About convection

I think air resistance should be considered.If there are two identical objects with different density,in the same volume,the one with a lower density occupies a larger space.Since the air resistance increases with the size of the object moving in the air,therefore the object with low density experiences a greater air resistance.If these 2 objects are released at the same time,the low-density object has lower acceleration due to greater air resistance.It lags behind another object,so it seeems object that has a lower density could rise

2008-12-08 22:50:41 補充：
如果有壺熱湯放在室溫下，湯上層既空氣會被加熱，expand.再上層既空氣就是未被加熱ge。由於重力影響，熱空氣和未被加熱的空氣會一齊fall down,但因為熱空氣受既air resistance 大D,熱空氣會fall 得慢D,於是未被加熱的空氣粒子會超前熱空氣粒子，熱空氣粒子會被逼走，所以the warm(expanded) air will rise and replaced by cold air

Your friend is wrong since all thing in the earth is due to the gravity(9.8ms^-2) neglect air resistance and disffusion force.
Since the density of the hot air is lower,it will go upward,the density of the cold air is higher,it will go downward.
在世界上愈高的地方氣壓愈低,愈冷,當熱空氣向上飛時,當接觸到高空的冷空氣,便會變成冷空氣,密度上升,便會向下沈.

2008-12-08 22:46:57 補充：
你可以試下當空氣係一杯水,水泡密度低所以會浮上面,就好似熱空氣密度比普通空氣低所以會向上升一樣.