Clouds form when the temperature is cool enough for certain molecules in a planet's atmosphere to form droplets (condense). Clouds are usually made from minor ingredients in a planet's atmosphere (for example, Earth's clouds are made of water droplets that makes up less than 1% of our air globally while the majority of Earth's atmosphere is nitrogen and oxygen).
Clouds are important because (1) they block input energy from the Sun (a cooling effect); and (2) they trap energy from the surface (a heating effect). It is difficult to figure out which effect has greater long-term significance in how the temperature will change over time periods of years. Clouds are also extremely hard to model because they are so variable and changeable. The formation of clouds is also part of an energy transfer process because of an energy called latent heat. When a liquid turns into a gas such as liquid water evaporating, it absorbs energy while the temperature remains constant. The absorbed energy is the latent heat. (This is why we sweat---as the droplets of sweat produced by our bodies evaporate, they absorb energy from our overheated body taking away the excess energy we produce during exercise.) That latent heat is released when the gas condenses into droplets (clouds form). On the Earth, water evaporation and condensation provide a major source of energy transfer to drive the winds and they are key parts of the water cycle (hydrological cycle) shown in the figure below.
Water from the oceans, rivers, and lakes evaporates to become water vapor. Warm air is able to hold more water vapor than cool air so as convection in the troposphere moves air upward, the water vapor will condense at the cooler altitudes. The cloud droplets (or crystals) will grow as they pick up more water vapor. Eventually, the cloud droplets grow so large that the cooler air cannot hold them anymore and they fall to the surface. The rainwater (or snowmelt during warm days) runs downhill starting out as streams that flow into small rivers that in turn flow into large river down to the oceans. Water also percolates into the soil to become part of the groundwater that totals approximately 10% of the mass of the oceans. Some of the groundwater collects in reservoirs underground called aquifers. Groundwater can remain in aquifers for a million years or more. Some aquifer water can reach the surface and flow out to the oceans. Water that falls as snow can also store water for long periods of time as ice sheets and glaciers near the poles or a high elevations.
A similar sort of evaporation-condensation-precipitation cycle is found on other planets but the molecule may be something other than water. The situation closest to what happens on the Earth is what is found on Titan, a moon of Saturn. There, methane is the key player, instead of water. In fact, Titan is so cold that water is in deep freeze, so water there plays the role of rock here.
The figure also shows the effect of mountains. The mountains on the Earth are high in comparison to the thickness of our troposphere so they can provide a significant barrier to the circulation of air or change the circulation patterns. They can also "speed up" the condensation-precipitation process by forcing side-ways moving air to rise upward leading to storms on the mountainsides that face the prevailing winds. Air on the other side of the mountains is much drier so the land on the other side has little rainfall---a "rain shadow". Some example rain shadows include the southern part of the San Joaquin valley in California (formed by the Diablo Range and the Tehachapi Mountains), the eastern half of Oregon and Washington (formed by the Cascade Range), and the Atacama Desert (the driest place on Earth formed by the Chilean Coast Range).
Our oceans also affect atmospheric circulation by transporting heat energy and water, heating or cooling the land and air. For example, the California Current is a current in the Pacific Ocean that moves southward along the western coast of North America, from Brithish Columbia to southern Baja California. The movement of the cooler northern waters southward makes the coastal waters cooler than the coastal areas of the eastern United States at the same latitudes. The Gulf Stream is a warm surface ocean current born in the Gulf of Mexico that flows up the eastern United States seaboard and then veers northeastward toward Europe at about 6 km/hour (4 mph). The North Atlantic Drift (aka North Atlantic Current) continues carrying the warm water north of the British Isles to Scandinavia. This warm water makes western Europe have climate more like the United States instead of Canada of which it shares the same latitude range. Northern Norway is near the Arctic Circle but most of its coast remains snow/ice-free, even during the winter.
These currents show up in this section of a MODIS image of the global sea surface temperature (select the image to bring up a larger version in another window). Red and yellow indicates warmer temperatures, green is an intermediate value, and blues and then purples are progressively cooler values.
Tropical storms form over warm waters and can gather enough energy and moisture from the warm waters to turn in hurricanes (or typhoons). The ocean currents play a key role in why the southeastern United States experiences hurricanes in the summer and fall. The oceans store a lot of heat energy and release it more slowly than land or the atmosphere, so they provide a moderating influence on local climates and even global climate changes. Air circulation also affects the surface ocean currents so the air and ocean circulation are constantly playing off each other, though the ocean moves much more slowly than the air so the oceans lag behind the atmosphere. This complex interplay between ocean circulation and air circulation is a key piece of the Earth's climate puzzle that still needs a lot of research! NASA created a nice set of videos called "Tides of Change" for Earth Science Week 2009 that provide good visualizations of the role played by the oceans in Earth's climate. Episode 4: Salt of the Earth is especially good for illustrating the Gulf Stream-North Atlantic Drift. Also, see the gallery in the Aquarius website and do a search on YouTube with search term "NASA | earth science week".
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last updated: August 26, 2016