This little blog is simply to act as a reminder for symbols you may commonly see on weather charts. As we know weather forecasting is not an exact science but a science it is, the information given below is a very simple summary of the common symbols used with the typical weather associated.
Warm Front – This symbol depicts the leading edge of a warmer air mass. The air ahead of this front will be cooler than the air behind it. Typical weather ahead of the warm front will be fine then heavier rain, as the warm front passes the rain will stop and become drizzle in the warm sector.
Cold Front – This symbol depicts the leading edge of a colder air mass. The air ahead of this front will be warmer than the air behind it. Typical weather just before and at the front itself will be heavy rain, hail, thunder, lightning, behind the front there will be sunny periods with scattered showers.
Occluded Front – An occluded front is formed when the cold front catches up the warm front, the symbol combines the shapes of the warm and the cold front. Typically underneath an occluded front there will be rain as the warm air is being pushed aloft and condenses.
Frontal System – This diagram shows a frontal system and the relationship between the fronts described above. This is a typical frontal system in the Northern Hemisphere and can be seen on good quality weather forecasts on television. The system is formed around an area of low pressure which will circulate in an anti-clockwise direction. This example already shows an occluded front where the cold front has already caught up with the warm front, the cold air in front of the warm front and behind the cold front with the area of warm air being squeezed in between.
Developing Warm / Cold Front (Frontogenesis) – This symbol depicts the formation or intensification of a meteorological front caused by the difference in temperature of adjacent air masses. No significant weather associated.
Weakening Warm / Cold Front (Frontolysis) – This symbol represents a front which is loosing its identity usually due to rising pressure and where air masses are diverging. Cloud and precipitation becomes increasingly fragmented.
High & Low Pressure Systems – This diagram depicts pressure systems in the Northern hemisphere, the arrows are showing us the wind direction, the wind moves anti-clockwise around an area of low pressure and clockwise around an area of high pressure. You will also note that the arrows around the low pressure are pointing slightly inwards toward the centre this is due to the wind converging around an area of low pressure, the opposite is true of high pressure, the wind diverges and therefore points slightly away from the centre.
Surface Pressure Chart – This surface pressure charts shows areas of high and low pressure depicted by the H and the L on the chart. You can also see lines all over the chart which are called isobars, these connect areas of the same pressure, some of these lines have numbers on them which is giving you the pressure at that particular isobar. Isobars are like contour lines on an ordnance survey map, the closer they are together the steeper the hill, with isobars the closer they are together the stronger the wind. With this type of chart you can actually measure the wind speed by measuring the space between the isobars and using the scale in the top left hand corner.
Wind symbols – If you use grib files or sites like Passage Weather you will see lines such as the ones on the left here showing you the wind direction and the wind speed in the area you are looking at. Wind is always described in relation to the direction it is coming from, in the UK our prevailing wind and weather comes from the South West and in wind terms would be described as a South Westerly. On the bars on the left there are a number of different lines or barbs, the short lines represent 5 knots and the long lines 10 knots of wind. You can now see combinations of these making up 15, 20, 25 etc until you reach the triangular shape which represents 50 knots. These lines will always be on the back of the arrow and this is where the wind is coming from so in all the examples on the diagram the wind is coming from the north east.
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