Question

a.) The relative humidity (RH) of surface air (2 m height) tends to decrease from morning into early afternoon, especially when the sun is shining and the weather is quiet and stable. As evening approaches, RH usually rises again. Explain the physical basis for this behavior.

b.) Explain the following statement using the concept of latent heat: Global thermostatic effects prevent wide variations in Earth temperature, thus moderating climate.

c.) please write the correct chemical and charged balanced reactions of the two processes. Keep in mind that you may need to combine two reactions to describe each type of process:

-carbon dioxide gas dissolving to form bicarbonate

-calcium carbonate skeleton dissolving to form bicarbonate

Answer 1

a) In the morning, the temperature is usually lower, and the air has a lower capacity to hold water vapor. As the air heats up during the day, in the early afternoon, the temperature rises, and with it the capacity to hold vapor. Relative humidity is defined as the ratio of water vapor content of the air to the maximum water vapor that the air can hold. Thus, at higher temperatures, since the capacity increases, the relative humidity will decrease (even if you account for evaporation, the amount of water vapor added is less than the increase in capacity). Towards the evening, the temperature reduces again and with it, the capacity of air to hold water vapor. Thus, the relative humidity increases.

b) Water requires heat to evaporate i.e. when water evaporates, it absorbs heat. This heat is known as latent heat of vaporization. When water condenses, it gives out this heat. Similarly, when water melts, it absorbs heat known as latent heat of fusion, and when it freezes, it gives out heat.

Thus, if temperatures rise, water absorbs surplus heat and evaporates or melts depending on its current state, causing latent heat to be taken out of the atmosphere and temperature to be regulated. Similarly, if it gets too cold, water will freeze and/ or condensate, giving out latent heat and regulating temperature again. This is the global thermostatic effect.

c) The chemical formula for carbon dioxide is CO₂ while that for bicarbonate is HCO₃^-.

Dissolving means that the carbon dioxide is reacting with water.

CO₂ + H₂O = H⁺ + HCO₃^-.

This is the dissolution reaction that occurs with water forming carbonic acid. This is why when large amount of CO₂ is  dissolved in water, it becomes acidic.

Dissolving calcium carbonate in water requires carbon dioxde to be present (acidic water).

CaCO₃ + CO₂ + H₂O = Ca²⁺ + 2HCO₃^-.

This reaction is why calcium carbonate (limestone, or organic skeletons) dissolve in water. Limestone terrains are vastly affected forming huge caverns.