By David Berry

  As this is being written, Tropical Storm Bonnie has just passed through and Hurricane Charley is heading for Southwest Florida, promising to dump more rain on the Chesapeake Bay sometime tomorrow or the next day. We brought a charter home a day early to avoid what was sounding like an ominous forecast. Last fall's experience with Hurricane Isabel is still fresh on everyone's mind so people are adding extra dock lines to their boat.
  A friend of mine said that you know you are a sailor if Heather Teasch of the Weather Channel is your favorite TV star. You tend to watch it for hours like most people view a sporting event. We have it on in the next room. As a sailor or power boater, it pays to understand something about the weather and its patterns to be safe out on the water.
  The Chesapeake Bay has some consistent summer weather patterns that make amateur predictions easier. The prevailing breezes during the boating months tend to be from the south, southeast or southwest. This is what brings the warm humid air that everyone complains about. Most days the wind speed is between five and 15 knots with less in the early morning and later in the day. Often the wind before ten in the morning is from the north. Then just before noon, it drops to nothing, and then swings around to come from a southerly direction that holds for the balance of the day.
  These warm days are a result of high-pressure systems that move slowly or even stall in our area. Forecasters often mention the term Bermuda High, which is a semi-permanent area in the Atlantic that is a factor in the large number of hot, humid days we get in this region. If you face the wind, the location of the center of a high is ahead and to your left. If you face a southerly direction on the Chesapeake, you will see the high pressure would be off the coast, in the Atlantic.
  The center of low pressure, when facing a breeze is behind and to your right. Winds blow from high to low pressure, which produces wind from the south in our case.
  Why is the early morning breeze from the north, or northwest? It is the effect of the temperature of the land and water. Land cools down faster than water after dark, which increases the pressure. The water retains its heat keeping the pressure lower. Because wind blows from highs to low and a lot of the land surrounding the Bay is to the north and northwest, we get the morning breeze from that direction. This is called a land or offshore breeze.
As the day progresses the land heats up more than the water, decreasing the pressure over shore resulting in the wind moving from the colder water to land, creating a sea or onshore breeze. This effect is minimal most days in this part of the country due somewhat to our relatively warm Bay waters. If you visit California or Hawaii which both have warm days, cool nights, and cold Pacific water, sea and land breezes can be dramatic.

Afternoon thunderstorms
It is a good rule to get to your destination on the Bay before 4 p.m. because of the thunderstorms that come in the late afternoon. The hot sun bakes the land all day. The warm air rises, expands, and therefore can hold more moisture. You see the anvil shaped clouds forming all day. The rising air pulls in moisture from the Bay and the Atlantic adding fuel to the fire. The air, saturated with water, cools as it rises to the point that the waters condense pulling more warm air upward, the pressure drops and boom, a classic thunderstorm. That sudden change creates some fierce winds that can be unpleasant if you're prepared, and dangerous if you're not.
  The good news is that these isolated cells tend to pass quickly, but can be lined up in a row that pounds you for several hours. After they pass, the temperature and pressure rise, with heat and humidity returning, setting off the same cycle the next day.
  If the barometric pressure drops slowly over several days, then it is a good sign that a weather system, usually a cold front, is coming, bringing sustained bad weather. The wind is light and variable until the actual front arrives. In the middle latitudes fronts move eastward. Cold fronts push cooler air under the warmer atmosphere, resulting in storms that last until it passes.
  Behind a cold front is cool, dry air with fair weather and strong breezes from the north and northwest. The low pressure associated with the front is now to the east and wind blows from high to low. Because of the extreme differences in pressure, the winds become strong and sustained until the atmosphere again begins to heat up and the wind swings to its normal southerly direction, bringing more weeks of hot, humid days with the classic afternoon squalls.
  A sustained wind from the east is rare here on the Bay and almost never accompanied by storms. Just rainy days, one after another. The weather is moving in a direction that seems to stall and sends all the moisture from the ocean and Bay down on our heads.
  Clouds can tell you a lot about the weather. After the cold front has passed, the sky is often cloudless, signaling fair weather. The fibrous looking cirrus clouds begin to form indicating continued fair weather. If the wind begins stretching these out into what are commonly called mares' tails, bad weather could be heading our way. Watch the barometer for changes.
  Sometimes the billowy, heaped cumulus clouds form, indicating fair weather. These come on the cool, sunny days that contain those floating cotton balls, the type of day everyone wishes was around all year. These, as the heat increases, can build into the moisture laden cumulonimbus clouds associated with thunderstorms. The low, gray clouds that fill the skies are Nimbo stratus. Nimbo means water, which translates to rain. See the paragraph above concerning east winds.
  With storms comes wind and rain, but also one of nature's most dangerous forces, lightning. We have friends that were motoring across the mouth of the Patuxent River when their sailboat mast was struck by a bolt. It was an unpleasant experience to say the least and expensive. No one was hurt, but with the exception of the batteries and 12-volt TV, all the electronics and most of the wiring were vaporized. The skipper headed into a marina where the boat was pulled by the travel lift to make sure none of the through hulls were damaged when the lightning struck. They have been known to blow out with the force.
  Your mast should be properly grounded. Make sure that has been checked. Stories are told about attaching an anchor chain to the mast and hanging it in the water as a type of ground, but I am not sure whether that would be considered safe and proper. The best thing is to head in when you see the storm rising in the distance.

