W E A T H E R > Honza Rejmanek
Good Glass-Off Gone Bad Originally published in Cross Country Magazine the troposphere. This is a much more dynamic state that : Carefree soaring in widespread lift in the late afternoon is a great way to end a day. However, sometimes
is no longer reliant on surface heating. Deep convection
and in some locations, the soaring situation can evolve
taps most of its energy from latent heat release due to
from carefree to worrisome in less than 20 minutes. This
the condensation of countless tiny droplets. In a sense,
is because a lot of energy has been put into the system
the atmosphere stops caring about what time of day it is.
throughout the day and, due to inertia, it is possible
In the vicinity of these towering clouds, late afternoon
that local winds can peak as much as six hours after
and evening might become the strongest part of the day.
peak heating. In the last issue, the phenomenon, or a
Mellow evening lift can increase to undesirable strength.
combination of phenomena referred to as glass-off, was discussed. It is important to examine situations when
of several cubic kilometers of air that has risen or is in
widespread late afternoon and evening lift is best left to
the process of rising. This volume has to be replaced
the birds.
from below, and this can lead to the strengthening of
Widespread lift with ascent rates that can match or
valley winds in the landing area at a time of day that they
slightly surpass the sink rate of soaring aircraft requires
would normally be decreasing. This increase in valley
significant low-level convergence and similar divergence
wind might not be evident up on launch.
at the top of lift. If there are cumulus clouds present,
Conversely, a cumulonimbus that has stopped growing
then their tops mark the top of the lift. It is important
and is dropping out can send a cool, dense outflow, or
to be vigilant of the height of the cloud tops. In the late
gust front, many kilometers down a valley. When air
afternoon, the scales of convection can change from a
quality is poor and the air is humid, the cloud producing
sky full of perfect fair-weather cumulus to a sky domi-
the gust front might be obscured by the haze. Whether
nated by a few cumulus congestus (towering cumulus).
and to what extent the gust front will affect your landing
Given enough humidity in the lower levels and a favor-
area depends on many factors, such as the strength and
able environmental lapse rate through the depth of the
distance of the storm. The surrounding topography can
troposphere, some of the cumulus congestus can blow
either channel or block the gust front.
up to full-blown cumulonimbus. Fighting to get down in
If no clouds are present, it is fairly safe to assume that
the vicinity of a large towering cloud at sunset is worri-
the top of lift will be near the boundary layer top or just
some at best.
below. In desert or dry mountainous regions, this can
Even if the towering evening cloud is not in your
28
A wide towering cumulus in the distance is evidence
still be several thousand meters above the surface. In
immediate vicinity, it is important to remain observant.
less arid regions, afternoon top of lift will usually be
The concept to keep in mind is that when even isolated
found 1,500-2,500m above the surface. However, the
cumulus congestus begin to appear, it is an indication
absence of clouds is not an excuse to be less attentive.
that the scale and depth of convection are starting to
Watching the sunset from 2,000m above the landing
transition from shallow to deep. Rather than the desired
zone while boating around in omnipresent lift can be
boundary layer convection, we are beginning to see
mesmerizing. Nonetheless, if you want to be on the
hints of deep convection that encompass the depth of
ground in less than half an hour, you will need to either
US HP A P I L O T
