Temperature :
Use a thermometer and read it to the nearest integer. Do not measure
temperature directly in the sun or very close to the ground.
Clouds :
Type - Cloud characteristics vary depending on their altitude as well
as the atmospheric conditions.
Altitude:
Low clouds: Stratus (dull gray, usually overcast,
associated with steady precipitation). Some other species of low clouds
include nimbostratus, cumulus, stratocumulus, mamatocumulus.
Middle clouds: Begin with the prefix alto. Examples include
altocumulus, altostratus.
High clouds: Cirrus, cirrocumulus, cirrostratus. Usually appear
"feathery".
Weather conditions:
Many times you can identify the current (or future) weather situation
by analysis of cloud type and coverage, for example: Cirrus clouds are
fair weather clouds, they usually indicate an approaching front.
Cumulonimbus clouds are indicative of significant upward motion (due to
warm humid air at the surface rising, then condensing) that leads to
thunderstorm development (convection). Stratus clouds are good indicators
that precipitation is approaching. Brief descriptions of some cloud types.
Stratus
The uniform dull gray colored clouds that often cause completely
overcast skies. The gray underside of the cloud is shapeless and may
resemble fog. The clouds are layered and formed largely from the nearly
horizontal motion of air with one air layer sliding over another. These
clouds are found at low altitudes.
Stratocumulus
Clouds that consist of masses or rolls of cumulus clouds, usually
arranged in a pattern with narrow breaks between the individual clouds.
They often form "cigar shaped" rolls. The bases of the clouds
form at low altitudes. They are common in the late fall and winter.
Cumulus
Detached clouds, generally dense and with sharp outlines, developing
vertically in the form of rising mounds, domes or towers, of which the
bulging upper part often resembles a cauliflower. The sunlit parts of
these clouds are mostly brilliant white; their base is relatively dark and
nearly horizontal. Expansion of rising air results in adiabatic cooling
thus forming cumulus clouds.
Nimbostratus
Grey cloud layer, often dark, the appearance of which is rendered
diffuse by more or less continuously falling rain or snow. The clouds are
shapeless and of a low altitude. Slow steady rain or snow may develop from
these clouds.
Altocumulus
Clouds found at middle altitudes that are similar in appearance to
cumulus clouds. Altocumulus clouds are composed of white or gray, rounded
patches with sky visible between the patches. These clouds are often
arranged in groups or lines.
Altostratus
Middle altitude stratus clouds. They are smooth on the underside
forming a uniform gray sheet. The faint image of the sun can sometimes be
seen through them. These clouds sometimes appear bluish.
Cumulonimbus
Heavy and dense cloud with a considerable vertical extent, in the form
of a mountain or huge towers. The upper portion of the cloud spreads out
in the shape of an anvil. Under the base of this cloud (which is often
very dark) there are frequently low ragged clouds either merged with it or
not, and precipitation sometimes in the form of virga.
Mamatocumulus
These clouds extend downward from a cumulonimbus cloud, they appear as
bubble like bulges. They are formed by a cold downdraft near the edge of a
thunderstorm and as a result can accompany severe weather conditions and
in particular tornadoes.
Coverage
Cloud coverage is classified into the following four categories.
- Overcast - The sky is completely (or greater than 90%)
covered by clouds.
- Broken - Mostly cloudy skies (50%-90% of the sky covered by
clouds).
- Scattered - Partly cloudy skies (10%-50% of the sky covered
by clouds).
- Clear - The sky is cloud free (or less than 10% cloud
coverage).
Movement
Clouds move in the direction the wind is blowing at the altitude of the
clouds. Clouds can be moving a different direction than the observed wind
at the surface. If a cold front just passed through the winds will be
backing with height (counterclockwise shift in wind direction). If a warm
front is approaching the winds will be veering with height(clockwise shift
in wind direction).
