How Much does Snow Evaporate?

Posted on February 13, 2011 by Weather Guys Editor

I notice that when the snow builds up in my patio, it starts to evaporate after a few days, even though the temperature is still below freezing.  On average, what percentage of our snowfall each year evaporates back into the cold, dry air?

The transition of water from the ice phase (or snow) to the gas phase (or water vapor) is called sublimation. Sublimation is a common way for snow to disappear in Wisconsin winters.

On warmer days, when temperatures are above freezing, we can see the melting process as snow leaves water behind on surfaces, which then evaporates or gets absorbed by the ground. We do not see the sublimation process because the snow goes directly into water vapor without first melting into liquid water.  However, we do notice that the snow amount is decreasing, so snow sometimes seems to disappear on cold winter days.

The rate of sublimation is a function of the weather conditions. It takes a lot of energy to turn ice into a gas called water vapor: about 7 times the amount of energy needed to boil that water. The energy needed to sublimate the snow off your patio comes primarily from the sun. So, sunny weather is the best weather for sublimating snow.  Windy days are also good, as the wind helps to remove the water molecules once they leave the snow and enter into the atmosphere. A low humidity also helps to increase the rate of snow loss.  So, the amount of snow that sublimates back into the air depends on the typical winter weather for a given location.

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Is shoveling snow dangerous?

Posted on February 7, 2011 by Weather Guys Editor

At least one death in Milwaukee and one death in Iowa were attributed to shoveling snow during this recent snowstorm. A study of the number and cause of deaths in Massachusetts immediately after six blizzards showed a 22 percent increase in heart related deaths following a blizzard. Published studies show that the number of heart attacks increases during the week following a blizzard as well,which suggests that it was blizzard-related activities that caused the deaths as opposed to exposure to the storm itself.

Heaving snow can raise your blood pressure, increasing the risk of a heart attack. Of course, snow shoveling itself may not be inherently dangerous. Any physically intense activity, such as running or skiing, could lead to increased deaths; however snow-shoveling is an outside activity that many people do after a big snow storm, including folks who may not otherwise do strenuous activities. If you don’t have a snow blower, you certainly know that shoveling is an intensive exercise.

Heart attacks aren’t the only problems associated with snow shoveling. One study found that in the US between 1990 and 2006, 195,000 people were treated in emergency rooms for snow shoveling related injuries. These treatments were for back problems, sore muscles, and broken bones. Some tips to avoid health related problems if you have to shovel snow include: warm-up and stretch your muscles before shoveling (good practice for any exercise), wear boots that won’t slip on the snow, pace yourself, and push the snow rather than heave it.

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What is the Coldest Time of the Year?

Posted on January 22, 2011 by Weather Guys Editor

The Earth revolves around the sun in a giant ellipse, and so is not always at the same distance from the Sun throughout the year.

Contrary to what you might expect, the Earth is closer to the Sun during our winter season. But, since the Earth’s axis (the imaginary stake that runs through the planet from the North to South Pole) is tilted at 23.5 degrees, the amount of sunshine in each season is more strongly regulated by this factor than by the distance to the Sun.

During winter, the North Pole is tilted dramatically away from the Sun, so the intensity of solar radiation is weaker and the daylight hours shorter in the Northern Hemisphere. At very high latitudes in the Northern Hemisphere, like northern Canada, the Sun is hardly ever above the horizon and the 24-hour day is dominated by darkness. The consequence of this darkness is extended periods of cooling and the production of extremely cold air masses near the surface.

By about the end of January, the very coldest air masses of the season have usually been produced in the high Arctic. These air masses gradually slide southward over the same period of time, usually bringing our coldest temperatures of the year in the last week of January or first week of February.

After that, the Earth’s continued orbit around the Sun produces a gradual increase in the intensity of solar radiation in the Northern Hemisphere. As a consequence, it becomes harder and harder to produce frigid arctic air masses to our north and the threat of a deep freeze is reduced.

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How are the Temperatures in the Higher Atmosphere Measured?

Posted on January 16, 2011 by Weather Guys Editor

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

The radiosondes measure vertical profiles of air temperature, relative humidity and pressure from the ground all the way up to about 19 miles. Temperature and relative humidity are measured electronically; a small aneroid barometer measures pressure. At low air pressures in the stratosphere, the balloon expands so much that it explodes and the radiosonde drifts back to the ground underneath a small parachute.

Wind speed and direction can also be determined by tracking the position of the balloon. When winds are also measured, the observation is called a rawinsonde. Rawinsonde measurements are made worldwide at several hundred locations twice each day. Rawinsondes are the workhorses of the weather data network above the ground.

However, they are usually launched only from land-based weather stations, which leaves out the 70 percent of the atmosphere that lies above the oceans. Therefore, modern-day meteorologists also use satellite observations to observe the upper atmosphere. Special satellite instruments measure temperature and humidity averaged through a layer of the atmosphere. The movement of clouds in a sequence of satellite pictures is used to infer wind speed and direction. The Global Positioning System satellite network used in cars to tell you where you are as you drive around, is also used to tell how much water vapor is in the atmosphere.

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What is Diamond Dust?

Posted on January 9, 2011 by Weather Guys Editor

Diamond dust is a cloud composed of tiny ice crystals that forms near the ground. It is often reported under clear sky conditions and so is also known as ‘clear-sky precipitation.’

The formation of diamond dust requires very cold temperatures, typically less than minus-13. When the air is very cold the water vapor that is always present condenses to form ice crystals.

Though it forms in air near the ground, diamond dust is not an ice fog. An ice fog initially forms as tiny drops of liquid water that then freeze. Diamond dust forms directly as a crystalline piece of ice. The shape of these ice crystals is similar to tiny, thin, six-sided pencils. The ice crystals are small and few in number so diamond dust is sometimes hard to see.

During daylight, bright sparks of light can be generated as the sun hits the ice crystals. This happens as the light bends as it passes through the crystals. This can make the air sparkle, much like a diamond ring can sparkle if the light hits the ring at the right angle. Because it is shaped like crystals, diamond dust can generate some beautiful optical phenomena, such as halos and sun dogs. We can see halos and sun dogs here in Wisconsin in high altitude cirrus clouds — which are also composed of ice crystals.

Because it needs cold temperatures to form, diamond dust is frequently observed in the interior of Antarctica. At the Antarctica Plateau, diamond dust is observed 316 days a year. Diamond dust also is frequent in the Arctic during winter and can occasionally occur in Wisconsin.

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