EATS 1011 3
Laboratory 4: Clouds, Radar and Satellite Images (Total 50 marks)
If you have a copy of the textbook, Meteorology Today, please remember to
it to your lab session. It contains some cloud identification charts which will be useful.
ing some coloured pencils (e.g. blue, red, green).
By the end of the laboratory you should understand:
⚫ cloud identification
⚫ how to interpret visible and infrared satellite images
⚫ how to interpret meteorological radar images
4.1 Cloud Classification and Coverage
In the first part of this exercise you will estimate cloud coverage, which meteorologists
ecord in eight. However, you may find it easier to first divide the sky up into half, quarter
or eighth sections, estimate the coverage in each section, and then convert to the nearest
tenth. You can also use your hands held at arm's length to assist in the estimate of cloud
coverage. Note that determining sky conditions toward the horizon can be tricky because
oken clouds often appear as though they completely cover that section of the
sky. Clouds on the horizon also tend to appear thicker and darker than they would if they
were directly overhead, because the light from a distant cloud travels through more
atmosphere and is therefore attenuated to a greater extent.
Note that water vapour is invisible, and that it is water droplets or ice crystals that
eflect light. Since these “hydrometeors” reflect light uniformly in all parts of the visible
spectrum they can appear as any shade of grey or white, depending on their
clouds are translucent and so appear
ighter than thicker clouds, which may shade their
own lower parts from the sun. Cloud tops in direct sunlight appear
ight white, while the
ases of those clouds are in the shadows and appear grey.
Sky conditions are classified as:
⚫ Clear (CLR): Less than 1/10 covered by clouds.
⚫ Scattered (SCT) or Partly Cloudy: from 1/10 to 5/10 covered by clouds.
⚫ Broken (BKN) or Mostly Cloudy: 6/10 to 9/10 sky covered by clouds.
⚫ Overcast (OVC): more than 9/10 sky covered by clouds.
⚫ Opacity: if the clouds are semi-transparent, they are classified as thin, e.g. thin OVC,
or thin BKN.
The cloud classification system we use today is based on one proposed by Howard in
1803 and later expanded by Abercromby and Hilde
andsson in 1887. Clouds are divided
into four major groups according to the height of the cloud base above Mean Sea Level
(MSL). These are (1) high, (2) middle (or “alto”), (3) low clouds, and (4) clouds with
vertical development. (Note that vertically developing clouds may cover the low, middle
and high ranges.) The heights given below are for the base of the clouds in mid-latitudes,
ut note that cloud bases are generally higher in the tropics (due to warmer temperatures)
and lower in polar regions (due to cooler temperatures).
Within these four height groups, clouds are classified according to their appearance with
Latin words, for e.g., stratus means “layered” while cumulus means “heap” or “pile”, and
us means “wispy” or “curly”. Nimbus means “rain cloud” and can be applied to any
cloud type that is precipitating. You may wish to refer to the book by Ahrens or Cloud
Chart link posted on eClass for pictures of the cloud types described below.
1. High Clouds: base at 5 km to 13 km altitude (in mid-latitudes).
Formed mostly of ice crystals (due to cold temperatures at high altitude.) Note that water
droplets suspended in air can be supercooled below the normal freezing point, and generally
don't freeze until well below –9oC: High clouds are rarely extensive or thick enough to
prevent the formation of shadows.
us (CI): Thin, white, silky clouds blown by strong winds. Generally have a wispy,
ous or feathery appearance. Often precede fair weather.
ocumulus (CC): Appear as small rounded white puffs or rippled rows (like waves).
Pattern often resembles “fish scales” and hence refe
ed to as “mackerel”.
ostratus (CS): A thin, transparent, veil-like, layer cloud which is smooth in
appearance. Produces halos around sun or moon. Often precede storms, and may indicate
ain or snow is on the way, particularly if followed by middle clouds.
2. Middle Clouds: base at 2km to 7km altitude (in mid-latitudes).
⚫ Altocumulus (AC): Grey and white puffy masses, often in parallel waves or bands.
Generally less than 1 km thick, composed mainly of water droplets. These clouds are
composed primarily of water droplets and hence have sharper edges than clouds composed
of ice crystals (e.g. ci
ocumulus). Altocumulus clouds show variations in colour from grey
to white, which also distinguishes them from pure white ci
ocumulus. Altocumulus puffs
generally appear larger than those in ci
ocumulus. The appearance of altocumulus in the
morning is an indication of high level conditional instability, which favors the development
of thunderstorms later in the day.
