How to Measure the Partial Pressure of Atmospheric Water

Before we delve into partial pressure, lets first examine what water vapor is and how it affects evaporation. The water vapor in the atmosphere is important because it affects our comfort. Except in cold weather, we sweat continuously. The water in the sweat evaporates, draws its latent heat of evaporation from the skin, and so keeps us cool. Beads of sweat appear only when the water cannot evaporate as fast as it reaches the surface of the skin; we then feel uncomfortably hot.

On the other hand, if water evaporates from the skin too rapidly, the skin feels parched and hard; around the mucous membranes-at the mouth and nose-it tends to crack.

The rate at which water evaporates, from the skin or anywhere else, depends on the pressure of the water vapor surrounding it. If the water vapor above the skin is far from saturated, evaporation is swift. If the vapor is already saturated, water reaching the skin comes immediately into dynamic equilibrium with it; individual molecules are exchanged between liquid and vapor, but no mass of liquid is lost, and water accumulates.

The Partial Pressure of Atmospheric Water

The atmosphere contains other gases besides water vapor, such as oxygen and nitrogen. In speaking of the water vapor, therefore, we must refer to its ‘partial pressure’ as explained.

Water vapor in the atmosphere is also important because it affects the weather. Let us suppose that the atmosphere has a temperature of 120⁰C- in a warm day and that the water vapor in it has a partial pressure of 12mm mercury. It will have a density of about 12mg per litre. The density of saturated water vapor at 20⁰C is 17.3mg per litre, and its pressure 17.5mm mercury. The water vapor in the atmosphere is therefore not saturated.

Now let us suppose that the atmosphere cools to 14⁰C, without changing its composition. The 6⁰C fall in temperature will hardly affect the density of the water vapor, but it will bring the atmosphere to saturation. For the pressure of saturated water vapor at 14⁰C is 12mm mercury, and its density about 12mg per litre. If the atmosphere cools liquid water-that is, of fog or cloud.

Relative Humidity

The dampness of the atmosphere, besides affecting the weather and our comfort, it important also in storage and manufacture of many substances-tobacco and cotton, for example. From what we have said already, we can see that the important factor is not the actual proportion of water vapor in the atmosphere, but its nearness to saturation. In the above example, the density of the vapor remained almost constant, but we would have felt the atmosphere becoming much damper as it cooled from 20⁰C to 14⁰C.

The dampness of the atmosphere is expressed by its relative humidity, R.H., which is defined as follows:  

In other words,

[RH= Density of water – Vapor in atmosphere/ Density of saturated water – Vapor at the same temperature]   

Because an unsaturated vapor roughly obeys Boyle’s law, its density is roughly proportional to its pressure; the relative humidity as defined above is therefore  given by 

[RH= Partial pressure of water/ S.V.P at temperature of atmosphere]

Where S.V.P. stands for ‘saturated vapor pressure’.

Before describing the methods of measurement, we must warm the reader against thinking that the atmosphere ‘takes up’ water vapor. The atmosphere is not a sponge. Water vapor exists in it in it sown right; and our knowledge of vapor makes us feel sure that, if we could live in an atmosphere of water-vapor alone, we would have just the same experiences of humidity as we now have in our happily richer surroundings.

The Vapor Pressure Water

The vapor pressure of water depends primarily on temperature. It increases with temperature and saturates when it reaches its boiling point. The kinetic energy of the water molecules also affects this. This is why humid air has a lower oxygen content than dry air. It can be estimated using psychrometric charts and Mollier diagrams.

Temperature

The temperature of water is a critical factor in the interaction between atmospheric water and air. It affects how fast gases can dissolve into the water and how easily water releases those gases back into the air. The temperature of water also affects the vapor pressure of the water. This vapor pressure influences the amount of oxygen in water at any given temperature. This is why it is important to know the vapor pressure of water at a particular temperature when doing a Winkler titration.

In general, a gas mixture’s total pressure is equal to the sum of its component gas pressures. This is known as Dalton’s lawDalton’s law. However, when it comes to the partial pressure of a gas, this rule does not always hold true. The partial pressure of a gas is defined as the pressure exerted by its molecules on a given surface area. The higher a gas’s concentration, the greater its partial pressure. For example, an oxygen gas exerts a stronger partial pressure on water than an argon gas.

When water is at a certain temperature, its molecules are in thermal equilibrium with the air around them. The molecular motions of the water molecules cause them to escape from the water to the air in a process called evaporation. As the water molecules escape, they displace air molecules and contribute their proportionate share to the overall atmospheric pressure. This portion of the atmospheric pressure is referred to as the vapor pressure of water.

Water vapor is present in the atmosphere at all temperatures, but it tends to be more pronounced in warmer air. The vapor pressure of water increases with the temperature. When the vapor pressure of water in an atmosphere is high enough to cause some of it to condense, the heat energy that was used for evaporation is liberated and warms the condensation surface. This causes some of the vapor molecules to re-enter the water surface and add to the vapor pressure of the water again.