Precipitation:
Rainfall is recorded by a rain gauge. Snowfall can be measured by using
a ruler to record the depth. Precipitation amount is always given as the
amount of precipitation that fell over a given time period (generally 24
hours, midnight to midnight). If the precipitation occurs while the wind
is strong precipitation measurements must be measured carefully. Rain
would fall more horizontally which means the instrument wouldn't
"catch" all the rain that actually falls. Snow depth would vary
considerably because of drifting so measure the depth at many flat, open
locations and take an average estimate. In making an observation while
there is precipitation, note the time precipitation began and ended.
Wind:
Wind has a speed and a direction. Wind speed is measured by an
anemometer. Wind direction can be estimated by noting which way a flag is
blowing. Remember wind direction is taken as the direction the wind is
coming from.
Present weather:
Weather is constantly changing and can change in many different ways.
Is it precipitating, if so, what kind of precipitation is it (rain,
snow, ice pellets, hail, mix) and what is the intensity of the
precipitation (light, moderate or heavy). Visibility is generally a good
indicator of precipitation intensity. Note the distance to landmarks near
the observation spot and determine what landmarks are visible when there
is significant precipitation.
Is the precipitation associated with a thunderstorm, if so the
intensity of the thunderstorm should be noted. Is there lightning
associated with the thunderstorm, if so how what is the intensity
(frequent or occasional, cloud to cloud or cloud to ground). Is there hail
with the thunderstorm, if so what is the size of the hail (its diameter).
Is there a tornado associated with the thunderstorm. Sources for weather
instruments.
Weather Map Symbols
Glossary of Important Terminology
Air Parcel -- An
imaginary small body of air that is used to explain the behavior of air. A
parcel is large enough to contain a very great number of molecules, but
small enough so that the properties assigned to it are approximately
uniform throughout.
Anvil Cloud -- The spreading of the upper
portion of a cumulonimbus cloud that takes on the form of an anvil,
typically fibrous or smooth in appearance.
Buoyancy -- That property of an object
that enables it to float on the surface of a liquid, or as in the case
with air parcels, to ascend and remain freely suspended in the atmosphere.
Cloud Base -- For a given cloud or cloud
layer, it is the lowest level in the atmosphere where cloud particles are
visible.
Cold Front -- A narrow transition zone
separating advancing colder air from retreating warmer air. The air behind
a cold front is cooler and typically drier than the air it is replacing.
Condensation -- The process by which
water vapor becomes a liquid; the opposite of evaporation, which is
the conversion of liquid to vapor. Sublimation is the process by
which a solid forms directly from vapor.
Convection -- In general, mass motions
within a fluid resulting in transport and mixing of properties of that
fluid.
- In Meteorology: Convection is referred to as the atmospheric
motions that are predominantly vertical, such as rising air currents
produced by surface heating.
- Forced Convection is the ascent of air induced by some
external force. An example of this convection is the lifting of
lighter, warmer air by an advancing cold front. Other examples include
convection resulting from orographic lifting and convergence.
- Free Convection is the rising of heated air and the sinking
of cooler air without the need of external forces.
Convergence -- An atmospheric
condition that exists when the winds cause a horizontal net inflow
of air into a specified region. Divergence is the opposite, where
winds cause a horizontal net outflow of air from a specified
region.
Eddy -- A small volume of air that behaves
differently from the predominant flow of the layer in which it exists,
seemingly having a life of its own. An example of such would be a tornado,
which has its own distinct rotation, but is different than the large-scale
flow of air surrounding the thunderstorm in which the tornado is born.
Evaporation -- The physical process by
which a liquid is transformed into a gaseous state.
Front -- The transition zone between two
distinct air masses. These air masses could be different in temperature or
moisture content.
Glaciation -- The transformation of
cloud particles from water droplets to ice crystals. Thus, a cumulonimbus
cloud is said to have a "glaciated" upper portion.
Halo -- A ring or arc that encircles the sun
or moon when seen through a cloud of ice crystals, such as cirrostratus.
