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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Another example of unprecedented weather extremes

Posted on September 26, 2022 by WeatherGuys Editor
Enhanced image of Hurricane Fiona from the GOES-East weather satellite, Sept 23, 2022. (Image credit: NOAA)

Another entry in the category of unprecedented weather extremes comes from the tropical Atlantic basin where, last week, Hurricane Fiona wrought devastation to the Commonwealth of Puerto Rico, still reeling from its assault by Hurricane Maria eerily precisely five years earlier.

Fiona dropped upwards of 30 inches of rain on the south shores of Puerto Rico before heading north into the Atlantic, where it systematically strengthened into a Category 4 hurricane with sustained winds of more than 130 mph.

As Fiona was strengthening, the weather here in southern Wisconsin underwent a welcome change from the extreme humidity of Monday and Tuesday to the much drier, fall-like conditions of the late part of the week. This change was afforded by the passage of a fairly strong autumn cold front — on the first full day of fall. That same cold front was part of a larger extratropical weather system over eastern North America that was heading eastward to a rendezvous with Fiona as she left the tropics.

Embedded within the extratropical storm was a feature known as a tropopause fold. Such features were first observed only in the mid-1950s and, for a long while thereafter, were thought to be very unusual features, bordering on curiosities. Subsequent work has shown that these folds are ubiquitous features of the mid-latitude flow and, in fact, account for the creation of some of the ingredients needed for the development of all extratropical weather systems.

The fold associated with the storm that cooled us off by Wednesday was unusually strong for this time of year. When it encroached on Fiona north of Bermuda, the combination led to a dramatic reinvigoration of Fiona, as an extratropical cyclone, that slammed into Nova Scotia on Saturday, recording the lowest sea-level pressure ever observed in Canada.

So, for two weeks in a row, the atmosphere has served up remarkable examples of the havoc that can result from vigorous interaction between the tropics and the extratropics.

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, Severe Weather, Tropical

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What can happen to hurricanes when they move into the mid-latitudes?

Posted on September 19, 2022 by WeatherGuys Editor
GOES West Air Mass RGB image of Ex-Typhoon Merbok in the Bering Sea on September 16th. (Image Credit: CIMSS Satellite Blog)

Hurricanes are large-scale, organized storms that form in the tropical latitudes.

They are fueled by the enormous amount of heat released when water vapor, evaporated off the warm tropical ocean surface, changes phase to liquid and ice in the thunderstorm clouds of the hurricane.

They are smaller in areal extent than the storms that commonly affect us in the mid-latitudes here in Madison.

The distribution of clouds and precipitation in a hurricane is usually symmetric about its center (the eye). This is vastly different from the characteristically asymmetric distribution of clouds in our more familiar extratropical cyclones.

Despite their tropical origin, hurricanes do find their way to the mid-latitudes with regularity, especially in the western Pacific Ocean basin. When they make this excursion they often undergo a dramatic restructuring that, in concert with interactions from the wavy flow of the mid-latitudes, can lead to powerful extratropical storms.

Such a situation occurred on Friday and Saturday when former Typhoon Merbok migrated from the tropics toward the central Bering Sea and became a devastating extratropical cyclone with 50 foot seas and wind gusts exceeding 80 mph.

Such extratropical transitions are most frequent in September and October in the Northern Hemisphere and can have global ramifications for the weather and its predictability. In the present case, there is a very good chance that extreme rainfall in central California in the middle of this week may be one result of Merbok’s dramatic conversion.

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, Meteorology, Severe Weather

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