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Category Archives: Meteorology
Why are clouds relatively flat on the bottom?
Most clouds, especially those with flat bottoms, form in rising air. The air can be forced to rise due to convection, frontal lifting, or when air near the surface flows together from different directions. As a volume, or parcel, of air rises, it expands and cools. In addition, the relative humidity of the rising air increases. As the parcel approaches the point of saturation, water vapor condenses to form tiny water droplets or ice particles, creating a cloud. Saturation occurs at a distinct altitude, which varies depending on the temperature and humidity structure of the atmosphere. Below this condensation level clouds do not form.
Often low clouds, like stratus and cumulus, appear to have flat bases. These clouds form as air near the ground is rising. As the air rises, it expands as pressure decreases with altitude. This expansion results in a cooling, which causes the relative humidity in the rising parcel to increase. The temperature of the rising air approaches the dew point temperature. When it reaches the height where those two temperatures are equal, the relative humidity is 100% and a cloud forms. Meteorologists call this altitude the lifting condensation level. Continue reading
What is the difference between sleet and freezing rain?
Rain, snow, freezing rain, and sleet all generate hazardous traffic conditions. Freezing rain, and the less intense freezing drizzle, can create the very treacherous road condition referred to as “black ice.” A freezing fog may similarly coat objects in ice while also reducing visibility. Black ice is so named because the affected roadway appears dark, just like wet pavement. Black ice creates nearly zero friction conditions with vehicle tires so that correcting a skid in such conditions can be nearly impossible. Continue reading
Category: Meteorology, Seasons
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How does factory exhaust contribute to snowfall?
An article in the Wisconsin State Journal reported that on November 26, 2024, a factory near Menomonie, Wisconsin contributed to a narrow band of snow that extended for nearly 100 miles. This can occur only with certain atmospheric conditions. A … Continue reading
How and when do Madison lakes freeze?
The surface of a lake exchanges energy with the air above. Cold air cools the lake surface through energy exchanges with the atmosphere, determined by the weather above. As cool surface water cools, it becomes denser than the warmer water below and so the cooled water sinks. Water from below then rises to the surface where it begins to cool.
What is unique about the H2O water molecule is that as liquid water cools, its density increases until about 39°F (4°C). At that point, the colder water becomes less dense, stays at the surface, and continues to cool. Once the surface water cools to approximately 32°F, the water molecules crystallize into interlocking lattice-like patterns and ice is formed. For a lake surface to freeze, the entire lake needs to be at a temperature of 39°F; only then as the surface cools will the temperature of the liquid water at the surface remain less dense than the water below and thus float and begin to form ice. Shallower lakes usually freeze before deeper lakes since shallower lakes contain less water that needs to be cooled.
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Do thunderstorms cause milk to spoil?
No, but it was once widely thought that summertime thunderstorms could cause fresh milk to curdle.
This thinking was based on observations that raw milk would spoil during lightning and thunderstorms. But it was neither the thunderstorms nor lightning that caused the spoiling; rather it was the atmospheric conditions. The conditions that are optimum for thunderstorm development are the same as those that would cause milk to spoil. Continue reading
