An observer measured the air temperature and the dewpoint and found the difference between them to be 12°C. One hour later, the difference between the air temperature and the dewpoint was found to be 4 C. What is occurring?

The simple answer to this question is the air is becoming more humid.

If you want to know why the air is becoming more humid, you need to first know that there are actually only 2 ways to change the properties of an air mass. You can change the temperature or you can change the amount of water in the air (also called the absolute humidity or the mixing ration).

In order for the difference between the temperature and dew point (called the dew point spread) to change one or both of the temperature or dew point need to change. In the example, since the dew point is always equal to or cooler than the temperature, the temperature could have dropped by 8 degrees or the dew point could have risen by 8 degrees, or some combination of the 2.

The mostly likely way for the temperature to drop is the sun setting. The most common way for the humidity to increase is precipitation. If neither of these is occurring, it is most likely the passage of a front causing both the temperature and dew point to change.

The decreasing difference between the air temperature and the dewpoint indicates that the air is becoming more saturated with moisture. This suggests that the relative humidity is increasing. When the air temperature and dewpoint are close together, it implies that the air is nearing saturation, which may lead to the formation of clouds, fog, or precipitation depending on other atmospheric conditions.