By observing and interpreting the formations of clouds in the sky above, it is entirely possible for the casual observer to predict what kind of weather to expect over the coming hours and days.
So, What are Clouds?
Clouds are the result of moisture-laden air cooling down to saturation point (its dew point) by one of two methods - either by decreasing the air pressure, when the air is forced to rise (known as adiabatic cooling), or when the air cools through conduction when it mixes with colder air.
When air cools down, it is less capable of holding onto water as vapour, and so the water vapour condenses, forming clouds.
When the water vapour condenses it forms water droplets and/or ice crystals which are held in the atmosphere by the turbulent, convective movement of the surrounding air and that in the cloud itself.
When these droplets coalesce into larger and heavier droplets, they become too heavy for the convective forces to keep them aloft. This leads to precipitation, either in the form of rain or ice (snow, ice- crystals etc).
Clouds are formed as a result of the following methods:
As the Earth radiates heat during a clear night, fog (which is simply ground-level cloud) forms at the surface as warm air is cooled by coming into contact with the colder ground.
When a cold front (the edge of an air mass of cold air - usually coming from Polar regions) pushes its way underneath a warmer air mass, the warm air is forced upwards (because the cold air is denser and heavier) and due to adiabatic cooling, the air reaches saturation point and clouds are formed.
Clouds can also be formed (from the same method of adiabatic cooling) when the warm air is forced upward when it comes into contact with mountains and has no option but to go up. This is why clouds and rain tend to form more on the windward side of mountains and hills, and as the air returns to ground on the leeward side the precipitation ceases and the weather improves.
Types of Clouds
A brief word on nomenclature. Four simple Latin terms are used to create the naming convention that describes the list of cloud types we see in our skies:
- cirrus - meaning 'curl, fringe'
- stratus - meaning 'spread over area'
- cumulus - meaning 'heap, pile'
- nimbus - meaning 'rain-bearing'.
These four terms are compounded together in a various ways (as described below) to form the well known list of high, medium and low cloud types we use today.
The terms cirrus, stratus and cumulus are used as primary cloud type designators, describing individual cloud types on their own. The term nimbus is purely a prefix or suffix (depending on the cloud) and add the meaning of 'rain' to that primary cloud e.g. cumulus becomes cumulonimbus - rain bearing cumulus.
Cirrus clouds are those dreamy, thin, wispy clouds, extremely high up in the atmosphere that can sometimes form a halo around the sun or moon. They are predominantly formed from ice crystals and are often said to be the precursor to bad weather as they tend to materialise on the leading edge of frontal activity.
Cumulus clouds, also known as heap clouds, typically are the fluffy, cotton wool style clouds that are often seen in our skies. They are undoubtedly the best known clouds, but by far the least understood.
There are six major types of Cumulus cloud:
Cirrocumulus: high level clouds, usually formed of ice crystals at around 25,000 feet. They are often embedded within blankets of cirrus clouds and are a good indication of instability aloft.
Altocumulus: clouds usually formed at a medium level, around 8,000 feet, and made of a combination of ice crystals and water droplets. When seen in the early morning, they are a good indicator that thunder or rain is on its way.
Cumulonimbus: massive, towering cumulus clouds that can reach from very low levels to extremely high levels. They are usually associated with heavy rain showers, hail, and thunder and lightning. The top of these clouds tends to be anvil shaped, due to it being flattened by the tropopause1.
Fair weather cumulus: these are low level clouds, forming at low levels in late morning. They are not very dense and tend to resemble clumps of cotton wool. They tend to form in stable air and are a result of ground heating and localised convection. Glider pilots love these formations as they tend to indicate where a thermal would be. Pilots can skip from onw fair weather cumulus to another and potentially stay up as long as they like.
Swelling cumulus: cumulus clouds formed at medium levels. They tend to resemble cauliflower heads. They are a definite mark of instability, and can often grow into cumulonimbus, rain-bearing clouds. They are common in hot regions, especially deserts.
Cumulus congestus: high level clouds formed by massive upward movement of air within very unstable air masses. Its top is more like traditional fair-weather cumulus rather than the anvil shape of cumulonimbus.
The term stratus comes from the Latin meaning layered. The distinction in this type of cloud is that it has no real defined base or top. It tends to appear as a sheet of cloud, covering a potion of the sky.
A common type of stratus cloud that we often don't appreciate is that of fog. There are a few different classifications of stratus cloud that should be noted:
Cirrostratus: extremely high level layered cloud, often forming halos around the sun and moon. They can often forewarn of rain within two days.
Altostratus: medium level, flat clouds, often dark grey in colour. A progressive observation of the cloud cover darkening can indicate rain could occur within two days.
Nimbostratus: dark, overbearing, low level, rain bearing cloud that can give rise to persistent precipitation. It is possible to see areas of instability within the could cover, displayed through embedded cumulus clouds.
Forecasting for the Lay-man
Identifying the cloud types listed above, and combining a number of observations, spaced over a few hours, can give an indication as to what the weather has in store over the coming two days.
Obviously the government has invested millions of pounds into weather forecasting, so don't expect it to be fool-proof, but at least with some cunning and good identification, it's possible to do a bit of off-the-cuff forecasting to get yourself out of trouble, if necessary.
How To Tell if the Weather Is Going to get Worse?
If more cloud is forming, especially at medium or low levels, or the cloud that is present is getting thicker and blacker.
Clouds that are racing across the sky begin to get fuller, blacker or fill in from cumulus, to stratocumulus, or cumulus clouds begin to grow taller and blacker at the base.
Clouds are moving in all directions. This indicates extreme instability.
If it is a hot day and the clouds begin to grow in early to mid-afternoon, there is a good chance of rain and possibly thunder, later that evening.
The cloud base begins to drop.
How Do I Tell if the Weather Will Get Better?
Fog disappears and is replaced by clear skies by mid-morning, or early afternoon.
The cloud base height is obviously rising, and possibly holes begin to appear in the covering.
And What Can We Tell From the Clouds Themselves?
If any of the cirrus-style clouds start to join up or thicken, or start to drop in height then it would be a good guess that it may rain within two days. Similarly, if they do not change, staying thin and wispy, then it would be a fair assumption that it will stay dry.
If the halo surrounding the moon, or sun begins to shrink, then it is a good sign that the cloud is thickening. Precipitation should be expected within twelve hours.
If altocumulus clouds are whipping around the sky, normally from west to northwest, it is a fair assumption that there will be a hard and heavy rain or hail storm very soon.
Altocumulus forms mid-morning on a summer's day, and wisps of cloud begin to stream from the top of it, you can expect to have thunder or heavy rain showers by mid-afternoon.
Stratocumulus clouds begin to merge into more of a stratus formation, expect persistent rain within twelve hours.
If cumulus clouds do not develop until into the afternoon, or simply hang around in the sky without much development, expect continued fair weather.
If cumulonimbus clouds form fierce black bases and suddenly an unusual light bulb-shaped protuberance appears at the bottom of it, run for cover as a tornado could be on its way.