Land and sea are different: One is solid, the other liquid; one is opaque, the other transparent; one is inhabited by walkers, the other by swimmers.

The clouds over land and sea are different, too. Because they're different, they respond differently to human activity.

Whether water is solid (ice), liquid (water), or gas (steam), it's composed of the same things: Identical molecules of water.

The differences among ice and water and steam stem from how tightly the molecules are bound together. In ice, they're bound tightly; in water, more loosely; in steam, they're hardly bound at all.

All liquids are loosely-bound, but some are looser than others, which makes it easier to convert them to gas. Water molecules, for instance, are bound more loosely than the atoms in liquid aluminum, so it takes less energy to boil water - to separate its molecules from each other - than it does to boil aluminum.

How easy it is to turn droplets of a liquid into gas also depends on the size of the droplets.

After you've taken a shower, the mirror is fogged by tiny water droplets; the drops you've splashed on the floor are much larger.

The tiny droplets on the mirror evaporate, or turn to gas, way before the drops on the floor. Small droplets evaporate faster than large ones.

When water vapor condenses to form cloud droplets, unless the droplets grow large quickly, they evaporate before a cloud can form.

Problem: When pure water condenses to form droplets, the droplets are so small they evaporate immediately. With only pure water, it's very hard to make a cloud.

Shower experiment: Sprinkle some salt on a dry piece of paper and place the paper somewhere in the bathroom where it won't get splashed. Then take a shower.

After your shower, the salt crystals look wet. Salt is hydrophilic, or "water-loving." Salt loves water so much, it pulls molecules of water right out of the air. Given the chance, salt can pull enough water out of the air to form a droplet in which it will dissolve.



Salty cloud, dusty cloud

When ocean spray is whipped up by the wind, or when bubbles pop at the surface, tiny droplets of seawater fly into the air. Still in mid-air, these droplets evaporate, leaving behind tiny particles of sea salt. Wafted high into the sky, the salt particles pull water molecules out of the air, forming droplets that grow too fast to evaporate.

Salt particles are cloud condensation nuclei. Gazillions of salt particles create gazillions of droplets, and a cloud forms over the ocean.

The land lacks salt, but there's plenty of hydrophilic dust. Dust particles act as cloud condensation nuclei, too.



The diesel connection

There's a lot more dust over the land than there is salt over the oceans, so land clouds contain more droplets than ocean clouds.

Since there are more of them, all competing for water molecules, the droplets in land clouds are smaller than those in ocean clouds.

With more, smaller droplets, clouds over the land reflect more light than clouds over the ocean. Land clouds are brighter - whiter - than sea clouds.

It's also harder to make raindrops out of the tiny land-cloud droplets than the larger droplets in sea-clouds.

Ships powered by diesel engines emit huge quantities of fine particles - artificial cloud condensation nuclei. As a ship's exhaust rises into the clouds over the ocean, it collects moisture and forms new droplets - lots of droplets - creating a cloud-within-a-cloud, a bright land cloud within a duller ocean cloud.

Seen from space, the paths of ships - ship tracks - show up as bright streaks of land-type cloud, blazed into the duller clouds of the ocean.