March 2012  Cloud Reading using Isobars

Former MetService Ambassador, Bob McDavitt, shares his expertise in the art and science of reading weather maps and clouds.

Weather forecasts follow the scientific principle of observe, theorise, experiment and report. We gather data, capture a pattern, and extrapolate its trends to inform weather users of the most likely next change. Computers help crunch numbers to run mathematical models based on known atmospheric physics and to extrapolate the captured pattern.

Despite the sophistication of modern forecasting models, real weather remains a mix of pattern and chaos. Things unravel chaotically in the real world, deviating from the extrapolated pattern. To evaluate the larger picture and take into account computer models, the season, and similarities with previous patterns, MetService uses a team of skilled and experienced meteorologists to produce the weather forecasts. By knowing how to tweak the words used in a forecast with your own local knowledge and your own eye-on-the-sky, you can enhance its usefulness.

MetService weather forecasts sometimes mention rain, and sometimes mention showers. By reading your own clouds and noting how they are changing, you can work out where and when any forecast rain or showers may eventuate in your area.

All clouds contain rising moist air but only a few produce rainfall. Basically there are three types of clouds: flat ones, wavy ones and bubbly ones. The factor that determines the type of cloud and any resulting rainfall is ‘upward motion’.

Flat clouds are the result of gentle large-scale upward motion created by air rising slantwise along a slope such as a frontal boundary. These layers of rising air have little and slow-changing buoyancy: a condition which is called stable air. They produce rain.

Bubbly clouds are a sign of rapid upward motions which occur on a small scale in time and space when the atmosphere has lots of triggered buoyancy, and is called unstable. They produce showers.

Wavy clouds are produced when strong winds blow over a range of mountains on a stable day. Supercooled cloud droplets form in the
lower pressure pockets that ripple downstream. These clouds, called Altocumulus lenticularis, are usually dry.

In order to be able to share our cloud watching experiences with others, we need to learn a few cloud names. The names we use were introduced in England by chemist Luke Howard in 1802. The two main Latin words are ‘Stratus’ for flat or stable and ‘Cumulus’ for lumpy or unstable. Combined, they describe a flat low layer of cloud with a bumpy base as stratocumulus.

The word ‘nimbus’ indicates rainfall. It’s used as a prefix for flat clouds, as in ‘nimbostratus’ but has the suffix ‘nimbus’ as in cumulonimbus.

Height above the ground is divided into thirds as follows: ‘Cirrus’ describes hairy looking clouds high in the sky, with the prefix ‘cirro’ attached to other clouds so high that they are made of ice crystals. The prefix ‘alto’ is added for middle clouds (the same meaning of the word is used for the alto voices in a choir). Middle clouds are composed of liquid droplets and some of these
may have a temperature below zero Celsius (supercooled). Low clouds carry no prefix, and are distinguished by being modified by the terrain or ‘feeling the ground’.

Average atmospheric pressure on Planet Earth is 1013.25hPa. The nearest isobar to this is the 1012, and indeed this is a magic isobar. It’s the dividing line; at any one time the isobars higher than 1012 will always be balanced by those lower than 1012. Robert FitzRoy selected the word ‘change’ to describe the 1000 to 1012 mean sea level pressure zone on a barometer. Usually the 1012 isobar is the straightest on the weather map. Isobars with lower numbers are associated with cyclones called ‘cyclonic’. Isobars with high numbers are associated with anticyclones called ‘anticyclonic’. Usually, but not always, anticyclonic isobars are associated with stable air, and cyclonic isobars are associated with unstable air.

In New Zealand flat, wavy, and bubbly clouds often co-exist, especially near the mountains. Even when the weather map and large-scale clouds indicate a stable day, a fresh breeze blowing onto a range of mountains from the sea can produce cumulus clouds and showers on the windward slopes. Stability can change during the day, often reflected in the clouds: a day may start with a flat deck of stratocumulus which has a lumpy underbelly, and by afternoon the cloud deck may have changed to a flat underbelly, indicating a change to unstable air. The clouds may then burn off and break up into cumulus clouds with lumpy tops. These changes add to the fascination of cloud watching.

Judging stability by reading weather maps and clouds is both an art and a science.

Bob McDavitt retired from the MetService in 2012. FMC thanks Bob for his valued contribution as the Bulletin weather columnist over many years. This column was originally published in the March 2012 FMC Bulletin