Why do clouds form over land?

I searched the various indexes on this site and could not find the answer to this question. Forgive me if it has been tackled before:

Why do clouds form readily over land and tend to disspiate over water?

A pilot friend of mine told me he could always find a lake by looking for an area with no clouds.

I have read that Pacific Islanders could find small specks of land on the other side of the horizon by rowing towards solitary, stationary clouds.

Satellite images on T.V. weather reports show clouds forming over Cuba and Puerto Rico every morning, but not over the Caribbean and Atlantic. (Hurricanes being an obvious exception to this less-than-iron-clad rule.)

And here on the western shore of Lake Michigan, I have often watched clouds roll in from the west, break up over the lake, and reform on the far shore (giving Grand Rapids one of the cloudiest climates in the lower 48).

So why is this? I would have thought that bodies of water would give off plenty of water vapor, and thus be more likely to generate clouds.

Any clues you can provide would be greatly appreciated.

Ooh! Ooh! Ooh! I know! I know!

Okay, the sun’s heat is reflected during the day, so it creates a little area of low pressure warm, dry air right above the land. This draws cooler, moister air from the sea inland, which rises to become hot, moist air - clouds! At least, that’s how it works for a seabreeze front here in Florida.

Anyone else got a meteorological question?


“I like Florida; everything is in the eighties. The temperatures, the ages, and the IQs.”
– George Carlin

Clouds don’t always dissipate over water. One time when I was flying over one of the great lakes, the clouds were only over the lake itself. It looked really cool.

Yer pal,

Although SanibelMan was right, the important point is that it is not just the amount of water vapor in the air that determines cloud formation, it is also the temperature.

The reason the clouds form over land and not over water – or vice versa, as Satan pointed out – is that the temperature and humidity at the one location are right for cloud formation and not at the other. Over small bodies of water I would guess it is more of a temperature thing, whereas at the ocean shore (the land-sea interface, as my AWACS friends like to call it) humidity probably plays a larger role, although temperature is still significant, as SanibelMan pointed out.

“non sunt multiplicanda entia praeter necessitatem”
– William of Ockham

Aren’t clouds formed when water particles stick to dirt particles?

Louie: basically, yes.

Well Louie, raindrops, and water vapor in clouds condense on dust particles (among other things). It isn’t a cause-effect thing though. Its safe to assume that there is dust everywhere all the time. The air conditions at the location dictates whether a cloud forms or not, the dust is just a vehicle.
Clouds form when the conditions are opportune. The necessary components are altitude, relative humdity, temperature, and pressure. All these factors combine to effect the dew point which determines if a cloud can form in a given location or not. Water plays a huge role in influencing these factors, but I doubt that it is nearly as cut and dry as you propose. In certain climates where the air conditions are fairly steady (tropical ocean, etc.) land-water interfaces will have a predicatble effect. But in most of the US the conditions vary greatly because of seasonal changes and high and low pressure systems.

If the temperature is quite high, the air can hold more water without condensing. Usually lakes are much colder than the land in spring, and when air moves out over the water the temperature drop sudddenly and that water needs to go somewhere, this creates clouds over water.

In contrast in the fall the water can be much warmer than land and therefore cause clouds to dissipate over water.

These variances can be seen over urban areas which dissipate heat much faster than rural areas.

In the winter the air is very dry and cold. The water is a bit warmer and will impart some moisture into the air. This causes clouds to form over water then as they move back over land causing lake effect snow when that moisture needs to be released back to land to re-establish the dry-cold equilibrium.

All these calculations are layed out on complicated charts that weathermen read to predict cloud cover. Basically, temperature change is more important, than the evaporation over water versus over land. Actually you’d be suprised how small the differences are between ocean evaporation and wet soil.

Any questions class?