A shunt is an extremely-low-resistance resistor, across which a small voltage is developed. The amount of voltage is dependent on the amount of current passing through the shunt, and can be calculated using Ohm's Law
. The developed voltage is read on a voltmeter that is calibrated and labeled to read "amps".
Given the extreme low impedance of this, I wouldn't think that it would be very affected by the elements, but you could always improve it's environment if you prefer.
Are the roads salted in winter where you go? Do you park on the beach? Salt water will have an effect.