In the troposphere, significant and variable amounts of water vapor affect temperature change with altitude. Above the tropopause, there is little (or no) water vapor, so the stratosphere behaves differently (and is more stable). Essentially all weather originates in the troposphere due to the presence of water vapor...I think the only Wx that ever gets up there would be giant CB's which originate in the troposphere but have enough upward momentum to carry them above it.
I imagine the tropopause is lower at higher latitudes due to colder temperatures and less sun...less water vapor is driven into the troposphere when it is colder.