How unusual was the past weekend’s fantastic weather?

Posted on October 31, 2022 by WeatherGuys Editor

It is a fair bet that we would get near universal agreement that this October has been pretty spectacular here in southern Wisconsin.

Through the 29th, the average temperature has been exactly normal, and we have had only 11 days on which precipitation fell — twice it was snow — for a total that is 1.75 inches below normal.

Oct. 21 began a streak of four consecutive days with a high temperature of 72 degrees or higher, with the warmth soaring to 76 and 77 on the 21st and 22nd. Amazingly, this is just the third such streak that has ended as late in October in Madison’s 150-plus years of weather records.

Previous streaks ending this late in the month ran five days and occurred Oct. 23-27, 1989, and Oct. 22-26, 1963, during which we recorded our latest 80-degree day ever in town (the 23rd).

One other such streak that is worthy of note, though it does not qualify for this distinction, is the 21-consecutive days above 72 degrees that ended on Oct. 17, 1878 — meaning there was an October that had 17 straight days above 72.

The coming week will also have some really nice days, so our collective sense of the fine weather this fall will surely continue for most of this week.

Inevitably, however, winter will either creep in or rush in to replace these golden days of late fall. As that happens, try to find solace in the fact that we have just experienced a record-breaking streak of benign weather.

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: History, Seasons

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What makes the wind blow?

Posted on October 24, 2022 by WeatherGuys Editor

Southern Wisconsin has certainly experienced some windy days this past week.

Wind is defined as the horizontal movement of air from one place to another. Wind exists because of differences in air pressure.

Any movement requires a force, and in the case of wind, it is the horizontal pressure gradient that supplies the force. The gradient of an atmospheric variable measures the difference of that variable over a given distance. The pressure gradient is the difference in pressure over a certain distance.

Pushed by the pressure gradient forces, gases in our atmosphere move from areas of high pressure to low pressure. The larger the difference between the pressures over a given distance, the greater the pressure gradient force and the faster the air will move from the high to the low pressure.

The pressure, temperature and density of a gas are related to one another through a mathematical formula known as the ideal gas law. A change in one of these variables will cause a change in one or both of the others. Unequal heating of the atmosphere leads to pressure gradients which drive the winds.

Large plumes of blowing dust across the central Plains on Sunday October 23rd as seen by the NOAA-20 satellite, lofted by strong southwest winds.

A wind gust is a sudden, short burst of strong wind that moves faster than the current wind. As the wind blows, a number of things can interrupt air’s motion, such as trees, cars and buildings. Whenever air is obstructed in this way, friction comes into play and slows the wind. Once wind passes the object, it flows freely again, and its speed can increase abruptly. The speed of the wind, and the size and shape of the object that the wind is blowing against define how strong the gust will be.

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 graupel?

Posted on October 17, 2022 by WeatherGuys Editor
Forms of frozen precipitation. L-R: hail, graupel, sleet, snow. (Photo credit: NOAA/NSSL)

Graupel is a type of frozen precipitation. Southern Wisconsin experienced this on Thursday afternoon.

Most clouds outside the tropical regions have temperatures that are below freezing somewhere in the cloud. These cold clouds are likely to have frozen cloud particles. They are also likely to include supercooled water drops, drops made of water that are below the freezing point.

Collisions between particles inside a cloud help create precipitation in cold clouds. When an ice crystal falls through a cloud it may collide with and collect supercooled water drops. This process of ice crystal growth by sweeping up supercooled water drops is called accretion, which can be thought of as a riming of the crystals. When ice crystals collide with supercooled drops, the drops freeze almost instantly. Accretion thus provides a mechanism for the particle to grow quickly, and when large enough fall out the bottom of the cloud.

An ice particle produced by the accretion process that has a size between 1 and 5 millimeters (0.04 to 0.2 inches) and no discernible crystal habit is called graupel (plural, graupeln). On collision and freezing, the supercooled water often traps air bubbles. Because of this trapped air, the density of a graupel is low, and it can easily be crushed, unlike a hailstone.

Hail is precipitation in the form of large balls or lumps of ice. Hail develops in the complex air motions inside a towering cumulonimbus cloud.

Aggregation is the process by which ice crystals collide and form a single larger ice particle. The probability that two crystals will stick together depends on the shape of the crystals and the temperature. A snowflake is an individual ice crystal or more likely an aggregate of ice crystals.

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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Will we hit 80 degrees again this year?

Posted on October 10, 2022 by WeatherGuys Editor

We will not see 80 degrees again this year.

The last time Madison was officially 80 degrees or warmer was Sept. 21, the last official day of summer. In fact, 13 of the first 21 days of last month we were at least that warm — fairly remarkable.

The weather has turned abruptly since then, culminating with our first really cold air of the year from Thursday night into Saturday. A number of locations in the area had their first night below freezing during this stretch, and temperatures dropped to 12 degrees in a couple of towns in southwestern North Dakota.

The dramatic about-face got us thinking about that 80-degree mark and whether it is likely to appear again this year.

The earliest day on which Madison has ever recorded its last 80-degree day of the year was Sept. 2, 1977 (and 2020). The all-time latest 80-degree day in Madison’s history was on Oct. 23, 1963. The average last such day (since 1939) is Sept. 29.

Within the 83 seasons (not including this one) since 1939 there have been 17 times when the last 80-degree day occurred after today’s date, Oct. 10 — just over 20% of the time. Though that might inspire hope that it is not terribly unusual to get that warm after today’s date, this year it seems unlikely that we will see that kind of warmth again as at least the next 10 days seem certain to be cooler than that.

So, enjoy the brilliant sunshine, light winds and dry conditions that have set in over us these last couple of weeks — but consider the summer officially over.

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 are clouds named and why are the bottoms flat?

Posted on October 3, 2022 by WeatherGuys Editor
Basic Cloud Chart (National Weather Service)

In 1803, British pharmacist and chemist Luke Howard devised a classification system for clouds. It has proved so successful that meteorologists have used Howard’s system ever since, with minor modifications.

According to his system, clouds are given Latin names corresponding to their appearance — layered or convective — and their altitude. Clouds are also categorized based on whether or not they are precipitating.

Layered clouds are much wider than they are tall. They generally have flat bases and tops and can extend from horizon to horizon. The Latin word “stratus” describes the layered cloud category.

Convective clouds are as tall, or taller, than they are wide. These clouds look lumpy and piled up, like a cauliflower. Convective cloud types are indicated by the root word “cumulo,” which means “heap” in Latin. Convective clouds may become very tall and are rounded on top.

Clouds are also classified by their altitude and their ability to create precipitation. The root word “cirro,” meaning “curl,” describes a high cloud that is usually composed of wispy ice crystals. The Latin word “alto,” or “high,” indicates a cloud in the middle of the troposphere that is below the high cirro-type clouds. The prefix or suffix “nimbus,” or “rain,” denotes a cloud that is causing precipitation.

Using the combination of appearance, altitude and ability to make precipitation, a wide range of cloud types can be identified. The 10 basic cloud types are cirrus, cirrostratus, cirrocumulus, altostratus, altocumulus, cumulus, stratus, stratocumulus, nimbostratus and cumulonimbus.

Many clouds, particularly convective and stratus clouds, have a flat bottom. The base of those clouds marks the lifted condensation level (LCL) of the rising moist air from below. As the warm moist air continues to rise from below, the base of the cloud remains the same.

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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