The actual water vapor concentration in the atmosphere at any time is determined by the mixing ratio of the atmosphere or by the humidity index, which is a combination of the actual vapor pressure and the vapor pressure necessary to saturate a given volume of air at a given temperature. In either case, the weight of the water vapor in the atmosphere only amounts to about one quarter of one percent of the total atmospheric pressure at sea level.

Pressure

As the atmosphere expands due to rising air temperatures, it also exerts a pressure on the surface of water. This is a direct result of the Clausius-Clapeyron equation. This pressure is referred to as the saturated vapor pressure, or WVP. The higher the temperature, the greater the pressure. The saturation vapor pressure is equal to the atmospheric pressure when the liquid is at its boiling point.

The water vapor pressure in the atmosphere is a small portion of the overall atmospheric pressure. The rest of the pressure is caused by gravity and other atmospheric forces. A typical value for the total pressure at sea level is about 1 bar, or 15 lbs per square inch (psi). This is known as one atmosphere. The weight of the atmospheric water vapor contributes only about a quarter of one percent to this pressure. If all of the water vapor in the atmosphere were to condense and fall as precipitation, it would cover less than five centimeters at the equator and about a tenth of that at the poles.

Dalton’s Law states that in a gas mixture, the total pressure is equal to the sum of the individual partial pressures of each gas. This is because each gas expands to uniformly occupy its container. The individual pressures are determined by the number of moles of each gas present. This law is commonly used in physics to determine the relative concentrations of gases in a solution.

Water vapor pressure increases with increasing temperature, which in turn causes the dew point to rise until the saturation vapor pressure is reached. This is an important factor in predicting climate change, since water vapor is also a greenhouse gas. Several studies have found that temperature and water vapor pressure trends are largely in agreement with predictions from global climate models. A study conducted at 309 stations throughout North America found that temperature and water vapor pressure increased over 1948-2010, although there were some inhomogeneities in the data. Statistically significant increases in water vapor pressure were most frequent for low and midlatitudes, while decreasing trends occurred mainly at high latitudes.

Density

The density of water is a physical property that describes the mass per volume of a substance. It can be measured using a density barometer, a device that measures pressure and the weight of a liquid or solid. The density of an object depends on its composition, size and temperature, but can be related to other properties as well. For example, a bulb that contains water at room temperature and atmospheric pressure has a lower density than a bulb filled with the same amount of dry air. The difference in density is due to the fact that water has more mass and is more dense than air.

A simple way to calculate the density of a substance is by measuring its vapor pressure at a given temperature. The vapor pressure is the force that molecules exert on the surface of the liquid, or in other words, the “pressure” that the vapor puts on the container. The vapor pressure of a liquid is directly proportional to its temperature, so as the temperature of a liquid increases its vapor pressure also increases.

When a liquid is saturated with vapor, the vapor pressure equals the ambient pressure. This condition is known as relative humidity (RH). The higher the RH, the more moisture in the atmosphere. The saturation vapor pressure of water varies with temperature, as illustrated in the graph below.

The vapor pressure of a substance can be determined from the Clausius-Clapeyron equation, P=TPV. A common approximation for this equation is Hn=p/RT, where p is the ambient pressure and T is the temperature.

A more accurate way to determine the vapor pressure of a substance is by dividing the saturation vapor pressure by its temperature. This gives the vapor pressure in millibars, which can be converted to atmospheric pressure using the inverse of the spherical law.

The vapor pressure of a substance is also affected by its surface area and the surrounding air. For instance, the vapor pressure of a lake at a certain elevation will be higher than the vapor pressure of a small stream flowing over a rock. This is because the lake has a larger surface area and is under greater pressure than the stream.

Moisture

When air is saturated with water vapor, the molecules of that vapor displace other molecules of the atmosphere and exert a pressure against them. This portion of the total atmospheric pressure is called the vapor pressure of water, and it is a key metric found in the formulas that define all other humidity parameters.

The vapor pressure of water depends on the temperature. If the temperature is low enough, water molecules move slowly and their escaping tendency is relatively small. In this condition the equilibrium vapor pressure of the liquid is about 0 kPa (or 29.9 inches of mercury).

As the temperature is increased, the molecular motion of water molecules increases and they become more active. As a result, the vapor pressure of the liquid increases and some of the molecules begin to evaporate. As they do, they release the heat energy that was used to cause them to evaporate to the surrounding air and thus contribute their proportionate share to the total pressure of the atmosphere. This is known as Dalton’s Law of Partial Pressures.

At some point the vapor pressure of the liquid will increase to the same value as the saturation vapor pressure of the air that it is in. This is referred to as the dew point temperature and it indicates that water in the air has reached its critical point and will start to condense into liquid or solid precipitation.

Moisture is a critical factor in global weather patterns and the formation of rain, snow, fog and frost. It is important to understand how the moisture content of the atmosphere varies and what causes this variation.

Air temperature, vapor pressure and the type and amount of particulate matter present in the air act together to determine the relative humidity. The higher the air temperature, the lower the relative humidity. At night, as the air temperature drops, the relative humidity increases until the dew point is reached and fog forms. Fog is a dense cloud of tiny water droplets suspended in the air close to the ground that reduces visibility.