OK, Chemistry lesson time.

Air is primarily Nitrogen (N2, 80%) and Oxygen (O2, 18%). PV=nRT. Where n is the number of molecules in a gas, P is Pressure, V is Volume, T is absolute temperature (in Kelvin) and R is a constant.

For a given volume of gas at a temperature and pressure, the number of molecules in that volume is constant. Whether those molecules are Hydrogen, Oxygen, Nitrogen, water, methane, or any other molecule that is gaseous at that temperature. Number of molecules is constant. In a mixture of gasses, each gas contributes to the pressure a "partial pressure" that relates to how many molecules of that gas are present.

Water at a specific temperature has a specific vapor pressure. That means that as the temperature rises, the vapor pressure will also rise. This is commonly mistakenly believed to be the amount of water the air can hold. Air does not "hold" the water the way a solvent holds a solute. In the atmosphere, we refer to humidity as a percentage. At 100 percent humidity, the partial pressure of the water is at its its maximum. Any less, and the partial pressure of water is less than its theoretical value in a stable system.

Now lets look at the weights of the gasses of concern. Oxygen as a diatomic gas has a molecular weight of 32. Nitrogen has a molecular weight of 28. Water, with one Oxygen Atom and 2 Hydrogen Atoms, has a molecular weight of 18, significantly lighter than Oxygen or Nitrogen.

Now the answer. As the temperature of the atmosphere rises, the number of molecules in a given volume is reduced, thus reducing the density. Further, as water contributes more to the partial pressure of the gas, futher reduces the density. Therefore, the least dense air is the hot, humid air experienced in the summer.

As the temperature drops, the number of molecules in a given volume increases. Further, the number of water molecules is reduced as the partial pressure of water is reduced. Consequently, the colder and drier the air, the denser it becomes.

It is this phenomenon that leads to golf balls travelling further on hot humid days. This is also why airplanes must reduce weight in order to take off on hot humid days, and why helicopters have to correct for "density altitude".

Back to landings. In cold weather, our planes don't stall as easily, but will also slow down faster due to the denser air.

Brad