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Apart from some ships of the Royal Navy and occasionally on ocean weather ships, cloud height at sea is obtained by estimation. The first step in estimating cloud height consists of identifying the cloud as a type belonging to one of the three classes, low, medium or high. Low clouds have their bases below 6500 feet (2000 m). Medium cloud layers usually occur at levels between 6500 and 18 000 feet (2000 and 5500 m), and high clouds are usually above 18 000 feet (5500 m). As a rough guide, the heights of the bases of the various types of low cloud may be expected to be between the following limits:

Stratus | Usually below 2000 feet (600 m) and sometimes nearly down to the surface.
Nimbostratus | 500 to 4000 feet (150 to 1200 m) usually below 2000 feet (600 m) in moderate rain or snow.
Cumulonimbus | 2000 to 5000 feet (600 to 1500 m).
Stratocumulus | 1500 to 4500 feet (450 to 1350 m).
Cumulus | 1500 to 5000 feet (450 to 1500 m).

These limits tend to be considerably higher in low latitudes; this applies particularly to high clouds.

It is difficult to estimate cloud height without much practice. The apparent size of the cloud elements is often an indication of height. For example, the lower the height of the individual cloudlets of an altocumulus layer, the larger they will normally appear. Layers having the appearance of altocumulus with large individual elements are often found at heights between 6000 feet (1800 m) and 10 000 feet (3000 m). The estimation of the height of stratified cloud, e.g. altostratus or nimbostratus, is particularly difficult. The lack of pronounced structure makes it easy to gain a false impression of height. Valuable experience can be gained on occasions when the observer knows that his ship is steaming towards a depression by watching the gradual lowering of the cloud base. The observer's impressions of the appearance of the sky in the successive stages of lowering will assist his judgement on future occasions. It is only by such experience that an observer can distinguish between a layer of nimbostratus in the lower middle band and a similar layer at, perhaps, only 2000 to 3000 feet (600 to 900 m).

Care must be taken before using the apparent speed of cloud as an index to its height. This apparent speed depends not only on the velocity of the wind at cloud level but also on the course and speed of the ship itself.

When coasting, cloud height may sometimes be estimated by comparison with the height of the mountains or hills in the background. In using this method, however, it should be remembered that cloud is usually lower over the hills than elsewhere and that it is the general level over the sea that is required.