Question

On a sunny day, large “bubbles” of air form on the sun-warmed earth, gradually expand, and finally break free to rise through the atmosphere. Soaring birds and glider pilots are fond of using these “thermals” to gain altitude easily. This expansion is essentially an adiabatic process. Why?

Short answer

The work done by the system will decrease the internal energy of the system and hence the temperature.

Hence, The air bubble attains the temperature in equilibrium with its surroundings.

Step-by-step solution

Adiabatic process explanation

On a sunny day, the air gets warmer and becomes less dense It rises like a bubble. When it reaches up, the surrounding air has even less density. It leads to the rapid expansion of this air bubble". Due to this expansion,the air-bubble system is doing work. Since this process is adiabatic, so there is no heat exchange.

In an adiabatic process, there is no heat exchange

$∆\underset{~}{O}=0$

According to the first law of thermodynamics, we have

$∆\underset{~}{O}=∆U+W$

Therefore, we get

$$\begin{gathered} 0=∆U+W \\ W=-∆U \end{gathered}$$

Conclusion

Therefore, the work done by the system will decrease the internal energy of the system and hence the temperature.

After this, the air bubble attains the temperature in equilibrium with its surroundings.