Halos are produced by the refraction of light.
Hot Spot -- Typically large areas of
pavement, these "hot spots" are heated much quicker by
the sun than surrounding grasses and forests. As a result, air rises
upwards from the relatively hot surface of the pavement, reaches its
condensation level, condenses, and forms a cloud above the "hot
spot".
Lapse Rate -- The rate at which atmospheric
variables decrease with height, (temperature for example).
Lifting Condensation Level -- The level in
the atmosphere where a lifted air parcel reaches its saturation point, and
as a result, the water vapor within condenses into water droplets.
Nucleus -- a particle of any nature upon
which molecules of water or ice accumulate.
Orographic Lifting -- It is the forced
ascent of air by topographical features, such as a mountain.
Refraction -- The bending of light as
it passes from one medium to another.
Relative Humidity -- The amount of water
vapor in the air compared to the amount of water vapor the air can
actually hold at a given temperature and pressure. When the air has a
relative humidity of 100%, it is saturated.
Saturation -- An atmospheric condition
whereby the level of water vapor is the maximum possible at the existing
temperature and pressure. If the pressure or temperature were to decrease,
the water vapor would then condense and form water droplets. Saturated air
has a relative humidity of 100%.
Stratosphere -- The layer of
atmosphere above the troposphere and below the mesosphere (between 10 km
and 50 km) generally characterized by an increase in temperature with
height.
Supercooled -- Liquid droplets that
exist in an environment where the temperature is colder than the freezing
point (0 degrees Celsius).
Thermal -- A small rising parcel of warm
air produced when the earth's surface is unevenly heated.
- How they form: Air molecules bounce against the earth's hot
surface, acquiring energy, and as a result, the heated air expands,
becoming lighter than the surrounding air, which allows it to rise.
These thermals "bubble" upwards and transfer energy to
higher levels. This is an example of convection.
Topography -- Generally, the lay-out of
the major natural and man-made physical features of the earth's surface.
Bridges, highways, trees, rivers and fields are all components that make
up this topography.
Tropopause -- The boundary between the
troposphere and stratosphere.
Updraft -- A current of ascending air.
Warm Front -- A narrow transitions zone
separating advancing warmer air from retreating cooler air. The air behind
a warm front is warmer and typically more humid than the air it is
replacing.
Wave Crest -- The highest point in a
wave.
Wave Trough -- The lowest point in a
wave.
Wind Shear -- The rate of change of wind
speed or wind direction over a given distance.
- Vertical Shear -- is simply the rate of change of wind speed
or direction, with a given change in height.
High Level Clouds
High clouds typically form above 20,000 ft. (6,000 m) and are composed
primarily of ice crystals, since the temperatures are so cold at such high
elevations. These clouds are rather thin and usually white in appearance.
However, during sunrise and sunset, these clouds can appear in a
magnificent array of colors, as unscattered components of sunlight (red,
yellow, and orange) are reflected by the underside of the clouds.
Low-Level Clouds
Low clouds are almost always comprised of water
droplets, since their bases generally lie below 6,500 ft (2000 m). The one
exception occurs when the temperatures are cold enough and these clouds
may contain ice particles and snow.
Types of low clouds include:
Nimbostratus Clouds
Stratocumulus Clouds
A Quick Glance at Hurricanes and Tornadoes
A hurricane is a storm of tropical origin, with sustaining winds in
excess of 74 (mi/hr). Fueled by heat from the warm ocean waters of the
tropics, hurricanes average about 300 miles in diameter, and winds
sometimes in excess of 180 (mi/hr), as was the case with hurricane Camille
1969, which resulted in the deaths of more than 200 people.
Tornadoes are very strong winds rapidly spinning around a small area of
intense low pressure. Spawning from cumulonimbus clouds, tornadoes are
much smaller than hurricanes, in that they have a range of diameters from
a few meters to about a mile, but winds are sometimes greater than 250
(mi/hr).
