Dew Point and Relative Humidity

Evaporation is a cooling process, so the wet-bulb temperature will always be lower than or equal to the dry-bulb temperature. It can only equal the dry-bulb temperature when the relative humidity is 100% and no evaporation occurs.

Dewpoint is the temperature at which air becomes saturated with water vapor. As the actual air temperature cools down toward the dewpoint temperature, the relative humidity increases toward 100%.

The difference between the dry-bulb temperature (20 degrees C) and the wet-bulb temperature (8 degrees C) is 12 degrees C. Looking at the Dewpoint Temperature chart in the Earth Science Reference Tables, a dry-bulb temperature of 20 degrees C and a difference of 12 degrees C corresponds to a dewpoint of -10 degrees C.

Using the Relative Humidity chart in the Earth Science Reference Tables, locate the dry-bulb temperature of 18 degrees C on the left axis. Follow this row to the column representing a difference of 7 degrees C, which gives a relative humidity of 40%.

Using the Dewpoint Temperature chart in the Earth Science Reference Tables, locate the dry-bulb temperature of -6 degrees C on the left axis. Follow this row to the column representing a difference of 1 degree C, which gives a dewpoint temperature of -10 degrees C.

When the air temperature and dewpoint temperature get closer together, the relative humidity increases toward 100% (saturation). As the air approaches saturation, the likelihood of condensation and precipitation increases.

Fog is a cloud that forms at ground level when water vapor condenses into tiny liquid droplets. This condensation occurs when the air is fully saturated, which means the relative humidity is 100%.

Dewpoint is a direct measure of the actual moisture content in the air. As the dewpoint temperature increases, it indicates that there is more water vapor present in the air sample.

Water vapor is the most important gas for day-to-day weather because it is responsible for condensation, cloud formation, precipitation, and latent heat transfer. While other gases like carbon dioxide and ozone play roles in long-term climate, water vapor directly drives daily weather phenomena.

Dewpoint is the temperature to which air must be cooled to become saturated with water vapor. Therefore, the dewpoint temperature is directly determined by the amount of water vapor (moisture) present in the atmosphere.

As the difference between the air temperature and the dew point increases, the air becomes drier and further from saturation. This causes the relative humidity (RH) to decrease. Consequently, with less moisture saturation in the air, the chance of precipitation also decreases.

A smaller difference between the air temperature and the dew point indicates that the air is closer to saturation, which corresponds to a higher relative humidity. Therefore, a very small difference like 2 degrees C offers the highest relative humidity and the best chance of precipitation.

In the Earth Science Reference Tables (ESRT), the relative humidity and dew point charts are structured as grids. To find a value, you locate the dry-bulb temperature along the vertical axis and the difference between the dry-bulb and wet-bulb temperatures along the horizontal axis. This intersection gives you the correct reading.

A psychrometer is a type of hygrometer that consists of two thermometers: a dry-bulb and a wet-bulb. By measuring the temperature difference between them as water evaporates from the wet bulb, meteorologists can determine relative humidity and dew point. Other instruments like barometers measure pressure, and anemometers measure wind speed.