It’s no secret that tropical storms and their bigger cousins do bad things to your boating pleasure. Any boat owner worth his salt knows that operating in these storm systems is extremely dangerous, if not suicidal. So, why do boaters get caught in these dangerous storms? Often it’s the boater’s lack of knowledge about the basics of weather.
Let’s take a brief look at basic atmospheric properties and how they create clouds, rain and thunderstorms. Three ingredients are required to produce rain and thunderstorms: moisture in the air, a lifting force, and instability or colder air above warmer air.
1. The atmosphere, which is a collection of gases, is held to the earth by gravity. This gives it the weight that we measure as pressure in inches of mercury or millibars. On the earth’s surface the atmosphere above us exerts greater pressure, just like deep water. And the higher we go, the less atmosphere we have exerting pressure on us.
2. Next, let’s see how that pressure affects the temperature of a gas (the air). The more pressure exerted on a gas, the more it’s compressed. That heats it up. The same gas cools down when it’s allowed to expand with less pressure on it. So when an air bubble is lifted higher in the air, it will expand and cool, because it has less atmosphere weighing on it. This type of cooling through lifting is called “adiabatic” cooling.
3. Remember your BS&S Course, which taught you that cooler air holds less moisture in a gaseous state than warmer air. Looking at our bubble of air, when it’s lifted we now know it will cool. When it cools to a temperature where it can no longer hold any more moisture, it is at the “dew point”. The air is 100% saturated and we have 100% relative humidity. Any further cooling will force the beginning of cloud formation. Remember, clouds are nothing more than very small water droplets.
4. Finally, when the water vapor forms clouds, it releases the energy in the form of heat. This heat release makes our bubble of air warmer than the surrounding air. Warmer air, being less dense than cooler air, continues to rise. As the air bubble rises it expands again and cools, condensing more moisture as cloud droplets. As long as there is moisture to keep condensing out and heating it up, the air bubble will continue to rise. The cloud will build as long as the heat release keeps it warmer than the surrounding air. Warmer air beneath cooler air creates an unstable air mass.
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| Hurricane Charley Heads for Florida in 2004 |
Tropical storms and hurricanes are this basic process on steroids. By early June each year the warm waters of the southern Atlantic and Gulf evaporate massive amounts of moisture into the air. This warm, moist air doesn’t need much lifting to expand, cool and become 100% saturated. It starts condensing out liquid water droplets and forming cumulus clouds. Once clouds begin to form, the brakes are off. The clouds release heat, building quickly into cumulonimbus, or rain clouds. A thunderstorm can occur within minutes.
Now all we need to begin forming a tropical depression and eventual hurricane is for the water temperature and atmospheric winds to cooperate.
If the water stays warm enough and there is low wind shear, more storms develop until sustained winds reach the tropical storm stage of 35 to 64 knots.
So long as conditions remain favorable to formation, the process continues to intensify and winds will begin to rotate as they move to the center of low pressure. The rotation of the surface inflow increases velocity much like a spinning ice skater as she pulls her arms inward. If this inflow continues to intensify and the pressure gets low enough to cause an inflow of winds exceeding 64 knots, we have a full-fledged Category I hurricane with a definite eye at the center.
A cluster of storms can change to a tropical depression in as little as half a day. Just as quickly it may also go from a tropical storm phase to a hurricane. This is a very rapid process and boaters should not delude themselves into thinking they can outrun them. Stay on shore or find a safe harbor as soon as possible.
Hurricanes are classified into five categories of intensity on the Saffir-Simpson Scale.
Category 1: 64-82 knots wind – some damage to trees, shrubbery, and unanchored mobile homes
Category 2: 83-95 knots wind – major damage to mobile homes; damage to roofs, and blow down trees
Category 3: 96-113 knots wind – destroy mobile homes; blow down large trees; damage small buildings
Category 4: 114-135 knots wind – completely destroy mobile homes; lower floors susceptible to flooding
Category 5: 135 knots wind – extensive damage to homes/industrial buildings; blow away small buildings
When these systems move over land and lose their moisture or move northward over cooler waters they will begin to dissipate and eventually extinguish themselves.
The best source of weather information is the National Hurricane Center of the National Weather Service at www.nhc.noaa.gov/. On the left side of this webpage you’ll find the Marine Forecasts section. If you go to the “Help with Marine” link you’ll find explanations for each forecasting and reporting product. Getting familiar with these pages will enhance your weather awareness and boating safety.
Safe boating and Semper Paratus.
