Is there any difference between ‘partly cloudy’ and ‘partly sunny’?

Posted on January 8, 2024 by WeatherGuys Editor

The National Weather Service denotes sky condition using the portion of sky covered by opaque cloud cover.

Cloud cover is indicated by the shading of the centre of the station circle. The table below shows the meanings of the different symbols. (Image credit: MetLink, Royal Meteorological Society)

In meteorology, an okta is a unit of measurement used to describe the amount of cloud cover at a given location. Cloud conditions are estimated in terms of how many eighths of the sky are covered with opaque cloud, ranging from 0 oktas (completely clear sky) to 8 oktas (completely overcast). It does not include transparent clouds, such as thin cirrus.

Mostly clear or mostly sunny is reported for coverage from 1 to 3 oktas, while partly cloudy or partly sunny refers to cloud coverage of 3 to 5 oktas. Mostly cloudy is reported if 5 to 7 oktas of the sky are covered with opaque clouds. When the sky is obscured by fog, 9 oktas is reported for total sky cover, indicating it could not be observed.

Trained weather observers make these estimates, and while the cloud cover measurements are subjective, they still provide useful and scientifically valuable data.

So, partly cloudy is the same as partly sunny, with 3 to 5 oktas of opaque cloud cover. Of course, partly sunny cannot be used in reporting nighttime conditions.

At any particular observation time, there may be different types of clouds at different heights above the ground. In such situations the total cloud amount is reported and defined as the fraction of the sky covered by any type of opaque cloud. Weather reports also will include the fraction of the sky covered by each type or layer of cloud as if it was the only type or layer of cloud in the sky.

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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What were the weather highlights of 2023?

Posted on January 2, 2024 by WeatherGuys Editor

As we begin a new year, let’s look back on the weather of 2023. The most recent, and odd, weather event of the year was the warm temperature on Christmas Day, with a high of 54 degrees in Madison. That was only the seventh time since 1869 that the maximum temperature exceeded 50 degrees on Christmas.

Daily temperature graph for 2023 (Image credit: Wisconsin State Climatology Office)

The state received less than one-third of its usual precipitation in November. The statewide average temperature for the meteorological autumn (September, October and November) was 2.5 degrees above normal, which made it the eighth-warmest autumn on record. In October, the 4.14 inches of statewide average precipitation was 1.13 inches above the 1991-2020 normal. This helped to alleviate our drought conditions.

We experienced heavy rain on Feb. 27. Both Milwaukee and Madison recorded more precipitation on that day (2.06 inches and 1.56 inches, respectively) than normally falls there during the entire month of February. That heavy rain contributed to the winter of 2022-23 being Wisconsin’s wettest meteorological winter (December through February) on record (since 1895). The winter also was warm overall, highlighted by a lack of temperatures at or below 0 degrees.

March was snowy. Total snowfall for the month exceeded normal conditions throughout Wisconsin. Madison recorded its snowiest day of the month on March 25, with 12.1 inches of snow, making it the snowiest March in Madison since 1959.

April was an interesting month. Temperatures for the state averaged 8 degrees warmer than normal from April 1 to April 15 and then were 8 degrees colder than normal from April 16 to April 30. Eau Claire, Wausau, Green Bay, La Crosse and Madison all had four straight days of 80 degrees or warmer in early April.

Our meteorological summer in 2023 was the fifth-driest summer on record (since 1895). An intense heat wave hit in August, peaking from Aug. 22 to Aug. 24. Temperatures reached more than 100 degrees in many areas across the state.

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, History, Meteorology

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What happens on the winter solstice?

Posted on December 26, 2023 by WeatherGuys Editor
The winter solstice marks the shortest day and longest night of the year. In the Northern Hemisphere, it occurs when the sun is directly over the Tropic of Capricorn, which is located at 23.5° south of the equator and runs through Australia, Chile, southern Brazil, and northern South Africa. (Image credit: NOAA/National Weather Service)

Astronomical winter begins at the winter solstice, which happened this year for the Northern Hemisphere at 9:27 p.m. Central Standard Time on Thursday, Dec. 21. Winter south of the equator begins in June.

The Northern Hemisphere winter solstice occurs at the time the sun is at its southernmost point in the sky. At this time, the sun is overhead at noon on the Tropic of Capricorn, approximately 23.4 degrees south of the equator.