A little foggy
  Unlike our sailing neighbors in New England, fog is not a big issue here on the Bay, at least during boating season. Maine and places such as Block Island get advection fog, which is when warm moist air passes over cold water. The Chesapeake rarely gets this type of fog, except in the off season or early in the season when the water is cold.
  A second type of advection fog commonly called sea smoke, steam or frost fog, is created when cold air passes over warm water. Both forms of advection fog require horizontal movement, which means wind. Too little wind and the fog will be shallow, and too much wind blows it upward.
  A more common type of fog on the Bay is called radiation fog where the land is cooled near the warm water and there is little wind. As the sun heats the land, the radiation fog dissipates, so day-long fogs are seldom a problem here.
  We forget proper procedures when faced with a foggy day because of their rarity. Turn on you running lights if you're sailing and your steaming lights if motoring. Slow down and keep a lookout posted. A sailboat underway should sound one prolonged (4-6 seconds) and two short (one second) blast of a horn or whistle every two minutes. If you anchor, ring a bell for five seconds every minute. Listen for other boats, the bells on navigation markers and bridge horns. Have your chart in the cockpit and ready for use.
  As this article is being finished, it is obvious from looking out the window that none of the wind and rain predictions from Bonnie have come true. We could have stayed out safely for the final day of the charter, but let's not forget the voice in the head we discussed in an earlier story. The voice said if we come back there will not be a reason to have come back, but if we stay out, all the predictions and more will come true and we will kick ourselves for not returning when we had the chance.
Being a weather professional must be the most frustrating job on the planet because you are almost never exactly right and if you are, people blame you. You can also look at three different forecasts and get three different answers, but they are generally in the ballpark and as a boater it is better to be safe than sorry.
  If you are a boater, you should both listen to your favorite broadcaster before you go out and then observe the conditions every time you are on the water. Do not depend on the NOAA channel on your VHF to be your only warning. It is in the best interest of you and your crew to study the subject until you can see when it's time to head for home or stay at the dock.
  I've got to go now. Heather is giving an update in the next room. 

The End

David Berry is an ASA certified sailing instructor who holds a US Coast Guard 100-ton OUPV license. He lives and works in Havre de Grace during the boating season. Prior to becoming a full time sailor he held a number of executive positions in the telecommunication industry. His hobby is sailing his own boat. You can reach him at themariner@chespub.com

                                               Photo by Chris Knauss
In the summer on the Chesapeake Bay, it's a good
 rule to get to your destination before 4 p.m. because
of thunderstorms that often come in late afternoon.

Cumulonimbus clouds with an anvil top mean severe weather is likely to occur. The anvil indicates that the
cloud has stopped growing vertically because of
stability above the anvil top.