Frequently Asked Met Questions.
Fort Regent (Jersey) used to fly weather signals, what did they mean?
The "Cone" and "Cone and Ball" signals are used when gale or strong winds are expected. A guide to the signals can be seen
here.What do the weather graphics used on this site mean?
The graphics show the most likely or significant weather for the forecast period. They are used in the "Summary for Today or Tonight" and in the "5 day Forecast". A decode for all the symbols used can be found
here.What is difference between a ground and air frost?
An air frost is when the air temperature falls below zero. Meteorologists measure the air temperature at a standard height of 1.4 metres above ground level, so when you hear the forecaster say ' a low of 3 degrees Celsius tonight', this means the temperature at around chest height. On a clear night the temperature at ground level falls further, and hence a ground frost can be recorded, with ground temperatures around 4 to 7 degrees below that of the measured air temperature.
So tender plants near the ground can often be burnt when the air temperature is several degrees above freezing, so gardeners need to watch out for ground frost even when a air frost is not forecast.
Why is Guernsey often colder than Jersey in the summer?
The answer here lies with the position of Guernsey. The sea is colder in the mid Channel and the Island slopes to the north, away from the sun. When a warm summer's wind blows from the southeast from France, Guernsey is further from the French coast and the warm air cools as it crosses the sea.
What is meant by a UV index?
The UV index is added to the forecast in the summer to show the risk of sunburn. Tables showing the risk and what it means to you can be found
here.When does a high pressure become a low pressure?
As the name says, a "high" is an area where the air's pressure is higher than the pressure of the surrounding air. A "low' is where it's lower. Meteorologists don't have any particular number that divides high from low pressure; it's the relative differences that count.
The pressure is high at the surface where air is slowly descending - much too slowly to feel. And, this is going on over a large area, maybe a few hundred square miles. As air descends, it warms, which inhibits the formation of clouds. This is why high pressure is generally - but not quite always - associated with good weather.
The air that descends in high-pressure areas has to get to high altitudes in some way, and its done by rising in areas where the pressure at the surface is low.
As air rises it cools. As the air cools, the humidity in it begins to condense into tiny drops of water, or if it's cold enough, into tiny ice crystals. If there's enough water or ice, rain or snow begin to fall. This is why low pressure is associated with bad weather.