Difficult meteorology concepts for high school students

E. Linacre

6/'99

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.

WIND SHEAR

1. 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).

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 shear;
• 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.

1. Example results of wind shear are these:

• 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.

DETERMINING ATMOSPHERIC STABILITY

1. 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.
2. 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).
3. 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?

1. 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 1.G).

2. 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 can happen:

• 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 (Section 12.4);

• 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'.

1. Consider these consequences:

• 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 strong winds. 

Reference

(1) Croft, P.J. 1999. Assessing 'The Excitement of Meteorology' for young scholars. Bull. Amer. Meteor. Soc., 80, 879-91.

(2) For more misconceptions in meteorology, see Alistair Fraser’s website.