Where is less dense air found in a hurricane

Hurricanes are the most awesome, violent storms on Earth. People call these storms by other names, such as typhoons or cyclones, depending on where they occur. The scientific term for all these storms is tropical cyclone.

Only tropical cyclones that form over the Atlantic Ocean or eastern Pacific Ocean are called "hurricanes. Tropical cyclones are like giant engines that use warm, moist air as fuel. That is why they form only over warm ocean waters near the equator. The warm, moist air over the ocean rises upward from near the surface. Because this air moves up and away from the surface, there is less air left near the surface. Another way to say the same thing is that the warm air rises, causing an area of lower air pressure below.

Air from surrounding areas with higher air pressure pushes in to the low pressure area. Then that "new" air becomes warm and moist and rises, too. As the warm air continues to rise, the surrounding air swirls in to take its place. As the warmed, moist air rises and cools off, the water in the air forms clouds. Some progress has been made in predicting the number and intensity of storms for the Atlantic Ocean by Dr.

William Gray of Colorado State University. He has shown that there is a correlation between the frequency of intense Atlantic hurricanes with the amount of rainfall in western Africa in the preceding year. This correlation has allowed fairly accurate forecasts of the number of storms of a given intensity that will form each year.

Nevertheless, Dr. Gray's predictions are closely watched, and have been otherwise fairly accurate. Reducing Hurricane Damage There is plenty of historical data on hurricane damage in the United States so that it is not difficult to see ways that damage from hurricanes can be reduced.

In terms of protection of human life, the best possible solution is to evacuate areas before a hurricane and its associated storm surge reaches coastal areas.

Other measures can be undertaken to reduce hurricane damage as well. The problem, however, is that it may not always be possible to issue such a warning in time for adequate evacuation of these areas. Because the storm surge and even gale force winds can reach an area many hours before the center of the storm, warnings must be issued long enough before the storm strikes that the surge and winds do not hinder the evacuation process.

The effectiveness of the warning systems also depends on the populace to heed the warning and evacuate the area rather than ride out the storm, and the state of preparedness of local government agencies in terms of evacuation and disaster planning. New Orleans is a particularly notable example. Since most of the city is at or below sea level, a storm surge of 6 meters 20 feet from a category 4 or 5 hurricane would most certainly flood the city and choke all evacuation routes.

Even with 24 hours notice of the approaching surge which would mean as soon as the storm entered the Gulf of Mexico it would be difficult to evacuate or convince people to evacuate within that 24 hour period.

A hurricane approaching New Orleans was a disaster waiting to happen as we can all testify. Hurricane Donna in shows the effects of the land decreasing the intensity of a hurricane. Donna hit the southern tip of Florida as a category 4 hurricane.

It then took a northeastward track across Florida, loosing strength as it crossed the land. On re-entering the Atlantic Ocean it again increased in intensity due to the warm ocean waters, took a track along the east coast and eventually hit Long Island, New York. Tropical Cyclones Hurricanes Fall Atmospheric Circulation The troposphere undergoes circulation because of convection.

If the Earth were not rotating, this would result in a convection cell, with warm moist air rising at the equator, spreading toward the poles along the top of the troposphere, cooling as it moves poleward, then descending at the poles, as shown in the diagram above.

Once back at the surface of the Earth, the dry cold air would circulate back toward the equator to become warmed once again.

The Coriolis Effect - Again, the diagram above would only apply to a non-rotating Earth. Since the Earth is in fact rotating, atmospheric circulation patterns are much more complex. The reason for this is the Coriolis Effect. The Coriolis Effect causes any body that moves on a rotating planet to turn to the right clockwise in the northern hemisphere and to the left counterclockwise in the southern hemisphere.

The effect is negligible at the equator and increases both north and south toward the poles. Low Pressure Centers - In zones where air ascends, the air is less dense than its surroundings and this creates a center of low atmospheric pressure, or low pressure center. Winds blow from areas of high pressure to areas of low pressure, and so the surface winds would tend to blow toward a low pressure center.

But, because of the Coriolis Effect, these winds are deflected. In the northern hemisphere they are deflected to toward the right, and fail to arrive at the low pressure center, but instead circulate around it in a counter clockwise fashion as shown here. In the southern hemisphere the circulation around a low pressure center would be clockwise. Such winds are called cyclonic winds. High Pressure Centers - In zones where air descends back to the surface, the air is more dense than its surroundings and this creates a center of high atmospheric pressure.

Since winds blow from areas of high pressure to areas of low pressure, winds spiral outward away from the high pressure. But, because of the Coriolis Effect, such winds, again will be deflected toward the right in the northern hemisphere and create a general clockwise rotation around the high pressure center. In the southern hemisphere the effect is just the opposite, and winds circulate in a counterclockwise rotation about the high pressure center. Such winds circulating around a high pressure center are called anticyclonic winds.

Because of the Coriolis Effect, the pattern of atmospheric circulation is broken into belts as shown here. The rising moist air at the equator creates a series of low pressure zones along the equator. Water vapor in the moist air rising at the equator condenses as it rises and cools causing clouds to form and rain to fall.

After this air has lost its moisture, it spreads to the north and south, continuing to cool, where it then descends at the mid-latitudes about 30 o North and South. Descending air creates zones of high pressure, known as subtropical high pressure areas. Because of the rotating Earth, these descending zones of high pressure veer in a clockwise direction in the northern hemisphere, creating winds that circulate clockwise about the high pressure areas, and giving rise to winds, called the trade winds , that blow from the northeast back towards the equator.

