Is there really a period of calm before a storm?

Hurricane Isabel hit the Baltimore/Washington D.C. area pretty hard last week. Though it was only a tropical storm by the time it made it to Maryland, thousands of trees were knocked down as a result of the strong winds, heavy rains and already saturated soils. Additionally, Isabel's tidal surge up the Chesapeake Bay was unprecedented in many places -- low lying areas in many shore communities were completely submerged.

About 2 million people were without power immediately following the passage of Isabel. I live in the Maryland suburbs of Washington D.C., and as of Thursday morning (7 days after Isabel), I'm still without electricity -- as are tens of thousands of others. However, our "crack" utility company is possibly as much at fault for this as is Isabel. Anyway, even though I'm now routinely wearing mismatched socks to work, one advantage of having to due without power is that my neighborhood is now sufficiently dark so that I can actually stargaze a bit. Last night, Mars was aglow, like an ember amongst the shoals of stars. Before dawn, Orion ruled the eastern sky, and a sliver of the Moon (waning crescent) flashed its Cheshire grin. It was all very peaceful, except for the constant drone of my neighbor's huge generator, which seemed to mock me for still being in the dark.

Post Isabel, things aren't yet exactly tranquil here. We had a deluge a few days after Isabel departed that actually dumped more rain than Izzy herself. However, was there a period of calm weather just before Isabel rudely ripped us up? Actually, the weather the two days preceding Isabel was absolutely gorgeous -- perhaps two of the nicest days of the year. Clear, azure blue skies gave no clue as to what was to follow.

The calm before the storm isn't a new concept by any means. It was recognized long ago that before a severe storm, the air is still and the birds stop singing and go to shelter. "Calm continueth not long without a storm." This quote from an unknown source was written in 1576.

During the Vietnam War, satellites were first used (in a war situation) to give bomber pilots an indication as to when and where it would be clear over targets. It was noted by meteorologists in the military that if a typhoon approached the coast a day or two before landfall was made, the weather inland could be exceptionally clear. In tropical areas such as Vietnam, it's rarely clear in the middle of the day, even during the dry season. Convection processes (upward atmospheric motion and energy transfer) produce cumulus and cumulonimbus clouds, which build during the heat of the day. The regularity and persistence of the clouds in southeast Asia meant that on many days pilots weren't able to visually lock-in their targets. Interestingly, those occasions when a tropical storm or typhoon moved close to Vietnam in the South China Sea, proved to be the best weather for bombing runs. It seems counterintuitive, but in order to be assured of finding the cleanest skies, pilots were basically instructed to wait until the typhoon moved nearer before they left on their missions.

Looking at visible satellite imagery of big storms, at first glance, it may appear that the clear areas are due to water vapor being sucked out of the sky and into the storms themselves. However, this isn't happening at all. A tremendous amount of vapor and heat are drawn toward the center of hurricanes, some of which is forced upward by powerful updrafts. These updrafts remove this saturated air and rain from the storm system by spilling it over the extended tops of the highest storm clouds (anvils). The anvils can be over 10 miles high (16 km). Descending air along the outside boundary of hurricanes and even large thunderstorm complexes, often results in clear skies, not only in the wake of the storm but ahead of it as well. Anytime air descends from aloft, it compresses, making it warmer and drier. This results in a stable air mass, which forms a "lid" over the lower troposphere and acts to dampen cloud formation.

and the Earth Science Picture of the Day for September 10, 2003 (http://epod.usra.edu/archive/epodviewer.php3?oid=155488)

It should be noted that in many cases, the perception that it seems calm is not actually true. We're perhaps so accustomed to hearing this maxim (calm before the storm) that when threatening weather is on our doorstep, we think that there will be a "mandatory" lull before the big blow. Storms connote suddenness -- meaning that weather conditions are drastically different before, during and after the storm. Most of us have experienced an eery silence before a storm strikes. Sometimes you'll hear the comment that "It came out of nowhere, and there wasn't a breath of air before all hell broke loose." On the other hand, with frontal storms, the weather may be tumultuous well before a line of thunderstorm approaches. Southwest winds ahead of an advancing cold front, for example, can be rather vigorous, even hours before cumulonimbus clouds begin to build.

Thanks to Dr. Robert Adler for his help with this question.

This week's question comes from Dr. James Foster. Dr. Foster originated this series and did it as a solo project for the GSFC website for SEVEN YEARS! Last year Dr. Foster decided to share the enthusiasm he has for this project with other Goddard scientists and will be posing questions on a semi-regular basis. This will be Dr. Foster's ninth year with Science Question of the Week!