When does spring begin?

Posted on March 16, 2015 by WeatherGuys Editor

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Spring marks the transition from winter to summer. Meteorologists often define the three months of spring as March through May. By that definition, spring would begin on March 1.

We might also define spring as the day on which, if there is precipitation, it is more likely to be in the form of rain than snow.

For southern Wisconsin, that occurs later in the month of March.

We may also define spring based on the appearance of a particular flower, the blooming of certain trees, or the return of specific migrating birds. These are the phenological signs of spring.

Some mark spring by the increase in the number of potholes.

According to the astronomical definition, spring occurs when the sun’s rays strike the equator at noon at an angle that is directly overhead.

This particular time varies from year to year due to variations in the Earth’s orbit about the Sun.

In the Northern Hemisphere the vernal (or spring) equinox (equi, “equal,” and nox, “night”) occurs sometime between March 19 and 23, but often on March 20 or 21.

This year, astronomical spring arrives on March 20 at around 5:45 p.m.

During the equinoxes all locations on Earth experience 12 hours of daylight and 12 hours of darkness. The sun rises due east and sets due west.

The seasons result from the tilt of the Earth and its yearly circling of the sun.

Whatever the definition, during spring the length of daylight hours is increasing and the air is warming.

That’s welcome news for many people, particularly after our cold February.

Category: Meteorology, Seasons

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Do snowballs in Washington, D.C., count as evidence against global warming?

Posted on March 9, 2015 by WeatherGuys Editor

Last week, Sen. James Inhofe of Oklahoma, the current chairman of the Senate Environment and Public Works Committee, brought a snowball fashioned from accumulated snow in Washington, D.C., onto the floor of the United States Senate.

 

Inhofe used the snowball as proxy “evidence” of, in his words, the “hysteria on global warming.”

It is undeniable that the second half of this winter has been unusually cold over a large portion of North America, with an especially brutal combination of snow and cold over the eastern seaboard.

However, it is also true that this past winter (December/January/February) registered the smallest average areal extent of lower tropospheric cold air since records began in 1948. In fact, the past two winters, cold though they have been over North America, have set that record in back-to-back years.

This fact makes for interesting scientific work ahead. On the one hand, we hope to find an explanation of the hemispheric circulation anomalies that have led to this unusual North American cold in the midst of a warm hemisphere. At the same time, this set of circumstances makes the point that local weather and global scale climate change are not always in step with one another at a specific location.

Rather than appropriate unusual local weather conditions in a lazy and willfully unscientific argument about an important environmental issue, we encourage our public servants to examine the observations and the science surrounding them, ask difficult questions, and endeavor to seek the truth through careful analysis of the evidence.

Category: Uncategorized

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Are there different types of snowfall?

Posted on March 2, 2015 by WeatherGuys Editor

Weather forecasters use different phrases to describe snowfall intensity:54f2af3399484_preview-300

• A light shower of snow that does not last long and covers a small region is called a snow flurry.

• When large amounts of snow fall, then the event is colloquially referred to as a snowstorm, though the term “large” is not officially defined.

• A snowstorm that lasts for three or more hours with sustained wind speeds of at least 35 mph and visibilities less than a quarter-mile is called a blizzard.

• A snowstorm that includes any occurrence of thunder is called a thundersnow event.

• A storm with freezing rain is referred to as an ice storm.

• Diamond dust, also called “clear-sky precipitation” is a cloud composed of tiny ice crystals that forms near the ground.

A weather watch informs us that current atmospheric conditions are favorable for hazardous weather such as a blizzard, heavy snow or heavy freezing rain. When the hazardous weather is occurring or will soon occur in an area, then a warning is issued. Weather warnings are issued when the event is a threat to life and property.

An advisory is a less urgent statement than a warning and is issued to bring the public’s attention to a situation that may cause some inconvenience or difficulty for travelers or people who have to be outdoors.

Category: Seasons, Severe Weather

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How is the cold weather affecting citrus crops?

Posted on February 23, 2015 by WeatherGuys Editor

54e963224008e_preview-300Last week was one of the coldest of the season in the southeast United States and such events have been known to present a substantial threat to the citrus industry in Florida.

The problem is that when the temperature gets below freezing, the fruit itself can freeze, rendering it substantially less valuable to the market.

How can citrus farmers combat the threat of cold on such nights? One of the most ingenious tricks they use takes advantage of the fact that phase changes of water involve substantial amounts of energy known as latent heats. Everyone knows that ice only melts into liquid water after some energy has been supplied to the ice – the latent heat of melting. In the process, the environment that surrounds the original ice becomes colder since it has been force to spend some energy to do the melting.

A classic, kitchen-ready example involves placing ice cubes in a glass of water. Two things happen — the ice melts and the original liquid water cools. The water cools because it has spent some of its energy in accomplishing the phase change.

The interesting thing is that if liquid water is converted to solid ice, the same amount of energy is released to the environment — this time it is called the latent heat of fusion.

So, when a freeze is forecast for a citrus farm, the farmers often spray the trees with liquid water. When the temperature drops, this water freezes and releases heat to its environment, some of which is the still ripening fruit. This heat is enough to preserve the fruit inside from freezing. So, paradoxically, causing ice to form on the fruit saves it from freezing.

Category: Seasons

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How severe has this winter been?

Posted on February 16, 2015 by WeatherGuys Editor

Last year we experienced a persistently cold winter (December-January-February) that ended up being the coldest since 1979.

This year has been different. The Midwest Regional Climate Center is experimenting with what they are calling the Accumulated Winter Season Severity Index (AWSSI), which incorporates temperature, snowfall and snow depth to rate the severity of winter.

Through Feb. 11, we are having a slightly colder than average winter locally.

Interestingly, it has been a remarkably “warm” winter when considered from the hemispheric perspective.

Four times each day we calculate the areal extent of air colder than minus 5 degrees Celsius (23 degrees Fahrenheit) 1 mile above the surface using weather data (not forecasts) 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.

Daily cold pool areaIn the accompanying chart, this daily value is plotted (thick solid line) against the 66-year average (broken line) for each calendar day from Dec. 1 to Feb. 11. Also plotted is last year’s daily record (thin solid line).

Last year was the “warmest” winter, by this measure, since records began in 1948-49. Remarkably, this season appears destined to break last year’s record.

Over the last 66 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 up as a result of changes in the chemical composition of the atmosphere induced by the burning of fossil fuels.

Despite the locally cold weather we are likely to experience over the next several days, the hemisphere is getting warmer every winter.

Category: Climate, Meteorology, Seasons

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