Since you probably don't have ceramic cylindrical xducer's used in the
Sensor's Journal article, Consider using piezo buzzers as bi-directional
transucers. Mount 3 or 5 (depending on the basis) in a box, with tubing, cut
to resonance, and bent at right angles at the ends towards each other. This
way, the transucers can be mounted next to the circuitry in an enclosure,
and metal tubing waveguides the sound out of the enclosure. And you can
attach you heater to the pipes. Mount upside-down to keep out rain (and
raindrop noises). Maybe the pipes could be cut to a combination
longitudinal-shear resonance with the ends closed so the pipe itself would
act as a resonant transducer. That would probably only be usefull for high
ultrasound freq. (>40KHz)as ringing in the tranducer would affect pulse
measurments.

A year or two ago, a Circuit Cellar Ink project was a PIC weather station
that featured a PIC as a single-chip weather station & video controller. A
large triangular dial appeared on a TV screen showing the wind direction.
Might make a nice interface.