How to make a cloud

Recipe for making a cloud in 5 easy steps.

1-The first ingredient is a mass of air to which a certain amount of water has been previously added. The air is said to have a relative humidity which is the percentage of water the air contains in relation to the water it would contain if it were completely saturated.

This first ingredient is in principle easy because all the air on Earth has a small amount of water in it so that its relative humidity is always greater than zero.

Obviously, the higher the relative humidity of the air parcel from which we start, the easier it will be to build the cloud.

2-Secondly, the air parcel is taken with its relative humidity and cooled. As the air cools, its capacity to hold water decreases. Thus, as the air always contains the same amount of water, i.e. its initial value, the relative humidity increases as the air cools down. We continue with this cooling procedure until the relative humidity rises to 100%. See the horizontal line is the graph.

If the cooling takes place in situ, i.e. typically because at dusk or nightfall the air parcel cools down in contact with the soil, which loses temperature by radiation, then the air will form a fog when it reaches saturation. These phenomena do not usually occur during the warm season because the soil never cools down enough to bring the air to saturation.

3- To form a cloud we will use the natural cooling that occurs as it air rises upwards. Unsaturated air loses 1 °C for every 100 metres of ascent. This cooling is due to the expansion of the air which, as it rises, encounters lower and lower pressures. From a certain height when the air reaches exactly its saturation temperature, also called dew temperature, the water will start to condense and visible micro-droplets will start to forming a cloud. It is important to know that these micro-droplets are formed around another essential ingredient which are aerosols. Aerosols are small microscopic solid particles that are suspended in the air. Without them, condensation would only occur at much lower temperatures.

4 How do we get the air to rise to form a cloud?

If our air parcel is lighter than the ambient air, it will naturally rise by buoyancy, like a hot air balloon. For our air parcel to weigh less than the ambient air it is simply necessary that the ambient air is cooler than our air parcel. This can happen when for example the sun heats the ground and the ground heats our air parcel until at a certain point the temperature of our air parcel exceeds the temperature of the ambient air. An upward air current will form, also called a convective current. As the air rises, it cools down as explained in section 3. These rising air currents are used by vultures and eagles to glide huge distances without doing any work. If the ascent is high enough, the air temperature will drop until it reaches the dew temperature at a certain altitude. From that height onwards, condensation will form and a cloud will form. This height is called the cloud ceiling and that is why when we look at a sky full of convective clouds they all seem to start from the same height.

5-When our parcel of air is not warmer than the ambient air it can still rise by two different methods.

Due to convergence, i.e. when the air moves from several peripheral points towards a central point where it converges, as for example in the case of a hurricane or cyclone. At the convergence zone or convergence point the air is forced to rise upwards in ascent, even though it is not lighter than the ambient air. This is because at the speeds at which air smove, the air is not compressed at the point of convergence, i.e. it functions as an incompressible fluid. Therefore at the point of convergernce the air has to "squeeze" upwards

6- The other way of forcing a parcel of air to rise is by orography. When a wind blows in a certain direction and encounters an obstacle in that direction, such as a mountain or mountain range, it is forced to rise parallel to the surface of that mountain or mountain range. This wind can carry a parcel of air upwards with it. It is this latter method that makes mountain peaks function as cloud generators.

What characteristics make a mountain or mountain range more effective in generating clouds?

a - the height of the mountain.

In fact, as the air parcel drops in temperature by 1oC for every 100 m of ascent, the height of the mountain determines the total temperature drop of our air parcel and will therefore be fundamental for reaching the dew temperature with the consequent process of condensation and cloud formation,

b- the relative humidity of the air currents approaching the mountain. A mountain near the coast in a tropical region with a warm sea will be affected by breezes laden with high humidity and therefore able to reach the saturation point more quickly, i.e. at a lower altitude; the same mountain in an extremely dry environment where the air has very low relative humidities may not be high enough to allow the air to reach saturation.

The photo shows the Rock of Gibraltar at the southern tip of the Iberian Peninsula. It is not an extremely high mountain (426 m), but it is washed to the east by the Mediterranean Sea which is relatively warm and therefore brings more moisture laden breezes. In the photo the wind blows from the east, pushing the humidity-laden air upwards. Above a certain height, condensation and cloud formation occur.

Not far from Gibraltar we have found another mountain over which a cloud often forms. This is the Sierra Nevada with its impressive heights of 3400 m only a few tens of kilometres from the coast. This is where we want to carry out our project. The breezes blow constantly during the day from the sea towards the peaks of the Sierra Nevada bringing 10 to 12 g of water per kilogram of air. From the coast to the mountains, irrigation and the sparse vegetation add a few more grams to the breezy air through the evapotranspiration of the plants. But this does not exceed the 21 g of water per kilogram of air that would be necessary according to Professor Millán Millán to trigger a summer storm.

We believe that by using efficient evaporation methods such as air rivers or the catabatic tower, we could exceed the threshold of 21 g of water per kilogram of air and favour the formation of summer storms, as was the case 60 years ago when the water cycle was healthy.