Was the Forecast of Irene a Success?

The emergency response and warnings prompted by the approach of Hurricane Irene along the East Coast last week has, sadly but predictably, become a subject of much controversy.

Much of the furor surrounds whether or not the decisions made by the mayors of New York City or Ocean City, Mary., were too extreme. New York’s Mayor, Michael Bloomberg, ordered the evacuation of lower Manhattan in the face of a threatened storm surge of eight feet that would have inundated the area.

Meanwhile, the mayor of Ocean City, a popular tourist destination on the eastern shore of Maryland, ordered businesses to shut down as the storm approached. Given the forecasts that were issued by the National Hurricane Center, staffed by the most knowledgeable hurricane scientists in the world, there was every reason to believe that a substantial hit from Irene was going to be delivered in these locations. In fact, the region was hit hard by the storm, though it was weaker than expected. Thus, these moves were wholly appropriate.

Luckily, Irene weakened enough before making landfall in the Northeast thus saving New York and Ocean City from the worst case scenario. The insistence that public officials overreacted to the threat posed by Irene represents a particularly dangerous type of Monday morning quarterbacking, perhaps engendering passivity in the face of future threats. Such an attitude would also suggest a distrust of science as a means to the end of protecting the lives and property of our fellow citizens.

It would be a shame if, given the outstanding progress that has been made in hurricane prediction in the last 25 years, the combination of media hype and a slightly errant forecast such as occurred with Irene might encourage such distrust and contribute to the willful disregard of future warnings.

Category: Tropical
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Why are Some Clouds Whiter than Others?

Light rays can change direction when they encounter small particles, a phenomenon called scattering.

If you smash a clear ice cube, the pile of small pieces appears brighter and whiter than the whole cube. This is because the shards scatter light more extensively than one whole cube. This kind of scattering also happens in clouds.

Clouds are made of small water drops and small ice crystals that scatter light from the sun in all directions. These individual drops and crystals are clear. It is the multitude of drops and crystals that make a cloud look white during the day because the small particles scatter the sun’s light in all directions.

The brightness of the cloud is a function of how much of the sun’s energy is leaving the part of the cloud you are viewing. If you are looking at a thin cloud, then there are few water particles to scatter the light, so the cloud will not look bright white.

The sides of a cloud often look bright white because lots of light is scattered out the sides. If the cloud is very deep, then only a small amount of light escapes out the bottom of the cloud, and so cloud bottoms often appear grayish.

If one cloud contains very large drops and another contains very small drops, the cloud containing the small particles will appear brighter. Again, much like the case of the smashed ice cube — small particles scatter the sun’s light in all directions more efficiently than do large particles.

Category: Phenomena
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When does the Overnight Low Temperature Occur on a Windless Day?

As the summer begins to fade we have begun to have the first cool nights of the season. When the sky is clear, the winds are calm, and the ground is dry as the sun sets, the conditions are perfect for an interesting phenomena to occur the next morning.

If you pay careful attention to the temperatures in the hour surrounding sunrise the next morning, you will most likely observe that the lowest temperature will be recorded some minutes after the sun rises. This repeatable observation requires explanation.

All objects both absorb and emit energy in the form of radiation. The surface of the Earth is one such object and it absorbs a lot of solar energy from sunlight during the daylight hours. We all know that this causes the surface temperature to increase. But, while the ground is absorbing energy from the sun, it is also emitting radiant energy (infra-red energy).

As long as the amount emitted is less than the amount absorbed, the temperature of the surface increases. After sunset, absorption ceases but emission continues. Consequently, since the emission is greater than the non-existent solar absorption, the surface temperature drops. This condition persists throughout the night.

When the sun finally rises and sends the first, low-intensity beams of radiation to the surface, it is still true (for a short time) that the meager amount of solar absorption is smaller than the emission from the surface.

Until the absorption is equal to the emission, the temperature continues to fall, even if only slightly.

Hence, the lowest temperature on such a morning occurs after sunrise.

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How Uncomfortable has this Summer Been?

After what seems like forever to most of us, this past week finally brought some relief from the almost uninterrupted heat and humidity of July and early August.

One way to measure the degree of discomfort of such a spell is to examine how many overnight low temperatures dropped below 60 degrees. In order for the overnight low to get below 60 degrees, the dewpoint temperature (which we have discussed as a measure of human comfort in this column before) must also get below 60 degrees.

Our sense is that most people begin to notice the humidity when the dewpoint is 60 degrees, are somewhat uncomfortable when it is 65 degrees, and are downright miserable when it is at or over 70 degrees. Perhaps not surprisingly, no one remembers June 2011 as particularly muggy, the month had 18 days on which the overnight low dropped below 60 degrees.

July, on the other hand, had only five such nights and, through Aug. 10, only one such night has occurred this month. Thus, beginning June 30, we have had only six out of the last 42 days with dewpoints low enough to drop the overnight low temperature to below 60 degrees. That is pretty darn oppressive!

During that same stretch, we have been 4.97 degrees above normal in temperature and 2.5 inches below normal in precipitation. That further testifies to the heat and the fact that it has not been broken up by many cooling thunderstorm events here in Madison.

Over the next 10 days or so it appears that we will continue to enjoy a relief from the hot July so the worst may be behind us.

Category: Seasons
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Is High Pressure Always Warm Air?

High pressure systems can be cold or warm, humid or dry. The origin of a high-pressure region determines its weather characteristics.

If a high-pressure system moves into Wisconsin from the south during the summer, the weather is usually warm and clear. If the high pressure originates from the north, it will generally bring cold or cooler weather.

When high pressures form, they adopt the characteristics of the source regions over which they form. Cold, high-pressure air masses form in polar regions, and are called polar air masses. Warm air masses are of subtropical or tropical origin; both are referred to as tropical air masses.

Air masses that form over water are referred to as maritime, whereas those generated over continents are referred to as continental. Maritime air masses are usually moister than continental air masses formed at the same latitude.

Atmospheric pressure is greater in a high-pressure system than the surrounding areas. High-pressure areas at ground level are normally caused by air above that is moving downward. This sinking air, called subsidence, is bad for cloud formation, so high pressure systems often have few clouds.

You may have also noticed that the winds are light when we are under the influence of a high pressure. The lack of wind causes pollutants to build up in the high, leading to reduced air quality. Few clouds also mean good sunshine that, along with the warm temperatures in the summer, is good for ozone formation at the ground.

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