Why don’t we see rainbows at noon?

Posted on August 6, 2014 by WeatherGuys Editor

The classic rainbow is a single, brightly colored arc. Red is the outermost color of this arc, and violet is always innermost.

On occasion, you may see two rainbows at once. The lower rainbow is the primary rainbow, and the higher, more faintly colored arc is the secondary rainbow. The color sequence of the secondary rainbow is opposite to the primary; red is on the inside of the arc and violet is on the outside.

To form a rainbow you need large drops of water, the sun at your back and at the correct angle. Raindrops act as prisms, bending and reflecting the sunlight that falls on them; just like a crystal hung in a sunny window.

As light enters water, the path it takes changes. How much the direction changes is a function of the color of the light. You probably noticed that a smooth water surface can act like a mirror and reflect light.

If the light beam entering the raindrop reaches the back of the drop at a certain angle, it undergoes a reflection and heads back toward the sun. Sometimes the light reflects twice off the back of the raindrop; this leads to the secondary rainbow. As the light exits the raindrop and re-enters the air, its path bends an amount that again depends on the color. This bending of the light as it enters and leaves the drop disperses the light of the sun into its spectrum of colors that form the rainbow.

A rainbow is located opposite to the sun; this explains why rainbows are not seen at noon with the sun overhead. There needs to be a clear path from the sun to the rain falling from the cloud. If the sun is overhead and raining, you are probably standing in the rain with the cloud obscuring the sun.

Category: Phenomena

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Is Lyme disease connected to the weather?

Posted on July 28, 2014 by WeatherGuys Editor

Scientists at the National Center for Atmospheric Research and the U.S. Centers for Disease Control recently explored the relationship between the reports of Lyme disease and weather observations. They found that warmer temperatures, higher humidity and less rain are correlated with an earlier start and peak of the Lyme disease season.

The start of the Lyme disease season begins in late May on average and lasts for about 14 weeks. An above average amount of precipitation from the start of the year tends to result in a later beginning of the Lyme disease season. An earlier start to the season is associated with more days with temperatures above 50 degrees, except for the most northern regions of the U.S.

Deer ticks carry Lyme disease and can infect humans when they bite us. The disease is found predominately in Wisconsin, Minnesota and the northeastern United States.

The ticks that commonly spread the disease develop faster with warmer temperatures, and they are more active in feeding with warmer temperatures, higher humidity and a lack of heavy precipitation. Of course, these are times many people seek the outdoors.

Weather conditions not only affect the tick life cycle and our outdoor habits, but also the population of the ticks’ primary host — the white-footed mouse. A dry summer can result in less vegetation that is the food supply for the mice. This can result in a reduced population of mice, reducing the tick population and thus the cases of Lyme disease.

The correlation between weather and the start of Lyme disease season is strong enough that one can forecast the start of the season by analyzing the daily temperatures for the first 10 weeks of the year. Unfortunately, there are few correlations that support predicting the end of a particular Lyme disease season.

Category: Seasons

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Does meteorological science have an impact on public policy?

Posted on July 21, 2014 by WeatherGuys Editor

One key piece of the world’s evolution toward nuclear sanity during the height of the Cold War was motivated by growing understanding of a fundamental meteorological phenomenon: the development of what’s now known as upper-level frontal systems.

The first atomic bomb was tested on July 16, 1945, in New Mexico, ushering in the nuclear age. Over the next decade and a half, continuously bigger bombs were tested in our atmosphere and oceans. Most of these bombs were exploded in the Earth’s stratosphere under the assumption that the air never mixed downward into the troposphere, where we all live.

During this time, meteorologists at MIT began to find evidence poking holes in this assumption. Professor Richard Reed was discovering that upper-level fronts were a possible pathway by which stratospheric air could mix into the troposphere.

His ideas were initially met with derision. However, evidence for the ubiquity of these upper fronts and the efficiency of the stratospheric/tropospheric mixing that they encouraged grew in proportion to the strength of the bombs that were being tested.

Eventually, the evidence was accepted by the meteorological community at large and this new scientific insight was employed in the shaping of important public policy.

In a June 1963 address at American University, President John F. Kennedy announced a new round of high-level arms negotiations with the Russians. This speech was considered so important by Premier Nikita Khrushchev that the Soviet press allowed it to be printed in its entirety.

On July 25, 1963, after only 12 days of negotiations, the United States and the Soviet Union agreed to the Nuclear Test Ban Treaty ending atmospheric tests of nuclear devices; it was signed 11 days later.

Category: Meteorology

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Is there any trace of winter left in the Northern Hemisphere?

Posted on July 14, 2014 by WeatherGuys Editor

We have made reference a few times in this column to the areal extent of cold air over the Northern Hemisphere as a measure of wintertime severity, that is, the geographic reach of air of a certain temperature.

Specifically, we have reported on the 23-degree air at 1 mile above the ground (where the atmospheric pressure is just 85 percent of its near-surface value). By mid-July it is impossible to find air that cold at that elevation in the Northern Hemisphere.

In other words, the cold air that in mid-winter covers over 26.2 million square miles disappears entirely in midsummer.

This is vivid testimony to the power of the increased sun angle and the longer days that characterize our summer versus the winter.

As we move past the summer solstice toward the autumnal equinox, both the sun angle and the length of the day begin to decline, with the length of day shortening dramatically faster at high latitudes than it does here in Madison.

The shorter day means, of course, a longer night, which allows more time for the ground to radiate energy away to space. This energy loss eventually and inevitably leads to the production of cold air near the surface of the planet.

As we head further into the late summer and early fall, high-latitude cold air production continues to benefit from the short day and low sun angle. The cold air builds up to the point where it is exported regularly from high latitudes to lower latitudes and the areal coverage of the 23-degree air grows, on average, at a uniform rate from Sept. 1 to Dec. 1. It reaches its peak near late January and then shrinks uniformly from around April 1 to late June.

Our longest day of the year has already come and gone. In the next week or two we will begin to see the inevitable return of cold air to the Northern Hemisphere, a harbinger of winter.

Category: Meteorology, Seasons

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What is the difference between a ‘warning’ and a ‘watch’?

Posted on July 7, 2014 by WeatherGuys Editor

The difference is that a weather watch indicates that hazardous weather may occur, while a warning is issued when hazardous weather is occurring, is about to occur or has a very high probability of occurring.

A warning indicates that conditions pose a threat to life or property, and people in the area of the warning should take action to protect themselves. A watch is intended to provide people with enough time to set safety plans in motion for possible hazardous weather.

Watches and warnings outline areas where the weather may occur. Pinpointing the location of hazardous weather in advance is extremely difficult. For this reason, watches are usually issued for large regions, sometimes covering several states. Warnings are issued for much smaller areas, often only a county or two, because they are based on actual observations of hazardous weather.

The National Weather Service issues weather watches and warnings under specific weather conditions.

A severe thunderstorm watch means that conditions are favorable for the development of severe thunderstorms in and close to the watch area.

A warning means that a severe thunderstorm has been sighted visually or indicated by radar, and that the thunderstorm is producing hail at least 3/4 of an inch in diameter and/or has winds equal to or exceeding 58 mph.

Then there are weather advisories, which may be issued when actual or expected weather conditions are not hazardous but may cause inconvenience or concern.

Category: Severe Weather, Weather Dangers

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