Thursday, 23 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 [...]

Watch for steam devils in late November/early December Steam Devils in Wisconsin One of the many advantages of living near large lakes in temperate climates is the steam fog that shrouds them in fall and early winter. Fog is essentially a ground-hugging cloud, composed of tiny liquid water droplets. Steam fog will develop at this [...]

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 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 [...]

How long have satellites been used to study Earth’s weather? NASA image of Explorer VII satellite, 1964 The first successful meteorological experiment conducted from a satellite was launched on Explorer VII on October 13, 1959, just over 50 years ago. Explorer VII carried an instrument that measured Earth’s heat balance. The thermal radiation experiment was [...]

What is the jet stream? Photo: NOAA The jet stream is a narrow current of strong wind that blows through the upper regions of the troposphere, about seven miles above ground. The jet stream generally moves from west to east, but it can meander like a river. The winds can move up to 200 mph [...]

Numerical Weather Forecasting is younger than rock ‘n roll!! From NOAA Before 1960, the idea that a reasonably accurate two-day weather forecast could be made routinely was a pipe dream – now it’s a routine reality. In fact, it was not until just after World War I that a theory concerning the structure, life cycle [...]

How accurate are weather forecasts? Image: NOAA In general, weather forecasts are getting better, due to improvements in computer models, observations and our understanding of atmospheric. Accuracy depends on the purpose of the forecast and how far out it extends. Next-day forecasts of maximum temperature are good to within 3 or 4 degrees; however, the [...]

Is there a relationship between sun spots and climate? Since the invention of the telescope in the 1600s, observers have recorded variations in the numbers of dark spots – “sunspots” – on the Sun’s surface. These variations normally follow a regular cycle with peaks 11 years apart. This cycle coincides with a small oscillation in [...]

What do heating degree-days tell us? Each degree that the mean temperature is below 65 degrees F is one heating degree-day. So if today’s average temperature is 55 degrees F, the day accounts for 10 heating degree-days. Engineers determined that when the mean outdoor temperature drops below 65 degrees F, most buildings require heating to [...]

What is space weather? Space weather describes the conditions in space that affect Earth and its technological systems. Space weather storms originate from the sun and occur in space near Earth or in the atmosphere. Space weather, like weather here on Earth, is continually moving and changing. Space weather phenomena include the Northern lights and [...]

What is the dewpoint temperature? Dew on evergreens, by Michael Theberge at NOAA Think of a morning when you walked on a grass lawn or through a field. Did your shoes get wet? If yes, that is because the grass was wet with dew. The dewpoint temperature is the temperature to which the air must [...]