SACRAMENTO - Monday's temperature was initially forecast to reach 102 degrees. As clouds moved into the region, by 9:30 a.m. it was clear that temperatures were not going to be a warm as previously thought.
When forecasting the maximum temperature for the day, or the "daytime high", there are several factors forecasters use. We look at satellite images to see if clouds are expected. Forecasters use data from weather balloons, which include upper-level temperatures and wind speeds. We also use information from short-term computer models.
In order to get the most accurate air temperature possible, the thermometer is in located in the shade. If the thermometer is in direct sunlight, the readings will be influenced by the sun.
Timing is another factor if temperatures will be warmer or cooler than the forecast. That was the case of Monday's changing forecast. Clouds moved in earlier than expected.
The timing of the delta breeze can also affect temperatures. For example, the forecast is calling for temperatures around 100 degrees. If the delta breeze moves in before the high is recorded, temperatures will likely not reach 100 because of the cooler marine air that moves into the valley as a result of the breeze.
There are several weather reporting stations for Sacramento, Sacramento International Airport, Sacramento Executive Airport, and downtown Sacramento.
The National Weather Service uses downtown Sacramento as the official reporting station. The weather station is actually located at the Sacramento Water Treatment Plant, east of Sac State. Temperature and precipitation readings have been taken there since Sept. 28, 1964.