Are there different types of snowfall?

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?

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?

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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Is the ground temperature changing?

First, let’s consider air temperature.

The Wisconsin Initiative for Climate Change Impacts, or WICCI, analyzed data from 1950 to 2006, a 57-year period. Wisconsin has warmed during this period.

The observed warming since 1950 has been greatest in winter, with an average increase of 2.5 degrees across Wisconsin. Winter temperatures in northwestern Wisconsin have increased by 3.5 to 4.5 degrees.

The WICCI report also showed that from 1950 to 2006, the frequency at which daily low temperatures have fallen below zero has diminished by about four to five days across southern Wisconsin and 14 to 20 days across northwestern and central Wisconsin.

A separate report that compared minimum temperatures averaged from 1961 to 1990 with the average minimum temperature during the period 1971 to 2000 demonstrated that the average minimum temperatures in winter have increased at nearly all locations in the continental U.S.

Given these observed warming temperatures, we might hypothesize that the ground would show a warming trend due to energy changes between the air and ground.

In addition to measuring air temperatures, there are locations that also measure ground temperature. One study explored ground temperatures for counties in Wisconsin.

The winter conditions in Wisconsin between 1948 and 2012 showed reductions in the length of the frozen ground season, defined as the start of frozen ground to end of frozen ground. There is a two- to three-week shortening of frozen ground conditions.

The logging industry is impacted by this. Frozen ground makes it easier for harvesting trees; a shorter frozen ground season could mean a muddier season making logging difficult as well as damaging forest roadways.

Category: Climate, Seasons

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How hard do weather forecasters work?

In the wake of a remarkable, but not perfect, forecast of a paralyzing blizzard that affected the Northeast last week, one may wonder how much effort is expended by an individual forecaster.

Since it takes only a minute or two to deliver a forecast on the evening news, it is easy to mistakenly view the process as taking very little effort or care.

In reality, the short broadcast time is supported by many hours of intense, professional effort divided among a multitude of difficult tasks.

Perhaps first on that list, the forecaster must gain a comprehensive understanding of the current observations throughout the atmosphere and over a large region of the country.

This process alerts her to the features in the atmosphere that will likely have a bearing on tomorrow’s weather in her region. She must then consult the numerical model guidance to incorporate the model forecast information into her evolving sense of what tomorrow will be like.

Then, she likely consults with other learned colleagues — all of whom are constantly putting their hard-won knowledge of how the atmosphere works into practice. Arguing about their separate interpretations of what is coming is among the most challenging and time-consuming aspects of the job.

By this point in the process a forecaster is likely to have spent several hours of preparation time. The forecaster has other duties as well, such as working with authorities to support their decision-making process, answering public questions and working with the producer, as well as administrative tasks.

Thus the final product — the public broadcast of the forecast — is actually only a small part of a much more elaborate and challenging process designed to produce a result with direct relevance to the lives of many thousands of people.  It is hard to judge what a given job really entails on the basis of uninformed assumptions that arise from one’s own limited experience with the nature of the task.

Category: Meteorology, Severe Weather

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