Weathering The Weather
Centers of surface high and low pressure areas are found within closed isobars on a surface weather analysis where there the absolute maxima and minima in the pressure field, and can tell a user in a glance what the general weather is in their vicinity. Weather maps will depict their highs as Hs and lows as Ls.
Low pressure systems, also known as cyclones, are located in minima in the pressure field. Rotation is inward and counterclockwise in the northern hemisphere as opposed to inward and clockwise in the southern hemisphere due to the coriolis force. Over land, thermal lows are indicative of hot weather during the summer.
High pressure systems, also known as anticyclones, rotate outward and clockwise in the northern hemisphere as opposed to outward and counterclockwise in the southern hemisphere. Under surface highs, sinking motion leads to skies that are clearer, winds that are lighter, and there is a reduced chance of precipitation. There is normally a greater range between high and low temperature due to the drier air mass present.
Fronts in meteorology are the leading edges of air masses with different density (e.g., air temperature and/or humidity). When a front passes over an area, it is marked by changes in temperature, moisture, wind speed and direction, atmospheric pressure, and often a change in the precipitation pattern.
A cold front’s location is at the leading edge of the temperature drop off, which in an isotherm analysis shows up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. Cold fronts can move up to twice as fast as warm fronts and produce sharper changes in weather, since cold air is denser than warm air and rapidly replaces the warm air preceding the boundary. Cold fronts are typically accompanied by a narrow band of showers and thunderstorms. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel, and it is placed at the leading edge of the cooler air mass.
Warm fronts are at the leading edge of the temperature drop off, which is located on the equatorward edge of the gradient in isotherms, and lie within broader troughs of low pressure than cold fronts. Warm fronts move more slowly than the cold front that usually follows because cold air is denser, and harder to displace from the Earth’s surface. This also forces temperature differences across warm fronts to be broader in scale. Clouds ahead of the warm front are mostly stratiform and rainfall gradually increases as the front approaches. Fog can also occur preceding a warm frontal passage. Clearing and warming is usually rapid after frontal passage. If the warm air mass is unstable, thunderstorms may be embedded among the stratiform clouds ahead of the front, and after frontal passage, thundershowers may continue.
Other Fronts. A wide variety of weather can be found along an occluded front, with thunderstorms possible, but usually their passage is associated with a drying of the air mass. A stationary front is a non-moving boundary between two different air masses, neither of which is strong enough to replace the other. They tend to remain in the same area for long periods of time, usually moving in waves.
By drawing Isobar lines between points of equal pressure on a map or chart a contour map showing variations in pressure is produced. When this is combined with wind and other observations and the changes over time are observed, reasonably accurate forecasts can be made. The aneroid type of barometer which is most commonly found on small boats provides a valuable indication pressure tendency, forecasting short term changes in the weather.
Generally high pressure is an indicator of fair weather while low pressure means that storms are more likely. A large change in pressure will not only herald a large change in weather it will also mean an increase in the wind strength. A drop in pressure is a sign of a low pressure system approaching bringing with it a good chance of rain. On the other hand a rise in pressure will generally accompany clearing skies and an improvement.Short Term Local Forcasting
By combining all the information from the latest computer forecasting output with our own observations of today’s marine weather, we should be able to accurately predict the local conditions for the following few hours.
The effect on the prevailing marine weather caused by the local topography can be dramatic.Wind speed can dramatically increase and be deflected from the prevailing direction as it is funneled through narrow channels. Even along open stretches wind direction can be deflected.
Cliffs & valleys can produce localized turbulence, and gusts can form unexpectedly. Wind over tide can cause short steep waves. Sea breezes will modify the prevailing conditions depending on the time of day. A good sense of observation will help you keep track of weather shifts over the short term.
The wind barb indicates the wind direction and wind speed. Wind barbs point in the direction “from” which the wind is blowing. The term easterly means that the winds are from the east. On the otherhand, the term “eastward” means that the winds are blowing towards the east.
Wind speed is given here in the units of “knots”.
A “Knot” is a nautical mile per hour. 1 Knot = 1.15 Miles Per Hour. Each short barb represents 5 knots, each long barb 10 knots. A long barb and a short barb is 15 knots, simply by adding the value of each barb together (10 knots + 5 knots = 15 knots). If only a station circle is plotted, the winds are calm.