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To understand weather systems it is important to understand some basic principles that affect weather.  The first is that dark surfaces absorb heat, while light surfaces reflect heat.  Therefore darker surfaces will be warmer than light areas.  The angle of the sun also affects the absorption of heat, so the more direct the sun’s rays are the more heat will be absorbed.  The second principle is that warm air or water rises while cooler air or water falls.  This creates convection currents in the atmosphere and in the oceans.  Warm air rises at the equator while cooler air falls at the poles leading to wind belts.  Likewise, warm water rises at the equator and cooler water falls at the poles, resulting in our ocean currents. 

Another important concept is that air has weight and pushes on objects.  This is called air pressure.  At sea level, air pressure is equal to 1013 millibars, which is 14.7 pounds per square inch or 29.92 inches of mercury.  Weather systems are either high pressure or low pressure.   In the Northern Hemisphere a high pressure area spins clockwise moving outward.  This usually denotes fair weather, low humidity, rising barometric pressure, clear and cooling.  A low pressure area like a hurricane spins counterclockwise and inward.  This usually denotes stormy weather, high humidity, and lower barometric pressure, wet, unsettled, and backing winds.  The most extreme low pressure areas are hurricanes.   The upper right portion of a hurricane or low pressure area is usually the strongest.  This is because the hurricane’s wind speed is added to the forward motion of the storm.  Likewise, the lower left portion of a hurricane is usually weaker as the forward motion of the storm is subtracted from the hurricane’s wind speed.  Take hurricanes seriously and prepare. 

The Buys Ballet Law states that in the Northern Hemisphere the center of a low pressure is found by facing into the wind.  The center of the low is to the right and behind you. This would be important to know in the event that you needed to move your vessel away from a storm.

An aneroid barometer is the instrument used to measure air pressure.  The most important information obtained from an aneroid barometer is the rate at which the air pressure rises or falls.  The faster the air pressure drops the more intense a storm is becoming.  On weather maps, isobars are lines connecting areas of equal air pressure.  Isobars close together indicate the presence of strong winds.

An anemometer measures  wind speed.  The Beaufort Scale is a numerical scale for indicating wind speed.  The numbers range from 0 (calm) to 12 (hurricane).  Wind blowing across open water is called fetch.  Apparent wind is the direction and speed of the wind relative to a moving vessel.  The motion of a vessel underway can make the effective wind vary from the actual wind direction and speed.  True wind is the actual direction and speed of the wind. 

Backing wind
 is a wind changing its direction, counter-clockwise in the Northern Hemisphere, clockwise in the Southern Hemisphere.  Veering wind is a clockwise shift in the wind in the Northern Hemisphere.

The most common local winds are land breezesand sea breezes.  Sea breezes occur during the day where the land heats up faster than the adjacent ocean.  The warm air rises over the land and the cool air from the ocean moves in to replace it.  A land breeze occurs at night when the land cools more quickly than the adjacent ocean.  The cool air is forced out to sea creating the land breeze.  These are usually not as strong as the sea breezes.

The Coriolis Effect
 is an apparent force caused by the rotation of the earth.   An object in motion in the Northern Hemisphere will deflect to the right; an object in motion in the Southern Hemisphere will deflect to the left.  As air moves from high to low pressure in the northern hemisphere, it is deflected to the right by the Coriolis force. In the southern hemisphere, air moving from high to low pressure is deflected to the left by the Coriolis force.  The amount of deflection the air makes is directly related to both the speed at which the air is moving and its latitude. Therefore, slowly blowing winds will be deflected only a small amount, while stronger winds will be deflected more. Likewise, winds blowing closer to the poles will be deflected more than winds at the same speed closer to the equator. The Coriolis force is zero right at the equator.  The Coriolis Force affects air (wind) and water (currents).

The earth has wind belts, or areas of prevailing wind.  Most of the United States lies in the prevailing westerly windbelt.  In this belt the general pattern of weather moves from west to east. The following weather proverb illustrates this concept:

“Winds that swing against the sun and winds that bring the rain are one Winds that swing around the sun keep the rain storm on the run”

Since the sun rises in the east and sets in the west, winds coming from the west usually denote a front and bring rainy weather. 

There are four major types of weather fronts.  A front is the boundary where two air masses of different density (temperature) meet.  A cold front is a cold air mass hitting a warm air mass, producing storms.  A squall line is a line of thunderstorms ahead of a cold front.  After a cold front passes the barometric pressure usually rises, often quite rapidly, with skies clearing.  A warm front occurs when an advancing warm air mass reaches colder air and rides up over it.   A stationary front is a front that is not moving or stalls.  An occluded front is formed when a cold front overtakes a warm front or stationary front.

On a weather map, isotherms are lines that denote equal temperatures.  Weather forcasters often use colors to denote temperature differences on a weather map.

is the amount of moisture in the air.  Relative humidity is the ratio, expressed in percent, of the amount of water vapor in the air compared to the maximum amount of water vapor the air can hold at that temperature.  A Wet and Dry Bulb Thermometer measures relative humidity.  Air is saturated at 100% humidity.  Dew point is a measure of atmospheric moisture.  It is the temperature to which air must be cooled to reach saturation.  At this point water will condense and form dew.  Radiation fog forms upward from the ground when air is chilled by the cooling ground and becomes saturated with water vapor to its dew point.  This fog usually forms on a calm night when a shallow layer of moist air is covered by drier air.  Advection fog is formed when warm moist air moves over a colder surface.   This often occurs in the New England area where Gulf Stream waters blow warm air towards cool coastal waters until the air is saturated with moisture.  Steam fog is cold air over warm water.  This occurs on the Ohio and Mississippi Rivers.   You must know the dew point to predict fog.  Wind dissipates fog.  Make sure that you know how to safely proceed with your vessel in fog.

It is important to know cloud types and the type of weather to expect with each.  Cirrus are the highest clouds and are usually composed of ice crystals.  Alto clouds are middle clouds and contain water droplets.  Stratusclouds are low and fog-like.  Cumulonimbus clouds, which are also called thunderheads, are anvil-shaped and usually contain lightning and squalls.  They should be avoided as you may encounter high winds, poor visibility, rough seas, squally weather and they may contain water spouts.

The National Weather Service provides marine weather forecasts and warnings for the United States coastal waters, the Great Lakes, offshore waters, and the high seas.  The principle means of disseminating marine weather is through the NOAA Weather Radio.  Mariners need to be familiar with the Coastal Storm Warnings, which are listed below and can be found in Coast Pilot publications.

Small Craft Advisory
 – Usually 18 knots of wind (less than 18 knots in some dangerous waters) or hazardous wave conditions.   Remember that a “small craft” indicates a vessel under 20 meters.  One red pennant displayed by day and a red light above a white light at night is used to alert mariners to sustained (more than two hours) weather or sea conditions that might be hazardous to small boats.

Gale Warning
 – Two red pennants displayed by day and a white light above a red light at night indicates that winds within the range of 34 to 47 knots are forecast for the area.

Storm Warning
 – A single square red flag with a black center displayed during the daytime and two red lights at night indicate winds 48 knots to 63 knots are forecast for the area.

Hurricane Warning
-Two square red flags with black centers displayed by day and a white light between two red lights indicates that winds 64 knots or above are forecast for the area.  A hurricane watch is an announcement made by the National Weather Service when a hurricane is near enough to keep vigilant and take precautions.

Weather Proverb
“Red sky in the morning, sailors take warning, red sky toward night, sailors delight”