[Unknownrash]

OK peeps, can someone explain why during the day when I'm riding my bike there is a stiff headwind that constantly blows and while I was just out with the dog at night not even a single leaf was moving because the air was so still?

 

[bbgobie]

Wind is created by the air heating during the day. Hot air rises, and currents are created. There isn't much wind at night...

 

[Legbacon]

There is less wind at night because the Earth's surface cools and causes cooler temperatures near the surface. The wind is still blowing but can't get down to the surface because the warmer air aloft can't sink down to the surface. This is why in calm air hill tops are warmer than valley's.

 

[Ultra Magnus]

Not totally relevent but reminds me of an incident from our group lunchtime road ride yesterday.

The wind was blowing full hurricane force. We are having wierd weather, hot, but very dark and cloudy, with tunderstorms in the forecast. This all makes for rapidly changing unpredictable weather (wind, especially). I mean, it's 95F out, and you can't even see the sun, the clouds are so thick... Anyway, there's this one part of the ride, an out and back, towards the end of the "out" part, where we climb the dam hill. It's the Mojave River Forks Reservior Dam. It doesn't look that steep, or that long, but in 39/25, it's steep and long, especially for me. For the 10 miles leading up to the dam hill, the lead guys in the group were setting a brutal pace, against the wind. When ever I fell of the back and lost the draft, I was done until they slowed for me to catch up... So anyway, we get to the dam hill, and we all start downshifting. The guys in the group, even my 54yr old boss, are all way stronger than me, especailly climbing, so they all pull away. As we go up the hill, all wind dies, probably due to the topography of the area. I'm grinding away, legs are burning, lungs aching, and finally as I get near the crest of the hill I stand up and pedal my heart out to power (I use that term loosely) over the top. Just when I think I'm finally going to get a break from all this effort- WHAM! I'm hit by a wall of hurcane force winds. It almost blew me over, with my deep aero rims and all. Talk about getting jipped... I had to pedal even harder to fight the wind than I did climbing the hill! At least on the way back we cruised. It was a rocket ride back to the office, spinning hapily along in 53/11 (or 12? don't know)...

BM

 

[Legbacon]

The local topography has can have a big effect on the wind. Large bodies of water set up what is called a land/sea(lake) breeze. When I lived in Ontario I went for a road ride to the North away from Lake Ontario into the wind for about 30 km. When I turned around the earth had warmed enough to steup the land breeze and it was back into the wind the whole way. When the land warms it causes the air near the surface to rise and forms a small local area of low pressure. The air over the water does not warm because the water temp remains constant and the cool air over the lake flows inland to try to fill the low pressure. The reverse can happen at night.