Due to being virtually always present clouds are one of the most common objects affected by the attacks or powers of characters. As such how to calculate feats involving the creation, destruction or movement of clouds is of common interest.
The first step necessary for any cloud calculation is determining how much of a certain cloud was affected (created, destroyed or moved). Pixel scaling is the most reliable way to do so and the first thing one should try to do. However, in case of clouds there often is nothing one could scale from. Hence other practices are often relevant.
If all clouds visible in the sky are affected and the view to the horizon is not obstructed by obstacles, through for example buildings or trees, one can use the maximum viewing distance to figure out the radius in which the clouds are affected. There are two things that limit how far one can see. One is the curvature of earth, that is the horizon, and the other is the visibility, that is how much fog, air pollution and similar limit the maximum viewing distance.
In practice the limit is almost always determined by the visibility, as clouds, due to being high up in the sky, are usually in the line of sight for over a hundred kilometers beyond the horizon.
On clear days (no rain, fog or snow) the visibility is about 20km. On very clear days it can be around 50km and on exceptionally clear days even 280km. On slightly hazy days it can also just be 10km.
The standard value to use is hence 20km. Higher or lower values can be used if there is a reason for it.
Characteristic Cloud Thickness
Different types of clouds have a characteristic thickness to them. The thickness is the distance from the lower border to the upper border of the clouds. If clouds can't be directly scaled this is usually the only way to figure out the height to use for the calculation of the affected cloud volume.
|Cloud Type||Description||Characteristic Thickness||Appearance|
|cirrus||Generally characterized by thin, wispy strands. They are usually white or light gray in colour. Since cirrus clouds arrive in advance of the frontal system or tropical cyclone, it indicates that weather conditions may soon deteriorate. While it indicates the arrival of rain, cirrus clouds only produce fall streaks (falling ice crystals that evaporate before landing on the ground).||100m to 8000m, with 1500m on average.|
|stratus||Low-level clouds characterized by horizontal layering with a uniform base. They vary from dark gray to nearly white. Stratus clouds may produce a light drizzle or a small amount of snow. These clouds are essentially above-ground fog formed either through the lifting of morning fog or through cold air moving at low altitudes over a region.||Less than 1000 meters|
|cumulus||Clouds which have flat bases and are often described as "puffy", "cotton-like" or "fluffy" in appearance. Cumulus clouds are often precursors of other types of clouds, such as cumulonimbus. Normally, cumulus clouds produce little or no precipitation.||600m to 2000m|
|stratocumulus||Clouds characterized by large dark, rounded masses, usually in groups, lines, or waves. They look much like cumulus clouds, except lumped together and bigger.||Less than 1000 meters, of the order of 100m|
|Nimbostratus||A cloud with a diffuse cloud base. Although usually dark at its base, it often appears illuminated from within to a surface observer. Typical rain clouds.||2000m to 4000m|
|cumulonimbus||Dense, towering vertical clouds. They are the clouds that usually accompany heavy rain, storms and thunderstorms.||Usually between 8000m to 11800m. Sometimes as few as 2000m in polar air.|
Using the two values from above one can approximate the volume of the clouds. A simple acceptable approximation is given by pi*(viewing distance)^2*(cloud thickness). There are better, but more complicated, formula model the clouds as the intersection of a spherical shell and a spherical sector. However, usage of those usually isn't necessary with the normal distances.
In order to determine the cloud mass from its volume one simply has to multiply it with the clouds density. However, what a clouds density is can be defined in two different ways
- The density of the water in the cloud.
- The density of the water and air in the cloud together.
Which to use depends on which way the cloud was affected. Chemical changes, like vaporization or condensation of water use the first density, while movement changes like the creation of instabilities or changing the clouds position or shape use the second density.
The density of the water and air in the cloud together is approximately 1.003 kg/m3.
The density of just the water in the cloud is the liquid water content. The liquid water content depends on the cloud type. Following values can be used as orientation:
|Cloud Type||Liquid Water Content (g/m3)|
|stratus||0.25 to 0.3|
|cumulus||0.25 to 0.3|
|cumulonimbus||1 to 3|
In order to get the final result you need to find out which method was used to affect the clouds. There are 3 methods that are typical:
- Condensation, for creation, or vaporization, for destruction, of clouds. This should be used if clouds are created or destroyed, by condensing / vaporizing the water that makes up the cloud. If the cloud creation includes the creation of great amounts of natural winds, in other words storms, CAPE should be used instead.
- Kinetic Energy for moving of clouds. Should only be used if clouds are moved from one place to another.
- Convective available potential energy (CAPE) for creation of storms. Should be used if clouds are created together with a lot of wind. If the CAPE value is lower than the condensation value, use condensation instead.
If it can not be said, within reasonable certainty, which of the three methods applies to the created clouds all three can be calculated and the lowest result used.
Condensation / Vaporization
To calculate the energy necessary to create/destroy the clouds via condensation/vaporisation simply take the mass of the clouds (water mass, without air), in kilogram, and multiply it by the latent heat of vaporization for water, which is 2264705 J/kg. An even better result can be reached if one estimates the temperature and uses the formula here to get the latent heat of condensation for water in clouds and multiplies that with the mass of the clouds (in gram).
To calculate the energy via kinetic energy one needs to additionally know the speed at which the clouds were moved. Since speed is distance over time, one needs to somehow figure out these two values. If that it done it can simply be calculated through the formula "kinetic energy = 0.5 * cloud mass * (Speed of cloud movement)2". The cloud mass is here the mass with the air.
One should keep in mind that not all clouds in a cloud movement feat necessarily move with the same speed. In such cases one has to calculate the kinetic energy with the different speeds of the different parts in mind. An example of this would be creating clouds and spreading them omnidirectionally around a center point. In that case the speed with which the clouds move may increase the further from the center they are. In the case of such an omnidirectional expansion one could use the formula "kinetic energy = 0.25 * cloud mass * (Speed of cloud movement)2" to account for the different speeds involved.
For pulling in clouds omnidirectional towards a center point, or dispersing them from a center point, the formula "kinetic energy = 1/12 * cloud mass * (Speed of cloud movement)2" can be used.
The following values assume that the clouds in question cover a radius of 20km and are cumulonimbus clouds and hence 8000m thick and have a liquid water content of at least 1 g/m^3. For the kinetic energy values it is assumed that the storms are pulled from 20km so that in the end the sky is completely covered within 1 minute. That means this values should not be used, if the timeframe might be longer, the viewing distance is limited to below 20km or the clouds in question are not cumulonimbus clouds.
|Condensation||2.277e+16 J||Small City level|
|Kinetic Energy||9.336+16 J||City level|
Common results for CAPE can be found on the Standard Storm Calculations page.