*Posted with permission of Metal Detector Engineer Dave Johnson*
First Texas Head Engineer Dave Johnson has always done an awesome job "educating" the metal detecting masses. Thanks again for sharing your knowledge Mr. J.!
Time of day influences the amount and type of electrical interference, not just local sources but also longwave radio communications (mostly military) from thousands of miles away.
Also influences the occurrence of thunderstorms, which tend to peak in the afternoon in most geographic regions. Sometimes it is possible using a metal detector to RDF (radio direction find) major storm centers hundreds of miles away if you know how to do it. But this doesn't usually impact metal detecting because holding the searchcoil horizontally puts it in a null plane with respect to the magnetic field of beyond-the-horizon lightning.
We usually think of electrical interference coming through the air, but it can also come through the ground, traveling along electrically conductive utilities such as buried power and phone lines and metallic water and gas pipes. These utilities may be sources of electrical interference in their own right, they may be carriers of powerline harmonic current because they're used as earth grounds, or (being huge antennas)they may be re-radiating signals that they picked up and concentrated from the air. All that is influenced by time of day.
In areas (such as deserts) where the soil is exposed to the sun, from late morning till past sunset there are sharp thermal gradients in the upper several inches of the soil. The ground balance point of soil minerals varies with temperature, so when you've got those thermal gradients, the ground is "noisier" in all metals mode. This usually won't affect depth in discrimination mode.
The discrimination point of metal targets also varies with temperature. The effect depends on the alloy, but with most metals and on most products' discrimination scales (typically 100 to 200 numerical values) there can be a difference of several points between 100 F (38 C) summer weather and subfreezing winter weather, the numbers being higher in cold weather. But nobody goes beeping in that kind of weather, do they?