Weather by Malcolm Riley - Forecasting from the clouds, Continuing Malcolm Riley’s* top five weather indicators by cloud type for Australian conditions.

3. Cirrostratus

Cirrostratus invading the sky as a veil usually from the west (southern states) can be the indication of a cold front upstream from the location.

The cirrostratus is caused by strong winds at high levels in the atmospheres blowing the cloud formed at the top of storms that are located on the cold front. These strong winds blow the cloud well ahead of the front and are often shown on a satellite picture in the shape of an inverted comma.

Southern Australian ocean areas often have a swell from the south to west quarter. The strong northerly/northwesterly on the eastern side of a cold front can reduce the size of these swells ahead of the front. Cirrostratus cloud, a falling glass and swell is a good indication of an approaching cold front.

However, cirrostratus is not always a good indicator of the weather to be expected. The presence of the cloud indicates a cold front upstream of your location but gives it no indication of strength of winds, amount of rain or weather that can be expected behind the front.

Cirrostratus Two vessels A and B would both see cirrostratus cloud thickening from the west. Both vessels have strengthening northwesterly winds and falling pressures. In this case with a low pressure moving directly to the east, vessel B would experience much more severe conditions than vessel A. If, however, the system is moving to the northeast Vessel A would be likely to experience the more adverse conditions.

4. Cumulonimbus

This cloud starts of as a ‘fluffy chicken’ type cumulus cloud. It grows often due to the heating of the day of convergent air masses. With further instability in the middle layers (castellanus) it can grow to incredible heights of greater than 60,000 feet. These clouds can produce lightning, heavy rain and flash flooding, hail and strong and gusty winds and possibly tornados.

Cumulonimbus with further instability in middle layers castellanus.  / Shelf or Arcus cloud approaching the coast.The satellite picture shows a series of large thunderstorm with the upper winds blowing the tops of the storms towards the northeast. This is the process, repeated all along a cold front for a day or two that causes the veil of cirrostratus cloud to form.

Satellite picture of large thunderstorms. / Cirrus Clouds.You may see the cumulonimbus from quite a distance away. Sometimes the cloud is obscured by other clouds and the mariner can only see the bottom of the clouds. A fairly scary indication of a large and active cumulonimbus is the shelf or arcus cloud. If you see this coming towards you batten down the hatches a large storm with potentially severe weather is very close.

clouds5. Cirrus

Let’s all relax; with this type of cloud it is usually the portent of a fine sunny day and a good day out on the water. Oh, except for sunburn!

Then of course there is always that strange looking cloud where you cannot figure out what the atmosphere is trying to tell you. 

 

*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.