Photo: IPhone Screen Capture Photo: Weather Underground Photo: Weather.com Photo: SFGATE/Weather Underground

The skies were clear, the sun shone brightly and everyone was complaining about the heat in San Francisco at about 1 p.m. on Tuesday.

Just how hot was it? The correct answer might be more than 30 degrees off.

Weather.com said it was 70 degrees, while the Apple iPhone Weather app had the temp a bit higher at 73 degrees — though both services us Weather Channel data. KCBS reported 88, while SFGATE reported 90, and the National Weather service reported 33 degrees lower than that at the San Francisco Zoo.

Huh?

If you look to the various apps and online resources for a single temperature in San Francisco, you'll often find huge and confusing variances in temperatures. This is because the city has microclimates and temperatures from one side of the city to the other can easily vary 10 to 20 degrees on a normal day, and more on a hot day like yesterday.

The sites and apps reporting temperatures are pulling data from different locations around the city, often taking averages, and offering a general cumulative look at the city's temperature. And in many cases, they're pulling data from the nearest airport, which are common sites for weather stations.

"This touches on a really interesting underbelly of the problem with temperature reporting," said Jan Null, a meteorologist for Golden Gate Weather Services. "All these different apps and sites hire developers and they're pulling data from all these different sources. Most likely you have an automated app that's pulling data from the nearest airport.

"San Francisco Airport is a 30-minute drive from downtown San Francisco. If you're in San Mateo and look up the temperature, you might get a reading from San Carlos Airport or the San Francisco Airport and if you're in the San Mateo hills you're not close to either of those places."

An online service and app called Weather Underground, which shares its ownership with Weather.com, aims to tackle this problem. It has a massive network of people sharing data from personal weather stations they've rigged in their backyards. It's a great resource for a city like San Francisco that's filled with microclimates. The site reported temperatures of 91 in Dogpatch, 85 in Union Square, and 60 at Crissy Field on Tuesday. Weather Underground also powers SFGATE's weather page.

"A lot of the other services are pulling data from SFO ," said Weather Underground spokesperson Andria Stark. "We know the airport is 30 minutes south of the city. Our [information] is based on granular hyperlocal stations."

Null says Weather Underground can be a great resource, but it also has its problems and you need to use common sense with it.

"Most of those weather stations are in people's backyards. Some may have good backyard setups not in direct sunlight. Often times people compromise and the gauge is mounted on the south facing side of their condo. So while the actual air temp is 82, the gauge reading is 92."

Null likes to use the National Weather Service (NWS) data. The stations are limited but they have been vetted.

What temperature data did the NWS offer on Tuesday around 1 p.m.?

A station on Mint Hill recorded 82 degrees, while another at the San Francisco Zoo was at 57 degrees. The San Francisco Airport gauge read 79 degrees. More evidence that temperatures in San Francisco are all over the map no matter how you look at them.