Case is a fit, balding man in his mid-fifties. A satellite photo of a hurricane about to clobber the coast of Maryland hangs in his office. He is responsible for issuing regional forecasts based on satellite imagery and a nationwide system called the Limited Fine Mesh, a grid superimposed on a map of the country where the corners represent data-collection points. Twice a day hundreds of LFM weather balloons are released to measure temperature, dewpoint, barometric pressure, and windspeed, and relay the information back by theodolite. The balloons rise to 60,000 feet and then burst, allowing the instruments to float back to earth on parachutes. When people find them, they mail them back to the Weather Service. The data from the LFM, plus input from a thousand or so other ground sites around the country, is fed into huge Cray computers at the National Meteorological Center in Camp Springs, Maryland. The computers run numerical models of the atmosphere and then spit forecasts back out to regional offices, where they are amended by local meteorologists. Humans still "add value" to a forecast, as meteorologists say. There is an intuitive element to forecasting that even the most powerful computers cannot duplicate.
Since early the previous day, Case has been watching something called a "short-wave trough aloft" slide eastward from the Great Lakes. On satellite photos it looks like an S-curve in the line of clear dry air moving south from Canada. Cold air is denser than warm air, and huge, slow undulations develop along the boundary between them and roll eastward—on their side, as it were—much like an ocean swell. The undulation gets more and more pronounced until the "crest" gets separated from the rest of the warm front and just starts to spin around itself. This is called a cutoff low, or an occluded front. Air gets sucked in toward the center, the system spins faster and faster, and within hours you have a storm.
The mechanics of a hurricane are fundamentally the same as a cutoff low, but their origins differ: hurricanes brew in the lukewarm waters around the equator. When the sun hits the equator it hits it dead-on, a square-foot beam of light heating up exactly one square foot of water. The farther north or south you are, the lower the angle of the sun and the more water a square foot of sunlight must heat up; as a result the water doesn't heat up as much. The equatorial sea cooks all summer and evaporates huge amounts of water into the air. Evaporated water is unstable and contains energy in the same way that a boulder on top of a hill does—one small push unleashes a huge destructive force. Likewise, a drop in air temperature causes water vapor to precipitate out as rain and release its latent energy back into the atmosphere. The air above one square foot of equatorial water contains enough latent energy to drive a car two miles. A single thunderstorm could supply four days' worth of the electrical power needed by the United States.
Warm air is less dense than cool air; it rises off the surface of the ocean, cools in the upper atmosphere, and then dumps its moisture before rushing back to earth. Huge cumulus clouds develop over the zones of rising air, with thunder, lightning, and terrifically strong rain. As long as there's a supply of warm water, the thunderstorm sustains itself, converting moisture into sheeting rain and downdraft winds. Other thunderclouds might line up along the leading edge of a cold front into a "squall line," a towering convective engine that stretches from horizon to horizon.
Hurricanes start when a slight kink—a disturbance in the trade winds, a dust storm blowing out to sea off the Sahara— develops in the upper-level air. The squall line starts to rotate around the kink, drawing in warm, volatile air and sending it up the gathering vortex at its center. The more air that gets drawn in, the faster it spins, and the more water is evaporated off the ocean. The water vapor rises up the core of the system and releases rain and latent heat. Eventually the system starts spinning so fast that inward-spiralling air can no longer overcome the centrifugal force and make it into the center. The eye of the storm has formed, a column of dry air surrounded by a solid wall of wind. Tropical birds get trapped inside and cannot escape. A week later, after the system has fallen apart, frigate birds and egrets might find themselves over Newfoundland, say, or New Jersey.