When the weather is colder in NYC, there are fewer rat sightings. But don't be fooled -- it's not because there are magically fewer rats, but instead likely due to several confounding factors such as being darker more often while people are rat-watching, colder weather driving rats to stay indoors more often, and more rain/snow/slush making the furry rodents harder to spot.

trash (rats' food source) is usually picked up twice a week in most places in the city, it never lines up perfectly to 'half-way through the week', and sometimes the 'other' collection is 'just recycling' (less organic matter to feast on)

I'm guessing the most frequent submitters send consistent reports when they can see rats on their block going after the trash (twice-ish a week); if more people in different areas reported as consistently, it might even out (some places have trash pickup monday, some tuesday, some wednesday...) (assuming all areas have equal concentrations of rat)

I live in Brooklyn and we have seen alot of construction around our block, which has also led to more rat sightings and a huge jump in trash can/bag attacks from hungry rats.