Barometers and Atmospheric Pressure
Barometers measure atmospheric pressure. This is the force exerted by the earth’s atmosphere or the ‘weight of the air’. A high pressure is usually associated with good weather and low pressure with grim weather. Wind circulates around pressure systems, in the southern hemisphere as follows;
High pressure anti-clockwise
Low pressure clockwise
In the northern hemisphere the wind directions for highs and lows are reversed.
Barometric pressure is measured in hectoPascals (formerly millibars), abbreviated to hPa. Barometers may be electronic or mechanical. Many mariners enter regular barometric readings in the log, and often carry a barograph, which traces changes in atmospheric pressure over a seven day period.
Isobars are the lines on a weather map which connect places with the same pressure. The spaces between the isobars indicate the direction and strength of the wind, where the closer together they are, the stronger the wind is, in a direction nearly parallel to the isobars.
Wind Direction and Speed
Wind direction is the direction the wind is coming from and is expressed in one of 16 compass points. Wind speed is derived from the average speed over a 10 minute period at a height of 10 metres above the surface, and is stated in knots, where 1 knot is equivalent to 1.85 km per hour.
Weather forecasts give the average direction and speed and it is important to keep in mind that gusts can be up to 40% more than this.
The Howard system of 1803, which is still used today, divides clouds into three categories based on the height of the base of the cloud. These heights can vary considerably between latitudes, and make forecasting clouds difficult and often inaccurate.
Low clouds are where the base is less than 2 km above the horizon, such as Cumulus. These are typically puffy looking and can be quite tall, but the base remains low. A cumulus cloud can, at times, become a cumulonimbus or thunderstorm cloud. Cumulonimbus clouds are tall and shaped like a cauliflower with a distinctive ‘anvil’ top.
Medium clouds, such as Alto Cumulus and Alto Stratus clouds, have their bases between 2 and 6 kms above the horizon. They are associated with unstable weather ahead of a cold front.
High clouds have a base over 6 km above the horizon, such as Cirrus. They are thin, streaky,white clouds and no rain falls from them.
Two air masses of different temperature, etc. will not mix when they come together. They clash causing bad weather. This is known as a ‘front’. Typically cold fronts occur in the southern hemisphere while warm fronts occur in the northern hemisphere.
When a mass of cold air meets a warm mass, the heavier cold air pushes under the warm air, forcing it to rise almost vertical. This can cause violent weather for a short period.
When a mass of warm air meets a cold mass, the warm air rises over it in a slope extending 300-400 kms horizontally. This can cause the sky to become overcast, maybe with rain, drizzle or fog.
Sea and Land Breezes
Where the land and sea meet the air is heated unevenly causing local winds known as sea and land breezes.
These occur during the day when the land heats up quicker than the surface of the ocean and consequently the pressure on the land is lower than the sea. The surface wind blows from the sea (high pressure) towards the land (low pressure). Sea breezes extend up to 20 kms each way from the coast and generally attain maximum strength in the mid-afternoon and, unless affected by trade winds or pressure, their speed is approximately 15 knots.
These occur during the night when the land cools faster than the sea, and therefore its atmospheric pressure is higher. Land breezes only extend to approximately 10 kms from the coast and are not as strong as a sea breeze. The speed of a land breeze is around 5 knots and strengthen during the night, dropping off not long before dawn.
When a front approaches, the local sea breeze weakens and swings back to the east (west on the west coast), and the gradient (synoptic) wind of the front backs to the south during the front’s passage.
|Sea Breeze||Land Breeze|
The Bureau of Meteorology provides routine weather and coastal forecasts including weather warnings, available by phone or via the internet. These forecasts are updated twice daily and cover up to 60 nautical miles seaward.