Question

Why does moisture condense on the outside of a cool glass of water in the summertime?

Short answer

Moisture condenses on the outside of a cool glass of water in the summertime because the cool surface of the glass lowers the temperature of the surrounding warm air below its dew point, causing water vapor to condense into liquid droplets on the glass.

Step-by-step solution

Understanding Condensation

Condensation occurs when water vapor in the air comes into contact with a surface that is cooler than the surrounding air. The cool surface causes the water vapor to lose energy and change back into liquid form, creating droplets on the surface.

Identifying the Conditions in the Exercise

In the summertime, the air is often warm and can hold a high level of water vapor (humidity). When a glass is filled with a cool liquid, the temperature of the glass decreases. The warm air around the glass meets the cooler surface of the glass.

Explaining the Process on the Surface of the Glass

As the warm, moist air comes into contact with the cooler glass, the air is chilled to below its dew point. This causes water vapor to condense into liquid water. The droplets that form on the outside of the glass are a result of this condensation process.

Key concepts

Water Vapor

Water vapor is the gaseous state of water and is invisible. It is always present in the atmosphere to some degree. When water evaporates from oceans, rivers, or even a wet surface, it turns into water vapor and rises into the air. This process is crucial for the water cycle and weather patterns.

Considering moisture condensation, water vapor plays a vital role because it is the source of the liquid water that forms as condensation. This can be particularly noticeable on a cool glass of water during a hot day, as explained in the original exercise. Warm air can hold more water vapor than cool air; hence, when warm air containing water vapor hits the cold surface of a glass, the air cools down, and the capacity to hold water vapor decreases, leading to condensation.

Humidity

Humidity refers to the amount of water vapor present in the air. When we say the air is humid, we mean that there is a high concentration of water vapor. There are different ways to measure humidity, but the most common are relative humidity and absolute humidity.

Relative humidity, expressed as a percentage, measures the current amount of water vapor in the air relative to the maximum amount the air can hold at the same temperature. Absolute humidity, on the other hand, is the measure of water vapor present in the air regardless of the air's temperature. In the context of the original problem, when air with high humidity comes in contact with a cold surface, like a glass of water, condensation occurs more readily because the air's ability to hold water vapor decreases as it cools.

Dew Point

The dew point is the temperature to which air must be cooled for water vapor in it to condense into liquid water. It is directly connected to the concept of humidity; the higher the humidity, the closer the dew point is to the current air temperature. As air is cooled and approaches the dew point, it becomes saturated, and water starts to condense out of it.

In the summertime scenario from the textbook exercise, the outside of a cool glass lowers the temperature of the surrounding air to the dew point, and water begins to condense on the glass. This is because the glass cools the air around it below its dew point, causing the water vapor to change phases into visible droplets—what we recognize as condensation. This concept is essential in understanding why glasses sweat on a hot day, causing rings of water on tables and requiring coasters to protect furniture.