Why Tomorrow's High Is Usually in the Afternoon
Tomorrow's high is the warmest air temperature expected during that day. On a typical clear day it does not happen at sunrise, and it often does not happen at noon either. The ground keeps absorbing energy through the late morning, and the air near the ground usually reaches its warmest point sometime in the mid to late afternoon, after the sun has been up for hours.
That lag is why a morning temperature can look far below the advertised high and still be on track. If you dress only for the 7 a.m. number, you may be overdressed by 3 p.m. The daypart line exists so you can see the climb instead of treating the high as if it lasted all day.
The usual shape
The low is often near dawn. Temperatures rise through the morning, peak in the afternoon, and ease in the evening after the sun drops and the ground starts losing heat faster than it gains any. The range bar on the page places morning, afternoon, and evening along that span from low to high when the hourly values support it. Afternoon should sit toward the warm end on a normal day. Evening slides back. Night is cooler still.
Clouds flatten the shape. A thick overcast keeps the afternoon from warming as much, so the high and the morning can be closer together. Strong wind mixes the air and can also narrow the range. A front can break the pattern entirely: the warmest hour might be in the morning, before cooler air arrives, and the afternoon can be the cooler part of the day. When that happens, the dayparts will not climb. Trust the blocks over the habit.
How to use the high
Compare the high with the part of tomorrow you will actually be outside. If your time outside is morning, the morning temperature is the one that matters, and the high is context for later. If you will be out in the afternoon, the high is close to what you will feel, especially if the afternoon block sits near it. If you will be out after sunset, the high is already behind you and the evening block is the better guide.
The high is rounded for display. A forecast of 72.4 and 72.6 can both show as 72. Pay attention when the high moves by a few degrees between updates, not when it sits still. A small morning revision of one degree does not change a picnic. A revision of eight degrees, or a change from clear to rain, does.
High is not feels-like
The large high and low are air temperatures. The feels-like value, when the forecast includes it, adjusts for humidity and wind. A mild high can still feel heavier if the air is humid, and a modest evening temperature can feel cooler in a breeze. Use the high to describe the day. Use feels-like when you are choosing clothing or deciding whether midday exertion is wise.
Tomorrow's high is also not today's current temperature plus a guess. A hot afternoon today can be followed by a cooler tomorrow if the air mass changes overnight. Read tomorrow on its own line, then use the comparison sentence only as a bridge between the two days.