Weathering the Monsoons

After a long hot summer, the dark clouds gather and the skies ruffles and then comes the rain. The monsoons of India are one of the world’s premier weather phenomenons. Every schoolchild is taught that the monsoons are caused by the change in wind patterns when a large body of warm air rises.  Cool, moist air comes flooding in over the oceans, the weather gets unstable, triggering spectacular thunderstorms and torrential rain.

Monsoons however, are not just an Indian phenomenon. The monsoon season happens in countless other places, notably in southeast Asia, Australia, some parts of Africa and many location in North and South America. The technical definition of a monsoon is a seasonal reversal in the prevailing wind direction. For example in a weather system where the wind blows in from the ocean for six months and then reverses direction for the next six months there are two monsoon seasons—the approaching monsoon when the wind comes from the ocean and the retreating monsoon when the wind comes from the interior. In Northern India, the monsoon wind blows in from the southwest carrying copious amounts of moisture from the Bay of Bengal, during the period mid-June to October. In November (till February), dry air from the interior travels to the ocean, causing the dry cool weather of the retreating monsoon. In the southeastern United States, the same phenomenon is noted, specially over Florida, where it is called the “Hurricane Season”.

A spectacular part of the southwestern United States is the high rugged desert straddling Arizona and New Mexico. The monsoon phenomenon is particularly active here. Most of the year the wind direction is from the interior and hence the weather is predominantly dry and clear. In summer the temperatures soar dramatically, days with temperatures of 110F (43C) are common. Then comes the monsoons—moisture laden air get blown in from the Gulf of Mexico, starting about July.

In Arizona, the bark of the monsoons is far worse than the bite. Every day the weather forecasters whine about possible thunderstorms rolling into the valleys. Occasionally they are right. Spectacular lightning follows accumulating dark clouds on a backdrop of magenta and purple hues caused by the setting sun. As winds pick up into mini-tornado-like formations, dust storms blanket the city, making visibility nonexistent. Cars crash into one another; roofs fly off houses and the temperature drops. And then, suddenly its over as quick as it started. Once in a while there is rain. Sometimes, the rain is quite heavy and the roads turn into torrential rivers.

Weather is an integral part of human life. Bad weather not only can cause untold misery, normal weather is also almost the staple of conversations. It is now well known that continuous “gloomy” weather causes high incidences of depression in humans. Populations of the warmer belts tend to be a bit “laid-back” while populations from harsh colder climates tend to be more aggressive.

In many parts of the US the weather is often quite unstable. A very warm day can be followed by a very cold day. Snowstorms can form overnight and wipe out transportation and communication. Pouring rain can suddenly appear. Prior knowledge of future weather conditions saves money, time, property, lives and commerce.

Weather forecasting is a comedy of errors. We all know of countless occurrences of wrong forecasts. Since older times, people of all types, from scientists to snake charmers have tried to forecast weather. Until the twentieth century they were all about equally correct.

The modern time brought two important tools that has made weather forecasting almost possible. These are computers and communications. Every day, at noon and midnight GMT (Greenwich mean time, also called Zulu time) weather balloons are launched from hundreds of points on the earth. These balloons rise to about 100,000 feet and provide a detailed snapshot of temperatures and pressures and wind speeds from ground level to the upper atmosphere. All the data collected from the balloons are then instantly transmitted via computer networks to many weather centers around the world. Thousands of weather sensors are scattered on the ground mainly at airports around the world and this data is collected. Finally, tens of geo-stationary satellites take high-resolution pictures of the Earth.

Every day, this huge amount of data is fed into computer models, running programs called numerical weather prediction algorithms. The programs create a model of the entire atmosphere in the computer’s memory and then decides how the temperature, humidity and wind speed at each of the points over the earth is going to change in, say one hour, given the surroundings. As we work the numbers over and over again, the future conditions emerge. As we go further into the future, the accuracy decreases, as many errors creep in.

Today, in the United States, weather forecasters regularly issue 10-day forecasts. More often than not, the forecasts are quite good. Of course, there are instances of unforecastable weather, for example, the monsoons of Arizona, the snowstorms in Colorado and the rains on the island of Hawaii.

The crowning glory of weather forecasting is the story of Hurricane Andrew. In August 14^th 1992, observers noted a “tropical wave” off of the coast of western Africa. By the 16^th the wave had transformed into a “depression” and the next day the depression had taken the shape of a swirling storm and then the folks at the Tropical Prediction Center in Miami, Florida gave it a name. Tropical storm “Andrew” was just born. Four days later, Andrew has crossed the Atlantic and had gathered speed and vigor and had to be reclassified. Tropical storm Andrew had grown up, and was now Hurricane Andrew. The job of keeping track of Andrew now moved to the good folks of the National Hurricane Center (NHC), also in Miami.

As Andrew flew over the islands of Bahamas, it became a strong hurricane. On August 22^nd the alarms went off in Florida. The NHC predicted Andrew would come crashing in with winds blowing at 125 mils/hour in about two days. The coastal communities braced, people were evacuated, air traffic was diverted, ships moved off of the harbors. A whole machinery of disaster prevention and evacuation swung into action.

Satellites and the hurricane hunters followed Andrew heavily. Hurricane hunters are daring people in old aircraft, who fly into the center of hurricanes providing valuable data about the conditions “at the eye” along with fascinating pictures. The NHC people kept dishing out charts and predictions of path and voracity of Andrew.

Andrew showed up as expected, though uninvited, crashing the party near the southern suburbs of Miami at 05:00 on 22^nd of August, 1992, showing off his 145 miles/hour winds that gusted to an unimaginable 175 miles/hour. Andrew left a track of devastation and then, as Andrew’s fury weakened (hurricanes loose power as they travel over land) the forecasters sent out another ominous warning. “It is not over yet” they said.

Instead of going north, as most hurricanes do, Andrew veered west. He crossed Florida and went out into the ocean. Here he picked up steam and rode over the Gulf of Mexico and came back to land in Louisiana for a double-whammy. Andrew did $25 billion in property damage, making it the costliest hurricane, ever.

Human deaths totaled 16 in Florida and 8 in Louisiana. The low number of deaths is a testament to a combination of good weather forecasting, good communications and good planning of the evacuation machinery. Hurricane names are reused, every six years, but Hurricane Andrew has joined 24 others as a name with a legacy, a name that will never again be used for a hurricane.

Partha Dasgupta is on the faculty of the Computer Science and Engineering Department at Arizona State University in Tempe. His specializations are in the areas of Operating Systems, Cryptography and Networking. His homepage is at http://cactus.eas.asu.edu/partha.