Relationship between dewpoint and absolute humidity

Discussion on Humidity

relationship between dewpoint and absolute humidity

The dew point is the temperature to which air must be cooled to become saturated with water Specific concepts The measurement of the dew point is related to humidity. .. For every 1 °C difference in the dew point and dry bulb temperatures, the relative humidity decreases by 5%, starting with RH = % when the dew. Compared to relative humidity, dew point is frequently cited as a way of measuring the humidity and comfort of air, since it is an absolute. For a given temperature and pressure, the more vapour per cubic metre the higher the absolute humidity and the higher the relative humidity (RH). Adding water vapour increases the dew point temperature. Is dew point anything besides a measure of absolute (not relative) humidity?.

It simply means that the maximum amount of moisture is in the air at the particular temperature the air is at. Saturation may result in fog at the surface and clouds aloft which consist of tiny water droplets suspended in the air. Droplets grow through a process called "collision-coalescence" whereby droplets of varying sizes collide and fuse together coalesce.

Ice crystal processes including deposition and aggregation also are important for particle growth. In thunderstorms, hail also can develop. Once the suspended precipitation particles grow to sufficient size, the air can no longer support their weight and precipitation falls from the clouds. In humid climates, thunderstorms often cause heavier rain than general wintertime rainfall since moisture content in the air typically is higher in the spring and summer, and since air usually rises at a much more rapid rate within developing thunderstorms than in general winter systems.

Meteorologists are not just interested in dewpoint or absolute humidity at the surface, but aloft as well.

Discussion on Humidity

Precipitable water PW is a measure of the total amount of water vapor contained in a small vertical column extending from the surface to the top of the atmosphere.

However, as mentioned above, the majority of moisture in the atmosphere is contained roughly within the lowest 10, feet.

relationship between dewpoint and absolute humidity

Precipitable water values around or above 1 inch are common in the spring and summer east of the Rocky Mountains including Kentucky. Values of 2 inches in the summer indicate a very high moisture content in the atmosphere, typical of a tropical air mass. In general, the higher the PW, the higher the potential for very heavy rain from thunderstorms if they were to develop. However, another very important consideration is not only the amount of ambient moisture in a particular location, but also the amount of moisture advection and convergence which provides additional moisture to an area.

If significant, these added factors help explain why rainfall totals from thunderstorms can exceed actual PW values of the air in which the storms are occurring.

About Dewpoint Temperature, Humidity and The Moisture Content of Gases

The movement of thunderstorms, called propagation, also is very important in determining the actual amount of rainfall in any one location. The slower the movement of thunderstorms, the higher the rainfall potential in one area.

relationship between dewpoint and absolute humidity

In the winter, if the air temperature was 40 F and the dewpoint was also 40, what would the relative humidity be? Now, in the spring, if the air temperature was 70 and the dewpoint was 70, what would the relative humidity be?

In which situation would if feel more humid? What does this tell you about relative humidity? If the air temperature was 95 F with a dewpoint of 70, would the air's relative humidity be higher or lower than if the air temperature was 70 degrees with a dewpoint of 55? Which air mass would feel more uncomfortable to you?

If the air temperature was 90 degrees with a relative humidity of 60 percent in the afternoon, would it feel more uncomfortable to a person than if it was 75 outside with a relative humidity of percent in the morning? Answer to Question 3 These examples show how relative humidity can be quite misleading.

relationship between dewpoint and absolute humidity

In general, assuming the dewpoint or absolute humidity does not change, the relative humidity will be highest in the early morning when the air temperature is coolest, and lowest in the afternoon when the air temperature is highest.

While dewpoint is a more definitive measure of moisture content, it is the relative humidity that commonly is used to determine how hot and humid it "feels" to us in the spring and summer based on the combined effect of air temperature and humidity. This combined effect is called the " Heat Index. In the winter, there is another index we use to determine how cold our bodies feel when we are outside.

Fog and Dew Disappear If you lower the temperature, dew will form as fog or condense on a cooler surface faster than it evaporates. This phenomenon is observed, for instance in very moist air when the dew appears as fine water droplets suspended in air fog and on cool beverage containers in hot weather conditions. Conversely, if the temperature increases above the saturation temperature, dew will evaporate faster than it condenses.

  • How is relative humidity related to the dew point?

Fog in air and dew on surfaces will dissapear under such conditions. This is commonly seen when dew on the ground vanishes as the air warms during the day after a cool night.

The dewpoint temperature depends on the air temperature, since hotter air can hold more water vapor per unit volume than can colder air. Likewise, the value of the dewpoint at a given air temperature is also a function of air pressure. Denver isn't called the Mile-High City without reason and Philadelphia sits nearly at sea level.

The RH of air is defined technically as the ratio of the water vapor pressure to the vapor pressure of saturated air at the same temperature. But it is measured in many different ways, ranging from horse hairs to special electronic sensors. The moisture content of air can also be characterized in terms of the absolute humidity AH or humidity ratio, the weight of water vapor in the air per unit weight of dry air at the same temperature pounds per pound or kilos per kilo.

Just from the brief descriptions above it can be appreciated that there are three different measurement variables described, dewpoint temperature, relative humidity and absolute humidity, which can each independently represent the same water-vapor-in-air condition.

To complicate matters even further, a time-honored method to measure the moisture conditions in air has been to measure the dry bulb and the wet bulb temperature of the air.

Good Question: What’s The Difference Between Dew Point And Humidity?

Since it takes heat input to evaporate liquid water and since it water vapor releases heat when it condenses, there is an obvious energy content in a given mass of moist air.

The energy content depends upon the temperature and the amount of moisture present. This involves costs when one is trying to remove the moisture from something like grain or a wet web of paper using a flow of air.

relationship between dewpoint and absolute humidity

Increasing the air temperature can enable faster drying, but it requires heat to increase the air temperature. That means some fuel must be expended at a cost. Similarly when, for comfort reasons, one needs to cool the air below the dewpoint temperature by flowing air over a cold surface.