Hurricanes
The counter-clockwise rotation of the swirling rainbands is very
evident in this satellite picture taken of a hurricane making landfall in
Mississippi and Louisiana. In the center of the storm is a relatively
cloud free region known as the eye, where the winds are calm and
typically no precipitation is falling.
A photograph taken from the Apollo 7 spacecraft of hurricane
Gladys, which was positioned about 150 km. southwest of Tampa, Florida. At
the time the of the photograph, Gladys had a central pressure around 986
mb, with surface winds of 80 knots.
Cumulonimbus Clouds
The Cumulonimbus Cloud is much larger and more vertically
developed than the Fair Weather Cumulus. It can exist as a single
towering cloud, or can even develop into a line of such towers, also known
as a "squall line". Fueled by vigorous convection,
which leads to strong vertical updrafts,
sometimes in excess 50 (mi/hr), these clouds can climb vertically in the
atmosphere to an altitude of 39,000 ft (12,000 m) or higher.
The lower portion of the cloud is made up of water droplets, while at
the cloud top, where temperatures are well below 0 degrees Celsius, there
are primarily ice crystals. These clouds are capable of developing into
great thunderheads which contain all forms of precipitation-- snowflakes,
snow pellets, large raindrops, and sometimes hailstones. Lightning,
thunder, and even violent tornadoes sometimes accompany intense
cumulonimbus clouds.
More on Cumulonimbus Clouds
Some cumulus clouds, which may initially appear as harmless floating
puffs of cotton, can quickly develop into powerful and potentially fatal
thunderstorms known as supercells.
The sun is setting behind this developing cumulonimbus tower, revealing
distinct outlines that mark the leading edge of the rising air.
Supercell
A supercell is a large thunderstorm whose updrafts and downdrafts are
so closely in balance that such a storm can have a lifetime of several
hours. Fueled by updrafts
occasionally as strong as 90 m.p.h., these storms have the capability of
producing large hail, damaging winds, and tornadoes.
Supercells are typically characterized by strong vertical
shear and steep lapse
rates. These storms can occur in isolation or develop into a line of
multiple supercells, also known as a squall line.
Lifting Mechanisms
The process of air rising, the water vapor in the air cooling to its
saturation point, condensing and becoming visible as a cloud, is basically
how clouds develop. The following mechanisms are responsible for the
development of many clouds we see:
When the earth is heated by the sun, bubbles of hot air, or "thermals",
rise up from the warm surface, cooling and expanding as they ascend. The
thermal becomes diluted as it mixes with the surrounding air and begins to
loose its buoyancy.
However, should other thermals follow the same path, successive
thermals usually rise higher than previous ones. If a thermal is able to
rise high enough to cool to its saturation
point, the moisture within condenses
and becomes visible as a cloud.
Convergence Lifting
When the horizontal flow of air converges
along the earth's surface, it must go somewhere. Since it can't go into
the ground, it therefore rises. Air converging into a large area, will
lift a layer of air, (sometimes hundreds of kilometers across), to higher
levels, cooling the air as it rises. If the lifted air is moist enough,
the water vapor will condense out to form cloud droplets.
The vertical motions associated with this type of cloud development are
typically much less than the stronger, smaller-scale vertical motions
associated with clouds formed by
covis.atmos.uiuc.edu/guide/clouds/cloud.lifting/html/convective.lifting.
It is also important to note, however, that convergence can occur, not
only at the surface, but at higher-levels as well.
An example of clouds formed by convergence lifting is: Cirrostratus
Cirrostratus are sheetlike clouds composed of ice crystals. Even though
cirrostratus can cover the entire sky, and can be up to several thousand
feet deep, they are relatively transparent, as the sun or the moon can be
easily seen through them. Sometimes, the only indication of their presence
is given by an observed halo
(around the sun or moon). Halos result from the refraction
of light by the ice crystals which make up the cloud.
