Statement I : The decrease of air temperature with increasing altitudes in the atmosphere is called the vertical temperature gradient. Statement II: In Troposphere, air temperature decreases with increasing altitude due to radiation from the Earth.

Statement I is true as the vertical temperature gradient, or lapse rate, defines the rate of temperature decrease with increasing altitude [4]. In the troposphere, this average rate is approximately 6.5°C per kilometer [3]. Statement II is also true and provides the correct explanation. The atmosphere is largely transparent to incoming shortwave solar radiation but absorbs outgoing longwave terrestrial radiation [2]. Greenhouse gases like CO2 and water vapor, which are most concentrated near the Earth's surface, absorb this terrestrial heat, making the lower layers warmer [1]. As altitude increases, the concentration of these gases and the pressure decrease, leading to a drop in temperature [4]. Furthermore, the atmosphere cools via radiation to space while the surface is heated from below, establishing the gradient that drives convection.

Sources

1. [4] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > 22.3. Adiabatic Lapse Rate (ALR) > p. 296
2. [2] Environment and Ecology, Majid Hussain (Access publishing 3rd ed.) > Chapter 1: BASIC CONCEPTS OF ENVIRONMENT AND ECOLOGY > Structure of the Atmosphere > p. 7
3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 20: Earths Atmosphere > Troposphere (0 to 12 km) > p. 275
4. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > 22.2. Lapse Rate > p. 295