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Old 07-21-05, 05:35 PM   #1
Digital Gee
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Question about temperature and heat

The heat wave has struck San Diego as well as the rest of the country. I have a question about the heat. Seems I heard somewhere the actual temperature is higher on a road than somewhere else, because of heat radiating from the pavement. is that true? And if it's true, is there a rule of thumb about how much hotter it actually is when out riding on the streets? For instance, today it was in the mid-90's (according to the weather service) with (by San Diego standards) quite a bit of humidity. Was the real temperature higher on my bike? Or is it like wind chill, and it just SEEMS higher?
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Old 07-21-05, 05:50 PM   #2
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While the temperature may be a few degrees higher along the road than the official tremperature, there's more to it than simply reradiated heat. When you'r out riding, you are unprotected and the solar radiation stikes you directly (assuming a sunny day). The direct radiation increases the temperature that you feel since you are absorbing the sun's energy directly.

The temperature sensor used for the official readings (and any other good outdoor thermometer) is housed inside a pagoda looking stucture coprised of multiple layers with airspace between them. This allows airflow to carry away the heat absorbed from the sun so that it does not directly affect the temperature sensor.

Some bike computers have a built-in thermometer. Any unit that has an altimeter should have one to help correct the altimeter reading. However, these sensors are pretty worthless for giving you an accurate reading of the outside air temperature since they are within an unventilated airspace that is sitting in the sun.

Finally, humidity affects the perception of heat. Although this may not be a problem in San Diego, when the humidity goes up you feel hotter because the body's evaporative cooling system is not as efficient.
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Old 07-21-05, 05:50 PM   #3
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Depends on if you're climbing a hill or coasting down it.

Yeah there is some re-radiation from the black pavement (sorry not a perfect black body absorber). But at the same time there is a wind chill component that you, moving, create for yourself.

Stay hydrated and don't worry about it.
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Old 07-21-05, 05:55 PM   #4
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Someone on here actually set up a thermometer on his bike that was shielded from the sun, and had all sorts of graphs of the temperature vs. the temperature at a base station (outside his office I think?).

I believe the basic conclusion was that in the MORNING (cool air temps) the on-the-road temp was 2-4 degrees higher (presumably due to retained heat from the previous day being radiated) but in the afternoon (hot air temps) the on-the-road temp was not significantly different.

Either way, 2-4 degrees isn't nearly as much as the perception of heat you'll get from being in direct sun and riding in high humidity.
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Old 07-21-05, 06:54 PM   #5
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Around here it's around mid 90's and I never really notice it, ethier I'm used to it or I just don't notice it. I can go for about an hour or so without feeling the effects, althought he other day I did get sick and puked a couple times. Other then that, just keep some water and stay hydrated.
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Old 07-21-05, 07:01 PM   #6
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Direct sunlight, humidity, vehicle exhaust, it all makes the roadways seem hotter to me.
Every summer I change my commute route to avoid stoplights.
My commute is longer, but I only encounter stopsigns, which don't take nearly as long to wait for.
The streets are more narrow, and shaded (lots of big trees around here).
Sitting at traffic lights in the summer makes my brain cook.
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Old 07-21-05, 07:18 PM   #7
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Let's not forget "heat soak" that all roads in direct sun have. The surface
temp can be a lot higher than the ambient air temp. The road will also
hold the heat a lot longer due to it's mass.
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Old 07-21-05, 08:44 PM   #8
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Since it has been so hot in San Diego these last few days, I am trying to ride in the afternoon, starting at 4:30 knowing it will cool off (I hope) as the day wears on. I have been riding to Mission Trails so I can enjoy the cool shade of the trees and hills. Works for me ! I must admit though, today was just to darn hot to even attempt that.
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Old 07-22-05, 01:50 AM   #9
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The road surface temperature can easily be 10-20C higher than the ambient air temperature.
That's why high-performance cars generally have the air intake mounted as high up as possible, to get as cool - and thus as dense - air as possible.

But on your bike, it's more likely to be direct radiation from the sun that makes you heat up.
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Old 07-22-05, 08:43 PM   #10
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Can't answer your question but noticed your from San Diego. Have you had any experiences with a shop called Custom Bike Pros? I'm unable to contact them concerning an internet order.
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Old 07-22-05, 09:19 PM   #11
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Quote:
Originally Posted by BeeJay
Can't answer your question but noticed your from San Diego. Have you had any experiences with a shop called Custom Bike Pros? I'm unable to contact them concerning an internet order.
Nope, sorry, never heard of them. Here's a link to their website, if that helps: Custom Bike Pros
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Old 07-23-05, 12:01 PM   #12
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My husband and I are from San Diego too and we have never heard of them, we know just about every bike shop around here.
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Old 07-23-05, 10:02 PM   #13
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I had a thermometer mounted on my handlebars for years. In the summer it was often a little warmer on the pavement, (road or bike path) than the grass. At times during the summer it would be as high as 10 degrees different, on a sunny day. I never recorded anything, I was just interested in what temp I was riding in at the time.
The black pavement does absorb solar energy better than a lawn and retain it longer. That's why they use black water barrels or black stone, bricks, etc. for passive solar systems sometimes.
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