NW/5 kts SW/20 kts
E/35 kts N/50 kts W/105 kts
Calm
Figure 12-18. Depiction of winds on a surface weather chart.
1028
1024
1020
1016
1012
1008
Isobars
Isobars Closely spaced isobars mean a steep
pressure gradient and strong winds.
Widely spaced isobars
mean a shallow pressure
gradient and relatively
light winds.
L
Figure 12-19. Isobars reveal the pressure gradient of an area of
high- or low-pressure areas.
Wind and Pressure Representation on Surface
Weather Maps
Surface weather maps provide information about fronts, areas
of high and low pressure, and surface winds and pressures
for each station. This type of weather map allows pilots to
see the locations of fronts and pressure systems, but more
importantly, it depicts the wind and pressure at the surface
for each location. For more information on surface analysis
and weather depiction charts, see Chapter 13, Aviation
Weather Services.
Wind conditions are reported by an arrow attached to the
station location circle. [Figure 12-18] The station circle
represents the head of the arrow, with the arrow pointing
in the direction from which the wind is blowing. Winds
are described by the direction from which they blow, thus
a northwest wind means that the wind is blowing from the
northwest toward the southeast. The speed of the wind is
depicted by barbs or pennants placed on the wind line. Each
barb represents a speed of ten knots, while half a barb is equal
to five knots, and a pennant is equal to 50 knots.
The pressure for each station is recorded on the weather chart
and is shown in mb. Isobars are lines drawn on the chart to
depict lines of equal pressure. These lines result in a pattern
that reveals the pressure gradient or change in pressure over
distance. [Figure 12-19] Isobars are similar to contour lines
on a topographic map that indicate terrain altitudes and
slope steepness. For example, isobars that are closely spaced
indicate a steep pressure gradient and strong winds prevail.
Shallow gradients, on the other hand, are represented by
isobars that are spaced far apart and are indicative of light
winds. Isobars help identify low- and high-pressure systems,
as well as the location of ridges and troughs. A high is an
area of high pressure surrounded by lower pressure; a low
is an area of low pressure surrounded by higher pressure. A
ridge is an elongated area of high pressure, and a trough is
an elongated area of low pressure.
Isobars furnish valuable information about winds in the first
few thousand feet above the surface. Close to the ground,
wind direction is modified by the friction and wind speed
decreases due to friction with the surface. At levels 2,000 to
3,000 feet above the surface, however, the speed is greater
and the direction becomes more parallel to the isobars.
Generally, the wind 2,000 feet above ground level (AGL) is
20° to 40° to the right of surface winds, and the wind speed is
greater. The change of wind direction is greatest over rough
terrain and least over flat surfaces, such as open water. In the
absence of winds aloft information, this rule of thumb allows
for a rough estimate of the wind conditions a few thousand
feet above the surface.
Atmospheric Stability
The stability of the atmosphere depends on its ability to
resist vertical motion. A stable atmosphere makes vertical
movement difficult, and small vertical disturbances dampen
out and disappear. In an unstable atmosphere, small vertical air
movements tend to become larger, resulting in turbulent airflow
and convective activity. Instability can lead to significant
turbulence, extensive vertical clouds, and severe weather.
Rising air expands and cools due to the decrease in air
pressure as altitude increases. The opposite is true of
descending air; as atmospheric pressure increases, the
temperature of descending air increases as it is compressed.
Adiabatic heating and adiabatic cooling are terms used to
describe this temperature change.
The adiabatic process takes place in all upward and
downward moving air. When air rises into an area of lower
pressure, it expands to a larger volume. As the molecules
of air expand, the temperature of the air lowers. As a result,
when a parcel of air rises, pressure decreases, volume
increases, and temperature decreases. When air descends,
the opposite is true. The rate at which temperature decreases
with an increase in altitude is referred to as its lapse rate.
As air ascends through the atmosphere, the average rate of
temperature change is 2 °C (3.5 °F) per 1,000 feet.
Since water vapor is lighter than air, moisture decreases air
density, causing it to rise. Conversely, as moisture decreases,
air becomes denser and tends to sink. Since moist air cools
at a slower rate, it is generally less stable than dry air since
the moist air must rise higher before its temperature cools
to that of the surrounding air. The dry adiabatic lapse rate
(unsaturated air) is 3 °C (5.4 °F) per 1,000 feet. The moist
adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to
5 °F) per 1,000 feet.
