However, such locations are quite rare. Isobars are almost always curved and are very rarely evenly spaced. This changes the geostrophic winds so that they are no longer geostrophic but are instead in gradient wind balance. They still blow parallel to the isobars, but are no longer balanced by only the pressure gradient and Coriolis forces, and do not have the same velocity as geostrophic winds. In this case, the centrifugal force acts in the same direction as the Coriolis force.
As the parcel moves north, it moves slightly away from the center -- decreases the centrifugal force. The pressure gradient force becomes slightly more dominant and the parcel moves back to the original radius. Open V-shaped isobars with low pressure inside mark a trough of low pressure.
In contrast N-shaped isobars with high pressure inside mark a ridge of high pressure. A col. When isobaric patterns are plotted with information gathered at weather stations in the form of weather symbols , the weather forecaster can use his skill to predict the weather over the next few hours or days.
Round the edge of a High, the winds are sometimes strong. Intense Highs tend to squeeze the isobars together creating areas of strong winds. Winter Highs often bring frost; summer Highs may bring thunderstorms and hail.
The bigger Highs are, the slower they tend to move, sometimes 'blocking' the fronts that are trying to follow them. Isobars make shapes and patterns. When they enclose an area of low pressure this is called a 'Low' or 'depression' and its centre is labelled on a weather map with an 'L'.
The term depression is a bit of meteorological jargon. A low pressure system is like a giant funnel of wind spiralling inwards and upwards forcing warmish air in the centre to rise. As air rises it cools and clouds form. The central pressure of a shallow Low is above hPa, of a moderate Low hPa, and of a deep or intense Low below hPa.
If there are two or more centres the Low is said to be complex. If the central pressure is rising the Low is said to be filling or weakening. If the central pressure is falling the Low is said to be intensifying or deepening. The satellite picture to the upper right from the satellite GMS-5, courtesy of the Japan Meteorological Agency shows cloud patterns fairly typical of those associated with Highs and Lows.
An air flow originating from a prescribed location warm, cold,moist or dry is called an air-mass. Air-masses are named according to where they have come from and each has its own characteristic temperature and humidity.
A front marks the boundary between two air-masses, and appears on the weather map as a line with triangles or semicircles attached. A cold front is the leading edge of a invading colder air-mass and is marked by a line with triangles pointing to where it is moving. Cold fronts push in underneath the warmer air ahead of them, forcing the warm air upwards and making cloud and areas of rain.
The cloud band is usually about 50 to kilometres 30 to nautical miles wide. As a cold front passes by: any rain clears but showers may appear, humidity drops, air temperature usually drops, pressure rises and thewind changes direction.
A warm front is the leading edge of an invasion of warmer air. Its surface position is marked by a line with semicircles pointing to where it is moving.
The advancing warm air rises over a zone of retreating cooler air, making a cloud bank that slopes forwards from ground level upwards, often bringing prolonged steady rain. This cloud bank can be to kilometres to nautical miles wide. As a warm front passes by: any rain becomes patchy but humidity remains high, air temperature may rise a little, pressure steadies, and the wind changes direction.
An occluded front or occlusion occurs when a cold front overtakes a warm front, so that all that remains of the original warm air is trapped above, where it cools making dense cloud and rain.
It is marked by a line with triangles and semicircles on the same side, pointing to where the front is moving. As an occluded front passes by: any rain becomes patchy, wind eases, the rate of pressure fall may level outbut air temperature does not change much. A stationary front is one which has lost its impetus for movement, so that neither air-mass is making much progress.
It is marked by a line with alternate triangles and semicircles on opposite sides It takes a while for a stationary front to pass by: any rain clears only slowly and temperature and pressure do not changemuch. When isobars make a sharp bend around a Low, this bend area is calleda trough of low pressure or simply a trough. Troughs are also often shaped like tongues and usually contain weather similar to lows and fronts.
Horizontal convergence is the coming together of two flows of air.
0コメント