|
Click here for a printable version of this page.
The rising and
falling of waves of fog in “Sea of Clouds” is caused by gravity. The cooler, denser, vapor-filled air flows
down, displacing the warmer, lighter air below, which then rises. (This is
similar to
the mechanism responsible for convection in “Convection Cells.”) Reminiscent of the way fog flows over the
coastal range from the Pacific Ocean, this pattern occurs in many places on the
Earth and on Mars (and — with fogs of substances other than water — on the moons
Titan, Io, and Triton).
Photo Credit: NASA
Canyons
on the Planet Mars Filled with Morning Ice-Fog
Fog is created
by condensation, the formation of liquid droplets as a gas changes from a
vaporous to a liquid state. When the gas in a particular area approaches
a temperature that favors its liquid form, a “condensation nucleus” (usually a
small solid or liquid particle) triggers the collection of gas molecules
together and stimulates the formation of a droplet. As air flows over such an obstacle its temperature may drop,
initiating the formation of a cloud.
Sometimes clouds cast shadows whose cooling effect can stimulate the
formation of other clouds. Fogs are
also generated by the partial vaporizing of a liquid. This is the process that creates “Sea of Clouds.” Click
here to experiment with a Cloud Chamber simulation.
These
processes are the source of all clouds.
Mars is the least cloudy planet of those that have an atmosphere. The Earth is often half covered with
clouds. Venus and the gas giants are totally swathed in clouds.
Photo Credit: NASA
The
Planet Venus
Light cannot pass through a liquid as easily as
it can travel through a gas. Thus, the
liquid in fog reflects light and makes the fog visible. The closer you are to the fog, the less
distinct is its edge; clouds sometimes look very solid and crisp only because
they are far away. As you pass your
hands through ”Sea of Clouds” you can see the formation of waves (as in
“Tectonic Basin”) and vortices (as in “Dust Devil”). Clouds reveal the flow of fluid, but it is important to remember
that the flows are present even if the fog is not there to make them
visible.
|
