Final answer:
The question deals with the Physics concept of temperature and the specific heat capacities of water and air. Water has a higher specific heat capacity than air and thus changes temperature more slowly, influencing climates and ecosystems. The difference in the observed average temperature of Earth compared to theoretical calculations is due to the greenhouse effect, primarily from water vapor.
Step-by-step explanation:
The question provided pertains to the temperature differences between water and air, which is a concept in Physics, particularly thermodynamics. The given temperatures, 5°C for water and 21°C for air, can help us understand how heat transfer and environmental factors such as specific heat can affect these two different mediums.
It is known that water has a higher specific heat than air, meaning it requires more energy to change its temperature. For instance, the final temperature after heat transfer will be closer to the temperature of the substance with higher specific heat when two substances are in contact, as illustrated by the provided example where the final temperature is closer to 20.0°C rather than to 150°C. This property explains why large bodies of water like lakes have relatively stable temperatures. Moreover, the temperature of water can affect nearby climates due to its slower rate of heating and cooling compared to land.
Additionally, it is worth noting that the average temperature of the Earth is warmer than what would be predicted solely based on solar radiation due to the effect of greenhouse gases, mainly water vapor (H₂O), which trap heat within the atmosphere, leading to a higher observed average temperature. Water temperature also plays a significant role in natural ecosystems, as seen in the example of lakes in winter where the water at the bottom is warmer and denser at around 4°C to 5°C.