The combination of moisture and temperature determine the
stability of the air and the resulting weather. Cool, dry air
is very stable and resists vertical movement, which leads to
good and generally clear weather. The greatest instability
occurs when the air is moist and warm, as it is in the tropical
regions in the summer. Typically, thunderstorms appear on
a daily basis in these regions due to the instability of the
surrounding air.
Inversion
As air rises and expands in the atmosphere, the temperature
decreases. There is an atmospheric anomaly that can occur;
however, that changes this typical pattern of atmospheric
behavior. When the temperature of the air rises with altitude, a
temperature inversion exists. Inversion layers are commonly
shallow layers of smooth, stable air close to the ground. The
temperature of the air increases with altitude to a certain
point, which is the top of the inversion. The air at the top
of the layer acts as a lid, keeping weather and pollutants
trapped below. If the relative humidity of the air is high, it
can contribute to the formation of clouds, fog, haze, or smoke
resulting in diminished visibility in the inversion layer.
Surface-based temperature inversions occur on clear, cool
nights when the air close to the ground is cooled by the
lowering temperature of the ground. The air within a few
hundred feet of the surface becomes cooler than the air above
it. Frontal inversions occur when warm air spreads over a
layer of cooler air, or cooler air is forced under a layer of
warmer air.
Moisture and Temperature
The atmosphere, by nature, contains moisture in the form
of water vapor. The amount of moisture present in the
atmosphere is dependent upon the temperature of the air.
Every 20 °F increase in temperature doubles the amount of
moisture the air can hold. Conversely, a decrease of 20 °F
cuts the capacity in half.
Water is present in the atmosphere in three states: liquid,
solid, and gaseous. All three forms can readily change to
another, and all are present within the temperature ranges of
the atmosphere. As water changes from one state to another,
an exchange of heat takes place. These changes occur through
the processes of evaporation, sublimation, condensation,
deposition, melting, or freezing. However, water vapor
is added into the atmosphere only by the processes of
evaporation and sublimation.
Evaporation is the changing of liquid water to water vapor.
As water vapor forms, it absorbs heat from the nearest
available source. This heat exchange is known as the latent
heat of evaporation. A good example is the evaporation of
human perspiration. The net effect is a cooling sensation
as heat is extracted from the body. Similarly, sublimation
is the changing of ice directly to water vapor, completely
bypassing the liquid stage. Though dry ice is not made of
water, but rather carbon dioxide, it demonstrates the principle
of sublimation when a solid turns directly into vapor.
Relative Humidity
Humidity refers to the amount of water vapor present in the
atmosphere at a given time. Relative humidity is the actual
amount of moisture in the air compared to the total amount of
moisture the air could hold at that temperature. For example,
if the current relative humidity is 65 percent, the air is
holding 65 percent of the total amount of moisture that it is
capable of holding at that temperature and pressure. While
much of the western United States rarely sees days of high
humidity, relative humidity readings of 75 to 90 percent are
not uncommon in the southern United States during warmer
months. [Figure 12-20]
Temperature/Dew Point Relationship
The relationship between dew point and temperature defines
the concept of relative humidity. The dew point, given in
degrees, is the temperature at which the air can hold no
more moisture. When the temperature of the air is reduced
to the dew point, the air is completely saturated and moisture
begins to condense out of the air in the form of fog, dew,
frost, clouds, rain, or snow.
A cubic meter of air with 17g of water
vapor at 20 °C is at saturation
or 100% relative humidity. Any further
cooling will cause condensation (fog,
clouds, dew) to form. Thus, 20 °C is
the dew point for this situation.
If the same cubic meter of air warms
to 30 °C, the 17 g of water vapor will
produce a relative humidity of 56%.
(17 g is 56% of the 30 g the air could
hold at this temperature.)
If the temperature is lowered to 10 °C,
the air can hold only 9 g of water
vapor, and 8 g of water will condense
as water droplets. The relative
humidity will still be at 100%.
At sea level pressure, air can hold
9 g H2O/cubic meter of air at 10 °C
17 g H2O/cubic meter of air at 20 °C
30 g H2O/cubic meter of air at 30 °C
Figure 12-20. Relationship between relative humidity, temperature, and dewpoint.
