Abstract
It is shown that so far as pure advection is concerned, entities like density, density discontinuity (front) and humidity mixing ratio move with the wind at the level considered. Any disparity between actual movement and streamline movement must be put down to development.
A pressureโfield at a given level is advected in a direction and with a speed given by the average wind at will levels above that level. Similarly the difference in pressure between two fixed levels is advected by the average wind between those levels. Any disparity between actual movement and that determined by the average wind must be attributed to development.
The advection of contours and thickness lines can be inferred from advection of isobars and pressureโdifferences in the absence of development.
Winds are normally available up to pressure levels of 60 mb. in the British radiosonde ascents, so that over 90% of the atmosphere can be utilised in computing average winds. There is reason to believe that averages based on this data will be sufficiently accurate to give a satisfactory measure of advection of tropospheric phenomena.
The average wind may also be taken as a measure of frontal advection, in the sence that the frontal through of pressure must be advected in the same way as other pressure fields when development is not of importance.
A measure of upward motion might be obtained on some occasions from a consideration of the conflicting advections at different levels.
The theory of the pressureโfield being advected by the average wind at all levels accounts satisfactory for the fact that systems with โsolidโ wind currents do not move. An explanation of โthermal steeringโ is given.