E3 Physics of Water Vapor, Clouds and Microphysics

Topic

Cloud microphysics describes how hydrometeors—the liquid water droplets, ice crystals, snowflakes, and hailstones that form within a cloud—originate, grow, and eventually fall as precipitation. This entire process begins with the amount of water vapor the air can hold before becoming saturated, a quantity governed by the Clausius-Clapeyron equation: the air's capacity to hold water vapor increases exponentially with temperature, meaning that for every degree the temperature rises, the air can hold about seven percent more water vapor than before.

When the air's relative humidity just exceeds one hundred percent—that is, when the air is slightly supersaturated—water vapor begins to condense onto condensation nuclei. These are tiny particles suspended in the air that provide a surface for vapor molecules to cluster together; this process is known as heterogeneous nucleation, the initial step that transforms invisible water vapor into the cloud's first liquid droplet.

Once formed, that initial droplet grows through vapor diffusion, following Maxwell's equation: larger droplets grow faster than smaller ones because they possess more surface area upon which water vapor can continue to condense. This advantage held by large droplets becomes decisive when the collision-coalescence process—the dominant mechanism for rain formation in warm clouds—comes into play. Larger droplets fall faster than smaller ones, allowing them to catch up, collide, and merge into a single, even larger droplet. As a droplet grows, it falls more rapidly and captures more small droplets along its path; thus, a droplet that collides today becomes larger by tomorrow, leading to even more forceful collisions. Eventually, the droplet's increasing size drives its own continued growth, ultimately resulting in the formation of a raindrop. In mixed-phase clouds—those that are only partially glaciated—supercooled liquid droplets coexist with ice crystals, bringing a third growth mechanism into play: the Bergeron-Findeisen process. Because the saturation vapor pressure over ice is lower than that over liquid water, air in equilibrium with supercooled liquid droplets becomes supersaturated with respect to ice; consequently, water vapor preferentially deposits onto the ice crystals, causing them to grow, while the surrounding supercooled droplets evaporate to replenish the vapor consumed by the crystals. Thus, in a mixed-phase cloud, ice crystals grow at the expense of the surrounding supercooled liquid droplets.

To represent these processes in a numerical model without tracking individual droplets or crystals, cloud microphysics is parameterized by describing the size distribution of all droplets or crystals in the cloud simultaneously. This involves using characteristic size distributions and summarizing the collective behavior of the entire distribution through a few key quantities: total mass, total particle count, and the total reflectivity produced by the ensemble.

The expected outcome of this sequence of events—from the initial droplet nucleation on a condensation nucleus to the growth driven by collision-coalescence or the Bergeron-Findeisen process—is that the cloud begins to precipitate between twenty and forty minutes after formation. The precipitation rate is proportional to the reflectivity measured by weather radar: the higher the reflectivity produced by the cloud's hydrometeors, the higher the rate of precipitation falling from the cloud. Thus, cloud microphysics connects, in a single chain, the invisible moisture in the air with the rain, snow, or hail that ultimately reaches the ground.

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