Dew point is growing in its use by meteorologists as a measure of humidity. This is probably because it is the most accurate reflection of what weather conditions feel like, as it combines temperature and humidity. Dew point is the atmospheric temperature at which dew can form. The measure marks the point to which the air must be cooled to reach a saturation point, as water vapor in the air condenses to form dew. Water vapor is the gaseous state of water or water in a gas form. The higher the dew point, the more moisture there is in the air. Have you ever noticed your windows mist up on a freezing cold day? Or perhaps you have noticed droplets of water forming on the outside of an ice-cold glass? This happens because of a difference in relative temperature. The temperature of the glass is colder than the air around the glass, and so, when the air, which contains water vapor makes contact with the glass, the vapor cools and begins to change state to become liquid. Similarly, changing states from water to ice is known as the frost point, as this is where frost would form. Of course, ice is water in its solid form. Humidity is also a measure of the amount of water vapor in the air. This can vary depending on various climate conditions. Some areas are far drier than others, humidity can change depending on the time of day, and moisture in the air can be blown in or away by the wind. It is usually thought that air containing a higher humidity is denser than dry air, but this is not the case. The average molecular mass of air is around 29, and the molecular mass of water vapor is around 18. So, when water vapor is in the air it displaces the air and lowers the overall molecular mass. It is counter intuitive, since water is certainly heavier than air! But water in a gas form is not. Water vapor is a gas lighter than nitrogen or oxygen – the primary components of air.
Humidity is either measured in absolute terms, relative terms, or as specific humidity. It is worth exploring these measures to better understand dew point and how to measure it relative to temperature, and humidity. Dew point is essentially the temperature at which air saturates to form vapor, this is equal to 100% relative humidity. Absolute Humidity: Absolute Humidity is a measure of water vapor per cubic meter of air. This represents the actual amount of water vapor present in a given quantity of air regardless of the temperature. This is important because the dew point is dependent on relative humidity and temperature. Absolute humidity ranges, with the highest humidity at roughly 30 grams per cubic meter. Absolute Humidity is equivalent to the mass of the water vapor divided by the volume of the air and water vapor combination: AH=m_H20/V_net Absolute humidity is affected by temperature or pressure changes, yet this is not accounted for in the formula. Relative Humidity: Relative Humidity is the ratio of the absolute humidity and the maximum possible absolute humidity for the temperature at the time. This is expressed as a percentage with a higher value indicating a more humid air-water mixture. You define the percentage as a ratio of the partial pressure of water vapor in the equilibrium vapor pressure of water over a flat surface of pure water at a given temperature. Releative Humidity=100% × (current absolute humidity)/max〖absolute hummidity〗 Shown differently, the relative humidity is the ratio of the water vapor pressure (Pw) to the same at saturation (Pws) at a given temperature. Relative Humidity=100% ×Pw/Pws This term is relative, because there is a sticky factor, in that warmer air can simply hold more water vapor. This makes humidity a complex measure of what we’re trying to capture in reporting it – how the air feels. Specific Humidity: This is also known as moisture content, and it is the ratio of the mass of the vapor to the total mass of the air. This is called the mixing ratio and as temperature decreases, so does the amount of vapor needed to reach saturation. It is all quite complicated but our calculator will enable you to make the computation painlessly.
Because of the effects of temperature and pressure, many of the equations for relative humidity are imperfect. Calculators use the Magnus-Tetens formula (Sonntag90) which allows for accurate results within a reasonable margin of error, for temperatures ranging from -45°C to 60°C. The Magnus-Tetens Formula: Ts=((b×α(T,RH))) / ((a- α(T,RH))) Where Ts = Dew Point T = Temperature RH = Relative Humidity And a and b are coefficients. For Sonntag90, the coefficients are a = 17.62°C and b = 243.12°C α(T,RH)=ln(RH/100)+a ×(T/(b+T)) This calculator uses this formula and will easily calculate dew point for you, given that you have the relative humidity and current temperature.
A high dew point can feel uncomfortable for some. Humans sweat to cool down, and in cases of high humidity, the relative pressures (humidity) make us less able to feel this relief! A dew point of <10°C (<50°F) is relatively dry, while between 10 and 16°C (50 - 60°F) is dry, but comfortable. Dew point of 16 - 18°C (60 - 65°F) is quite humid, and 18 - 21°C (65 - 70°F) is fairly unpleasant with a high humidity. A dew point over >21°C (>70°F) is extremely uncomfortable and can even be dangerous.
Relative Humidity is the amount of moisture in the air compared to the amount of moisture the air can ‘’hold’ before it starts condensing, and the dew point is the temperature at which dew would form. Thus, these are different measures of the same point, and while both measure the air’s water content, dew point provides a more accurate measure. Because relative humidity does not take temperature into account, the dew point is a more concrete measure of moisture in the air, as a propensity. Although these measures are not the same, they are closely related. If the relative humidity is high, the dew point will be closer to the current air temperature. Where the relative humidity is 100% and the air is maximally saturated, then the dew point is equal to the current temperature.
As the sun sets, the ground temperature cools, especially the surface. As the surface of the earth is cooled to a temperature below the dew point, vapor in the air condenses to form dew. When the sun rises, these droplets evaporate.
In various desert regions, dew is an important source of water. Desert plants obtain up to 50% of their total water intake from dew. Dew can be collected on shade cloth, or other this and exposed surfaces with poor thermal conductivity. Dew forms best under a clear night sky after a warm day. A calm night with no strong winds allows for the right balance of pressures and temperature for the formation of dew.