* Temperature Inversion: The stratosphere has a temperature inversion, meaning temperature increases with altitude. This is due to the absorption of UV radiation by ozone. The upper stratosphere is extremely cold, reaching temperatures well below freezing.

* Low Water Vapor Content: The air in the stratosphere is very dry. Water vapor is transported upwards from the troposphere, but it rapidly freezes and precipitates out in the lower stratosphere.

* Chemical Reactions: The upper stratosphere is also a region where chemical reactions involving water vapor occur, further reducing its concentration.

While there are trace amounts of water vapor in the upper stratosphere, it's significantly lower than in the lower atmosphere.