As moist, unstable air rises, clouds often form at the altitude
where temperature and dew point reach the same value. When
lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet
and the dew point temperature decreases at a rate of 1 °F per
1,000 feet. This results in a convergence of temperature and
dew point at a rate of 4.4 °F. Apply the convergence rate
to the reported temperature and dew point to determine the
height of the cloud base.
Given:
Temperature (T) = 85 °F
Dew point (DP) = 71 °F
Convergence Rate (CR) = 4.4°
T – DP = Temperature Dew Point Spread (TDS)
TDS ÷ CR = X
X × 1,000 feet = height of cloud base AGL
Example:
85 °F – 71 °F = 14 °F
14 °F ÷ 4.4 °F = 3.18
3.18 × 1,000 = 3,180 feet AGL
The height of the cloud base is 3,180 feet AGL.
Explanation:
With an outside air temperature (OAT) of 85 °F at the surface
and dew point at the surface of 71 °F, the spread is 14°. Divide
the temperature dew point spread by the convergence rate of
4.4 °F, and multiply by 1,000 to determine the approximate
height of the cloud base.
Methods by Which Air Reaches the Saturation Point
If air reaches the saturation point while temperature and
dew point are close together, it is highly likely that fog, low
clouds, and precipitation will form. There are four methods
by which air can reach the saturation point. First, when warm
air moves over a cold surface, the air temperature drops and
reaches the saturation point. Second, the saturation point may
be reached when cold air and warm air mix. Third, when air
cools at night through contact with the cooler ground, air
reaches its saturation point. The fourth method occurs when
air is lifted or is forced upward in the atmosphere.
As air rises, it uses heat energy to expand. As a result, the rising
air loses heat rapidly. Unsaturated air loses heat at a rate of
3.0 °C (5.4 °F) for every 1,000 feet of altitude gain. No matter
what causes the air to reach its saturation point, saturated air
brings clouds, rain, and other critical weather situations.
Figure 12-21. Radiation fog.
Dew and Frost
On cool, clear, calm nights, the temperature of the ground
and objects on the surface can cause temperatures of the
surrounding air to drop below the dew point. When this
occurs, the moisture in the air condenses and deposits itself on
the ground, buildings, and other objects like cars and aircraft.
This moisture is known as dew and sometimes can be seen
on grass and other objects in the morning. If the temperature
is below freezing, the moisture is deposited in the form of
frost. While dew poses no threat to an aircraft, frost poses a
definite flight safety hazard. Frost disrupts the flow of air over
the wing and can drastically reduce the production of lift. It
also increases drag, which when combined with lowered lift
production, can adversely affect the ability to take off. An
aircraft must be thoroughly cleaned and free of frost prior
to beginning a flight.
Fog
Fog is a cloud that is on the surface. It typically occurs when
the temperature of air near the ground is cooled to the air’s
dew point. At this point, water vapor in the air condenses and
becomes visible in the form of fog. Fog is classified according
to the manner in which it forms and is dependent upon the
current temperature and the amount of water vapor in the air.
On clear nights, with relatively little to no wind present,
radiation fog may develop. [Figure 12-21] Usually, it forms
in low-lying areas like mountain valleys. This type of fog
occurs when the ground cools rapidly due to terrestrial
radiation, and the surrounding air temperature reaches its
dew point. As the sun rises and the temperature increases,
radiation fog lifts and eventually burns off. Any increase in
wind also speeds the dissipation of radiation fog. If radiation
fog is less than 20 feet thick, it is known as ground fog.
When a layer of warm, moist air moves over a cold surface,
advection fog is likely to occur. Unlike radiation fog, wind
is required to form advection fog. Winds of up to 15 knots
allow the fog to form and intensify; above a speed of 15 knots,
the fog usually lifts and forms low stratus clouds. Advection
fog is common in coastal areas where sea breezes can blow
the air over cooler landmasses.
Upslope fog occurs when moist, stable air is forced up sloping
land features like a mountain range. This type of fog also
requires wind for formation and continued existence. Upslope
and advection fog, unlike radiation fog, may not burn off with
the morning sun but instead can persist for days. They can
also extend to greater heights than radiation fog.
