Friday, September 26, 2008

Lesson 13: Weather


Clouds – Clouds are formed when warm, moist air rises and is cooled to the dew point. The water vapor condenses around tiny particles of dust to form droplets. Although the variety of clouds are almost endless, the following are the main types.

High Clouds – occur above 20,000 feet and are composed mainly of ice crystals. The major types of high clouds are Cirrus (ci), Cirrocumulus (cc), and Cirrostratus (cs).

Middle Clouds – occur between 6500 and 20,000 feet. Included in the middle cloud group are

Altocumulus (ac) and Altostratus (as).

Low Clouds – are found below 6,500 feet. Nimbostratus (ns), Stratus, Stratocumulus (sc), and Cumulus (cu) are types of low clouds.

Vertical Development Clouds – This type of cloud may, form at low level and grow as high as 45,000 feet. The “thunderhead” (cumulonimbus, cb) cloud that is often shaped like an anvil is an example of a cloud with vertical development.

Atmospheric Pressure – The pressure exerted by the weight of the earth’s atmosphere which is about 14.7 pounds per square inch. Atmospheric pressure is generally measured in either inches of mercury or in millibars (mb). Millibars are generally used in marine weather and inches of mercury are generally used for land weather in the U.S.. The average pressure worldwide is about 1013mb. Extreeme values are 950 and 1050 – though greater extremes are possible during severe weather such as Hurricanes. Atmospheric Pressure is measured using a barometer. Low pressure is usually associated with “bad weather” – stormy conditions. High pressure is usually associated with “good weather” – clear and cool.

Barometer - the most common type used is the aneroid barometer which determines atmospheric pressure by the effect of such pressure on a thin metal cylinder from which the air has been partly evacuated. Aneroid means without liquid, as opposed to a mercury barometer which contains liquid mercury.

Wind – can be caused by many different factors. On the large scale, the main factor is the movement of air from a high pressure area to a low pressure area. On the earth’s surface, this flow from high to low pressure does not follow a direct path. It is deflected by the earths rotation. Surface winds encounter friction from geographical features or rough seas which tend to deflect the surface winds. An “anemometer” is used to measure wind speed, while a “wind vane” is used to measure wind direction. An installed anemometer is an instrument fixed somewhere aloft, usually at the masthead. The winds blow the propeller attached to one end of the vane that pivots. The whirling propeller revolves a spindle, attached with a synchro-repeater in the pilothouse or chart house bulkhead.

As a high or low pressure system moves through an area, there is always a noticeable change in wind direction. When the wind changes direction in a counterclockwise direction it is called a “backing” wind. A wind shifting in a clockwise direction is called a “veering” wind.
Radiation Fog - Radiation fog is formed on clear, still nights when the ground loses heat by radiation, and cools. The ground in turn cools the nearby air to saturation point, thus forming fog. Often the fog remains patchy and is confined to low ground, but sometimes it becomes more dense and widespread through the night.
After dawn, fog tends to disperse because it is 'burnt off' by the incoming solar radiation, some of which penetrates the fog and reaches the ground. The ground heats up, as does the layer of air near it. Eventually, the air reaches a temperature where the minute fog droplets evaporate and the visibility improves. However, in winter fogs can be very persistent.

Advection Fog - Advection fog is formed when very mild moist air moves over a cold ground. This can often happen in early spring when mild southwesterly winds moving across the country over snowy or icy ground. The lower layers of the air get cooled down rapidly to below the temperature at which fog forms.

Coastal Fog - Some coastal regions suffer from 'sea fog' which forms when moist air is cooled to saturation point by traveling over a cooler sea. The wind may then take the fog into coastal regions. This type of fog tends to occur in spring and summer, and particularly affects inland waters.

Steam Fog - 'Steam fog or sea smoke' is sometimes seen when cold air moves over much warmer water especially in the morning or late evenings on large rivers and or lakes. When cold air passes over much warmer water the lowest layer of air is rapidly supplied with heat and water vapor. Mixing of this lower layer with unmodified cold air above, can, under certain conditions, produce a super saturated (foggy) mixture.

Freezing Fog - Freezing fog is composed of super-cooled water droplets (i.e. ones which remain liquid even though the temperature is below freezing-point). One of the characteristics of freezing fog is that rime - composed of feathery ice crystals - is deposited on the windward side of vertical surfaces such as superstructure, rigging, masts, stanchions, and radio antennas.

Need To Know

Cold air is denser than warm air and when pushed by weather systems forces a wedge under the warm air ahead of it. The denser air exerted higher pressure in the atmosphere reflected in a rising reading on a barometer. The reverse is true of warm air following a cold air mass. In each case the change in barometer indicate an instability which can cause bad weather conditions: high winds, reduced visibility in fog or rain, and lightening storms. A falling barometer often means the approach of a weather front or deteriorating weather, as a rising barometer forecasts good weather. The faster the barometer changes the more dramatic the weather.
If air is warm it can hold more water vapor than if cold. Capacity varies with temperature and when air has all the water vapor it can hold, the atmosphere is said to be saturated.

The temperature at which air is completely saturated is called the dew point, below that mark, condensation can take place.

The measure of the water vapor in the atmosphere is called absolute humidity. When we compare that amount with what the air can hold, the term is called relative humidity.

Fog is like a cloud, the product of condensed water vapor.

Air masses do not tend to mix and dilute each other. Rather they collide along a battle line called a “front “and push each other around until one has nudged the other along by either undermining or by overriding.

Warm Front: Warm air slowly pushes over cold air. This front moves slowly, 10 to 15 knots, and the weather slowly changes to showers. However, this front can also bring strong winds and thunderstorms.

Cold Front: Cold air rapidly pushes beneath warm air. This front can move fast, up to 25-30 knots, and weather deteriorates with rain, strong winds and thunderstorms.

Backing winds run counter-clockwise. Veering winds run clockwise.
Weather patterns in the mid-latitudes of the northern hemisphere generally track westward. Though this pattern does not seem to hold true on the west coast of the U.S.

Hurricanes have a navigable and un-navigable or dangerous semicircle. Ahead of the storm track (northern hemisphere), wind and sea carries vessels into the path of the storm, so don’t cross ahead of a storm unless you are more than 300 nm out. In the un-navigable semicircle, the wind is greater due to the pressure augmented by the forward motion of the storm. In the navigable side, wind decreases by the storm’s forward motion. If you find your vessel in the dangerous semicircle, place the wind and sea on the starboard bow and hold it. In the navigable semicircle, place the wind and sea on the starboard quarter.

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