grade 8 Humidity & Clouds

Humidity and Clouds

Humidity measures how much water vapor is in the air. The maximum vapor air can hold grows with temperature, and relative humidity of 100% means saturation. When air rises and cools, that capacity falls, vapor condenses on nuclei, and clouds form. Change temperature and moisture and relative humidity and the path from vapor to cloud appear.

What is Humidity?

💧 Water Vapor in Air

①Humidity = amount of water vapor in air

②Saturation amount = maximum vapor air can hold

③Higher temperature → larger saturation amount

④At 20°C this sketch uses 17.3 g/m³.

Relative Humidity

Temperature20°C

Water vapor10 g/m³

☁️ How Clouds Form

①Rising air cools down

②Temperature↓ → saturation amount↓

③When RH reaches 100%, vapor condenses

④Droplets form on condensation nuclei → clouds

Summary

Relative Humidity

RH(%) = current vaporsaturation vapor × 100

Percentage of vapor compared to saturation amount

Dew Point

Dew point = temperature at which condensation begins

At the dew point, RH = 100%

🎯 Exam Key Points

①RH = (current vapor / saturation) × 100

②Temp↑ → saturation↑ → RH↓

③Temp↓ → RH↑; at 100%, condensation

④Cloud formation: air rises → cools → condenses

⑤Dew point = temperature where condensation begins

Examples and Unit Test

Example 1

On a day when the saturation vapor amount is 20 g/m³ and the actual vapor amount is 15 g/m³, what is the relative humidity of the air?

Relative humidity = (actual vapor amount ÷ saturation vapor amount) × 100.

(15 ÷ 20) × 100 = 75 %.

▸ Relative humidity = (15 ÷ 20) × 100 = 75 %

Relative humidity tells what percent the current vapor amount is of the saturation amount.

Example 2

When a parcel of air with a fixed vapor amount rises, how do its relative humidity and clouds change?

As the air rises its temperature drops, and the lower the temperature, the smaller the saturation vapor amount.

The vapor amount stays the same but the saturation amount shrinks, so relative humidity rises; at 100% (the dew point) the vapor condenses into a cloud.

▸ Temperature down → saturation amount down → relative humidity up → condenses at 100% (cloud)

Even with the same amount of vapor, relative humidity changes when the saturation amount changes.

Unit Test

Which statement about humidity and clouds is correct?

①When temperature rises, the saturation vapor amount becomes smaller

②At the dew point the relative humidity is 50%

③Relative humidity is the current vapor amount divided by the saturation amount, times 100

④When air rises its temperature goes up and clouds scatter

⑤With the same vapor amount, relative humidity stays fixed even if temperature changes

▸ ③ Relative humidity is the current vapor amount divided by the saturation amount, times 100

When temperature rises the saturation amount grows (① wrong), and at the dew point relative humidity is 100% (② wrong).

When air rises its temperature falls and condensation occurs (④ wrong), and with the same vapor amount a change in temperature changes the relative humidity (⑤ wrong); relative humidity = (current ÷ saturation) × 100 (③ correct).

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Layers of the Atmosphere

Air Masses & Fronts

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