The Condensation Line

There is a great mystery concerning the sky above us that I have never seen referred to before but for which I believe I have a simple solution.

There are several different types of cloud that regularly form in the atmosphere. The fluffy white low clouds are known as cumulus. The high wispy clouds across a blue sky are called cirrus. The shapeless sheets of low cloud are stratus. The three basic types of cloud can combine to form several others, such as cirrocumulus. There are also the mid-level clouds with the alto- prefix, which means high. This gives us altocumulus and altostratus.

Clouds are relatively simple phenomenon. When the earth's surface is heated and significant vertical currents of air rise, cumulus clouds will form. If water evaporates and then condenses on the tiny particles of dust in the air without a strong vertical motion, stratus (meaning layer) clouds will form.

These low clouds are thicker than the higher altostratus and altocumulus clouds simply because the air is denser at lower altitudes and can thus hold more condensed water vapor (vapour). As at lower altitudes, whether stratus or cumulus form depend on whether there are significant vertical current in the air (which would form cumulus) or not (which would form stratus).

When clouds form at very high altitudes, five miles or eight km or so, the condensing water vapor forms ice crystals instead of water droplets, creating cirrus clouds. These high wispy clouds tend to be aligned in the direction of the often high winds at such altitudes.

We can see that the factors in cloud formation are relatively simple; the altitude of condensation and movement of the air at that point, as well as the temperature where the water condenses. The different types of cloud found at different altitudes make it easier to forecast the weather. At a warm or cold front, where an air mass of one temperature is advancing while the other is retreating, the heavier cold air will tend to form a wedge under the lighter warm air.

Sailors of centuries ago learned that when they saw high cirrus clouds one day and lower altocumulus or altostratus the next, that a warm front would arrive soon. Since warm air can hold more water than cold air, that meant rain because the vapour in the warm would condense and fall as precipitation when it came into contact with cold air. Likewise, when the cloud types got higher from one day to the next, that meant that a cold front had passed by.

The great mystery concerning the atmosphere that I have never seen referred to before is WHY do different types of cloud form? The process of cloud formation and operation are well-understood. But why does one batch of water form a cumulus cloud when it condenses in the atmosphere and another batch forms a cirrus cloud?

Humidity is not really a factor in which type of cloud forms. All clouds form when water evaporates, goes up into the atmosphere where the temperature drops as we get higher, known as the adiabatic rate, and condenses on particles of dust. The small droplets of condensed water on dust that form clouds can be "supercooled", meaning that such water can remain liquid even in freezing temperatures.

This is why there are clouds in winter. Altitude of condensation is an important factor because the air gets thinner and colder as we go higher, as is the presence or lack of strong vertical currents of air, but that still does not explain why different types of cloud form from water that evaporates.

I have found a simple answer to this question. The time of day that evaporated water condenses on dust particles in the atmosphere is what determines what type of cloud will be the result. There is a line that is usually at or near the earth's surface during the night. I will call it the condensation line. It is the line in the air parallel to the earth's surface above which some of the evaporated water will condense due to the cooler temperature that comes with altitude.

We know that fog tends to form at night and often dissipates when the sun rises and begins to warm the earth. Fog is basically a stratus cloud at ground level. Dew, or frost in cooler weather, is water that condensed during the night. This is because the condensation line was low enough during the night for the fog to form but rose when the sun heated the ground, causing the fog to evaporate.

We also know that cumulus clouds tend to form in the morning. This is because the sun heats the earth after rising, causing vertical air currents, as some pieces of ground warm more than sorrounding land. These vertical air currents carry a lot of water vapor, which condenses upon reaching about 4,000 feet altitude (about 1.2 km).

You will notice how the bases of cumulus clouds tend to form straight lines, which are at the same level as the bases of nearby clouds. This line represents the condensation line at the time the clouds formed early in the day. Cumulus clouds, unlike stratus clouds, are spaced because if they are formed by vertical air currents, obviously there has to be compensating downdrafts or a vacuum would form at ground level. As the day goes on, the condensation line rises.

Remember that the sun does not heat the air directly. Rather, the sun heats the earth which then heats the air. Logically, the longer the sun shines on the earth, the more it will warm the air.

