II. The Atmosphere

6. Cloud and weather

Relative altitudes of cloud layers

Fig. 7: Relative altitudes of cloud layers.

Clouds are a sign of changing weather. Since ancient times, farmers and fishermen had often tried to forecast weather by observing clouds.

In Taiwan, on a fine winter or summer day, there will often be clouds with white round tops and flat bottoms. Their altitude will be above 1 kilometer and they are called fair-weather cumulus clouds.

At the same time, in the sky 3 to 6 kilometers above will be white clouds in the shape of cauliflower and a wad of cotton, respectively called altocumulus and cirrocumulus. When these types of cumuliform clouds appear, it means that the air flow is very steady and the weather is good.

Bad winter weather is usually related to nearing fronts or low pressure. Clouds are usually horizontal and placed throughout the sky and are called stratiform clouds. According to differences in altitude they can be divided into cirrostratus, altostratus, nimbostratus, and stratus. The highest which are cirrostratus, will appear at an altitude of 6 km and consist of ice crystals. This type of cloud is sparklingly white, and when backlit by the sun or the moon, a colorful circle of light will appear which is called a halo. An old Chinese saying states, "Sun halo brings wind, moon halo brings rain". A halo is a sign of bad weather because it means there is already a front or low pressure system coming from far away.

When a low pressure system draws near, altostratus clouds will appear at about 3 km in altitude. These types of clouds consist of a mix of ice crystals and water drops. The cloud is relatively thicker, spanning horizontally up to several hundred kilometers, and it is capable of blocking sunlight. Precipitation will fall from this type of cloud. Nimbostratus clouds appear near low pressure systems, usually at around 800 meters in altitude, with dark hue and close to the ground. They will bring continuous rain.

During a hazy, summer afternoon before a thundershower, the clouds you will most probably see are high, dark-grey cumulonimbus. They indicate that there are abundant upward motions of air inside the clouds; the speed of which can reach as much as 600 meters per minute. In as short as ten minutes, a cloud pillar over 6000 meters in height can be formed.

In these cloud pillars are ice crystals and water drops. The ice crystals near the top of the cloud are usually dragged by the wind into the shape of an anvil known as cumulonimbus clouds. Due to the strong convection within the cumulonimbus cloud, when the rainfall begins it will usually be accompanied by gusty winds created by the rapid descent of cold air. Wind speed can reach 40 to 45 knots in a short time. That's why when there is a thunderstorm there are also strong wind changes.

This type of cumulonimbus cloud also comes in springtime alongside cold fronts. However, the creation of a springtime cumulonimbus is different from that of the summer time ones. They are formed by the uplifting of warm and wet air being adjacent to the fronts.