These high level clouds typically form when a broad layer of air is
lifted to its Lifting
Condensation Level by large-scale convergence.
Cirrostratus clouds, however, tend to thicken as a warm front approaches,
signifying an increased production of ice crystals. As a result, the halo
is no longer seen and the sun (or moon by night) becomes less visible.
Lifting Along Frontal Zones
Since warm air is less dense than cold air for parcels of air at the
same pressure, the warm air will be lifted upwards when confronted by a
mass of cooler air. The lifted air cools as it rises, and if the air is
moist enough, the water vapor will condense out to form cloud droplets.
Lifting along cold fronts
Towering cumulonimbus clouds are found commonly
along or in advance of a cold front is:
Warm Fronts:
Lifting
along Warm Fronts
A cloud type commonly found as a result of lifting along
a warm front:
Cirrostratus Clouds
Orographic Lifting
When air is confronted by mountains, it can't simply go through them,
it must go over. If there is enough moisture in the air...the air ascends
the mountain, the water vapor in the air cools, condenses, and form
clouds. These are called "orographic clouds, which develop in
response to the lifting forced by the topography
of the earth.
For an example of clouds resulting from orographic forcing:
Orographic Clouds
Orographic Clouds formed by
Lee Mountain Waves
 Possible
Air Trajectories Forced by Mountains
Air that passes over a mountain will oscillate up and down as it moves
downstream. During the initial stage, stable air encounters a mountain,
and is lifted upwards. The air
parcels undergo cooling through expansion, and eventually grow heavier
than the environment. If a parcel cools to its saturation
point during this process, the water vapor within will condense and
become visible as a cloud.
Upon reaching the mountain top, the air is heavier than the environment
and will sink down the other side. Warming as it descends, the air will
once again return to its original height. At this point in time, it has
the same temperature and same buoyancy
as the surrounding air, but the air is moving downward and does not stop
immediately.
With continued descent, the air becomes warmer than the surroundings,
and begins to accelerate upwards. As a result, the air slows its downward
path, and begins to rise once again, accelerating toward its original
height, beginning another cycle. It is during the upper-most ascent phase
of the cycle where the clouds develop, (provided, of course, that the air
is moist enough). As in the diagram above, the lifting of moist air can
result in the generation of clouds, while in contrast, the lifting of
drier air may not produce any clouds at all. Where there is rising motion,
clouds will form (if they form at all) and where the air is moving
downwards, skies are likely to be clear.
An example of clouds developing at the wave crest,
with clear skies in between at the wave trough
is given below:
 Click
here for picture
These oscillations continue as the air moves further downstream from
the mountains and are eventually damped out by mixing and friction.
Other
examples of clouds formed by lee mountain waves
Mid Level Clouds
Mid-level clouds have cloud bases typically between 6,500 to 23,000 ft
(2,000 to 6,000 m). Because of their lower altitudes, they are composed
primarily of water droplets, however, when the temperatures are cold
enough, they can be composed of ice crystals as well.
An example of a mid-level cloud is:
Altocumulus Clouds
Altocumulus Clouds
Altocumulus clouds generally appear as puffy masses,
Click here for picture
or are sometimes aligned in parallel waves or bands.
Click here for
picture
One part of the cloud is usually darker than the rest, which makes it
distinguishable from the higher cirrocumulus. These clouds are a
result of slow lifting that is common ahead of an advancing cold
front, and are typically located about 3-4 km. above the ground.
In the presence of rising air at cloud level, altocumulus take on the
appearance of "little castles", and these clouds are often seen
on warm, humid summer mornings, sometimes followed by thunderstorms later
in the afternoon.
Convective Clouds
Probably the most familiar of the classifications is the Cumulus
Cloud, and the variety of shapes, sizes, and colors it is able to
attain. Generated most commonly through either thermal convection
or frontal
lifting, what may begin the day as harmless-looking cumulus clouds,
exists the potential for tremendous vertical development come afternoon.
