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Sept-Oct 2009

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Weather Queries

Why do hurricanes favor some areas but not others? For example, Florida and Louisiana seem prone to hurricanes, but California does not.

Jeremy DiBattista McDonald

 I’ve reproduced a figure from the National Hurricane Center (NHC) Web site that shows how many hurricanes have struck coastal counties along the U.S. Atlantic seaboard and the Gulf Coast since 1900 (Figure 1). The numbers are color-coded, and it’s important to keep in mind that large counties are likely to see more hurricanes than small counties, all other things being equal. The map shows that southern Florida is the most vulnerable to hurricane landfall, followed by the Gulf Coast from Texas to the Florida Panhandle and the Carolinas.

The shape of the coastline influences the number of landfalls. Southern Florida stands in the way of many hurricanes passing from the Caribbean Sea into the Gulf of Mexico. Once in the Gulf of Mexico, provided the hurricane doesn’t dissipate over water or strike Mexico, it can hardly avoid striking one of the Gulf Coast states. Recurving hurricanes along the Atlantic seaboard often strike the Carolinas because their coastlines jut northeastward. Hurricanes that pass just to the east of Cape Hatteras, North Carolina, often bypass the mid-Atlantic Coast and New England.

Figure 2, also from NHC, shows the tracks of all North Atlantic hurricanes since the 1800s and of eastern North Pacific hurricanes since 1949. At least somewhere along each storm track, a hurricane existed for a time. Despite the tangle of tracks, you should readily discern the general westward progression of hurricanes at low latitudes, as well as recurvature, generally toward the northeast. A few eastern Pacific hurricanes head north, but all of them weaken or die before reaching the California coastline or the southwest United States. Only 1 tropical storm has ever hit the California Coast, near Long Beach in September 1939, with 50-mph winds.

Hurricanes will neither form nor strengthen unless the sea surface temperature is about 80°F (27°C) or higher. They derive their energy from the heat and moisture passing from the ocean to the atmosphere. Figures 3 and 4 are from Scott Woodruff at NOAA’s Earth System Research Laboratory. They show the long-term mean sea surface temperatures (in °C) in the Gulf of Mexico, along the East Coast, and along the western Mexican Coast and California for the month of September, when the water is warmest. Note that surface waters are warm enough for hurricane formation throughout the Gulf of Mexico and along the Gulf Stream, which courses up the East Coast from Florida to near Cape Hatteras before heading out into the Atlantic. By contrast, sea surface temperatures are much lower along the West Coast from central Baja northward. Upwelling brings nutrient-rich, chilly water to the surface from below—good for sea life but death to hurricanes. Some hurricanes survive passage over cooler ocean waters, but mainly because steering winds are pushing them along quickly before they weaken to a tropical storm or make the transition to a mid-latitude low-pressure system.

Finally, note that warm surface water is a necessary condition for hurricane formation, although alone it is not enough. Another necessary condition is that winds throughout the troposphere (where clouds and weather occur) should be light. Strong winds in the upper troposphere, or significant shear at any altitude (change in wind direction or speed with height) tend to destroy would-be hurricanes.

What exactly is dew, and why does it not count toward precipitation?

Chad Comer
Red Boiling Springs, Tennessee

The American Meteorological Society Glossary of Meteorology defines precipitation as all liquid or solid phase aqueous particles that originate in the atmosphere and fall to the earth’s surface. Thus, precipitation must be H2O (rain, freezing rain, sleet, snow, partially melted snow, or hail), and it must fall from the sky. Dew qualifies on one count (H2O) but not the other, in that it forms on chilled surfaces when water vapor condenses from the air.

Dew forms at night, almost always under clear and windless conditions. The ground is constantly losing energy to the sky through infrared radiation. Late in the day, as the sun sets, the input of solar energy at the ground is less than the loss of energy by infrared radiation, so the surface begins to cool. If the wind isn’t blowing, the air in contact with the ground cools as well, creating a very shallow inversion. Any breeze will destroy this inversion and mix warmer air from above with the cooler air in contact with the ground. In addition, if the sky is clear, much of the infrared radiation from the ground escapes to space. Some is absorbed by water vapor, carbon dioxide, and other gases in the atmosphere, but the rest can pass through without being absorbed. A thick cloud layer absorbs almost all the radiation coming from the ground and reradiates it back toward the ground with an intensity that depends strongly upon the temperature at cloud base. Warm (low) clouds radiate much more energy than cold (high) clouds. This explains why dew formation is quite rare on nights with thick clouds: the surface cooling is hindered.

The dewpoint is the temperature to which air (or any surface exposed to air) must be cooled (at constant pressure) in order for water vapor to condense. The dewpoint is part of most surface weather reports. Because clear, calm nights result in maximal surface cooling, the only requirement is that the temperature on the grass, the roof of your automobile, or any other surface should reach the dewpoint temperature. If and when that happens, dew begins to form. You might have noticed that dew doesn’t form under trees. That is because grass beneath a tree cannot “see” the sky. The infrared radiation from the grass is captured by the tree, which is at nearly the same temperature, and sent right back down to the grass. For this reason, the grass temperature under a tree won’t drop nearly as much as it will in an open field on a clear, calm night.

I took the accompanying photo early in the morning following a wet spell, so the humidity was already high even on the evening before. The sky had cleared very early in the morning, and there was no wind. The photo shows fairly large drops of water, many of them on the tips of blades of grass. If you look closely, you can also see tiny drops, probably 50 times smaller, coating some of the blades of grass. The tiny drops come from dew. The larger drops are pushed out through the blades of grass from the roots when the ground is moist. This is called guttation. Neither dew nor guttation comes from the sky, so neither is precipitation, but it is useful to be able to tell the two apart.          

Weatherwise Contributing Editor THOMAS SCHLATTER is a retired meteorologist and volunteer at NOAA’s Earth System Research Laboratory in Boulder, Colorado. Submit queries to the author attn: Weatherwise; Taylor & Francis LLC; 325 Chestnut Street, Suite 800; Philadelphia, Pennsylvania, 19106; or by email to

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