Definitions of Humidity and VPD Control
Water Vapor
Water can exist in several different forms. It can exist as a liquid, solid (ice), and gas (water vapor). Water vapor is always present in the atmosphere. The main water vapor increases in the atmosphere is through evaporation. Liquid water evaporates from oceans, lakes, rivers, plants, the ground, and fallen rain. A lot or a little water vapor can be present in the air. Water vapor also is called moisture.
Relative Humidity
Humidity is the amount of water vapor that is suspended in the air. There are a number of different ways of describing how much water vapor is actually in the air. By far, the most common way is referred to as relative humidity. Relative humidity is calculated as a percentage, where 0% means that there is no water vapor in the air and 100% means that there is no more water vapor that can be added to the air. So if someone states that the relative humidity is at 50%, what they mean is that the air currently contains half (50%) of the water vapor that it can contain.
There are a number of ways that the relative humidity can change. Obviously if water vapor is added to the air, the relative humidity will go up. If water vapor is removed from the air, the humidity goes down. But humidity can also be changed by adjusting the temperature of the air. If you raise the temperature of the air, it can hold more water vapor. If air is at 100% relative humidity (it can hold no more water vapor) and you raise the temperature of the air so that it could potentially hold more water vapor, the relative humidity will go down.
If the relative humidity is at 100% and we lower the air temperature, the air will no longer be able to hold as much water vapor. The water vapor will start to condense and will start to form condensation or dew.
Dew Point
The dew point temperature is the temperature that air would have to be cooled to in order for saturation (100% relative humidity) to occur. The dew point temperature assumes there is no change in the amount of water vapor in the air. When the temperature drops to the dew point, condensation or dew starts.
Dry Bulb Temperature
The dry bulb temperature sensor measures the actual air temperature of the greenhouse. This measurement is best taken as close to the plants as possible.
Wet Bulb Temperature
The wet bulb temperature sensor measures the lowest temperature that can be obtained by evaporating water into the air. In order to measure this, we put a wet cloth over the bulb of a thermometer and then blow air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. When we measure the temperature of the wet bulb, we will find that it is a lower temperature than the dry bulb. This lower temperature is the wet bulb measurement. Using the difference between the dry bulb and wet bulb temperature, we can calculate how much water vapor is in the air.
Absolute Humidity
Absolute humidity is the amount of water vapor that is currently present in the atmosphere. It is measured in grams per cubic meter of air.
Saturation Humidity
The atmosphere can only hold a certain amount of water vapor. The maximum amount of water vapor that the air can contain is called the saturation humidity. This limit is set by natural laws and the saturation humidity changes with temperature. If air contains this maximum (saturation) humidity and the temperature drops, it will begin to deposit its moisture. The excess moisture condenses on the coldest available surfaces. In nature, dew forms because the atmosphere is cooling. In the greenhouses, this forms as condensation on surfaces that may include the glazing, structure, and even the plants.
Relative Humidity
Once we know what the saturation humidity is (the amount of water vapor the air can hold at a certain temperature) and we know what the absolute humidity is (the amount of water vapor that is currently in the air), we can figure out what the relative humidity is. To calculate the relative humidity, divide the absolute by the saturation humidity. Multiply the result by 100% to express it as a percent.
Humidity Deficit
Another term used in discussing humidity phenomena is humidity deficit. Humidity deficit is the difference between the saturation humidity and the absolute humidity, expressed in grams of water per kilogram of air. Calculated as saturation humidity minus absolute humidity, it is the amount of additional moisture that the air could carry at any particular pressure and temperature.
Vapor Pressure
Up to now, we have been talking about measuring the amount of water vapor by its mass (how much it weighs). We can also measure it by the amount of pressure water vapor exerts.
Air pressure at sea level is around 1000 millibars (14.7 psi). Most of this air pressure is caused by nitrogen and oxygen that make up the majority of atmospheric gases; some of that pressure, however, comes from other gases found in smaller quantities. Water vapor in the atmosphere produces only a small amount of this pressure but the pressure can be used to describe how much water in the form of water vapor is in the air at any given time.
If the air temperature is 30°C and the relative humidity is 100%, the vapor pressure of the air is 42.3 millibars (or hectopascals). This is equal to 0.622 psi.
If the relative humidity is 50%, the vapor pressure would be half of 42.3 millibars which is 21.15 millibars.
Theoretically, if the relative humidity is 0%, there would be no water vapor in the air. The vapor pressure would be 0 millibars.
Vapor Pressure Deficit
Vapor pressure deficit (VPD) is the difference between the amount of vapor pressure inside the plant leaf and the vapor pressure of the air around the leaf. Since the air inside the leaf is always at 100% humidity, the vapor pressure inside the leaf can be calculated if we measure the leaf temperature.
If the leaf temperature is 25°C, we can calculate that the vapor pressure inside the leaf is 31.61 millibars. If the air temperature is 22°C and the measured relative humidity of the air is 75%, we can calculate that the vapor pressure of the air is 19.79 millibars. The vapor pressure deficit would then be 11.82 millibars.