It would seem the ATC outputs pretty low power!
This is from a General Dynamics Air Traffic Control radio data sheet
RF Output Power:
Low Power Transmitter 2-12 Watts with co-site filter,
2-20 Watts with out co-site filter
High Power Transmitter 12-35 Watts with co-site
filter, 12-50 without filter
I don't know if this would be a radio for an airport control tower, which talks to in a range of about 10-15 miles, or an Approach radio which talks to you over a wider range, like 30 miles or more.
The datasheet does say "these rack
mounted transmitter and receiver systems are specifically designed to
meet the dynamic mission requirements of air traffic control centers,
commercial airports," so perhaps both?
When I flew from just west of Boston south to Nantucket, a distance of 98 nautical miles as the crow flies, I started with Boston Approach for flight following, got switched to a 2nd Boston Approach, got switched to Providence Approach (I flew a little more southerly to go over Martha's Vineyard and less over the ocean), then a 3rd Boston Approach, and finally Nantucket Tower when about 8-10 miles from the airport. Same going back.