&gt; ended up learning which days were sunny and which were overcast.<p>The earliest source I can find for this is <a href="https:&#x2F;&#x2F;neil.fraser.name&#x2F;writing&#x2F;tank&#x2F;" rel="nofollow">https:&#x2F;&#x2F;neil.fraser.name&#x2F;writing&#x2F;tank&#x2F;</a> but it says it &quot;might be apocryphal&quot;.