Section: Science

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During subfreezing days in many parts of the country, the indoor relative humidity decreases when homes are heated. Furniture and skin dry out, and static electricity increases. For health reasons, doctors recommend the use of humidifiers. Which of the following best explains the lack of humidity in the air indoors?

1. The amount of water vapor in the air goes down.
2. The water vapor in the air evaporates.
3. The humidity in winter is lower.
4. The cold temperatures prevent humidity.
5. Dry air can only occur in warm air.

At a given time, water vapor in the air is constant. Cold air can hold less than warm air; and when air is heated, it has a greater capacity to hold moisture. If that moisture is not added ot the air, the humidity level goes down.