Tropical Storms and Hurricanes need deep layers of moist air in order to develop. That is why they only form over water and not over land. Their enegy is derived from the warm, moist, unstable air that eixsts over the tropical oceans during the summer months.

Since plenty of warm water often prevails throughout the tropical Atlantic, Caribbean and Gulf of Mexico from June through October, the basic ingredients usually exist for such storms. Fortunately, there are a few counteacting forces that put a lid on storm formation despite the abundance of warm water. One of these, is the wind shear that can often prevail across the tropics. Wind shear is strong winds (usually blowing from west to east) in the upper-levels of the atmosphere. Most people have heard that tropical storms and hurricanes do best with low levels of wind shear, since high levels tend to tear (or shear) the tops of the storms apart.

A lesser know factor inhibiting tropical storm formation is the Saharan Air Layer (or SAL). This is hot, dry, stable air that blows off of the deserts of north Africa and flows into the eastern and central Atlantic. Sometimes, dust and dry air from this wind pattern reach into the Caribbean and western Atlantic. This year, the flow off hot, dry air off the deserts of Africa has been stronger than normal, pushing dry air far into the Atlantic. It has been one reason why we have not seen any storms so far this season. While the SAL is still fairly active, there are some signs that it may be weakening. At any rate, I monitor this pattern fairly closely as it will help to keep the storms at bay as long as it persists.

Below are a couple of maps (coutesy of the Naval Research Laboratory and the Univeristy of Wisconsin) showing the current extent of the dry air from SAL. The first, shows moisture levels throughout the tropics. Drier air is depicted by the blue, green and purple hues. The second is a water vapor map of the same region. Again, the lighter blues show the areas with decreased or lower water vapor levels.