Identifying clouds
(Section 2.7.2)Clouds originating in the troposphere are identified by genus, and where possible species, varieties, supplementary features, accessory clouds, mother-cloud and any other meteors associated with the cloud.
When identifying clouds, observers must:
Wear sunglasses
1. Ideally, by day, wear polarized (prescription) sunglasses, preferably those with opaque side wings to shut out light from the sides, especially when viewing high cloud. If the polarized lenses make an insignificant difference to how you see the sky, it is likely the lenses are not properly oriented.
Polarized lenses minimize the dazzling effect of bright sunshine and protect your eyes from ultraviolet radiation. They also reveal the presence of cirriform cloud when very thin, such as Cirrostratus nebulosus, by creating greater contrast between cirriform clouds and the blue sky, and when the clouds are veiled by haze.
Conduct night observation
2. At night, perform the observation from as dark a place as possible, well away from lights. Let your eyes adjust to the darkness − this will take at least 5 minutes. Night vision works best when using peripheral vision; moving your head from side to side will reveal more detail than a fixed gaze.
Maintain a continuous watch
3. Accurate observing relies on noting the constant evolution of clouds. Has the Altocumulus formed due to the spreading of tops of the Cumulus congestus observed 15 minutes earlier? Distant lightning may be revealed where it has not been visible at routine times of observations.
In particular, observe the sky during sunrise and sunset. The systematic changes in colour of clouds in these transition periods may confirm or bring to your attention the presence of multiple cloud layers.
Observe the whole sky
4. Observe the sky in its entirety, including the complete horizon:
- Look towards the upstream horizon to see if the invading Cirrus uncinus is thickening to Cirrostratus
- Look towards the Sun may reveal a 22° halo, confirming the presence of thin Cirrostratus nebulosus that you observed because you were wearing polarized sunglasses
- Look all around the horizon − you may have an expectation that thunderstorms always come from the west, but on this day, they may come from the east
Observing the sky in its entirety can assist in determining the general character of the sky. Isolated areas of virga beneath an extensive layer of Altostratus translucidus may indicate slow transition to Altostratus opacus; conversely, extensive areas of virga may indicate rapid transition to Altostratus opacus and even transition to Nimbostratus.
Watch out for varying appearances
5. Be aware that clouds of the same genera, species and varieties may appear differently in different meteorological situations. For example, cumuliform clouds during an outbreak of cold polar air may look different to cumuliform clouds during an incursion of unstable tropical air. During the cold outbreak, the clouds appear sharper in outline, broader and with little vertical extent; they are less sharp, more turreted and of great vertical extent in tropical air.
Other factors such as vertical wind shear can affect the appearance of clouds. A few examples are:
- Cumulonimbus capillatus incus tops are often asymmetrical (spread downstream), but are occasionally symmetrical (spread out in all directions) when there is minimal wind shear at the cloud top.
- Cirrus uncinus usually has a trail (falling ice crystals) falling below and behind the top of the tuft (head) as the wind speed usually increases (positive shear) with height from the trail to the tuft (head). When there is no difference in wind speed (nil shear) with height through this layer, the trail appears to fall directly beneath the tuft (head). When the wind speed decreases (negative shear) with height through this layer, the trail falls below and ahead of the tuft (head). The positive and negative sheared Cirrus uncinus are mirror images of each other, although even an experienced observer may see them as being completely different.