Sometimes to a height in excess of 39,000 ft (12,000 m), releasing
incredible amounts of energy by the condensation
of water vapor within the cloud itself.
For further insight into the different types of cumulus clouds,
choose from the following list:
Fair
Weather Cumulus
Cumulonimbus
Fair Weather Cumulus
Click
here for picture
These clouds have the appearance of floating cotton with a flat base
and distinct outlines, while having a lifetime anywhere from 5-40 minutes.
The color shading ranges from white to light gray, with bases that can be
as low as 1,000 m above the ground, and have a diameter of about a
kilometer or so. The cloud tops, likewise, are usually not very high and
designate the limit of the rising air.
Called cumulus humilis, these clouds show only slight vertical
growth. However, given suitable conditions, what may initially begin as
harmless Fair Weather Cumulus, can later develop into towering
cumulus clouds that have an appearance resembling that of a cauliflower,
with strong updrafts
and possibly precipitation.
Click here for
cauliflower example
Fair weather cumulus are distinguishable from stratocumulus by the fact
that they are detached, while stratocumulus
generally appear in patches or groups. The tops of fair weather cumulus
have a domed-shape to them while in contrast, the tops of stratocumulus
are flat.
More on Fair Weather Cumulus
Fair weather cumulus are fueled by buoyant
bubbles of air, or thermals,
that rise upwards from the earth's surface. As they rise, the air within
the thermal cools and the water vapor condenses,
forming cloud droplets. The height above the ground where this process
takes place is known as the lifting
condensation level, and the location of this level is easily
identified by the distinct flat cloud bases commonly associated with fair
weather cumulus.
Since air is primarily rising within the clouds themselves, there must
be "subsidence", or downward motion of air between the
clouds. Subsiding air slowly descends to replace the warmer air that has
ascended to higher levels from below. A valuable rule of thumb to remember
is that: if clouds appear, they are most likely to develop where there
is some type of rising motion. In contrast, sinking air, or downward
motions, inhibit cloud development, and as a result, clear skies are
common in areas of subsidence.
Clouds in their early stages of development have sharply defined edges,
while in older clouds, their bases and edges are more ragged in
appearance. This reveals the process of cloud erosion. The evaporation
of cloud edges cools the air surrounding the cloud, making it heavier and
also resulting in sinking motion. This downward motion inhibits further
convection and the growth of additional thermals from below, which is why
fair weather cumulus typically have expanses of clear sky between them.
Without a continued supply of rising air into the cloud, the erosion
process will continue, eventually resulting in the cloud's eventual
dissipation.
Stratocumulus Clouds
A low, lumpy layer of clouds that typically appears near sunset, and
are the spreading remains of much larger cumulus clouds. They range in
color from dark to light grey and can appear in rows, patches, or as
rounded masses with breaks of clear sky in between.
Rain or snow rarely fall from these clouds and they are different from
altocumulus clouds since their individual elements are larger than those
of their altocumulus counterparts.
You can easily decipher the difference between the two cloud types by
holding your hand at arm's length and pointing it towards the sky.
Altocumulus elements are about the size of your thumb nail, while
stratocumulus are about the size of your fist.
Other Cloud Types
 Contrails
A contrail, also known as a condensation trail, is a cirrus-like
trail of condensed vapor that is produced by jet aircraft flying at high
altitudes.
Kelvin-Helmholtz
Clouds
Wave-like clouds that form as a result of a strong change in wind speed
across two adjacent layers of air.
Mammatus
Clouds
Named after their "pouch-like" appearance, mammatus
are most commonly seen on the underside of cumulonimbus clouds, though
they have been observed to be associated with other cloud types as well.
Orographic
Clouds
The forced lifting of air by topographical
features, for example mountains, produces a special classification of
clouds known as orographic clouds.
Pileus Clouds
A smooth cloud that is found above or attached to either the top of a
topographical feature, such as a mountain, or a growing cumulus tower.
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