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 [...]

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 [...]

What happens when people throw boiling water in the air on a cold day? You can watch videos on the web, but this is a great experiment to try at home, if you do it carefully and don’t burn yourself! On a day with very cold, dry air, throw a cup of boiling water into [...]

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 [...]

What is ‘black ice’? The term ‘black ice’ refers to either a new layer of transparent ice on water, which allows us to see the deep water below, or a layer of clear ice on a roadway, which makes for hazardous driving. In both cases, the ice is transparent, not black, and so it shows [...]

When does winter really start in Wisconsin? Photo: MODIS Image Gallery by Liam Gumley, SSEC (Space Science and Engineering Center, UW, Madison). State outline barely visible under snowcover (click image to enlarge), this is Winter in Wisconsin. The beginning of astronomical winter doesn’t occur until the Winter Solstice, which is usually between December 20-22. On [...]

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 [...]