Bob Gardner wrote:
In the section "VFR Aeronautical Charts," under "Culture," we find that the
yellow areas represent "Cities and Large Towns," with a larger blotch of
yellow identified as "Large Cities." Nothing about lights or anything other
than population.

True, but they seem to follow the Army Field Manual on Topographic
Symbols:

b. Populated Places. A larger populated place is shown, generally true
to shape, by an outlined and [yellow] tinted area. Within the outline,
the only features usually shown are the mainline railroads and
through-route roads.

c. Use of Tinted Squares. In many areas, there is insufficient
information available to plot the correct outlines of populated places.
In such cases, tinted squares of varying sizes are used as symbols,
with the size depending upon the population or importance.

I tell my students to pay no attention to the boundaries of the yellow
blotches, because there is no way to identify city limits from the air, and
towns/cities simply change too fast for chart designations to make any
sense.

I dunno, some of the isolated smaller cities here in the Northeast do
have matching outlines at night. I've even matched up some outlines
during the day.

The only practical use of the yellow blotches is, in my opinion, to
identify places where it is going to be difficult to find a place to land
(in-town airports excluded).

Now that's definitely good advice, and a great use of the outline!

Interestingly, I ran across an ultralight website where they described
the yellow outlines as being off-limits to them, because they're
obviously congested areas.

Best regards,
Kev