Low Clouds Made Really Simple

To begin, let's define what we mean by low clouds. There are three basic types of cloud: stratus, cumulus and, cirrus. The cirrus clouds are those high, wispy strands that are usually seen as aligned parallel to one another along the wind direction at that altitude. It gets colder as altitude increases and cirrus clouds are so high, in the upper troposphere, that they are composed of ice crystals rather than water droplets.

It is the other two types that we are concerned with here, the stratus and cumulus. These are composed of water droplets that have condensed on particles of dust in the atmosphere. These are the low clouds because they are found at altitudes much lower than cirrus clouds. Cumulus are those fluffy clouds that are seen especially in the summer and can reach great heights. Stratus is a dull sheet of cloud that covers much, or all, of the sky.

(By the way if you have ever wondered why we can have water-based clouds in winter, the posting "Water Made Really Simple", on the meteorology and biology blog, gives my explanation of it).

Most books about weather and the atmosphere explain the difference between the formation of stratus and cumulus cloud as being the motion of air. If there is a strong vertical motion to the air, often caused by the heating of the ground in the morning, we get cumulus clouds at interval with spaces in between them. The updrafts are what forms these clouds, and the returning downdrafts take place in the spaces between the clouds.

The layers in hail stones form in towering cumulo-nimbus storm clouds when water gets pulled into a powerful updraft, is carried high enough in the atmosphere to freeze, falls back down before being caught in another updraft and repeating the circuit. This forms another layer and continues until the hailstone is heavy enough to fall to the ground.

If such a vertical motion to the air is lacking, we get a simple layer of continuous cloud that is known as stratus. The word "stratus" means layer.

But there is a profound difference between stratus and cumulus cloud beyond the vertical or horizontal alignment of the cloud. This difference lies in the altitude of the base of the cloud.

A layer of stratus cloud can be found at any altitude from the ground up the the level of cirrus cloud, which might be 8 km (about 5 miles) above the ground. Stratus cloud at ground level is known as fog.

Cumulus cloud, in sharp contrast, always seems to have it's base right around 4,000 feet (about 1200 meters) or so. In fact, this is such a strict rule that larger cumulus clouds usually have a bottom that appears as flat as a floor or ceiling. How many times have you seen cumulus clouds across the sky with their bases appearing as neatly parallel straight lines?

I got to wondering what makes stratus and cumulus clouds so different in form.

As we gain in altitude, there are two primary factors that change. There is a drop both in temperature and in atmospheric pressure, a drop in either decreases the ability of the air to hold water vapor (vapour). The change in pressure is much more precise than the change in temperature. The change in temperature as altitude increases can be affected by any number of other factors, from wind to the terrain or water below.

But the change in pressure tends to be so constant and precise that it can be used to measure the altitude. The altimeters used on smaller aircraft are basically barometers calibrated at ground level and adapted to measure the altitude. Regardless of whether we are near a high or a low pressure area, in terms of the weather, pressure still tends to decrease at a virtually constant rate with increasing altitude.

It became clear to me that, instead of defining stratus cloud as a result of horizontal air motion and cumulus as vertical, it would be more accurate to state that stratus cloud is based on temperature while cumulus cloud is based on pressure.

Stratus cloud forms when water evaporates, moves upward in altitude, and then condenses when it reaches an altitude that cannot hold as much water vapour (vapor). This can take place at a very wide range of altitudes, starting at ground level.

But cumulus cloud is based on change in pressure, not on temperature. An updraft of warm air carries it's heat with it, so temperature ceases to be as much of a factor. A column of air, rising upward from the ground, reaches a height with lower pressure. This causes the section of air to expand, so that it cannot hold as much vapor (vapour). The water condenses into a cumulus cloud.

This is why the bases of larger cumulus clouds can form such nearly-perfect straight lines. It is because the change in pressure with altitude is so much more precise than the change in temperature.