In the southern hemisphere the air circulating around a high pressure center is veered toward the left, causing circulation in a counterclockwise direction, and giving rise to the southeast trade winds blowing toward the equator. Near the equator, where the trade winds converge, is the Intertropical Convergence Zone ITCZ Air circulating north and south of the subtropical high pressure zones generally blows in a westerly direction in both hemispheres, giving rise to the prevailing westerly winds.

These westerly moving air masses again become heated and start to rise creating belts of subpolar lows. Meeting of the air mass circulating down from the poles and up from the subtropical highs creates a polar front which gives rise to storms where the two air masses meet. In general, the surface along which a cold air mass meets a warm air mass is called a front. The position of the polar fronts continually shifts slightly north and south, bringing different weather patterns across the land.

In the northern hemisphere, the polar fronts shift southward to bring winter storms to much of the U. In the summer months, the polar fronts shift northward, and warmer subtropical air circulates farther north. The convection cells circulating upward from the equator and then back to surface at the mid-latitudes are called Hadley cells.

Circulation upward at high latitudes with descending air at the poles are called Polar cells. In between are cells referred to as Ferrel cells. At high altitudes in the atmosphere narrow bands of high velocity winds flowing from west to east are called the jet streams. The polar jet occurs above the rising air between the Polar cells and the Ferrel cells. The subtropical jet occurs above the descending air between the Ferrel cells and the Hadley cells.

These jet streams meander above the earth's surface in narrow belts. In the northern hemisphere, where the jet streams meanders to the south it brings low pressure centers and associated storms further to the south. Where it meanders to the north, the high pressure centers move to the north.

Water and Heat Water has one of the highest heat capacities of all known substances. Further energy is absorbed by water vapor as the latent heat of vaporization, which is the heat required to evaporate water or change it from a liquid to a vapor. This latent heat of vaporization is given up to the atmosphere when water condenses to form liquid water as rain.

If the rain changes to a solid in the form of snow or ice, it also releases a quantity of heat known as the latent heat of fusion. Due to general atmospheric circulation patterns, air masses containing differing amounts of heat and moisture move into and across North America. Polar air masses, containing little moisture and low temperatures move downward from the poles.

Air masses that form over water are generally moist, and those that form over the tropical oceans are both moist and warm. Because of the Coriolis effect due to the Earth's rotation, air masses generally move across North America from west to east. But, because of the differences in moisture and heat, the collision of these air masses can cause instability in the atmosphere. This rising moist air cools as it rises causing the condensation of water vapor to form rain or snow.

Note that the cold air masses tend to circulate around a low pressure center in a counterclockwise fashion in the northern hemisphere. Such circulation around a low pressure center is called a mid-latitude cyclone.

When warm air moving northward meets the cooler air to the north, a warm front forms. As the warm air rises along a gently inclined warm front, clouds tend to form, and can also cause rain, but rain is less likely because the warm front is not as steep as a cold front.

If the rapidly moving cold front overtakes the warm front, an occluded front forms, trapping warm air above a layer of cold and cool air.

Mid-latitude cyclones and their associated fronts are responsible for such severe weather conditions as thunderstorms, snow storms and associated hail, lightening, and occasional tornadoes. Because the converging winds spiral inward toward the central low pressure area, the winds rotate in a counterclockwise direction around the central low in the northern hemisphere clockwise in the southern hemisphere.

As these winds spiral inward they draw in the thunderclouds around the storm, creating the spiral rain bands that are clearly visible on satellite images of the storm recall satellite images of hurricanes that are seen frequently on TV during hurricane season.

As the winds converge toward the central core, they spiral upwards, sending warm moist air upwards. As this air rises, it cools and releases its latent heat into the atmosphere to add further energy to the storm. The winds spiraling around this central core create the eye of the tropical cyclone and eventually spread out at high altitudes.

Eventually, cool air above the eye begins to sink into the central core. This dry descending air within the eye gives the core a clear, cloud free sky, with little to no wind. Hurricane Intensity and Frequency Once a hurricane develops, the Saffir-Simpson Scale is used to classify a hurricane's intensity and damage potential. There are five possible categories. Category 1 storms are more common than category 5 storms.

In a typical year, there may be many category 1 storms, but category 5 storms occur very infrequently. Central Pressure mb inches of mercury. Again, because a hurricane derives its energy from the warm ocean waters in the topics and subtropics, hurricanes are more frequent in the late summer months. Thus, as seen in the graph, hurricanes in the Atlantic ocean are more frequent in the months of August, September and October.

The peak occurs on September A high pressure system has higher pressure at its center than the areas around it. Winds blow away from high pressure. Swirling in the opposite direction from a low pressure system, the winds of a high pressure system rotate clockwise north of the equator and counterclockwise south of the equator.

This is called anticyclonic flow. Air from higher in the atmosphere sinks down to fill the space left as air is blown outward. On a weather map, you may notice a blue H, denoting the location of a high pressure system. How do we know what the pressure is? How do we know how it changes over time? Today, electronic sensors in weather stations measure air pressure. These sensors are able to make continuous measurements of pressure over time. In the past, barometers were used and measured how much air pushed on a fluid, such as mercury.

When you inflate a balloon, the air molecules inside the balloon get packed more closely together than air molecules outside the balloon. This means the density of air is high inside the balloon. When the density of air is high, the air pressure is high.