During a rain shower, a water droplet is formed by condensation of water vapour in a cloud. The shape of the drop varies as it falls through the air. Some drops are spherical, while others are more oblate. There are several factors that influence the size and shape of a raindrop. These factors include the surface relative humidity, the air pressure, and the surface tension.
The surface tension force decreases as the mass of the raindrop increases. As the air pressure pushes up on the front of the drop, it flattens out. This creates a spherical shape. The size of the drop is a function of these two forces. The smallest raindrops are the spherical type, whereas the larger ones are oblate. This may affect the kinetic energy of the drop, as well as its wash-off process.
The smallest raindrops fall at speeds of 2 to 9 m/s, while the larger drops fall at speeds of 13 m/s and 42 ft/s. This range of precipitation sizes is important for determining rainfall rates, estimating surface concentrations of pollutants, and for many other commercial and scientific applications. There are several challenges to measuring the size and shape of raindrops, however. Manual techniques are often used for small drops, while automated methods are limited to drops of 0.3 mm diameter and above.
The surface relative humidity changes due to latent heat required for the liquid-to-gas phase transition. This increases the relative humidity in the area adjacent to the drop’s surface. This causes the height of the zLCL to change. The height of the zLCL can be calculated by dividing the surface RH by the radius of the drop. For example, for a raindrop with a radius of 0.1 mm, the zLCL will be less than a kilometer if the surface RH is high.
The mass distribution of a raindrop is also influenced by planetary parameters. The temperature and humidity of the atmosphere determine whether a cloud is saturated or not. In addition, the size of the drop is a function of a drop’s cross-sectional area. Compared to the growth processes, the size of these two factors have very little impact on the size of a raindrop.
The terminal velocity of a raindrop is determined by a combination of its weight, its gravitational force, and its aerodynamic drag force. The drag force is calculated using the CD method. The effective gravitational force is the sum of the forces, which is derived after accounting for the drop’s buoyancy.
For raindrops larger than 4 millimeters in diameter, the air pressure causes the raindrop to flatten out at the base. This flattened shape is called the parachute shape. These larger raindrops tend to coalesce into one drop, while the smaller ones continue to exist as individual droplets. This process occurs when the cloud is unable to contain the large number of droplets.
As the mass of the raindrop decreases, the height of the zLCL also decreases. This effect is inversely related to the surface RH, so the higher the surface RH, the lower the zLCL.