Since water in the air condenses to form clouds when the air temperature drops to a certain point with altitude, the longer the sun has been warming the earth, the higher the water vapour will have to go to condense and form clouds. The line that I am defining as the condensation line of cloud formation will thus get higher and higher as the day goes on and then drop back toward the earth at night. This explains why different types of cloud form from the same kind of water in the air.

Cloud type depends on altitude because the air gets thinner as we go higher. Since the condensation line rises from morning until evening, the time of condensation determines the altitude and thus the cloud type. This does not mean that the cloud types visible can be used to tell the time of day, it only shows what time of day those clouds formed. Low clouds form in the morning, middle clouds in early afternoon and high cirrus clouds in late afternoon or evening.

I also believe there to be a significant amount of evaporation from low clouds that formed earlier in the day to rise to higher altitudes and then re-condense. We can see small cumulus clouds gradually diminishing in size in the afternoon with no water falling as precipitation.

Obviously, this model of cloud formation operates differently over water as compared to land. Water is slower to heat up during the day but holds more heat overnight than land. This could mean that middle clouds are more likely to form over water than over land.

This concept opens the possibility of simplifying our description of the weather.

There are several parameters which we express in order to describe the weather. These include temperature, wind speed and direction, relative humidity, barometric pressure and sometimes, cloud ceiling.

Barometric pressure is useful in weather forecasting because, since water is actually lighter than air by molecule, wet air is lighter than dry air. As a storm approaches, the barometer drops because the air gets more and more full of water until water falls as precipitation when the air cannot hold any more. The relative humidity is the amount of water in the air in comparison to the amount that the air is capable of holding at that temperature. Warmer air can support more water vapor (vapour) than cold air. Winds bring in weather conditions from other places.

Water vapour (vapor) reaches a certain altitude and then condenses because air gets both thinner and colder with increasing altitude. This water condenses into tiny droplets on particles of dust, salt and, smoke which act as condensation nuclei, and the result is the formation of clouds.

Meanwhile, air near the surface is warmer and denser so water keeps evaporating, even when the air higher up is at it's limit for holding water. This is the reason for precipitation, if the air was uniform in density and temperature, it would absorb all the water it could and then evaporation would cease. But as it is, precipitation is to correct the imbalance of water capacity of the air against the total water which has evaporated into the air.

Condensation is somewhat more complex than this. If there is a strong updraft, it can carry vapor (vapour) to greater heights where there is still water-holding capacity. This is how towering cumulo-nimbus thunderclouds form. Water that has condensed into cloud may re-evaporate and re-condense at a higher altitude and falling rain may re-evaporate before reaching the ground.

Notice that it very rarely rains heavily when there is blue sky showing. This is because it indicates that there are areas nearby which still have room for more water to condense and so there is no reason yet for precipitation to fall.

The Condensation Line is the line at which water which has evaporated condenses to form cloud. The line rises as the day progresses and the temperature increases. But often, the condensation line will reach a certain altitude and then effectively halt. This will be due to the inability of the air above the line to hold more water because of either cold or humidity already present.

The Mean Condensation Altitude which I am presenting here is simply the mean (average) altitude at which water vapor (vapour) condenses into cloud on a given day. Obviously, if there is a clear blue sky with more room available to hold water, then there will be no Mean Condensation Altitude.

The Mean Condensation Altitude would be an excellent predictor of precipitation. Notice that clouds are always low during heavy rain. It can rain from high clouds, but only lightly.

The chance of precipitation and the intensity of that precipitation would be inversely proportional to the Mean Condensation Altitude. When water does not rise very far before condensing, it can only mean that the air is reaching the limit of it's ability to hold water.

Weather reports give the temperature, barometric pressure and, relative humidity. But what it all comes down to is the altitude to which water vapour (vapor) is rising before it condenses. The Mean Condensation Altitude is a function of all three of these parameters, it is simply the average altitude to which evaporated water is rising before condensing into cloud. The more full of water the air is, the lower that altitude will be and the more likely it is to rain. If there is a dense layer of cloud, the Mean Condensation Altitude would be a horizontal line halfway through the cloud.

Let's start making the MCA a regular parameter of weather reports.