Question

When air cools at night, water often condenses out of the air to form dew. Explain dew in terms of relative humidity. (Hint: Water will condense out of air when the relative humidity nears \(100 \%\).)

Short answer

Dew forms when the relative humidity reaches nearly 100% because the cooler night air can no longer hold all of the water vapor, leading to condensation.

Step-by-step solution

Understanding Relative Humidity

Relative humidity is a measure of the current amount of water vapor in the air compared to the maximum amount of water vapor that the air can hold at a given temperature. It is expressed in percent.

Relationship Between Temperature and Air's Capacity to Hold Water

As temperature decreases, the capacity of air to hold water vapor also decreases. This means that if the temperature drops and the amount of water vapor stays the same, the relative humidity will increase.

Explanation of Dew Formation

At night, the air cools and can hold less water vapor. If the air cools enough such that the relative humidity reaches nearly 100%, the excess water vapor will condense into liquid to form dew.

Key concepts

Humidity and Temperature Relationship

Imagine filling a glass with water until it's nearly brimming. In this analogy, the glass represents the air and the water represents water vapor. Relative humidity measures how full that glass is in percentage. When the air's temperature drops, it's as if the size of the glass shrinks but the amount of water stays the same, resulting in a fuller glass, or higher relative humidity.

At a certain temperature, the air reaches its saturation point, meaning the relative humidity is at 100%. Any cooling beyond this point can't be accommodated within the glass - any additional cooling forces some water out, which is similar to how dew forms when the air nears this saturation point during the night.

The Effect of Temperature Drops

As the temperature declines during the nighttime, so does the air's ability to retain moisture. This interplay between temperature and humidity is critical in predicting and understanding weather patterns, as well as the formation of dew, frost, or fog.

Water Vapor Condensation

Water vapor condensation is the process that transforms gas (water vapor) into liquid (water) and it occurs when air is saturated with water vapor. Saturation happens when air holds as much water vapor as possible at a given temperature and pressure. Imagine air as a sponge that absorbs water; when it's completely soaked, any additional water will lead to dripping — which is akin to condensation in the atmosphere.

Think of a cold soda can taken out of the fridge on a warm day; the can quickly becomes wet on the outside. This is a small-scale version of condensation: the can's surface cools the surrounding warm air to the point where water vapor condenses into liquid droplets.

Condensation and Dew Point

The dew point is a specific temperature at which air becomes saturated and condensation begins. When the air temperature reaches the dew point, relative humidity is at 100%, and any further drop in temperature forces water vapor to leave the air as dew.

Atmospheric Water Cycle

The atmospheric water cycle is the constant movement of water on, above, and below the surface of the Earth. It includes various processes such as evaporation, condensation, precipitation, and runoff. The cycle is crucial to the distribution of water resources across the globe and dictates weather and climate.

Over oceans and other bodies of water, the sun's heat enables the evaporation of water into water vapor, which rises and cools in the atmosphere, potentially forming clouds. These clouds move over the landscape due to wind. When the water droplets in clouds combine and grow larger, they may precipitate as rain, snow, or other forms if the air cools enough.

The Cycle Continues

After precipitation, the cycle continues: some of the water will infiltrate into the ground, recharging aquifers, while some will run off and return to the oceans, rivers, and lakes. Meanwhile, vegetation absorbs water through roots and releases it into the air through transpiration. This continuous process replenishes fresh water on Earth and is essential for supporting life.