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Rain Forests and High Humidity: How to Measure Relative Humidity?

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Author: Janice VanCleave

All forests in the tropics are tropical forests, but they are not all rain forests. But, most rain forests are in the tropics. For a forest to be a rain forest, it must receive more than 80 inches (200 cm) of rain each year. Most tropical rain forests receive about 200 inches (500 cm) of rain every year, and a few get more than 400 inches (1,000 cm).

While the rainfall in most tropical rain forests is evenly spaced throughout the year, the tropical areas of India, Myanmar (formerly known as Burma), and Southeast Asia along the northern Indian Ocean coasts experience a cycle of seasonal changes related to rain called wet seasons and dry seasons. Wet seasons are times of abundant rain, and dry seasons are times when there is a lack of rain. Tropical rain forests in these areas are called monsoon rain forests (areas that experience pronounced wet and dry seasons). Unlike most tropical rain forests, which are evergreen forests, many plants in monsoon rain forests are deciduous, which means they lose all their leaves during part of the year. The leaves are lost during the dry season.

The consistently high temperatures and abundant rainfall in rain forests result in another characteristic of this type of forest—high humidity. Humidity is the measure of the amount of water vapor (gaseous state of a substance, such as water, that is normally in a liquid state) in the air. The air in a rain forest is like a sponge; it holds lots of water vapor. While rainfall is a major contributor to humidity, transpiration also affects humidity. Transpiration is the evaporation (change from a liquid to a gas) of water from a plant's stomata (tiny surface openings that are especially abundant on the undersides of leaves, the main food producing part of a plant).

Purpose

To measure relative humidity.

Materials

  • cotton ball
  • tap water
  • 2 outdoor thermometers that measure in Celsius degrees
  • transparent tape
  • index card (handheld battery-operated fan can be used)

Procedure

  1. Wet the cotton ball with water and wrap it around the bulb of one of the thermometers. This is your wet-bulb thermometer. Leave the second thermometer uncovered. This is your dry-bulb thermometer.
  2. Lay the two thermometers on a table with their bulbs extended over the table edge. Tape the other ends of the thermometers to the table.
  3. Use the index card to fan the air near the bulbs of the two thermometers. Do not hit the bulbs with the card. Note: If a battery operated fan is used, hold it so that the blades of the fan are about 4 inches (10 cm) from the thermometer bulbs.
  4. Rain Forest Humidity

  5. Continue to fan the bulbs until the temperature on the wet-bulb thermometer stops decreasing. Then record the Celsius temperatures from both thermometers.
  6. Use the following example and the relative humidity table to determine the relative humidity from your temperature readings.

Example:

What is the relative humidity if the dry bulb reading is 16°C and the wet-bulb reading is 13°C?

  • Subtract the wet-bulb temperature from the dry-bulb temperature:
      16°C – 13°C = 3°C
  • Find the dry-bulb temperature (16°), in the column on the left side of the Relative Humidity in Air table. Now find the difference between the two thermometer readings (3°) in the horizontal row at the top of the table. Where the column and the row meet is the number for the relative humidity. For this example, the number is 71; thus the relative humidity is 71 percent.

Rain Forest Humidity

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