The Weather Guys

How do you measure weather far above the ground?

Posted on April 4, 2016 by WeatherGuys Editor

A monitor displays temperature and humidity data being sent back from a weather balloon at the Cooperative Institute of Meteorological Satellite Studies at UW-Madison. (Photo credit: Andy Manis)

Temperature and relative humidity are measured electronically; a small aneroid barometer measures pressure. Wind speed and direction are determined by tracking the position of the balloon.

When we also measure the winds, the observation is called a rawinsonde. The vertical distribution of rawinsonde observations above a location is known as a sounding.

Rawinsonde measurements are made worldwide at several hundred locations twice each day at the same time. They are only launched from land-based weather stations, which leaves much of the world unmeasured.

Because of this limitation of observations in space and time, scientists also use satellite observations to monitor the weather far above the ground. A few satellites can make global observations many times a day. Winds are determined by tracking the movement of clouds and water vapor features in the atmosphere. Temperature and humidity can be inferred under clear-sky conditions by detailed measurements in the infrared.

All these observations become critical measurements for weather forecasting. They provide a measurement of the initial state of the atmosphere at the start of the forecast.

Category: Meteorology

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Why are there different types of precipitation?

Posted on March 28, 2016 by WeatherGuys Editor

Tuyet Cullen shields herself from the sleet on Thursday as she takes the path along Atwood Avenue to a friend’s house. (Photo credit: Amber Arnold, State Journal)

When particles fall from clouds and reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet.

The main difference between these different types of precipitation is the temperature variations between the cloud base and the ground. Last week, Madison experienced all four of these precipitation types.

Almost all precipitation particles that fall in Madison begin as ice particles. If the particles completely melt, they reach the ground as raindrops. If they remain frozen, we have snowfall.

Ice storms occur when particles falling from the cloud melt and then fall through a layer of below-freezing air near the ground. The two precipitation types most common during ice storms are freezing rain and sleet.

Freezing rain forms when a very thin layer of cold air near the surface causes melted precipitation (raindrops) to become supercooled – a condition in which the drops remain in the liquid state even though the temperature of the liquid is below freezing. Such supercooled raindrops then freeze on contact with exposed objects on the ground. Freezing rain covers everything in a sheet of ice, creating shimmering landscapes.

Sleet consists of translucent balls of ice that are frozen raindrops. It occurs when the layer of subfreezing air near the surface is deeper — deep enough for the freshly melted raindrop to refreeze before reaching the surface.

When sleet hits the surface, it bounces and does not coat objects with a sheet of ice, as freezing rain does. Instead, it covers flat surfaces such as roads and driveways like millions of icy ball bearings.

Category: Meteorology, Seasons

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What forced last week’s strong winds?

Posted on March 21, 2016 by WeatherGuys Editor

Lesley Davidson, of Madison, shelters her 8-month-old daughter, Eleanor, from the wind as she walks with her 4-year-old daughter, Lily, to the Chazen Museum of Art on the UW-Madison campus on Wednesday. (Photo credit: Amber Arnold, State Journal)

Our windy Wednesday last week was a notable departure from a winter without many high-wind events.

The strong winds from Tuesday and Wednesday were a result of the passage of an intensifying low-pressure center (mid-latitude cyclone) nearly directly over Madison overnight Tuesday into Wednesday.

The fundamental circumstance that forces the wind to blow is the existence of horizontal differences in pressure from one location to another. Such differences are known as pressure gradients.

Since mid-latitude cyclones have lowest pressure at their centers, regions within a few hundred miles of the storm center have strong pressure gradients and, therefore, strong winds.

If, as was the case on Tuesday night and Wednesday, the mid-latitude cyclone is intensifying, the central pressure continues to get lower. This serves to strengthen the pressure gradients associated with the storm and, thus, increases the wind speeds. That was precisely the situation that led to our strong wind event on Wednesday.

Also, since the force that is imposed on objects caught in the wind varies as the square of the wind speed, the destructive power of 30 mph winds (such as we had on Wednesday) is nine times stronger than for more run-of-the-mill winds of 10 mph. Thus, many locations endured broken tree limbs and other such damage.

The force of the wind also depends on the surface area of the objects through which the wind is blowing.

Had the same trees that lost limbs on Wednesday been in full leaf, the area available to catch the wind would have been greatly increased and more than likely much more damage would have resulted.

Category: Meteorology, Severe Weather

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Did we have a normal winter this year?

Posted on March 14, 2016 by WeatherGuys Editor

Mild, spring-like weather encourages members of the UW-Madison campus community to take to the outdoors March 7th, including these visitors to the university’s Library Mall. (Photo credit – John Hart, State Journal)

In fact, 46 states had a winter temperature that was above average. The contiguous U.S., or the lower 48 states, had its warmest winter temperature in 121 years of record keeping.

We did have our cold spells, one of which occurred in the second week of February.

Early January temperatures were cold, causing the regional lakes to freeze over. The total precipitation for most of southern Wisconsin was above normal, about 130 percent of normal.

Southern Wisconsin, however, was about 5 to 10 inches short of its climatological winter snowfall.

Another way to look at our winter temperature is to consider the number of days between the last day with a 60-degree daily average temperature and the first such day of the year.

The daily average temperature is determined by adding the daily high to the daily low and dividing by 2.

Last Tuesday we had a high of 68 and low of 52, making March 8 the first day of the year with a 60-degree daily average temperature.

The previous such day was Nov. 5, which was 123 days before.

This was 58 days shorter than the mean interval between consecutive days at or above 60 degrees, and the second shortest interval of all time – the record shortest being 118 days from Nov. 9, 1999 to March 7, 2000.

The average interval, determined from records that date back to 1871, stretches from Oct. 17 to April 17. So, almost no matter how you slice it, this past winter was quite abnormal in its mildness.

Category: Climate, Meteorology, Seasons

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How severe has this winter been?

Posted on March 8, 2016 by WeatherGuys Editor

Posts intended as wall supports for an ice rink at Vilas Park were still standing in liquid water in early December (Photo credit: John Hart, State Journal archives)

It may not surprise anyone that the average temperature from Dec. 1 to Feb. 29 this season in Madison was 5.67 degrees above normal, with most of that surplus accumulated during an extremely warm December that was 12 degrees above average.

There are other ways to assess the winter severity that are less local in nature. Four times each day we calculate the areal extent of air colder than minus 5 degrees Celsius at 1 mile above the surface using weather data supplied by the National Center for Environmental Prediction. Averaging the four measurements per day together creates a daily value of the areal extent of this “cold pool.”

Despite the fact that our last two winters were either normal (last year) or well below normal (2013-14) in Madison, around the entire Northern Hemisphere those winters set back-to-back records for the smallest average cold pool areas (warmest winters) in the last 67 years.

Though we did not set a record, this season’s cold pool was the seventh smallest on record. Six of those seven years have occurred since 2000-01 indicating a trend toward overall hemispheric winter warming. In fact, over the last 67 winter seasons, the 90-day December-January-February average areal extent of this low-level cold air has systematically decreased.

The best explanation for this long-term trend is that the Earth is warming as a result of changes in the chemical composition of the atmosphere induced by the burning of fossil fuels.