Conditions associated with Caribbean weather systems – Part 2
FOCUS QUESTION
What are the weather conditions associated with (cold fronts and anticyclones) Caribbean weather systems and formation? A front is the boundary between air masses of different temperatures. A front bringing in a cold air mass is referred to as a cold front.
COLD FRONT
This is defined as the transition zone where a cold air mass is replacing a warmer air mass.
It is the leading edge of a mass of cooler air that is pushing warmer air ahead of it out of the way. Cold fronts generally move from northwest to southeast.
They tend to be strongest in the spring and the fall, and weakest in the summer.
The air behind a cold front is noticeably colder and drier than the air ahead of it.
As a cold front moves into an area, it displaces warmer air at ground level.
When a cold front passes through, temperatures can drop more than 15° within the first hour.
Cold fronts move along the ground where they encounter friction; they move slower at ground level than they do further up in the atmosphere. For this reason, cold fronts tend to be more sloped than warm fronts.
Typically, cold fronts move faster than their warmer counterparts.
Colder air is denser than warm air, so it pushes the warm air higher into the atmosphere. Temperature changes along the boundary of the cold/warm air can be in excess of 50° F (10° C).
As the warm air rises, it causes an area of low pressure along the cold front. The warm air cools as it rises and moisture begins to condense.
If the cold front boundary is unstable, thunderstorms are more likely. Stable systems often just bring steady rains.
Very unstable cold fronts can generate hail storms and even tornadoes.
The combination of higher speed and slope pushes warm air masses upwards very quickly.
There is usually an obvious temperature change from one side of a cold front to the other.
Temperatures east of a cold front could be approximately 55°F, while at a short distance behind the cold front, the temperature can go down to 38°.
An abrupt temperature change over a short distance is a good indicator that a front is located somewhere in-between.
If enough moisture is present, a narrow line of thunderstorms and rain can form along the edge of the cold front. A cold front is represented by a solid line with blue triangles along the front pointing towards the warmer air and in the direction of movement (pointing in the direction the cold front is moving).
ANTICYCLONES
They are an area of high atmospheric pressure, where the air is sinking.
They are areas of intense high pressure (above 1020mb), where air molecules descend to the earth’s surface from the upper troposphere.
They can occur in both winter and summer with varying effects, but both are characterised by low winds due to a weak pressure gradient and stable conditions with no clouds.
The absence of clouds is because as the air molecules descend through the troposphere, they warm at the dry adiabatic lapse rate (DALR). This means that air cools or warms at 9.8° C for every 1,000m ascent or descent through the atmosphere. Ultimately, it means condensation does not occur.
They only involve one type of air mass which usually covers large areas and does not have any fronts.
They are high-pressure systems in which the air moves downwards towards the earth’s surface.
As the air descends, the molecules become compressed, the pressure increases and it warms.
When air is warming, any moisture in the atmosphere is evaporated, so no clouds can form. The sky is clear.
They can be very large, typically at least 3,000km wide, which is much larger than depressions. Once they become established, they can give several days of settled weather.
Winds are very gentle or even calm in an anticyclone, they move clockwise, and this is shown on a synoptic chart by widely spaced isobars Judith Henry teaches at Ardenne High School. Send comments to kerry-ann. hepburn@gleanerjm.com.