Try putting other materials (wood, a piece of an apple, cardboard, plastic, a stone, whatever comes to mind ...) On a tray with one cm of tinted water.

Check which one moistens above water level. Can you explain the reason why?

Water in the atmosphere

Although we are used to thinking of water as a liquid, in the atmosphere we can find it in solid, liquid or gaseous states.

Water is only 1% of the mass of the atmosphere, but is essential for the climate as it is an important greenhouse gas responsible for global albedo.

Global map of cloud coverage


Animated map made by PhD students at the University of Delaware, which shows the annual evolution of cloud coverage over the continents.

The molecules of water are heavier than N2 or O2, which are the majority of gases, and they therefore tend to accumulate in the lower layers of the atmosphere.  50% of atmospheric moisture is in the lowest 2,000 metres.  Here, therefore, is where most clouds are situatedAt the poles, however, water-ice clouds can form that reach heights of 85 km above the surface.  Cloud currents we see in any other place on Earth are formed at heights of 12 km, and their characteristic forms depend on humidity, pressure and temperature.

Water in the atmosphere is in a gaseous state, but water in clouds form tiny water droplets or ice crystals, so small that they remain in suspension as dust particles.  These minuscule droplets are hundreds of thousands of times smaller than a normal raindrop, and in fact rain does not occur until the water drops reach sufficient mass to overcome the force of air friction, when they then fall due to gravity.   Interestingly, air friction also prevents the droplets from increasing in size indefinitely.  Having acquired a diameter of about 4 - 5 mm, droplets split into two. This is why raindrops are so homogeneous in size.


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