⚫ Altostratus (AS): A grey (never white), striated veil of cloud, composed of both ice
crystals and water droplets. Often hard to distinguish from ci
ostratus. The sun may appea
through altostratus as a dim round disk, but halos only occur with ci
us clouds (ice
crystals). Altostratus is generally thick enough to prevent shadows, whereas faint shadows
are usually present with ci
3. Low Clouds: base at surface to 2km altitude (in mid-latitudes)
⚫ Nimbostratus (NS): A dark grey cloud layer which is thick enough to block the sun,
and produces fairly continuous light to moderate (but never heavy) precipitation. The ai
elow nimbostratus is usually quite humid due to the evaporation of precipitation. This
often leads to the condensation of ragged i
egular shreds of clouds, called low-lying scud.
⚫ Stratocumulus (SC): Ragged or lumpy, fair-weather clouds a
anged in rows, patches,
or rolls. They range in colour from white to dark grey. The individual clouds appear much
larger than altocumulus. May be produced by the
eakup of stratus clouds.
⚫ Stratus (ST): An extensive grayish cloud layer with a uniform base and appearance.
Stratus often covers the entire sky, and sometimes produces precipitation in the form of
drizzle (very light rain). An extensive fog layer is really just a stratus cloud with its base at
4. Clouds with vertical development: base at 0 km to 4 km altitude
⚫ Cumulus (CU): Detached billowy clouds which, owing to their vertical development,
usually have rounder tops, flatter bases and sharper outlines than stratocumulus. Small, fair-
weather cumulus clouds may occur in rows or patches, but are generally spaced much
farther apart than stratocumulus. Occasionally cumulus clouds have ragged edges, but
stratocumulus always occur in closely spaced groups.
⚫ Cumulus Congestus or Towering Cumulus (TCU): These are larger cumulus clouds
that possess rising domes or towers with very sharp outlines and a shape that resembles
cauliflower. Their sunlit tops are
ight white, while their bases are relatively dark. They
may occur in isolation, or in rows or lines. They often produce showers.
⚫ Cumulonimbus (CB): Extremely large cumulus clouds with extensive vertical
development, often reaching the tropopause, where the top spreads into a flattened “anvil".
The tops of cumulonimbus clouds are high enough to reach low temperatures where
virtually all water droplets freeze. These ice crystals give the top of a cumulonimbus cloud
ous and somewhat fuzzy texture compared to the more sharply defined congestus
variety. Lightning, hail, thunder, and tornados are all only associated with cumulonimbus
clouds (rare exceptions exist, for example during a snowsquall). Heavy showers are usually,
ut not exclusively, produced by cumulonimbus. Cumulonimbus may be isolated or form
part of a “squall-line" or multi cell thunderstorm.
4.1.1 Exercise: Cloud Observations… ........................................... XXXXXXXXXX10 marks
Observe the cu
ent sky conditions outside. Use the “View of Sky” box below to mark the
egions of the sky occupied by various cloud types (i.e. construct a view of the sky, looking
upward). You are not required to provide a detailed sketch of the clouds, just the type and
extent. Then record your analysis in the “Sky Conditions” Table. Note that this exercise
will be very quick if the sky is clear or completely overcast!
1. Record the date and time in the Sky Condition Table.
2. Divide your “View” box into sections by cloud type, and label each section accordingly
(e.g. NS, CU, AC, CLR, etc). Also indicate cloud shape (layered or puffy) and opacity (thin
or thick) as appropriate.
3. You can use your “View" box or other means to estimate total cloud coverage in tenths,
and record the sky conditions (CLR, SCT, BKN, or OVC) in the Table. Also assess cloud
opacity in the sky condition row.
4. Identify and record the prevalent cloud types in the Table. Provide
ief statements o
sketches indicating shape, extent, opacity and colour (or
5. Record the strength and type of any precipitation, (e.g. light, moderate or heavy; rain o
Cloud Coverage and Sky Conditions
Date and Time
NB: The entire
sky at the time
Main Cloud Type
Other Cloud Types
4.1.2 Exercise: Regional Cloud patterns… ................................... XXXXXXXXXX10 marks
In this exercise, you will obtain and analyze data from the cu
ent surface map (either on
the computer or from a print-out), in order to improve your understanding of the clouds in
the region. Use the blank map of Eastern North America to record the information specified
elow. Transfer the station circles from the surface map to the co
ect location on your blank
1. Mark the cloud amounts in each station circle. (i.e. fill in the appropriate amount, 15-20
2. Sketch a scalloped