How to Read Clouds to Forecast Weather
High Clouds – 20,000 – 50,000 feet; are all made of microscopic ice crystals
Cirrus clouds are thin and wispy, often resembling white “horse tails”
and cotton threads
A few Cirrus clouds usually predict fair, cool weather after a front passes.
Lots of cirrus clouds may mean a warm front is coming: weather may change
in the next 24 hours, especially if followed by cirrostratus and/or altostratus.
Cirrocumulus clouds are tiny, “fingernail-sized”, puffy clouds (“popcorn”).
Cirrocumulus clouds indicate that fair weather is imminent (right now)..
When they follow after cirrus, they indicate a warm front is coming with
snow or rain. In the tropics, they may indicate a hurricane is coming.
Cirrostratus clouds cover the entire sky but are sometimes so thin that you
can see the sun or even the moon through them, which produce a halo effect.
As Cirrostratus clouds thicken, there is a greater chance of rain or snow in
the next 24 hours.
When they follow after cirrocumulus, they may indicate that rain (or snow) is
coming later that day (next 12 – 24 hours) or the next. If some mid-level
(alto-) clouds follow behind them, you can count on precipitation.
Mid-Level Clouds – 6,500 – 20,000 feet
made of small water droplets low turning to ice crystals higher
Altocumulus clouds are small, “thumb-sized” puffy clouds; often found in
waves or rows. As Altocumulus clouds build, they may predict
thunderstorms later that afternoon or the next day. If you see them on a
warm morning, expect storms in late afternoon or next day.
Altostratus clouds are translucent gray with a blue tinge. Sun appears as a
white disk and shadows are fuzzy and indistinct.
Altostratus clouds predict that a warm front storm is due within a few hours.
Low-Level Clouds – 0 - 6,500 feet
made of large water droplets (microscopic droplets for fog)
Stratus clouds are low, layered clouds that can “blanket” the entire sky.
They are uniform gray in color and opaque. The sun is not visible at all, so no
shadows will be visible on the ground.
Stratus clouds are more likely to produce a slow, steady drizzle than heavy
rain. If temperatures are freezing, light snow may occur. Sometimes, stratus
clouds produce no precipitation at all!
Stratocumulus clouds are both layered and puffy. They may not cover the entire
sky and simply gather in patches or rows. Shading varies from light gray to dark
gray (with maybe a little blue sky). Skies remain overcast or slow clearing - no
precipitation: Stratocumulus clouds seldom produce rain.
Nimbostratus clouds are very dark gray and are a harbinger of rain (“rain is on
the way”) & are associated with large areas of continuous rain, snow or sleet.
A steady, continuous rain will be falling really soon or may be falling right
now! (The Latin root “-nimbo-” means “rain”.)
Fog: low lying stratus clouds – Cloud base near ground level; made of
microscopic water droplets. Visibility is poor, but there is generally no rain!
Convective Clouds - Show Vertical Development – 0 - 67,000 feet
made of large water droplets (& maybe hailstones in thunderheads)
Cumulus clouds are the ones we drew in grade school. They look like fluffy
cotton balls and begin at about 4000 feet.
Cumulus clouds usually indicate fair, sunny weather.
When they start to build vertically into towering Cumulus congestus clouds, they
are building up to stormy weather.
Towering Cumulus (Cumulus congestus) clouds grow when the air is unstable,
producing strong convection currents. As these clouds billow upward into the
middle altitudes, they predict showers and thunderstorms in a few hours.
Cumulonimbus clouds are classic thunderclouds, with flat bases and anvil tops.
Strong updrafts and convection send the cloud tops up to 67,000 feet.
Shearing winds stretch the top of the cloud into
a distinctive anvil that points in the direction
that the storm is moving.
Cumulonimbus clouds predict a strong
thunderstorm is imminent, with lighting, rain,
possible flash floods and tornadoes.
Strong updrafts and downdrafts create large hail stones.