The salinity of ocean water is not uniform everywhere. Under which one of the following conditions does ocean salinity generally decrease?

Introduction / Context:
Ocean salinity is a key concept in physical oceanography and geography. Understanding the factors that increase or decrease salinity is important for explaining ocean circulation, climate, and marine ecosystems. Examinations often ask how processes such as evaporation, rainfall, and river inflow affect salinity levels.

Given Data / Assumptions:

• The question asks under which condition ocean salinity decreases.
• Options include more evaporation, high wind velocity, heavy rainfall, and high humidity.
• We need to select the condition that leads to dilution of salt concentration.

Concept / Approach:
Salinity refers to the concentration of dissolved salts in water. If additional fresh water is added, for example from rainfall or river inflow, the same amount of salt is dissolved in a larger volume of water, so salinity decreases. On the other hand, if water is removed by evaporation, leaving salt behind, salinity increases. Wind velocity and humidity can influence evaporation and mixing, but the most direct and clear condition that decreases salinity is heavy rainfall, which adds fresh water to the surface.

Step-by-Step Solution:
Step 1: Recall that heavy rainfall adds fresh water to the ocean surface.Step 2: Understand that adding fresh water dilutes the salt concentration, lowering salinity.Step 3: Recognise that increased evaporation removes water and leaves salts behind, raising salinity rather than reducing it.Step 4: Note that high wind velocity mainly affects surface mixing and waves, not salinity by itself.Step 5: Observe that high humidity tends to reduce evaporation but does not directly add fresh water, and its effect on salinity is indirect and weaker than rainfall.Step 6: Conclude that heavy rainfall is the most direct and correct condition for decreasing salinity.

Verification / Alternative check:
Equatorial and tropical regions with heavy rainfall often have lower surface salinity compared to hot, dry subtropical regions where evaporation is strong and rainfall is low. This pattern is visible in oceanographic maps of surface salinity and confirms that heavy rainfall is associated with reduced salinity values.

Why Other Options Are Wrong:
When evaporation is more, water molecules leave the surface and salts are left behind in a smaller volume of water, increasing salinity. High wind velocity may distribute surface water and mix layers, but it does not automatically reduce salinity. High humidity can limit evaporation but does not by itself add fresh water to dilute the ocean; its impact is indirect and not as decisive as heavy rainfall.

Common Pitfalls:
Some learners may think that reducing evaporation automatically reduces salinity, overlooking the need for actual dilution. Others may confuse strong winds with storms that bring rain, but wind alone without precipitation does not change salt concentration. Keeping a clear distinction between processes that add fresh water and those that simply move or remove water helps avoid these errors.

Final Answer:
Ocean salinity generally decreases when rainfall is heavy, because the fresh water input dilutes the salt concentration, making this the correct option.