Batman is a fictional superhero appearing in American comic books published by DC Comics. He saves Gotham City—similar to New York City—from crime. Batman is summoned by Gotham City Police Department with the Bat-Signal—a signal made at night against the clouds.
   
“Good thing Gotham’s supervillains never thought to attack on a clear, cloudless night” was posted on the newsgroup alt.tv.simpsons on January 6, 1997. “How does Gotham City summon Batman on a cloudless night?” was asked on the Straight Dope Message Board on October 4, 2010. “Gotham criminals should only do crime on cloudless nights” was posted on Reddit—Shower Thoughts on February 1, 2018.
   
   
Wikipedia: Bat-Signal
The Bat-Signal is a distress signal device appearing in American comic books published by DC Comics, as a means to call for the superhero Batman. It is a specially modified searchlight with a stylized emblem of a bat affixed to the light, allowing it to project a large bat symbol onto the skies over Gotham City.
 
The signal is used by the Gotham City Police Department as a method of contacting and summoning Batman in the event his help is needed and as a weapon of psychological intimidation to the numerous criminals of Gotham City.
 
It doubles as the primary logo for the Batman series of comic books, TV shows and movies.
           
Google Groups: alt.tv.simpsons
A review of “El Viaje Misterioso de Nuestro Homer (3F24)”
Benjamin Jay Robinson
1/6/97
(...)
When Homer stands in front of the lighthouse beam, Lisa notes that either her father is casting the shadow, or Batman has let himself go lately.  No doubt she’s thinking of the world-famous Bat Signal (tm). In the days before wireless pagers, the commissioner of Gotham City needed a way to alert Batman that the city needed his services.  The solution was a searchlight with a bat logo (not Batman himself) painted on the lens.  When criminal masterminds struck, the commish would shine the light in the sky, projecting the logo on a cloud.  Batman would come to the rescue.  Good thing Gotham’s supervillains never thought to attack on a clear, cloudless night.
 
The Geek Twins
WHAT HAPPENS TO THE BAT-SIGNAL ON A CLEAR NIGHT? [COMIC QUESTIONS]
Nigel G. Mitchell 12/06/2009 05:00:00 AM Batman, comic-questions, Comics
(...)
In the comics, this is solved by always drawing Gotham City under a perpetual blanket of clouds. This is good for drama, but makes about as much sense as Gotham City’s criminals only committing crimes at night. Unless Gotham City is some sort of meteorological freak of nature, it has to have a clear night sometime. If I were a criminal, I would plan my jobs on a cloudless night. You could hold half the city hostage and kill the other half and never worry about Batman, because he would never find out. Bruce Wayne’s always tucked away in that mysterious castle miles from the city, brooding by a window.
 
Straight Dope Message Board   
How does Gotham City summon Batman on a cloudless night?
Shalmanese
10-04-2010, 03:15 AM
Given that the Bat Signal relies on reflection from cloud cover?
 
Twitter
Rick Lecoat‏
@ricklecoat
Knowing how the bat signal worked, why didn’t Gotham’s criminals just commit crimes on cloudless nights? #stupidcriminals
11:04 AM - 31 Oct 2011
       
Twitter
Johann Ménage Artois‏
@JayStr1ng
If cops used the Bat Signal to tell Batman when shit was going down, does that mean criminals got away with it on cloudless nights?
10:43 PM - 22 Sep 2012
 
Quora
How does Gotham City summon Batman on a cloudless night?
Bharat Jakati, When I’m done reading a good book, I pick up another one.
Answered Mar 13, 2013
Gotham needs to summon Batman only on cloudy nights.
   
Twitter
K U R I A N‏
@notnotpeanut
Criminals in Gotham could just wait for a cloudless night, so Batman can’t use his signal
6:15 PM - 8 Aug 2016
 
Twitter
Schmuckaroo Banzai‏
@henrysfingers
#ThatMomentWhen you realize the Bat Signal couldn’t possibly work in real life. Gotham’s criminals can run wild on cloudless nights.
8:18 PM - 18 Nov 2016
   
Twitter
Ben Kreucher‏
@BenKreuch
Why don’t criminals in Gotham only go out on cloudless nights?
No clouds, no Bat-signal
8:23 PM - 1 Mar 2017
   
Twitter
flinnie 🐻‏
@flinnie
Replying to @Brittany_207 @OwenBenjamin
I actually remember him from Sullivan and Sons.  But he told a joke on @scrowder about criminals committing crimes in Gotham on cloudless nights(bat signal won’t work) and I was hooked.
6:14 PM - 29 Jan 2018
(Owen Benjamin.—ed.)
 
Reddit—Shower Thoughts
Gotham criminals should only do crime on cloudless nights.
submitted February 1, 2018 by gobblecok