Monday, 20 May, 2013

What is sleet? ENLARGE Photo: Generation X-Ray When precipitation droplets refreeze before hitting the ground, you get sleet. Sleet is translucent balls of ice that are frozen raindrops. The most common forms of precipitation are rain, snow, freezing rain, and sleet. In Wisconsin, precipitation usually begins as ice particles in a cloud. The temperature conditions [...]

Where does the water come from in Midwestern snow storms? ENLARGE Photo: NOAA/NASA GOES Project Water can travel a long way to dump onto the Midwest as snow. This picture shows the storm system that cause the massive February 2011 storm. Last week we were visited for the second time this winter by a sizable [...]

Do abundant snowstorms suggest global warming is not occurring? ENLARGE Graphic: Rutgers University Global Snow Lab This graphs shows the area of land covered by snow over the past few decades in North America. No. These storms are individual weather events, which cannot be used to support or refute climate trends. Which also means that [...]

Why do bridges ice before the road? People in cold climates are used to signs warning that “bridge freezes before road.” Water on surface will freeze once the surface becomes cold enough, but why does the road cool faster? ENLARGE Photo: Petelewisr Beware of the bridge on an icy day! Warming and cooling result from [...]

What is freezing rain? ENLARGE Photo: Mike Giovinazzo Freezing rain covers everything with a sheet of ice. When water particles fall from clouds and reach the surface as precipitation, they do so primarily as rain, snow, freezing rain or sleet. The many types of precipitation occur because water can change phase as it falls. In [...]

Why does snow disappear? ENLARGE Photo: Amanda Graham Sublimation in action. When the snow builds up on 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 to the air? The transition of water from the [...]

How do ice ridges form on a lake? ENLARGE Photo: Jeff Miller Pressure ridge near shore on Lake Mendota in Madison, WI Lake ice formation is dynamic. Even when a lake is completely frozen, the ice is not stagnant. It expands and contracts as it warms and cools. Differences in day and night temperatures can [...]

Why do I feel colder on a damp day? © S.V. Medaris A damp, January morning brings hoarfrost to a Wisconsin farm. Let’s define a damp day as one with high relative humidity but without precipitation or fog. High relative humidity reduces evaporation, which can be dangerous in high temperatures. Although air temperatures are too [...]

How do snow fences work? Photo: flickr via Wikimedia Commons, by John Talbot. Snow fence might not have helped here (Ottawa, Canada). Snow carried by wind can reduce visibility and cover roads. We cannot “switch off” the wind; but we can slow it with obstacles. Obstacles like trees and fences break wind into a swirl [...]

Is Madison, Wisconsin in a snow belt? What is the origin of the term? While the lakes around Madison provide many winter recreation activities, the local lakes do not yield a snow belt. Map by Pierre cb We refer to agricultural regions in the United States as ‘belts’, such as the cotton belt and wheat [...]

Do all snowflakes have unique shapes? It is highly unlikely that you’ll find two identical snowflakes. Feel free to try to prove us wrong, but your search may involve some cold feet! If you compared 1 million snowflakes (a minuscule fraction of the flakes in a single snowstorm), performing two comparisons per second, you’d need [...]

How does a lake freeze? Enlarge Photo: NOAA Ice forming around the shore of Lake Michigan Lakes freeze from the top down. Ice is less dense than water, which is why ice floats. The density of liquid water is determined by its temperature, and water is most dense at about 40 F. Why is that [...]

How does frost form? Heavy frost on Burr Oak tree in Wisconsin Frost on objects is just water vapor in the air that has condensed as ice onto a surface. Frost forms on objects close to the ground, such as blades of grass. At night, a blade of grass loses energy by emitting a non-lethal [...]

Why does El Niño often lead to a warmer winter in Madison? Image: NASA Forecasts of a relatively mild winter are being tied to a phenomenon called “El Niño” in the tropical Pacific Ocean. El Niño is so often invoked to support predictions of a warm winter that it begins to sound like Greek mythology. [...]

Is global warming real? Image: NASA Over the past two decades, the global average surface temperature has increased noticeably. This observed warming trend indicates a significant global change and is consistent with other observed changes on our planet: There is a widespread retreat of non-polar glaciers. Arctic sea-ice has thinned by 40 percent in recent [...]