If you want rain, you’ll need at least two things. One is moisture in the air — and the other is some way to lift the air so the moisture cools and condenses into precipitation.
We know how clouds form — lots of moist, rising air, and condensation nuclei for the moisture to condense around. So which ingredient is usually missing when there’s a drought? The condensation nuclei? The moist air? Something else?
You do have to have moist air for there to be rain — and you have to have a way to lift that air to a high enough altitude for water vapor to condense out and fall as rain. So maybe you can see that a change in the circulation of the atmosphere can be a factor in causing a drought. A change in large-scale air currents might cut off the normal flow of moisture to an area for a time. Or a strong mass of high pressure air might settle over an area. Since high pressure air sinks, the sinking air warms, dries out and doesn’t produce rain.
Scientists think that some of the most persistent localized droughts in recent years are linked to air pollution. Many of the clouds that form in polluted areas are made of abnormally small droplets – often too small to produce rain. “Drought” is defined as an unusually long and dry period. If you lived in south Florida, you’d consider a year with 75 centimeters — that’s about 30 inches — of rain a serious drought.
The world’s largest deserts are located around thirty degrees north and south of the equator. That’s because the Earth’s air circulation pattern keeps this area under constant high pressure air.