Symbols on Weather Charts

I recently gave a weather talk where the group were interested in the symbols that are on the synoptic charts (weather maps).

Many are familiar with the depiction of cold front; a barbed thick line. However, less were certain of warm fronts, occluded fronts and decaying or developing fronts. (Cold fronts were covered in Afloat Dec’12 and Jan’13).

Warm fronts form when warm air meets up with and ‘overtakes’ slower moving colder air.

As the warmer air is less dense it rises on top of and over the colder slower moving air. As the warm air is forced to rise over the colder air it cools, cloud will form and rain will often develop. This rainfall can be heavy and fall for many hours. The symbol on the chart for a warm front is a line with half circles.

Occluded fronts form when a cold front catches up to the warm front. The warm air is then trapped between cold air ahead of it and very cold air behind it.

As the cold air is denser than both of the other airmasses it forces both to rise ahead of it. The warm air is then trapped and keeps on being forced higher in the atmosphere. This leads to greater cloud formation and heavy rain. Occluded fronts are shown with a mixture of cold front triangular barbs and warm front half circles.

Depending on the temperature of the two cooler airmasses an ice storm can develop. These ice storms are usually seen at high latitudes. The two cool airmasses are both below freezing and the trapped warm air is not. As rain falls through the cold layers it can become supercooled. A supercooled raindrop is liquid despite being well below freezing.

When the supercooled drop lands on anything solid it freezes. This can lead to huge build up of ice on trees and man made structures. If the weight of the ice gets too much for the strength of the tree or structure they can collapse. These ice storms can also cause havoc in the marine world.

For vessels underway ice can form up high on the vessel making them unstable and may build up to such an extent the vessel becomes top heavy and may capsize.

Vessels that are moored and are unattended may build up such a weight of ice that they can sink under the extra weight.

Decaying and developing fronts have the symbols of the individual type of front (cold or warm) with interspaced dots or plus signs between the frontal symbols. Dots indicate a developing front and plus signs a decaying front.

*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.