Relative humidity is a measure of the amount of moisture present in the air compared to the maximum amount of moisture the air could hold at a specific temperature. It is expressed as a percentage and is a key factor in weather forecasting, agriculture, and various other fields.

The correct answer is option 'A', percent, because relative humidity is always expressed as a percentage.

Why is relative humidity expressed as a percentage?

Relative humidity is expressed as a percentage because it represents the ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air could hold at that temperature. By expressing it as a percentage, it allows for easy comparison and understanding of the moisture content in the atmosphere.

How is relative humidity measured?

Relative humidity is measured using a device called a hygrometer. There are different types of hygrometers, but they all work based on the principle of measuring the moisture content in the air. Some common types of hygrometers include:

1. Psychrometer: This type of hygrometer consists of two thermometers, one of which has a wet bulb covered with a wet cloth. As the water evaporates from the wet bulb, it cools the thermometer, and the difference in temperature between the wet and dry bulbs is used to calculate the relative humidity.

2. Hair hygrometer: This type of hygrometer uses human or animal hair that expands or contracts depending on the moisture content in the air. The hair is connected to a dial or pointer that indicates the relative humidity.

3. Capacitive hygrometer: This type of hygrometer uses a thin film capacitor that changes its electrical capacitance based on the moisture content in the air. The change in capacitance is measured and converted into a relative humidity reading.

Why is relative humidity important?

Relative humidity is an important parameter in various fields and activities. Here are a few reasons why it is significant:

1. Weather forecasting: Relative humidity plays a crucial role in weather forecasting as it affects cloud formation, precipitation, and the overall stability of the atmosphere. High humidity levels can lead to the formation of clouds and potentially severe weather conditions.

2. Agriculture: Relative humidity is important in agriculture as it affects plant growth, evaporation rates, and the spread of plant diseases. Different crops have different humidity requirements, and maintaining optimal humidity levels can help improve crop yields.

3. Indoor comfort: Relative humidity is important for maintaining indoor comfort levels. Low humidity levels can lead to dry skin, respiratory issues, and static electricity, while high humidity levels can cause discomfort, mold growth, and damage to furniture and electronic devices.

In conclusion, the unit of relative humidity is expressed as a percentage (option 'A'). It is measured using devices called hygrometers, and it is important in various fields such as weather forecasting, agriculture, and indoor comfort.