Download How Clouds Form To understand how clouds form, we need to take

How Clouds Form
To understand how clouds form, we need to take a step back and examine the
processes of evaporation and condensation. Picture a birdbath outside on a hot day.
When the temperature of the environment is warm, molecules of water (H2O) are
energetic and can move more, expanding the distances between them. More molecules
will leave the birdbath's mass of liquid and become water vapor in the air. On a cool
day, the molecules have less energy and are less able to separate themselves from the
larger mass of water. (On a very cold day, water molecules generally contract into
their solid form, ice, and don't have the level of heat energy needed to separate
themselves.) You can see the processes occurring in the first two cases. However, the
first scenario's result is net evaporation; and the second scenario's result is net
condensation. Other factors can affect these outcomes, but for our purposes, we'll just
focus on temperature.
Stephen Studd/Stone/Getty Images
Clouds get their moisture from a variety of sources, including evaporated ocean water.
As water molecules shift between vapor, liquid and solid phases, they move throughout
the air, even if we can't see them. However, when a parcel of air cools quickly and
reaches saturation, there's a chance water vapor will condense and appear as a cloud.
This could occur because of different factors, like a mountain pushing it upwards into
cooler air, or perhaps, because it enters a cold front.
Additionally, cloud formation happens easily when water vapor has something to cling
to, allowing the water vapor to change into its liquid or solid phases. A number of
particles can act in this function. Commonly called condensation nuclei or freezing
nuclei, these particles become the center of a water droplet or ice crystal. Typically,
these will be things like dust particles, sea salt particles and soot from wildfires, and the
water droplets or ice crystals form around them.
Clouds are, in essence, massive collections of tiny water droplets and crystallized water
molecules. The different shapes, textures and other features of clouds depend largely
on the conditions under which they form and later develop. For instance,
temperature, humidity and altitude are all factors that affect cloud formation.
Clouds and Precipitation
Clusters of water droplets (called cloud droplets) and crystallized frozen water
(called ice crystals or snow crystals) form clouds. A cloud can contain both of these,
depending on its temperature. For instance, a cloud's top may be cooler than the lower
regions, creating a mix of liquid and frozen water.
Gravity causes all the liquid water to fall as rain. The average size and volume of a
cloud droplet is tiny, but, if a cloud droplet manages to attract enough water, the
influence of gravity causes it to become a raindrop and fall.
David James/Riser/Getty Images
Cumulonimbus clouds often mean severe thunderstorms and other serious weather.
That being said, snow happens a lot like rain. As snow crystals condense and clump
together, snowflakes form. When they reach the point where they're too heavy to remain
aloft, they fall together as snow. Different surrounding temperatures affect what type of
snowflakes will develop. Sometimes on the way down, snowflakes melt into rain; other
times they fall intact.
Photo courtesy Kenneth G. Libbrecht and SnowCrystals.com
A photograph of a snowflake, taken with a special photomicroscope.
You may be asking, "If water droplets and snow crystals make up clouds, how do we
get hail, sleet and freezing rain?" The answer is that once cloud droplets and ice
crystals condense and reach critical falling mass, a few additional processes can occur.
 Freezing rain, also known as glaze, can occur where warm and cold air
fronts meet. A snowflake can fall into cold air, then pass through a layer of
warmer air and melt. As it continues to fall and right before it hits, the
snowflake passes through a layer of cold air and
becomes supercooled. This means that it won't refreeze, but upon impact
with a cold object, such as the street or a tree branch, it will immediately
turn to ice.
 Sleet starts the same way as freezing rain, but the melted snowflakes have
time to refreeze before they hit the ground.
 Hail forms during severe storms. The gusty updrafts produced by high winds
may knock snowflakes and raindrops up and down until the supercooled
water droplets collect themselves into chunks of ice. This can happen
repeatedly, until the heavy hail can no longer be lifted by the storm's
powerful updrafts. The resulting ice chunks can be quite large when they're
finally released and create quite an impact if they hit objects like the hood of
your car.
Homework
IN PENCIL, fill in a line on your summary table with “Clouds Reading”. There were
two main sections in this article, “How Clouds Form” and “Clouds and Precipitation”.
Write a main point IN PENCIL for each section, and complete the rest of the row IN
PENCIL.