Wednesday, 22 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 [...]

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 [...]

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 [...]

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 [...]

Why are clouds white? Photograph of Cumulus clouds in fair weather taken by Michael Jastremsk Clouds are made of water and clean water is clear. So why are clouds white? Because clouds are made of billions of small water droplets and ice crystals. When light beams interact with particles suspended in air, some of the [...]

How many lightning bolts hit Wisconsin each year? Multiple cloud-to-ground lightning captured by C. Clark for NOAA with time-lapse photography. Wisconsin gets hit by lightning about 300,000 times a year; most of that during the spring and summer. That’s about five flashes for each square mile in the state. For about 20 years, the continental [...]

What does a 60 percent chance of precipitation mean? The probability of precipitation (fondly known as PoP) has been part of weather forecasts since the late 1960s, and is the only forecast element that includes a probability. Unfortunately, there is confusion about the exact meaning of a “60 percent chance of precipitation.” Part of that [...]

Raindrops on water from Juni of Kyoto, Japan Are raindrops tear-shaped? No. While some cartoons and some science diagrams draw raindrops in that shape, raindrops are neither tear-shaped nor spherical. Due to the interaction of cohesion, surface tension, air resistance and gravity, large raindrops are shaped more like the top half of a hamburger bun. [...]