Thursday, 23 May, 2013

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 [...]

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 [...]

Do tides and seiches occur on lakes? Tides are changes in water level caused by the gravitational pull on water by the moon and, to a lesser extent, the sun. Tides are of interest mainly in oceans and their coastal estuaries, which are part of one big body of water that is free to move [...]

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 [...]

How can we determine how far away lightning is? Make your own lightning Because of the vast differences in the speed of light and the speed of sound, the flash of lightning precedes the rumble of thunder. It takes sound waves five seconds to travel one mile, whereas the flash of lightning travels the same [...]

Is humid air “heavier” than dry air? Broadcasters of baseball games in mid-summer commonly suggest that the air on a hot, muggy night is “heavy” and that, as a consequence, the ball will not travel as far when hit. But is humid (i.e. moist) air really heavier than dry air? First of all, “moist air” [...]

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 [...]

Does a waterspout over the ocean contain fresh or salt water? Waterspout photo from NOAA Photo Library A waterspout is a whirlwind that forms over water, beneath a cumulus cloud. Before you see the waterspout, you may see a funnel cloud hanging from the bottom of the cloud. This funnel cloud is made of pure [...]

What are growing degree days? The Growing Degree Day, or GDD, is a heat index that can be used to predict when a crop will reach maturity. Each day’s GDD is calculated by subtracting a reference temperature, which varies with plant species, from the daily mean temperature (we ignore values less than zero). The reference [...]

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 [...]

What is a landspout tornado? “Landspout” is slang for a tornado that, unlike most tornadoes, is not associated with the mesocyclone of a thunderstorm. The name reflects the fact that these tornadoes look “like a weak Florida Keys waterspout over land.” Landspout tornado over Hale County in Texas (NOAA). The official name, “dust-tube tornado,” comes [...]

Crane moves radome into position during installation of radar antennae at NOAA’s National Severe Storms Laboratory in Norman, Oklahoma Photo from NOAA How does weather radar work? Radar, an acronym for RAdio Detection And Ranging, was invented during World War II to detect aircraft, but precipitation frequently got in the way. The military’s noise is [...]

What are the different types of thunderstorms? Thunderstorms can be classified by severity or structure. For example, the National Weather Service defines a severe thunderstorm as one that produces one or more of the following: wind gusts of at least 58 mph, hail at least one inch in diameter, or a tornado. A supercell thunderstorm [...]