How Today's High and Low Are Calculated
Today’s high is the warmest air temperature expected during the day. Today’s low is the coolest. The current temperature is neither of those by default. It is the latest reading. The three numbers answer three questions, and the desk keeps them in separate places so the big figure is not confused with the day’s range.
On a typical clear day the low occurs around sunrise and the high falls in the mid to late afternoon. The current temperature climbs between those points and then eases. If you look at noon, the current reading should sit between the low and the high. If it does not, something else is going on: the high may already have happened, a front may be moving through, or the day’s range may have been revised.
A forecast high is not a promise about one minute
The high on this page comes from the daily forecast for the place you searched. It is the model’s expected maximum, not a reading already locked in, unless the warmest hour is behind you and the observations have caught it. A forecast high can still shift by a degree or two as clouds thicken or thin. A larger shift usually means the cloud cover or the air mass changed.
The low works the same way, with one timing quirk. For many forecasts the calendar day’s low is the coolest temperature from the recent night through the coming evening, which can mean this morning’s dawn reading is already the low, while tomorrow morning’s dawn reading belongs to the next day. That is why an afternoon “low” can look odd if you expect it to happen tonight. Tonight’s coldest hour often shows up as tomorrow’s low.
How to read the pair
Subtract the low from the high. A wide range, fifteen degrees or more, usually means a clear, dry pattern where the ground heats and cools freely. A narrow range often means clouds, humidity, or a steady wind keeping the temperature from moving much. The range is sometimes more useful than either number alone, because it tells you whether the day will feel like one temperature or two.
Compare the current temperature with the high. If you are within a couple of degrees of the high in the afternoon, the rest of the daylight will not get much warmer. If you are far below the high at 10 a.m., the day still has warming ahead, and dressing for the current number may be too much clothing by lunch. The hourly strip shows where that warming is supposed to happen.
Tomorrow’s high is a different day
The Today vs Tomorrow block repeats the exercise for the next day: expected high, expected low, condition, and rain probability. Tomorrow’s high is not an extension of today’s current temperature. A hot afternoon can be followed by a cooler day if a front passes overnight. Judge tomorrow on its own line.
Highs and lows are rounded for display. A forecast of 72.4 and 72.6 both become 72. That rounding is why two updates can look identical while the underlying value twitched. Pay attention when the high moves by a few degrees, not when it sits still.
Use the current temperature for right now, the high for the warmest part of the day, and the low for the coolest. If you only remember one, remember the one that matches the hour you will be outside. The range is the context. The current reading is the decision.