Clouds come in all shapes and sizes, but they are usually flat on the bottom.

Air is a mixture of gases, the most abundant of which are nitrogen and oxygen. The third most abundant gas in air, and the most variable, is water vapor. Clouds are made of microscopic water droplets that have condensed out of water vapor in rising air.

The amount of water vapor that a given volume of air can hold depends on the temperature of the air. The warmer the air, the more it can hold, much in the same way that warm water can dissolve more sugar than cold water. Humidity is a measure of the amount of water vapor in the air.

Usually we speak of relative humidity, which is the percent of saturation of the air with water vapor at a given temperature. When the air is saturated, tiny droplets of water will surround the grains of dust and salt that are always present in the air.

At 50% relative humidity, the air contains half of its capacity for water vapor at that temperature. At 75% haze begins to form as dust particles attract the vapor and swell with water like tiny sponges, like the salt in the salt shaker on muggy days.

Evaporation of water from the earth’s surface increases the humidity, but that is not the only way it can increase.

Warm air can hold more water vapor than cold air, so cooling the air increases its relative humidity. That is why the air gets muggier as the temperature falls around sunset.

Likewise, the morning haze dissipates as the rising sun heats the air and lowers its relative humidity.

When air rises, it encounters lower pressure and expands. Expanding causes it to cool, which increases its relative humidity. If it reaches an altitude where it cools below the saturation point, water vapor then condenses into many tiny micro-droplets of liquid water at that altitude.

The droplets are microscopic in size and weight, small enough to be held aloft by the rising air but just large enough to scatter light and make the collection of droplets appear from a distance as a white cloud.

The bottom of the cloud is at the condensation level. Below that level the air is too warm for water to condense; above it the air is too cold for water to remain as a vapor.

The base of the cloud at the condensation level marks a temperature boundary between saturated and unsaturated air. Since temperature decreases in a regular way with altitude, the condensation level is flat, like the boundary between oil and water in a jar.

So are the bottoms of the clouds.

Once the air has reached the condensation level, it will continue to rise if it is warmer than the surrounding air. Condensation releases latent heat from the water vapor and adds to the air’s temperature as it continues to rise, causing more condensation. Bubbles of warmer air might burst out of the top of existing clouds, causing cauliflower cumulus clouds.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.