A water droplet begins to fall as soon as the downward-directed gravitational force exceeds the upward-directed buoyancy force caused by instability. Some larger drops form when liquid water droplets in clouds merge and grow to a size that can be pulled down by gravity. The figure to the right illustrates what can happen to a moving water drop that interacts with smaller cloud droplets around it.Ĥ. Due to this role, such particles are called condensation nuclei.ģ.
It is energetically difficult for the tiny water droplets in clouds to just form by themselves, but it is easier for them to form if they condense around even tinier particles, such as dust, salt, and smoke. Raindrops are huge compared to the size of water droplets that form a cloud, as shown by their relative sizes in the properly scaled diagram below.Ģ. An important factor in how water droplets in a cloud grow to become raindrops is the immense differences in size between the various players. Clouds at intermediate ambient temperature, or intermediate altitudes, will contain a mix of drops and ice. Under cold ambient conditions, a cloud contains mostly ice throughout its vertical extent. Temperatures decrease upward within a cloud, however, so the upper levels can contain ice, while the lower levels contain drops.ĥ. If ambient temperatures near the cloud are warm, the cloud will contain mostly drops of liquid.
Whether a cloud contains drops of liquid, ice crystals, or some combination depends primarily on the temperature of the cloud. For precipitation, some processes are occurring within the cloud that make the water droplets or bits of ice heavy enough that the pull of gravity can overwhelm the buoyancy forces (atmospheric instability) that uplift air within the cloud (the rising air is how most clouds form).Ĥ. Next, the water vapor forms the various types of clouds, which can contain tiny drops of water, ice crystals, or some of each.ģ.
The cycle begins with evaporation of water in the oceans and other parts of Earth's surface, a process that puts water vapor into the atmosphere. Examine this figure and consider all the processes that have to occur to cause rain, snow, or hail.Ģ. Precipitation is the process whereby liquid droplets of water (raindrops), solid bits of ice (snowflakes and hail), or some combination of these fall from the sky. How does precipitation occur? What is going on inside clouds that forms raindrops, snowflakes, and hail? It turns out there are two important mechanisms by which precipitation droplets form, one of which is somewhat surprising. Precipitation is the ultimate source of all the fresh water on the planet, which we depend on in our daily lives. THE PROCESS OF PRECIPITATION is vital to life on Earth, helping to redistribute water from the oceans to the atmosphere to the land.
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