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Saturation, Condensation, relative humidity


This is a backup page, originally from https://uag-earthsci.blogspot.com/2018/09/day-011-giftionary-dewpoint.html

What is condensation/saturation?

Think of air as having an invisible container that shrinks or grows with the temperature. Saturation is the point that the air has gotten so cold and its “container” is so small, there is no more room for water and it starts to spill out.


If the temperature drops to the dewpoint, we are at saturation. In other words, the water must come out of the air and fall as rain. In the GIFs above, the dewpoint is 15°C, and when the air temperature reaches this, we are at 100% Relative Humidity.


When air gets to the saturation point, the air is at 100% relative humidity.
When this happens, we call it condensation. In other words, water from the air comes out of the air because it is too cold and cannot carry any more.


Do you remember the GIF of cold water from yesterday? Condensation doesn’t just happen in clouds! You deal with it every day!

011.2 What is relative humidity?

Relative humidity tells us how full is the air of water. Is it 100% full (at saturation) and cannot hold any more? Is it only 50% full and can hold quite a bit more? This is what is meant by the relative humidity.

Dewpoint humidity

Relative humidity changes as temperature changes. As the temperature drops, you can imagine that the air’s container for water shrinks. Because of this, air which used to have a low relative humidity can have a high relative humidity without adding any more water to the air. In this case (as in the GIF above), the amount of water stays the same, but the air loses its ability to hold as much water.

Dewpoint humidity 2

When relative humidity gets to 100%, that’s another way to say that the air is at saturation, or 100% full. The air is always at saturation when clouds form.

011.3 How do we use ESRT (R) and (S)?

When scientists want to calculate the relative humidity or dewpoint, they use a tool called a psychrometer. One of the bulbs on the psychrometer is dry, and the other one is kept wet with a piece of damp cloth.


Using the psychrometer, we can read the temperatures ESRT (R) and (S), we need to find two numbers and figure out where these two numbers meet on the reference table. For this, we use our fingers to follow where these things meet.

psychrometer table

Using the reference table with data from the psychrometer.
For example, in the GIF above, if the dry bulb temperature is 24°C and the difference between dry and wet bulb temperatures is 7°C, the dewpoint of the air is 17°C.

011.4 What are the steps for finding out relative humidity/dewpoint from beginning to end?

Here are a few steps to determining dewpoint or relative humidity using psychrometer data.
STEP 1: Read the question
STEP 2: Determine the dry bulb temp and the difference between the two bulbs. (Subtract to get this.)

psychrometer ESRT

STEP 3: Use your fingers to find the correct intersection.
STEP 4: Read the correct table in the correct square.

psychrometer ESRT 2

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