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… More
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… More
I saw a photo of the sun rising above Lake Michigan. Why does sun appear flat? ENLARGE Photo of moon partially obscured by earth’s atmosphere: NASA. The distortion of the moon’s lower section is caused by refraction. This flattening of… More
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… More
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… More
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… More
How much water is in the atmosphere? Water is an exceptionally interesting chemical with many important implications for life on Earth and the circulation of the atmosphere. It is the only chemical that naturally exists in all three phases (solid,… More
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… More
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… More
What’s a 100-year flood? Do they really occur only once in 100 years? ENLARGE Diagram: DNR. The name is misleading: a “100-year flood” does not happen only once every 100 years. The name describes the estimated probability of that particular… More
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… More
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… More
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… More
Where does the dew on blades of grass come from? As fall begins, we have lots of mornings with dew on the grass. To explain this, we start with the fact that air contains water in the gas phase, called… More
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… More