Kevin Myatt

Latest entries from the Weather Journal blog

• Weather Journal taking a long break
• Yes, there's still an Atlantic tropical season going on
• Freezing temperatures likely tonight

About Kevin

Kevin Myatt grew up in Arkansas to the tune of tornado sirens and the rhythm of hailstones, aspiring to be a meteorologist before his studies and career were turned to journalism instead. Though he often chases storms, he prefers living in the cooler, more tranquil weather of the Blue Ridge. He moved to Roanoke in 1999 to take a job on the copy desk of The Roanoke Times; writing headlines and editing copy is his principal work for the newspaper today.

Each May, Kevin assists Pulaski County High School / Virginia Tech meteorology instructor Dave Carroll in leading college and high school students to the Plains to observe severe weather firsthand. The accounts of many of his storm chases can be found here on the storm chasing page of his weather blog on roanoke.com.

Kevin was an editor for "Hurricanes and the Middle Atlantic States," a book written by D.C.-area weather enthusiast Rick Schwartz and published by Blue Diamond Books that documents hurricanes striking the mid-Atlantic states since colonial times.

The Weather Journal column began in 2003 and appears on Friday's Virginia section front in The Roanoke Times. The Weather Journal blog began in 2006 and follows weather day-by-day between the larger columns.

Frost on the pumpkin, salt in the air?

"Frost on the pumpkin" is a rite of fall, a sure sign that summer has been left behind and winter is on the horizon.

Some of you may have seen your first frost during the last two nights. For most places, it was still a bit too warm, a bit too cloudy, or both.

Frost does not fall. It is also not frozen dew. Dew and frost form through a similar process, but one does not cause the other.

After a fall cold front comes through, cooler, drier air settles over the area. Warm air has a greater ability to hold moisture than does cool air, and drier air has a greater ability to gain and lose heat than does moist air.

The dew point is the temperature at which the air would be saturated – or have 100 percent humidity. Following a fall cold front, with air moving in from the middle of the continent rather than more directly off an ocean, the dew point is quite low. That's because there's not much moisture in it, and the temperature would have to fall a long ways to shrink the air capacity enough to make that small amount of moisture enough to "fill" the air completely.

When the dew point is low, daytime sun often warms the temperature rapidly. The humidity — or percentage of the air filled with water — is low because the dew point is low while the temperature rises. As the air warms, its spacing expands, but the amount of water in the air does not change much, and therefore fills a smaller portion of it.

But at night, the temperature falls rapidly as the daytime heating is lost quickly in the dry air, and the temperature plunges toward the dew point. As it does so, the capacity of the air to hold water decreases, and the water more completely fills the air.

Some of this water in the air gets squeezed out, in a sense, and begins to condense on objects at ground level, pressed down by the sinking air of the high pressure system above. When tempeatures are well above freezing, it forms liquid water droplets known as dew. But near or below freezing, the condensing water vapor forms ice crystals on objects known as frost.

Clouds can inhibit frost because the clouds act like a blanket and can keep the temperatures up just enough to prevent the needed condensation from occurring. Winds also put a stop to frost by keeping the air and moisture stirred up, not allowing it to settle onto the surface.

Frost continues to recur throughout the cool months, but the National Weather Service only issues advisories for the first few frosts in the fall and any that occur once the spring growing season has resumed That's because these are the only times of year when frost is a danger to vegetation, like crops and flowers. Once a widespread "killing frost" has occurred, there's no more need to issue frost advisories, for the inevitable damage has been done.

Tropical rain isn't totally clear matter

A question I've got frequently of late is why rain water from hurricane remnants is not salty, since it's not drawn out of the ocean.

The main reason is that when water evaporates, it leaves the salt behind. You can try this experiment at home by boiling a pan of salt water. The water will get lower as it turns to vapor, and the salt will start to collect.

If salt did evaporate with the water, there would be no reason to expect more salt water to fall with tropical systems than any other kind of weather system. That's because oceans are the prime supplier of moisture for any and all of our precipitation events.

All that said, there is sometimes a small amount of salt residue following rains from a hurricane that has come inland. But this salt is carried aloft by the hurricane's strong winds, not drawn up in the evaporation process, and it's not thick enough to make real salt water fall.

Some blamed salt carried in by Hurricane Isabel for making the fall foliage less than spectacular in much of Virginia last year. That should not be a problem this year from the many tropical systems that have crossed our state. Isabel made a direct drive into Virginia off the Atlantic; Frances, Ivan and Jeanne came at us over a lot more land from the Gulf, their winds having settled down by the time they got here, and surely, any salt they might have been carrying settled with it.

Much residue after a tropical rain is not salt. Studies have shown that dust from Africa is a frequent passenger in tropical moisture. So that might be some of the Sahara Desert caked up in Victory Stadium.