Last month we looked broadly at cold fronts. This month it is the indicators from the atmosphere that you identify on your boat to gauge where the cold front may be.
Cold fronts are variable in their behaviour and this article concentrates on a fairly typical passage of a cold front.
The earliest indications on the ground of an approaching cold front would be a veil of cirrostratus cloud and a falling barometer. These two indicators tell you that there is a cold front upstream of you. The cirrostratus cloud has actually formed from the top of thunderclouds along the actual front. Strong winds that are blowing much faster than the front, blow the cloud (at high level) a long way ahead of the cold front.
This saying “Red Sky at night, sailor’s delight. Red sky in the morning, sailors take warning” uses this cloud formation. Another similar saying is “If a circle forms round the moon, ’twill rain soon.”
This circle around the moon is called a halo. It forms when light from the moon or sun pass through ice crystals in high cirrus or cirrostratus clouds. These clouds do not produce rain or snow, but they often mean that a cold front could be upstream of you and advancing, which may often mean that bad weather is approaching.
In conjunction with the cirrostratus moving over you, the falling barometer indicates that a high pressure system is moving away from you.
As the cold front moves closer to you the cloud may thicken and winds will strengthen from the north or northwest, while the air pressure is steadily falling. Remember the atmospheric tides may mask or enhance any fall in air pressure (see Afloat Jan’08).
As the air is coming from the north, temperatures should increase and certainly be warmer than average.
A rain band may develop as much as 200 kilometres ahead of the front. This rain is not actually caused by the front but by the movement of air southward ahead of the front in the middle levels of the atmosphere; as this air moves south it actually rises.
As this air rises it cools, if there is enough moisture in the middle levels, cloud will form. Depending on the amount of lifting and the amount of moisture anything from cloud through to light patchy rain up to heavy constant rain can occur.
As the front gets very close winds can become very strong and gusty and temperatures should be at their warmest. Cloud becomes more convective (cumulus, cumulonimbus) and showers and thunderstorms are the likely precipitations. Air pressure should be falling rapidly.
This is probably one of the dangerous periods for the mariner with gusty winds being at their strongest, waves at their highest and squalls caused by thunderstorms possibly either adding to the prevailing wind speed or coming from a non-prevailing direction. Also heavy shower activity reduces visibility and just makes deck work harder and more dangerous.
Immediately after the front the wind should back to become anywhere from west northwest through to southerly or even southeasterly.
Mostly winds do ease post frontal but this is not always the case; they can blow just as strongly from the new direction. Temperatures can fall rapidly. In a summertime front it is possible for temperatures to fall as much as twenty degrees but it is usually a lot less than this particularly at sea. The post frontal air is colder and can hold less water vapour, so the dewpoint/humidity will fall.
Air pressures should begin to rise. The precipitation will be shower activity and is caused by cold air being heated by a warmer sea as it moves northward. This is the speckled cold air field you see post frontal on satellite pictures. In fact this cold air cloud forms in hexagonal shapes; however, this is rarely seen due to the distortion of the cloud by the wind usually associated with the cold air field.
The satellite picture (left) shows the cold air as speckled cloud south and west of Tasmania. Thicker cloud over the western half of the island is a result of orographic lifting. The northeast is clear of cloud due to the southerly stream being blocked by the mountain ranges. The front would move off to the east in the Tasman Sea.
Conditions immediately after a front can cause problems for vessels that are anchored. An anchoring choice may have been made to be protected from the existing stronger northerly prefrontal winds. When the wind changes direction the anchorage may not be as sheltered, wave may get larger and you could see the change to a more dangerous lee shore.
Eventually the showers stop, the cumulus cloud reduces in height and becomes stratocumulus, winds drop and temperature and air pressure begin to rise as a new high pressure system moves in.
*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.