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Summertime VFR and haze
Indeed, haze can be problematic for pilots not
flying on
an instrument flight plan. Add nightfall or flight over a
large
body of water and you can easily become disoriented when the visibility
drops below 7 statute miles. The biggest concern is forward
visibility...not being able to see what's directly in front of
you. Obstructions like towers and mountain peaks may not be
visible until you are right on top of them...especially if you get
distracted. Other aircraft are essentially invisible at your
flight level once the haze restricts visibility to below six miles.
Traffic sensors and ATC radar advisories are critical when
flying VFR in hazy conditions.
Haze as it appears in the METAR is just an
obstruction to
visibility. That is, obscurations are not directly measured by
the
Automated Surface Observation System (ASOS). Instead, they
are
inferred by the reported visibility. The other three
obscurations
are FG (fog), FZFG (freezing fog) and BR (mist). What about
smoke, dust and sand? Other obscurations, such as DU
(dust),
FU (smoke), and SA (sand) are not
automatically reported by the ASOS, but may be augmented by a human
observer.
To understand the reporting of obscurations,
here's how
the ASOS automatically determines what to report. Once each
minute, the obscuration algorithm checks the reported
visibility.
When the visibility drops below 7SM the current dewpoint depression
(temperature-dewpoint spread) is checked to distinguish between FG, BR,
and HZ. If the dewpoint depression is less than or equal to 4°F (~2°C),
then FG or BR will be reported. Visibility will then be used
to
further differentiate between FG and BR (another subject of a future
e-Tip).
When the dewpoint depression is greater than 4°F and no precipitation is
reported, then HZ is reported as the obscuration. When
precipitation is reported, HZ is not
provided as the obscuration.
Even on the clearest of days, the atmosphere
contains
significant concentrations of micron and submicron size particles
(1,000 microns = 1 mm). These are very tiny and are
essentially
invisible to the naked eye. A small cloud drop is about 10
microns in diameter and the average human hair is about 100 microns in
diameter for comparison. Some of these particles become
wetted at
a relative humidity less than 100% and typically account for
the haze we see that impedes visibility. More hygroscopic
particles, called condensation nuclei have an affinity for water and
serve as centers for condensation
and may grow to cloud drop size as the relative humidity increases
or as the air rises, expands and cools to eventually reach saturation.
Negative lapse rates, also known as a temperature
inversion, will typically increase the concentrations of particles
trapping them in the unstable air below the inversion, thus  increasing
the haze in the layer below. The inversion prevents these
particles from "mixing
up" into the air
above the inversion keeping the air a bit cleaner aloft. The
picture to the left (click here
to view a larger picture) shows the haze between the cumuliform clouds
that is trapped in the mixed layer. The key is to identify
these
inversions and determine their approximate height. Flying
above the haze layer does two things.
It keeps you in stable (smooth) air and as you can see in the
picture, it keeps you above the haze with nearly unlimited
visibility. When
cumliform clouds like
this exist, flying above them is one way to identify the clear and
smooth air. However, on days where the sky is cloudlesss,
it's
actually very easy to find these inversions using a thermodynamic chart
called a Skew-T log (p) diagram (see below how you can learn
to read this powerful diagram).
More
particles due to dirtier air caused by pollution, forest fires,
construction, farming, burning, etc.
and higher moisture (higher relative humidity) will typically increase
the haze. A
stronger inversion will also increase the intensity of
the haze especially when it is confined to a thin layer just above the
surface. One of the
reasons the visibility is worse in the morning hours near the surface
is due to a nocturnal inversion and typically higher values of relative
humidity. The nocturnal inversion is normally very shallow.
One
particular inversion that traps pollutants is called a subsidence
inversion that occurs in areas of high pressure. As air
subsides
within the high pressure, it compresses and heats up a bit.
This
produces an inversion aloft. This subsidence inversion can
often
occur well into the teens (15,000 feet, for
example). Often
the water vapor satellite image is a also a good source determine how
"dry" the air is aloft. Dryer air aloft as shown in red in
this
color-enhanced water vapor satellite image generally leads to a less
hazy
atmosphere when a subsidence inverson exists.
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