Where do weather forecasters get their data?

Meteorology, like every other science, relies on careful and precise measurement of its subject. Weather observations are critical to both weather forecasters and computer models that predict the weather. These measurements are made at the ground level as well as in the atmosphere.

An important resource for weather observations near the ground is the Automated Surface Observing System, or ASOS (pronounced “A-sauce”).

There are about 2,000 ASOS stations located at airports across the country, and the instruments are maintained by the Federal Aviation Administration.

National Oceanic and Atmospheric Administration (NOAA) uses the Meteorological Assimilation Data Ingest System (MADIS) to collect and distribute weather observations from NOAA and non-NOAA organizations.

Meteorologists monitor the atmosphere above the surface by using a radio-equipped meteorological instrument package carried aloft by a helium-filled “weather balloon.”

These devices are launched twice a day by NOAA by over 100 locations across North America, the Pacific islands and the Caribbean. Radiosondes provide upper-air data that are essential for weather forecasts and research.

Weather observations of the upper atmosphere are also made by commercial aircraft flying passengers around the world and distributed to the National Weather Service to use in its computer models.

Satellites and radar systems provide a huge volume of observations.

Radar is used to track precipitating weather systems and are very valuable for short-term forecasts. Satellites track weather systems and make important observations for global weather prediction models.

Privately owned personal weather stations are also an important part of many private forecasting companies. Trained volunteers provide observations about precipitation and threatening weather conditions.

Category: Meteorology

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How are we doing with heating degree days this year?

How are we doing with heating degree days this year?

A heating degree day is a weather index designed to make weather data easier to use in planning.

The index estimates fuel consumption needs, and fuel distributors use it to schedule home deliveries. Electric and natural gas utilities predict power demands by adding up this index over time.

The following mathematical formula defines this index.

Each degree that the daily mean temperature is below 65 degrees is one heating degree day. So, if today’s average temperature is -5F, that is 70 heating degree days, or HDDs. The higher the HDD, the colder it is.

This baseline of 65 degrees was determined by engineers who found that when the mean outdoor temperature drops below 65, most buildings require heating to maintain an indoor temperature of 70.

The amount of heat required to maintain a building’s temperature is proportional to the accumulated heating degree days. So, heating degree day totals are usually reported each day, as well as the total sum for the season. This allows us to quickly judge whether the season is above, below or near normal in terms of heating bills.

Over a year, northern Wisconsin has about 9,000 HDDs, while southern Wisconsin has about 7,000 HDDs. It will be no surprise that as of Jan. 28, Madison was above normal for this time of year, at 4,418 HDDs compared with 4,173 HDDs, though significantly below the very cold 1976-77 winter record of 5,347 HDDs for Jan. 28.

So, Madison was frigid last month, but we have experienced colder winters.

Category: Climate

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What was that column of light extending above the setting sun?

A vertical shaft of light extending above a setting or rising sun is called a sun pillar. You could have seen this optical phenomenon if you were looking at the sunset on Jan. 18.

While sun pillars look like vertical beams of light projected above the sun, they are actually a collection of reflections of millions of ice crystals floating in the atmosphere.

This optical feature is caused by the reflection of sunlight off tiny ice crystals in a cloud that is near the horizon. Reflection takes place at the boundary of an object. Flat, six-sided, plate-like ice crystals floating in the atmosphere act like small mirrors, reflecting the sun’s light at their flat surfaces.

When in the right location between you and the sun, the ice crystals act like mirrors and reflect sun light off their surfaces toward your eyes. If the crystals were not there, the sunbeams that they are reflecting would never be seen by you; those beams of light would be traveling far above your head.

On this same day, the snow at times sparkled. As with the sun pillar, flat crystals resting on top of a blanket of snow can act like tiny mirrors, reflecting a portion of the sun’s image toward your eye.

Each “sparkle” is an individual ice crystal reflecting the sun’s image. Whether we see the sparkle depends on the angle formed between where the sun is, where we are looking and the angle at which the snowflake is resting. If all these conditions are right, occasionally as we walk by a field of snow, we’ll see it glitter because of how the sun is being reflected by the different crystals of snow.

Category: Phenomena

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What is the wind chill temperature?

On the cold and windy days of last week, you probably tried to keep yourself warm by wearing appropriate clothing and seeking shelter from the wind.

It feels colder in the wind because the wind sweeps away heated air in contact with your body and replaces it with colder air.

Whereas still air is a poor heat conductor (which is why storm windows have air trapped between glass panes), moving air is not!

The cooling power of the wind is measured by the wind chill factor. The wind chill describes the increased loss of heat by the movement of the air. The wind chill is relevant to humans and other animals needing to maintain a constant temperature that is higher than the surroundings.

The wind chill factor cannot be measured with a thermometer; it must be computed. The wind chill temperature translates your body’s heat losses under the current temperature and wind conditions into the air temperature with a 3-knot wind that would produce equivalent heat losses.

This is not an easy conversion. The original wind chill formula was devised by Antarctic explorer Paul Siple in 1945. More recent research has revealed some flaws in Siple’s work, such as assuming that the wind at face level is equal to the wind at 33 feet above the surface. The National Weather Service updated its wind chill temperature calculation in November 2001.

Dr. Ed Hopkins, Wisconsin’s assistant state climatologist, has computed the wind chill temperatures for all the hourly temperature and wind speed combinations available from Truax/Dane County Regional airport since January 1948. Madison’s lowest wind chill temperature was -54.3 at 4 a.m. and 5 a.m. on January 20, 1985 — the same day of President Ronald Reagan’s second inauguration, which was held indoors because of the cold Washington, D.C., weather and because it was a Sunday. In the cold air outbreak of last week, the coldest wind chill temperature was -43 at 9 a.m. on Jan. 6.

Category: Meteorology

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How unusual is this kind of cold ?

We are at the beginning of the most intense cold air outbreak of the winter thus far with high temperatures today not likely to rise above zero. It may be similarly cold tomorrow.

Having two consecutive days in a row with daytime high temperatures below zero is noteworthy in Madison. The last time that occurred in town was in early February 1996 (Feb. 2 and 3).

In fact, a daily high temperature colder than minus 5 has only occurred 12 times in the last 33 years, while a daily high lower than minus 10 has only been recorded on four occasions in that time period.

In addition, the morning low on Monday is likely to be near 20 below zero. The last time Madison saw a morning low temperature that low was on December 25, 2000, when the low was minus 21, part of a 12-day stretch during which 11 days had morning lows below zero.

Overnight lows of minus 20 or colder are not common in Madison, though not unprecedented. Since 1980 it has happened 26 times and since 1990, 12 times: Jan. 15, 16, and 18-20, and Dec. 31, 1994; Jan. 31, Feb. 2, 3 and 4, 1996; Jan. 5, 1999; and Dec. 25, 2000.

The low of minus 29 recorded on Feb. 3, 1996, tied for the third coldest of all time in Madison.

Whether the current cold outbreak meets or exceeds any of these records will be quite beside the point as we all try to bear the brunt of it over the next couple of days.

Category: Climate

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