The December solstice marks the longest night and shortest daylight of the year for those living in the Northern Hemisphere. For the Southern Hemisphere, it marks the shortest night and longest daylight. Although daylight hours increase during the winter months, temperatures continue to fall because Earth cools down and heats up slowly. The overall energy balance is still negative, with more heat lost at night than is regained during the daytime.

Meteorologists mark the start of our Northern Hemisphere winter as Dec. 1, with winter covering the three months of December, January, and February. These are the coldest months of the year. This means the time when daylight gets longer occurs during the Northern Hemisphere meteorological winter. Slowly at first, but at increasing daily intervals, as spring approaches.

For the Southern Hemisphere, meteorological winter covers the months of June, July, and August. For the Southern Hemisphere, the winter solstice occurs around June 21.

A day is not just the period of daylight. A day defined relative to the sun is called a solar day; it is the time from one local noon to the next. A sundial, correctly orientated, will indicate the precise moment of local solar noon, when the sun reaches its highest point in the sky each day. The longest solar days, as measured from one solar noon to the next, occur each year around the December solstice, no matter where you live.

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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What is the probability of a white Christmas in Madison?

Posted on December 18, 2023 by WeatherGuys Editor

Believe it or not, the National Weather Service has criteria for declaring a Christmas white: If there is at least 1 inch of snow on the ground at 6 a.m. on Christmas Day, the year has registered a white Christmas.

As might be expected, the probability of this condition being met varies widely across the state of Wisconsin. In the far north, the probability exceeds 90% and approaches a certainty in some locations, while in the south the historic probability runs at about 40%, which might strike Madisonians as unexpectedly low.

Last year, we had a fairly fresh 6 inches of snow on the ground on Christmas morning, courtesy of a crippling storm that affected the eastern half of the United States just days before the holiday. In fact, during the past 25 years we have done better than average in terms of white Christmases, as 15 of the past 25 years — 60% — have qualified. While 2004 met exactly that 1-inch threshold, both 2008 and 2000 welcomed Christmas Day with 15 inches of snow at Dane County Regional Airport, the latter in what was the snowiest December in Madison history.

Unfortunately, it appears this year will not qualify. Just a week out there is nothing promising in the forecast — that is, no storms on the horizon. This immediate circumstance, coupled with a robust El Niño ongoing in the equatorial Pacific Ocean may well doom any chance of a white Christmas this year — and may extend the unwelcome snowlessness through a good deal of the winter.

Typical winter weather patterns during El Niño. Credit: NOAA

So, we can hope for some snow before next Monday, but it is probably best to proceed toward the holiday with a realistic outlook in that regard.

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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Why is the wind often calmer at night than during the day?

Posted on December 11, 2023 by WeatherGuys Editor

The cycle of daytime heating and night time cooling explains why, under most circumstances, calm winds are near the surface at night.

The wind usually increases with height above Earth’s surface. The wind several thousand feet above the ground is almost always stronger than that experienced near the ground. Friction causes the wind close to the ground to move more slowly. Friction decelerates the wind in the same way a rough road surface slows down a bicycle.

When the sun sets, the ground loses the solar energy from the sun and continues to lose energy via emission of longwave radiation. This cooling ground conducts heat away from the air near the ground, causing that air layer to cool down faster than the layers higher in the atmosphere. This creates a stable area with cool air near the ground and warmer air above. As the word suggests, “stable” means it is difficult to move the air layer, keeping the fast-moving air above from mixing down to the surface. This is called a decoupling: This layer is no longer influenced by what is occurring above it.

Calm conditions due to decoupling are evident in this sunset photo of Lake Mendota taken by Jonathan Gero in 2010.

On many calm nights, there is still wind blowing far overhead. When the sun is up, it warms the surface of Earth, which in turn warms the atmosphere above it. The warm air rises and the displaced air is replaced by the air above. These warm thermals mix up the air, bringing the faster moving air from above down near the surface. As the daytime heating goes on, more air from above is mixed down and the wind speed picks up. This results in turbulence and mixing of the air near the ground.

Of course, if there is a low-pressure area or fronts in the region, the winds will likely blow day or night.

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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