Why does the electrical conductivity of the earth's atmosphere increase with altitude.

Electric conductivity of a semiconductor:

Electrical conductance of metals decreases with increase in temperature.

Why does zinc oxide exhibit enhanced electrical conductivity on heating?

It has been recorde that the temperature of the earth's atmosphere has increased by 0.6^@C What has caused this increase?

The electrical conductivity of metal decreases with rise in temperature, while that of a semiconductor increases. Explain