Clouds usually begin to form in the atmosphere when which of the following conditions is satisfied?

Introduction / Context:
Cloud formation is a key topic in climatology and basic physics of the atmosphere. Understanding how clouds form helps you interpret weather patterns, rainfall, and humidity. Competitive exams often ask conceptual questions about the conditions under which clouds appear. This question focuses on identifying the correct atmospheric condition that leads to the formation of clouds from water vapour.

Given Data / Assumptions:

• Clouds are formed when water vapour in the air condenses into tiny water droplets or ice crystals.
• Temperature, humidity, and the presence of condensation nuclei all influence cloud formation.
• The options describe different atmospheric situations.
• We assume typical tropospheric conditions where most weather phenomena occur.

Concept / Approach:
The dew point is the temperature at which air becomes saturated with water vapour for a given pressure, meaning it cannot hold any more vapour. When air is cooled to or below this dew point temperature, water vapour begins to condense on tiny particles called condensation nuclei, such as dust or salt. This process leads to the formation of cloud droplets. Therefore, cloud formation normally occurs when the air temperature falls to the dew point in a layer of the atmosphere and sufficient condensation nuclei are present.

Step-by-Step Solution:
Step 1: Recall that saturation occurs when relative humidity reaches 100 percent at the dew point temperature. Step 2: When air rises, it expands and cools; if it cools to the dew point, condensation begins. Step 3: Tiny droplets of water form around condensation nuclei and cluster together to make clouds visible. Step 4: Identify the option that describes this condition, which is air temperature reaching the dew point. Step 5: Confirm that option A matches this mechanism, making it the correct answer.

Verification / Alternative check:
Meteorology textbooks and weather training materials emphasise that the lifting condensation level is the height at which a rising parcel of air cools to its dew point and condensation starts, marking the base of a cloud. Weather reports often mention dew point to describe how close the air is to saturation. Experimental observations in labs and field studies confirm that cloud formation correlates strongly with air being cooled to its dew point with moisture and nuclei present.

Why Other Options Are Wrong:
Evaporation warming the surrounding air, described in option B, actually tends to increase the capacity of air to hold water vapour and does not directly cause cloud formation. Relative humidity of 0 percent in option C means completely dry air with no water vapour, which cannot condense into clouds. Removing condensation nuclei from the air, as in option D, would hinder droplet formation rather than encourage it, since droplets usually need a surface to form on. Therefore, none of these alternatives lead to cloud formation, leaving only the dew point condition as correct.

Common Pitfalls:
Some students think that maximum warmth or maximum humidity alone leads to clouds, without considering that cooling to the dew point is the key. Others may confuse the role of evaporation with condensation, assuming that more evaporation directly produces clouds. To avoid these errors, remember that clouds require both sufficient water vapour and cooling to the dew point so that condensation can take place on available nuclei.

Final Answer:
Clouds generally form when the air temperature reaches the dew point, allowing water vapour to condense into droplets or ice crystals.