We had an unusually wet April – 28.2 inches. The 16-year average for our area in April is 6.7 inches.
We have two seasons – rainy and dry. Where we live, the rainy season falls in months with more than 10 inches of rain and dry season in months with fewer than 10 inches. Therefore we consider May through November our rainy season (invierno or winter) and December through April our dry season (verano or summer).
This year our rainy season began in April. The blue line is the average, the red line is 2010.
The reason for the seasonal pattern is found in global atmospheric circulation. Because the Earth receives more heat from the sun at the equator than elsewhere, the warm air at the equator rises, leaving a low pressure zone behind. That air, having risen, moves away from the equator toward either pole, cools, and sinks back down to the Earth at the Horse Latitudes, creating a high pressure region. The surface air at the Horse Latitudes rushes back down toward the equator, creating the Trade Winds. [image from wikimedia commons]
The Earth is tilted, of course, and in its annual travel around the sun, regions north, then south of the equator receive the maximum heat from the sun. Therefore this low pressure zone, called the Intertropical Convergence Zone (because air masses from the northern and southern hemispheres converge there), moves up and down the latitudes of the earth. [image from wikimedia commons]
The nautical term for the Intertropical Convergence Zone (ITCZ) is The Doldrums. Sailing ships, having enjoyed the trade winds on their journey from, say, Europe, to the Caribbean, suddenly had no wind (that is, no horizontal movement of air – all the air in this region moves vertically upward) and plenty of rain.
The rain falls because the warm air, loaded with moisture, is unable to hold onto that moisture when it reaches the cooler upper atmosphere and therefore the moisture falls back to earth.
These days, the ITCZ is regularly tracked by NASA. The chart below is the Caribbean Sea graphic posted a few hours ago. The ITCZ appears as a red hatched line, crossing Costa Rica, just west and north of Panama. Earlier today, the red hatch crossed Panama itself. As I write, it is raining at the rate of 1.34 inches per hour.
So, we’re in the Doldrums, and forecasters say we’ll be there for at least the rest of the month. Of course, we should be there from sometime in May through sometime in November, so perhaps our rainfall will trend toward normal this month. Our rain right now, of course, is nowhere near the catastrophe now playing out due to the flooding in Tennessee and Kentucky, but it is predicted that we will have flooding in the weeks to come, if for no other reason than that the ground is prematurely saturated.
Next up, sometime this week: Can rainfall trigger the blooming of a tree?
