What Makes Clouds? (1965)

where do clouds come from what are they made of to try to answer these questions first take a look of cloud when a cloud hugs the ground we call it fog this fog is covering part of the Golden Gate Bridge and San Francisco Bay let's take a closer look at it yeah in a fog you can often see water dripping from trees and plants when you drive into a fog you often see water drops collecting on the windshield now let's see what happens when we fly into a cloud high above the ground water drops collect on the strut and on the windshield of the plane evidently clouds are made of water but what does this water look like in the cloud itself under a microscope water in a cloud looks like this these Cloud drops are magnified hundreds of times their normal size it would take thousands of them to make just one of the drops we saw on the strut of the plane and many trillions of them suspended in the air to make a cloud but where does the water that makes these tiny Cloud droplets come from let's try to answer that question by watching clouds form we'll speed up the action with a timelapse camera the water to make clouds obviously comes out of the air there must be a large supply of invisible water in the air where does this invisible water come from you may have observed what happens to water exposed to air on a clear day where did the water go it went into the air it evaporated it changed into water vapor which is an invisible gas the water in the oceans evaporates too it is estimated that about 10,000 billion gallons of water evaporate from the oceans each year watch what happens when we trap the air around this plant the plant must be giving off water vapor into the trapped air and some of it collects as drops on the inside of the jar a Big O tree gives off over 100 gallons of water per day an acre of corn gives off about 3 or 4,000 gallons of water per day the water that's given off by all growing things and the water that evaporates from all wet surfaces goes into the air we can't see it because it's in the form of invisible water vapor the problem now becomes what makes invisible water vapor turn into the drop that form clouds you may have noticed what happens when you make a pitcher much colder than the air around it water collects on the cold surface of the pitcher but no water collects on the other pieces of metal table wear which have not been cooled the invisible water vapor in the air changes back into liquid water again we say it condenses when the air is cooled by coming into contact with the cold sides of the pitcher so it appears that temperature has something to do with whether air holds or releases its water vapor let's investigate this further this plexiglass box is airtight it holds 8 cubic feet of air the air in it is very dry because we have pumped it through the cylinder of water absorbing chemicals this fan keeps the air in the Box thoroughly mixed and the clock will time the experiment the temperature of the air in the box is 70° now we'll put 40 cubic cm of water in the dish let's see how much water the dry air in the Box will take from the container we'll speed up time with the time-lapse camera after losing 5 cubic cm the water in the dish has stopped evaporating the air in the box has taken all the water vapor it can hold at 70° we say the air in the box is saturated now let's see what happens when we heat up the air in the Box again the water has stopped evaporating now the air in the Box holds 15 cubic cm of water the air at 100° has absorbed all the water vapor it can hold it is again saturated what will happen now if we cool the air as the saturated air close to the cylinder is cooled the water vapor in this cooler air condenses and it condenses onto the nearest available surface in this case the surface of the cylinder so to understand how clouds are made we'll have to answer two questions questions how are hugee masses of air cooled enough so that the water vapor in the air condenses and what surfaces are there in the air for water vapor to condense on let's look into the question of cooling first the air inside this tire is compressed it's under higher pressure than the air outside the tire the temperature of the air in the tire is 100° about the same as that in the room what do you think will happen to the temperature of the air in the tire if we let it expand as the compressed air in the tire rushes out it expands and when it expands it cools the temperature of the air has dropped more than 15° so one way to cool air is to let it expand here is a plexiglass box containing air attached to the top is a plastic bag let's see if we can make the air in the Box expand for [Music] [Music] we'll photograph this experiment with a time-lapse camera watch what happens to the bag as the plane climbs to 10,000 ft [Music] the air in the box is expanding into the bag because the pressure of the surrounding air grows less as the plane climbs higher at an altitude of 10,000 ft above sea level the air in the box is expanded enough to fill the bag the air in the Box expanded because the air pressure up here is much lower than it was on the ground where we started since we know air cools as it expands we can be sure that the air in the Box must have cooled as we carried it up to higher altitudes now we have a clue to one way in which large masses of air in nature can be cool to form clouds when heat from the Sun warms moist air near the surface of the Earth the air rises as this warm invisible air rises it expands as it expands it becomes progressively cooler finally the water vapor in the air begins to condense into billions of tiny water droplets and clouds form but we have left out one important factor earlier we saw that water vapor needs surfaces to condense on if there are any surfaces up here they must be very small because we can't see them let's investigate this further this is a centrifuge it can spin a sample of air at 24,000 revolutions per second fast enough to throw anything that may be in the air against this piece of specially treated foil inside this cone when this foil is photographed through an electron microscope its surface looks like this these are particles magnified more than 30,000 times they were contained in a sample of air collected off the California coast they are so small and light that they can remain suspended in air almost indefinitely most air contains such particles they could be the surface sures on which water vapor condenses let's investigate this idea the air inside this glass chamber is saturated because we have let it stand for several days it has taken all the water vapor it can hold from the water in the bottom of the chamber also the air has been filtered so that it is as clean as we can make it we know if we make the air in the chamber expand it will get colder let's make it expand with the pump as the air expands and cools some of the water vapor should condense but because there are no particles in the air no Cloud can form air in nature is never completely clean so let's add some invisible smoke particles to the air in the chamber now we have invisible smoke particles in the air inside the chamber what do you think will happen when we lower the temperature of the air this time a cloud forms because the tiny invisible smoke particles provide surfaces on which the water vapor can condense when the air in the chamber is allowed to contract back to its original volume the cloud disappears the tiny water droplets change back into water vapor again what do you think has happened to the Smoke particles the smoke particles are still there when the air expands again the water vapor condenses on them again and the cloud reappears most clouds in nature form in a similar way as a mass of air rises it expands and cools if it cools enough the water vapor it carries condenses billions of tiny water droplets form on the billions of particles in the air although we can't see these particles you can be sure they are there otherwise we would have no clouds at all if most clouds are produced by air which Rises expands and thus is cooled how then is fog formed near the ground and what happens when and clouds form at very high altitudes where the temperature is well below freezing what must these clouds be made of and what about rain it takes about a million Cloud droplets to make one average rain drop how these droplets collect to make rain is one of the many unsolved problems being investigated by meteorologists today [Applause]