Difficult meteorology concepts for high school students
Intense meteorology tuition of 30 high school students in Mississippi over
four weeks was followed by a test which showed that certain concepts proved
relatively hard to understand (1). The three most difficult are dealt with
here, in terms of a) the definition, b) the cause and c) some consequences.
- This is the difference
between adjacent wind vectors. The vector difference may be decomposed
into speed and direction. The winds being compared may be separated
vertically or horizontally. Vertical wind shear, for instance, is shown by
differences between winds in adjacent layers of the atmosphere. Wind shear
may be considered as occurring across a large region, or within an area as
small as near the corner of a building. Wind shear is a gradient really,
e.g. the wind difference per unit of height, and has units of s-1.
It can be estimated in a hodograph (showing wind
vector ends at various height intervals), a skew
T log p (with wind barbs on the side), or a wind speed profile (if the
wind direction changes little, e.g. Fig 14.2).
- Wind shear can be caused in
ways such as these:
- ground friction reduces the
surface wind to a speed below that at higher levels, and turns it a little
(Fig 12.9 c);
- horizontal temperature
gradient (thermal wind, Note 12.F), explaining jet streams, for instance;
- horizontal boundaries, such
as a cold front or a sea breeze front, are associated with horizontal
- instabilities, from
baroclinic to buoyant to shear instabilities,
all locally enhance shear while the flow is not balanced; the scale ranges
from a few hundred km to a few metres.
- Example results of wind shear
- a rising or descending
airplane encountering severe horizontal shear (as in a microburst) suffers a sudden change
of lift, which can be dangerous.
- horizontal wind shear at an
aerodrome creates unexpected abrupt forces sideways on a plane.
- shear induces a rolling
motion, turning the wind about the region of lower speed. Such rotation
near the surface leads to turbulence, which promotes the vertical exchange
of surface heat and moisture, for example, into the atmosphere.
- What is called 'static
stability' can be exemplified by a stack of air layers which shows no
signs of any spontaneous rearrangement of layers, no tendency for part of
a lower layer to rise, for instance. Any temporary upward displacement of
the lower part in such a stack is automatically followed by restoration of
the initial arrangement. It is characteristic of a dense fluid below a
lighter one, such as cold air beneath warm. So it is determined by
considering the variation of temperature with elevation within the fluid.
This is an example of the general phenomenon of 'stability', a tendency
for negative feedback to negate any perturbation.
- To consider the cause of the
stability of an inversion layer (where temperatures above are greater than
those below), consider what happens when a small volume of the lower
cooler air is momentarily raised into the warmer air above. The raised
volume is heavier (being colder), so it tends to sink back down again.
Likewise, any lowering of some of the upper layer puts it into the colder,
heavier air beneath, like pushing oil beneath water; there is an immediate
pressure on the lowered air to rise on account of its relative buoyancy.
In both cases, there is resistance to change. In other words, a
temperature profile (like Fig 1.9) determines where there is any layer of
stable air; it will certainly exist where there is an inversion. The
matter can be considered at greater depth by allowing for the cooling that
occurs when the air volume rises, or the heating when it descends,
according to the 'adiabatic lapse rate' of about 10 K/km. One should
compare the temperature of a displaced volume with its surroundings, after
making that allowance. When this is done, the criterion for stability is
either a 'lapse rate' (i.e. fall of temperature with increasing
elevation) less than 10 K/km, or else there is actually a rise with
elevation (i.e. an inversion).
- A notable consequence is
this: the inability of air to rise or fall within a stable layer makes it
impenetrable. So that air pollution is trapped beneath it, and wind at
upper levels is unable to share its energy with surface air. (That last
point is the reason why surface air is still at night, isolated from the
upper winds by the 'ground inversion' caused by nocturnal cooling of the
ground by radiation loss to the sky.)
HOW DO LOWS DEVELOP?
- A 'low' is a region some
hundreds of kilometres across, say, where air pressures at some specified
level (such as sea level) are less than outside the region. A low implies
less air's weight, i.e. less air in the column of atmosphere above (Note
- A low develops by air
flowing out of the column faster than the inflow rate, perhaps because the
air in the column expands by warming. There are several ways in which this
- the central atmosphere of a
tropical cyclone (Section 13.5) warms on account of the latent heating in
the eyewall and adiabatic subsidence in the eye - this only occurs when
the environment is conditionally (but not absolutely) unstable;
- spreading out of air from
the top of a column reduces the amount of air in the column, pressing onto
the ground - such upper divergence occurs on the poleward side of a jet
streak exit region, sometimes visible as a 'delta' on a satellite image
- a low forms when wind spins
rapidly around a vertical axis, as in a tornado - the pressure gradient
opposes the centrifugal force;
- a low may form in the lee
of a mountain range - here the warming is due to vortex stretching (Note
13.D) and adiabatic subsidence;
- heating of the ground warms
the air above, so decreasing its density and hence the surface pressure -
this is a 'heat low'.
- Consider these
- a low attracts air from the
region of higher pressure around, since winds result from pressure
difference. Then that converging air has nowhere to go but upwards. Rising
air cools, perhaps to its dewpoint temperature. In that case, cloud forms
and skies are overcast. Also, there is then the possibility of rain. So
lows imply 'bad' weather. But low-level convergence and ascent are a
negative feedback to cyclogenesis, unless the atmosphere is conditionally
unstable over the depth of the acsent.
- a falling barometric
pressure indicates increasing nearness to a mobile low or trough - these
are usually frontal. This implies the possibility of bad weather and
Croft, P.J. 1999. Assessing 'The Excitement of Meteorology'
for young scholars. Bull. Amer. Meteor. Soc., 80, 879-91.
For more misconceptions in meteorology, see Alistair Fraserís