Steam fog, or sea smoke, forms when cold, dry air moves over
warm water. As the water evaporates, it rises and resembles
smoke. This type of fog is common over bodies of water
during the coldest times of the year. Low-level turbulence
and icing are commonly associated with steam fog.
Ice fog occurs in cold weather when the temperature is
much below freezing and water vapor forms directly into ice
crystals. Conditions favorable for its formation are the same
as for radiation fog except for cold temperature, usually –25
°F or colder. It occurs mostly in the arctic regions but is not
unknown in middle latitudes during the cold season.
Clouds
Clouds are visible indicators and are often indicative of
future weather. For clouds to form, there must be adequate
water vapor and condensation nuclei, as well as a method by
which the air can be cooled. When the air cools and reaches
its saturation point, the invisible water vapor changes into
a visible state. Through the processes of deposition (also
referred to as sublimation) and condensation, moisture
condenses or sublimates onto miniscule particles of matter
like dust, salt, and smoke known as condensation nuclei. The
nuclei are important because they provide a means for the
moisture to change from one state to another.
Cloud type is determined by its height, shape, and
characteristics. They are classified according to the height of
their bases as low, middle, or high clouds, as well as clouds
with vertical development. [Figure 12-22]
Low clouds are those that form near the Earth’s surface and
extend up to about 6,500 feet AGL. They are made primarily
of water droplets but can include supercooled water droplets
that induce hazardous aircraft icing. Typical low clouds
are stratus, stratocumulus, and nimbostratus. Fog is also
classified as a type of low cloud formation. Clouds in this
family create low ceilings, hamper visibility, and can change
rapidly. Because of this, they influence flight planning and
can make visual flight rules (VFR) flight impossible.
20,000 AGL
6,500 AGL
Low clouds
Middle clouds
High clouds
Nimbostratus
Altocumulus
Cirrostratus
Cirrocumulus
Cirrus
Altostratus
Stratus
Stratocumulus
Cumulonimbus
Clouds with vertical development
Towering Cumulus
Figure 12-22. Basic cloud types.
Middle clouds form around 6,500 feet AGL and extend up to
20,000 feet AGL. They are composed of water, ice crystals,
and supercooled water droplets. Typical middle-level clouds
include altostratus and altocumulus. These types of clouds
may be encountered on cross-country flights at higher
altitudes. Altostratus clouds can produce turbulence and may
contain moderate icing. Altocumulus clouds, which usually
form when altostratus clouds are breaking apart, also may
contain light turbulence and icing.
High clouds form above 20,000 feet AGL and usually form
only in stable air. They are made up of ice crystals and pose
no real threat of turbulence or aircraft icing. Typical high
level clouds are cirrus, cirrostratus, and cirrocumulus.
Clouds with extensive vertical development are cumulus
clouds that build vertically into towering cumulus or
cumulonimbus clouds. The bases of these clouds form in
the low to middle cloud base region but can extend into high
altitude cloud levels. Towering cumulus clouds indicate areas
of instability in the atmosphere, and the air around and inside
them is turbulent. These types of clouds often develop into
cumulonimbus clouds or thunderstorms. Cumulonimbus
clouds contain large amounts of moisture and unstable air
and usually produce hazardous weather phenomena, such
as lightning, hail, tornadoes, gusty winds, and wind shear.
These extensive vertical clouds can be obscured by other
cloud formations and are not always visible from the ground
or while in flight. When this happens, these clouds are said
to be embedded, hence the term, embedded thunderstorms.
To pilots, the cumulonimbus cloud is perhaps the most
dangerous cloud type. It appears individually or in groups and
is known as either an air mass or orographic thunderstorm.
Heating of the air near the Earth’s surface creates an air mass
thunderstorm; the upslope motion of air in the mountainous
regions causes orographic thunderstorms. Cumulonimbus
clouds that form in a continuous line are nonfrontal bands
of thunderstorms or squall lines.
Since rising air currents cause cumulonimbus clouds, they
are extremely turbulent and pose a significant hazard to flight
safety. For example, if an aircraft enters a thunderstorm,
the aircraft could experience updrafts and downdrafts that
exceed 3,000 fpm. In addition, thunderstorms can produce
large hailstones, damaging lightning, tornadoes, and large
quantities of water, all of which are potentially hazardous
to aircraft.