I found the following passage from one of my engineering reference books
concerning microwave propagation:

"Effects of rain attenuation are considered negligible on frequencies
below 7000 Mhz. The susceptance of signals above 10,000 Mhz to
absorption and scattering by rainfall sometimes limits their application
to short path distances. This factor merits close attention in those
regions noted for heavy rainfall and, in general, when a high level of
reliability is necessary for transmission-line protective relaying,
remote supervisory control, and similar critical applications."

(Standard Handbook for Electrical Engineers, 11th Ed. Fink and Beatty,
pp. 16-99 to 16-100, F.R. Nelson, section author)

The above, as noted, is in the context of protective relaying for power
systems requiring at least a 99.99% reliability: that's at least a 40db
fade margin. These links are in operational fixed service. A street
wireless network, however, will find it's place in the land mobile
model. The environment surrounding the laptop will present attenuation
due to foliage and structures, multipath, and destructive fading. Fade
margins to achieve an acceptable level of service will have to approach
that of 40 db, else the loss of packets may be intolerable, and render
the AP useless at greater than very localized distances.

Achieving talk distances greater than a few hundred feet may be
problematic in an environment of concrete, steel, and trees at microwave
frequencies with power levels of 30 mW.

It is my opinion that precipitation should still be considered in
calculating propagation reliability for both major nodes (operational
fixed, i.e. Tower to Walker) and street downlinks (AP to mobile, i.e.
Walker to Loring Park laptop). With some of the downpours and
electromagnetic disturbances in the Cities, I would <not> rule out the
effects of these conditions.

IIRC, I read a propaganda sheet from a manufacturer of ACSB (Amplitude
Compandered Sideband) equipment that noted of the failure of an 800 Mhz
(trunked?) system due to rainfall in Florida.

Neal: nkras at nkras.dsl.visi.com