Assertion: Electromagnetic waves with frequencies more than the critical frequency of ionosphere cannot be used for communication using sky wave propagation.

Reason: The refractive index of the ionosphere becomes very high for frequencies higher than the critical frequency.

Assertion: The electrical conductivity of the earth's atmosphere increase with altitude

Reason: The high energy particles (i.e. $\gamma$ -rays and cosmic rays) coming from outer space while entering the earth's atmosphere cause ionization of the atoms of the gasses present in the atmosphere.