Hot soil — activity is dropping below the surface
Soil at 6cm is 82°F. That is warm enough to drive worms down, and the moles go with them. Surface ridging usually falls off, but the deep runs stay open and keep severing roots under the hottest part of the year — which is when a lawn can least afford it.
Quiet surface does not mean the mole left. It means it is feeding deeper than you can see. The runs under your lawn are still open, and they still open back up the moment conditions turn.
Summer heat
Dry, hot surface soil drives earthworms down, and the moles follow them. Surface ridging often goes quiet in midsummer, which homeowners routinely read as the moles leaving. They have not left. They are working deeper, and the deep runs are still cutting roots.
What that means for trapping: Summer trapping works, it just requires reading deep runs rather than fresh surface ridges. An irrigated lawn stays productive all summer and holds moles that the rest of the neighbourhood does not.