On my iPhone, I have about a half dozen of the cities I frequently visit on my Weather app and the temperatures are dead on with what my car reads when I hit those cities. When I'm home, my backyard thermometer is almost never more than a degree off then what the app is telling me from a station a few miles away.
So the science of being able to tell the current temperature in a given spot is pretty much perfected. Just about every car built since 2000 has an accurate current temperature displayed on the dashboard. Most people have those internal/external thermometers at home.
So even if Yahoo! got together with Google and the rest to try and fool us about the temperature, they would get outed pretty quickly.
And what would that "outing" look like?