Why do hurricanes form over the tropical oceans?

Posted on August 13, 2013 by Weather Guys Editor

The organized storms we experience here in Madison in fall and winter are known as mid-latitude cyclones. One of the most notable characteristics of these storms is the presence of strong temperature, humidity and wind contrasts at what are known as fronts. In fact, fronts are such an integral part of the structure of these storms that they are often referred to as frontal cyclones.

Unlike the mid-latitude cyclones, the air temperature is fairly uniform throughout a hurricane. This is the case even though the air sinks and is warmed by compression on the outer edge of the hurricane, while near its eyewall the air rises vigorously and cools by expansion.

Even though some of that cooling is counteracted by the enormous amount of latent heat release that occurs in the eyewall convection as water vapor is condensed into liquid water, one might still reasonably expect the eyewall region to be cooler than the periphery of the storm. This is not the case because the low-level air that is directed toward the center of the storm is greatly modified during that journey — making constant contact with a wildly windswept and very warm ocean surface.

As a consequence of the warm sea-surface temperature (SST), the inflow air is both warmed and moistened as it flows toward the eyewall — evaporation from the ocean being enhanced by the strong winds and disturbed ocean surface. For this reason, very warm, very moist air is the optimal fuel for the sustenance of a hurricane. This explains why warm, tropical ocean basins are where hurricanes form.

Category: Tropical

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What’s happening to Arctic sea ice this summer?

Posted on August 5, 2013 by Weather Guys Editor

During our mini heat wave of July 16-19, the Northern Hemisphere reached its warmest day of the year, by one measure, on July 17.

On that day, at about 1 mile above sea level, it was warmer than 23 degrees everywhere in the hemisphere. Just a day or two later, the buildup of cold air at that elevation in the atmosphere began again for the coming winter season.

Nearly coinciding with this peak of summer was the rather frightening development of a meltwater lake at the North Pole. The lake started forming about July 13 as a result of two weeks of unusually warm weather near the pole.

In fact, temperatures were 2 to 5 degrees warmer than average in early July over much of the Arctic Ocean.

Such meltwater ponds develop more easily on thin, young ice, which is now the general condition over much of the Arctic. More solar radiation energy is absorbed by a puddle than by ice or snow, so the more ponds that develop, the faster ice will melt in the Arctic.

In addition to the warm weather, an unusually strong cyclone just visited the North Pole region, bringing strong winds that further help to break up the thin ice. That storm was connected to the cold air that chilled us on the weekend of July 27-28.

The interaction of the warmth, which comes in some degree every July to the high Arctic, and the strong winds associated with this storm can leave the sea ice in a weakened state at the end of this summer.

That will mean that the coming winter freeze will have a harder time creating thick sea ice and the next summer will further erode it in similar fashion.

In this way, warming at very high latitudes is enhanced as the planet warms in the face of increased greenhouse gases.

Category: Climate

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Did Wisconsin just have a heat wave?

Posted on July 29, 2013 by Weather Guys Editor

Wisconsin experienced a small heat wave July 16-19, when heat indices around the state reached above 100 F.

The heat index, or apparent temperature index, indicates how hot it feels. When our bodies get hot, we cool down by sweating. It is not the sweating that cools our bodies; it is the evaporation of the sweat. If the air has a high humidity, then the rate of evaporation is reduced. This hampers the body’s ability to maintain a nearly constant internal body temperature. When the temperature is high and the relative humidity is high, the heat index is high and it seems hotter than it really is. In these cases, the heat index is greater than the actual temperature.

High heat indices can pose a threat to our health. When life and property are threatened, the National Weather Service, or NWS, will issue weather advisories, watches or warnings. A watch implies you should be aware a weather hazard may develop in your area. A warning is issued when the hazard is developing in your area, and you should take immediate action . An advisory is a less urgent statement to bring to the public’s attention a situation that may cause some inconvenience or difficulty for travelers or people who have to be outdoors.

The NWS issued a heat advisory during this recent hot weather event. When under a heat advisory, you are advised to limit vigorous outdoor activity and drink plenty of fluids.

Category: Weather Dangers

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What is a monsoon?

Posted on July 22, 2013 by Weather Guys Editor

A monsoon is a seasonal reversal in wind patterns over a region. The word “monsoon” comes from the Arabic word mausim, meaning “season.” The seasonal wind shift is usually accompanied by a dramatic change in precipitation.

The best known example of a monsoon occurs over the Indian subcontinent. In summer, the sun warms the land and the air above it. With cooler air lying over the oceans that surround the subcontinent, a horizontal pressure gradient is established that generates winds directed from the ocean to land.

The air flowing over the water remains over the ocean a long time, which causes it to gain moisture. This wind brings humid air inland. The solar heating over land triggers convection that produces rain. Orographic lifting of air by the Himalaya Mountains generates more rain.

The summer monsoon is a wet season over land. These rains can cause major flooding, but they are vital to agriculture and the economy.

During the winter season, the air above the land cools faster than the air over the water, establishing a pressure gradient force from land to sea. The winds are therefore reversed from the summer monsoon flow — from land to sea instead of from sea to land. Sinking air above the land suppresses cloud development and precipitation. The winter monsoon is a dry season.

Other parts of the world also have monsoons. The North American monsoon, also called the Arizona monsoon, occurs over the southwestern United States and northwestern Mexico. It results in a rainy season that begins in July and typically lasts until mid-September, and then the drier weather regime gets re-established.

Category: Meteorology

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What is heat lightning?

Posted on July 15, 2013 by Weather Guys Editor

In heat lightning you see the flash of light from the lightning, but you don’t hear the thunder the lightning produces. All lightning produces thunder when the bolt heats the air around it. This rapid heating causes the air to quickly expand and generate a sound wave known as thunder, which travels through the atmosphere. The temperature and wind structure of the atmosphere can lead to an acoustic shadow zone where you can see the source of the sound (the lightning bolt) but not hear the noise (the thunder). Topographical obstructions can also lead to an acoustic shadow.

Sound can interact with objects in multiple ways. As sound waves move through the atmosphere, they are absorbed by molecules in the air. The waves can bend around objects, such as trees and leaves, or be absorbed by those objects. This will muffle the sound. Rapid changes in wind speed or wind direction can influence the direction of a sound wave, which can also affect whether we hear it.

Sound waves travel at a speed of about 12 miles per minute. The speed is a function of the temperature of the air. In general, sound travels faster in warm air and more slowly in cold air. Since the temperature of the atmosphere changes from point to point, so does the speed of sound.

As sound waves travel through the open air, they can change the direction in which they are traveling. In the lower atmosphere, the temperature of the air generally is cooler with increasing height above the ground. Cooler air over warmer air causes the sound wave to bend upward away from the ground. In this condition, you may see the lightning but not hear the thunder.

Category: Phenomena

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