are expected. These weather conditions combine to produce an increased risk of a fire and the warning alerts fire officials and firefighters of potentially dangerous conditions within the next 12 to 24 hours. High winds also help spread the fires. Red flag conditions also alert the public to be cautious and not ignite a fire.

NWS issuance of a red flag warning is based on wind speed, humidity and how dry the ground is, but exact thresholds vary by region. In Wisconsin, these conditions generally occur in the springtime before plants green up and in the fall before there is snow on the ground.

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: Climate, Seasons, Weather Dangers

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How is air pollution measured?

Posted on April 10, 2023 by WeatherGuys Editor

There are different sources of air pollution, including human-generated emissions from the burning of fossil fuel, as well as natural sources such as wildfires, volcanic eruptions and dust storms.

Particle pollution is referred to particulate matter (PM) and is made up of small, suspended solid or liquid particles. The amount and density of pollutants in the air are converted into an Air Quality Index (AQI). An AQI of less than 50 is considered safe, while unhealthy conditions have an AQI above 100. The chemical composition of the pollutant can also be a health concern.

Some air quality monitors use lasers to determine the density of particulate matter. Other methods include passing air through filters and then measuring the material collected on the filter.

The Environment Protection Agency has a nationwide network of monitoring sites to measure conditions of particulate matter. PM2.5 describes fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller. Analysis of observations demonstrate that the average PM2.5 concentration has been decreasing nationally, an indication that air quality is improving.

Last week NASA launched a new satellite instrument to monitor air pollution. The Tropospheric Emissions: Monitoring of Pollution, or TEMPO, was launched into geostationary orbit on Thursday. TEMPO is the first space-based instrument to monitor major air pollutants hourly and at high spatial resolution of 4 square miles.

Observations of such temporal and spatial detail enable research studies of rush hour pollution, the movement of pollution from forest fires, and even the impact of fertilizer application. The TEMPO measurements will join a global satellite constellation of observations that will track pollution around the globe.

This animation demonstrates the east to west scanning strategy of TEMPO’s hourly profile of air pollutants over North America using proxy NO2 data. (Credit: Robert Carp, SSEC)

Scientists at UW-Madison’s Space Science and Engineering Center are members of NASA’s TEMPO science team and will monitor the presence of air pollutants over North America and help improve air quality forecasts.

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, Weather Dangers

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Is March weather susceptible to being unusual?

Posted on April 3, 2023 by WeatherGuys Editor

The adage about March weather coming “in like a lion, out like a lamb” was far off the mark for southern Wisconsin this year.

In Madison, it was not a particularly cold month as the average temperature was only 0.8 degrees below normal. However, we had the second-snowiest March ever — and snowiest since the record was set in 1959 — with 25.3 inches of accumulated snow. In addition, it snowed on 16 of the 31 days in the past month, so winter was unusually reluctant to loosen its grip.

Finally, on the last day of the month, though the temperature soared to 64 degrees — the first day at or above 60 since Nov. 10 — the state was visited by at least nine confirmed tornadoes.

Tornadoes in Green and Dane counties were EF1 tornadoes, with winds up to 100 mph. The others were EF0 tornadoes, with winds up to 80 mph. Some damage but no injuries were reported.

As we bid it adieu, we note that the month of March is distinguished by a couple of other meteorological oddities in our region. First, the greatest difference between the all-time warmest and all-time coldest temperatures for a calendar day in Madison is 88 and belongs to March 20 when in 2012 the high temperature was 81 degrees and in 1965 the low temperature was minus 7.

Along the same lines, if one compares the all-time warmest daily maximum temperature to the all-time coldest daily maximum temperature, the record difference is 72 on March 17 when the warmest high temperature was 80 in 2012 and the coldest high temperature was 8 in 1941.

These examples provide remarkable testimony to how variable the weather can be in southern Wisconsin around the time of the spring equinox. Here’s hoping for spring to finally make its presence known in April.

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: Climate, Meteorology, Seasons

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How did Madison get a foot of snow on Saturday?

Posted on March 28, 2023 by WeatherGuys Editor

The 12.1 inches of snow that fell over Madison in the first nine hours of Saturday morning was surely a surprise to many residents while setting the all-time monthly record.

Forecasts made through the late afternoon on Friday, consistent with those made on Wednesday and Thursday, were suggesting that the heaviest band of snow would run southwestward from Racine/Kenosha back to northwestern Illinois and leave Madison with an inch or two.

GOES East Visible satellite imagery from March 25th paired with Day Cloud Phase distinction RGB imagery showing where the heaviest snow fell and a sharp cutoff where no snow occurred.

The forecast was in error not because the heavy snow band was not foreseen, but rather because the band ended up occurring farther northwestward than the computer forecast models had predicted.

This error underscores how hard it is to pinpoint the location of heavy precipitation bands in winter storms, especially rather weak winter storms like the one that hit on Saturday morning.

Even such weak storms have circulations that impact millions of square kilometers, while the precipitation bands impact only a small fraction of that area. In this case, the important structures and dynamics that conspired to produce the snow were well represented in the computer forecasts, but their location was different than the forecasted location for reasons that can be determined only in an after-the-fact investigation of the event.

Though much of the stunning progress in forecasting that has occurred in the past half century has been a result of relentless advances in theory, observational capabilities and the expanding power of computers, an often overlooked ingredient is the grueling detective work that ensues in the aftermath of such forecast errors. Meteorologists undertake “case studies” of such incidents to determine what physical factors were responsible for the weather as it actually occurred and also to better understand how and why the computer models went astray.

This difficult but necessary work informs the future development of these forecast models with the hope of minimizing future forecast errors. Thus, though such incidents as our Saturday snow might inspire the cynical view that weather forecasting is not a scientific endeavor, the exact opposite is actually true.

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

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