We’ve had a warm spell for the past few weeks here in the London area (where “warm” means as high as the low 90s, for those of you who wish to mock), and looking at the temperature data the highest hourly readings seem to have come in anywhere between 2pm and 6pm. Usually the peak temperature seems to have been around 4pm BST, i.e. 3pm solar time. In London itself the effect seems to be intensified, with the sun warming up the buildings which then radiate heat in the early evening.
Last night, I was going to say what Dana Scully said today; that the sun has been up longer. Though Seattle, and Whatcom County where I am, are not ‘on the coast’ in the sense that coastal cities in California are, they are on the water. One would expect onshore flows to cool things off in the afternoon. Since the temperature peaks in the afternoon, and since we are on the water, I’m leaning toward the longer-days hypothesis.
Here in Montreal, the temperature generally peaks an hour or so before sundown. And reaches its low around sunup. This is true winter and summer. It is also true that the hottest part of the summer is right about now and late January has the coldest average temperature.
During the day, with the sun high overhead, features on the ground become directly heated by the sun. As the suns rays wane later in the day, heat energy from those heated ground features release heat into the air by convection. Places that accumulate high amounts of surface heat are called “heat islands” – typically concrete cities and asphalt roads and parking lots…
Another factor could be that the winds tend to diminish late in the afternoon, so in the evening, the still air will feel less refreshing, even though the same temperature. And, as the actual temperature drops, the dew point does not, so the amount of moisture in the air remains constant. The dew point, as much as the temperature, controls the comfort level of warm air. With a high dew point, your perspiration will not cool you by evaporating.
I am in the Willamette Valley now, and was living in the southern end of the SF Peninsula in California, so both valleys with any coastal wind being blocked by mountains.
It sounds like it might be the longer days up here, so that we’re being heated by the sun for more hours, along with streets and structures holding on to that heat until after dark when they’ve had enough time to cool. It just seems strange that it’s so different here. I spoke with an Oregon native today who thought it was weird that I was used to peak heat at midday, she said in her memory it’s always hottest around dinner time.
In meteorology, the solar day is called the dinural cycle and solar noon, the time when the sun is directly overhead, is the middle of the cycle. Solar noon for Portland right now is 1:17pm so right off the bat, you know 12pm won’t be the hottest part of the day. But there is always a temperature lag in dinural heating due to thermal inertia, meaning the peak air temp lags behind peak solar radiation.
As the sun warms the ground, the ground easily warms about an inch of air directly above it but that inch of air suffers poor heat exchange with the rest of the air and for much of the day, incoming radiation is warming things up at your feet more than the air at your feet is passing that on to the air higher up around your body where you’d actually feel it. There is a heat buildup at the ground that continues to warm the air above it for hours after the peak sun.
It doesn’t reach equilibrium until late afternoon when the air has finally soaked up all that heat that the ground has been trying to pass on all day. Unless you are an inch tall, that is the hottest part of the day.
So basically, the sun warms things up but you don’t get peak temperature until those “things” warm the air up.
That’s just background that still doesn’t answer the question. Here’s the closest thing to an answer I have. Peak temperature is usually thought to be around 3-5pm depending on local variations and dynamic variations too such as cloud cover, ground moisture, humidity, even soil type and other things are a factor in how well and fast heat is exchanged from sun to earth (or water or concrete) and then to air.
So, it’s quite possible for SF to reach it’s peak at 3pm and the winds come in and cool you off for the rest of the day, while OR may not see it’s peak until 5pm and stay that way for a while.
I don’t think it’s latitude:
http://www.weather.com/weather/hourbyhour/graph/Austin+TX+78745:4:US?pagenum=2&nextbeginIndex=6 (Peak temp 5pm)
http://www.weather.com/weather/hourbyhour/graph/Waco+TX+USTX1413:1:US?pagenum=2&nextbeginIndex=6 (4pm)
http://www.weather.com/weather/hourbyhour/graph/Mexico+City+MXDF0132:1:MX?pagenum=2&nextbeginIndex=6 (4pm)
The places I found that peaked around 1 or 2 were islands (San Juan, Honolulu) or coastal cities (San Diego, Corpus Christi, Daytona Beach)