Kevin Myatt is The Roanoke Times' weather columnist.

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When it rains, the moisture has to pour in from somewhere.

With our region entering a cloudy, damp period resulting from moisture originating in the Gulf of Mexico and Atlantic Ocean, here is a look at the five sources that provide almost all of the atmospheric moisture for Southwest Virginia's precipitation.

1. The Gulf of Mexico

Few significant precipitation events occur in Southwest Virginia without a heaping helping of Gulf moisture.

The Gulf of Mexico stays warm much of the year, and it is easy to tap. Prevailing winds over the continent are from the west. When that westerly wind develops a southerly slant, it carries moisture to our region from the Gulf, situated to our south and southwest. Also, any time the westerlies bring low pressure systems from the west, the counterclockwise circulation around the low sweeps in moisture from the Gulf of Mexico on south winds.

The thickest Gulf moisture comes from tropical systems that move inland. Hurricanes Camille (1969), Agnes (1972) and Juan (1985) are among a long list of hurricanes making landfall on the Gulf Coast that eventually caused extreme flooding in Virginia.

2. The Atlantic Ocean

The Atlantic is closer than the Gulf, but winds have to turn around against their more common westerly flow to move large quantities of Atlantic moisture into our region.

This can be accomplished by strong low pressure systems moving in from our west or southwest. These lows first sweep in abundant Gulf moisture, then bring southeast winds that add moisture off the Atlantic to the mix.

As is the case currently, low pressure positioned to the south and high pressure to the north can create the easterly fetch necessary to transport Atlantic moisture into our region.

Tropical systems hitting the Atlantic coast sometimes spread rainfall into our region, though many of them end up slipping by just to our east.

3. Local sources

Rivers, creeks, lakes, wet ground, snow cover, ice-coated surfaces and plants transpiring through their leaves add to the total moisture in the air, particularly near the surface.

Sunny days allow these sources to be tapped by evaporation, and they can be a significant source of moisture for the afternoon and evening showers and thunderstorms common on many summer days (and even snow showers triggered by weak instability in winter).

Local moisture sources often have a self-propagating cycle. A wet period leads to moist ground, greener trees and fuller streams and reservoirs, providing more moisture for additional rainfall. A dry period that leaves dusty ground, crunchy leaves and lower water levels provides less moisture, leading to drought.

4. The Pacific Ocean

The Pacific is thousands of miles away, with deserts, high mountains and wide plains between us and it, but prevailing westerly winds high over the continent do get some of its moisture into our region. Any strong low pressure system that has crossed the southern U.S. often carries a good amount of Pacific moisture before it taps the Gulf and possibly the Atlantic. On rare occasions, the remnants of Pacific tropical systems making landfall in Mexico move into our region, usually pushed along by a cold front.

5. The Great Lakes

Stiff northwest winds can transport Great Lakes moisture to our region, lifting it over the Appalachians and condensing it. This is most noticeable in winter during the rounds of intermittent snow showers that follow cold fronts.

Other sources like the Caribbean and Hudson Bay contribute negligible amounts of moisture, carried long distances by fronts and strong storm systems.

All this moisture naturally recycles. Rain falls, draining into creeks that form rivers, carrying the water back out to sea, where it is evaporated again into large rain-producing storm systems.

So you may see these same water molecules again someday.

Weather Journal runs Monday, Wednesday and Friday.