Level: Middle School to H.S.
Time involvement: 30-40 minutes.
Materials: Special clear plastic tank (3-4 gallons), blue and red food coloring, salt. This is a visual creation of what happens in the atmosphere. Because warm and cold air masses appear to be the same we can’t “see” actual frontal boundaries. Colored water in the tank simulates air masses of different temperatures. Addition of salt to the blue water simulates the greater density of cold air. Very clever! Amazing slow motion interaction when center divider is removed.
Discussion: Our atmosphere is in constant motion. Driven by the solar energy of the Sun, and shaped by the rotation of the earth, oceans, land masses, mountains, and the changing seasons, the occasional day or two of calm winds are the exception to the winds which at altitude may reach a hundred miles per hour or more. When these air masses “bump” into each other unsettled weather (clouds and precipitation) can occur.
A key concept in physics is Density. Density simply means how much weight or mass is found in a specific volume. For example a cubic inch of aluminum weighs 1.56 oz., a cubic inch of lead weighs 6.56 oz. Therefore, we would say lead is more dense. Since both are solids there is no possibility of the lead sinking through the aluminum. However, if we consider the density of a specific solid compared to specific liquid we can predict whether the object will sink or float. Most woods are less dense than water and will float. Most metals are more dense than water and will sink. (No wonder lead fishing weights are called “sinkers”.) When we consider the sinking/floating behavior of water and oil things get a bit more complicated. Because both are liquids the density of one compared to another is important.
Long ago it was observed that oil always floats on water and never the reverse. Hence it would be safe to state that water is more dense than oil. When we consider two air masses, both will have the same density if they have the same temperature. Strictly speaking, this is not precisely true because even if the temperatures are identical the amount of moisture present will slightly change the density due to the fact that a molecule of water H2O is lighter than a molecule of Oxygen O2. This is a bit contradictory to common sense because it would seem that adding water would make the air heavier!
How does one air mass become warmer or cooler than a neighboring air mass? The answer is the same as the answer to the question: “How would you warm or cool yourself?” For example if you wanted to warm up you would stand in the sunshine or go in a warm room. Likewise air masses in sunny areas tend to be warmer, Air masses in areas with less sunshine or a a latitude of low sun angle tend to be cooler. Air masses over warm water will become moist and warm. Air masses over cool land will become drier and cooler. When warm Gulf air masses encounter cold Canadian air masses get ready for action. If the Gulf air is stronger it will override the Canadian air creating Stratus clouds and rain. If the Cold air is stronger it will push under and lift the Gulf air creating Cumulus clouds and strong storms.
Frequently air masses are pushing against each other as a consequence of the general circulation patterns of the Earth. In simplest terms cold polar air is always heavy and is working it’s way toward the lighter warm equatorial air which is rising and working its way toward the poles. In reality this loop is thousands of mile long but breaks up into three smaller loops. As mentioned above, the boundary between air masses is called a Front. Depending on which air mass is pushing harder the frontal boundary will always move away from the higher pressure area to the lower pressure area. If the higher pressure area is colder air it will push under the warm air and be named a Cold Front. Sometimes the fronts will stall and become a Stationary Front. Stationary fronts bring low overcast skies, moderate rain, and can last for days!
By now it should be obvious that Fronts are a common weather maker. The tank with colored water and an “artificial” or simulation of dense cold air (salt was added) is a fascinating small scale animation of what nature produces on a scale of thousands of feet in height and many miles in width. The high altitude leading edge of a warm front may be hundreds of miles in advance of the location of an advancing warm front where the warm air actually contacts the ground. High thin wispy Cirrus clouds are frequently the telltale announcement of a distant but arriving warm front.
The tank simulator has always been a popular and remembered teaching aid. Because the tank is only two feet long it cannot completely simulate a phenomenon which may be hundreds of mile long. The complete demonstration may take several minutes for the red water to stabilize over the blue water. However, the moment of the lifting of the center barrier brings many reactions from students as they watch the colored water “masses” interact.
Learn more: This is an advanced topic for kids. Much discussion is possible by studying weather maps. Fronts are always clearly marked with unique and often colored symbols. TV weather personnel routinely explain forecast weather in relation to approaching fronts. Repeatable at home with a glass bread pan and cardboard divider.