What is rain?
A: Rain is liquid water in the form of droplets that have condensed from atmospheric water vapor and then become heavy enough to fall under gravity.
Rain is a major component of what?
A: The water cycle.
Rain is responsible for depositing most of the fresh
A: On the Earth.
It provides suitable conditions for many types of
ecosystems, as well as water for what?
A: Hydroelectric power plants and crop irrigation.
What is the major cause of rain production?
A: Moisture moving along three-dimensional zones of temperature and moisture contrasts known as weather fronts.
If enough moisture and upward motion is present,
precipitation falls from what?
A: Convective clouds (those with strong upward vertical motion) such as cumulonimbus (thunder clouds).
The urban heat island effect leads to increased what?
A: Rainfall, both in amounts and intensity, downwind of cities.
Global warming is also causing changes in the
precipitation pattern globally, including wetter conditions where?
A: Across eastern North America and drier conditions in the tropics.
Which continent is the driest?
What is the globally averaged annual precipitation
A: It is 715 mm (28.1 in).
What is the globally averaged annual precipitation over
the whole Earth?
A: 990 mm (39 in).
How is rainfall measured?
A: With rain gauges.
Rainfall amounts can be estimated by what?
A: Weather radar.
Rain is also known or suspected on other planets, where
it may be composed of what?
A: Methane, neon, sulfuric acid, or even iron rather than water.
Warmer air can contain more water vapor than cooler air
before becoming what?
Therefore, one way to saturate a parcel of air is to do
A: Cool it.
What is the dew point?
A: It is the temperature to which a parcel must be cooled to become saturated.
What are the four main mechanisms for cooling the air
to its dew point?
A: Adiabatic cooling, conductive cooling, radiational cooling, and evaporative cooling.
When does adiabatic cooling occur?
A: When air rises and expands.
When does conductive cooling occur?
A: When the air comes into contact with a colder surface, usually by being blown from one surface to another, for example from a liquid water surface to colder land.
Radiational cooling occurs due to the emission of what?
A: Infrared radiation, either by the air or by the surface underneath.
Evaporative cooling occurs when moisture is added to
the air through what?
A: Evaporation, which forces the air temperature to cool to its wet-bulb temperature, or until it reaches saturation.
Water vapor normally begins to condense on condensation
nuclei such as what?
A: Dust, ice, and salt in order to form clouds.
As drops become larger, they become what?
A: Flattened on the bottom, like a hamburger bun.
Very large raindrops are split into smaller ones by
A: Air resistance which makes them increasingly unstable.
Coalescence occurs when water droplets do what?
A: Fuse to create larger water droplets.
Air resistance typically causes the water droplets in a
cloud to remain what?
When air turbulence occurs, water droplets collide,
A: Larger droplets.