Saturday, 18 May, 2013

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. [...]

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, liquid, and gas) in our atmosphere. Diagram: NOAA At any moment, the atmosphere contains an [...]

Did a volcanic eruption in Alaska impact weather in the Midwest? Mt. Redoubt, May 8, 2009 The eruption of Mt. Redoubt in Alaska in April, 2009 did not impact weather in the Midwest. Moderate eruptions can lead to cooler temperatures near the mountain, as demonstrated by the Mount St. Helens eruption in Washington on May [...]

Why is the sky blue? Half Dome (in California’s Yosemite National Park) under a bright blue sky. To understand why the sky is blue, we need to understand a little about light. Light is a form of electromagnetic energy, which can propagate through empty space. We describe this energy by wavelength – the distance between [...]

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 water vapor. Dew is made of liquid water that has condensed from that water vapor. [...]

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 flood happening in any given year. A 100-year event has a 1 percent chance of [...]

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 the sun on the horizon is an example of a mirage. Mirages are not illusions; [...]

How does fog form? Photo: NASA Fog is a cloud in contact with the ground. When the relative humidity approaches 100 percent, water vapor condenses on tiny particles suspended in the air to form a suspension of small water drops. The air in contact with the ground can reach high relative humidity if it cools [...]