We’ve also had days where the dew point is around 70 along with a similar air temp and it’s been raining. You tend to get at least patchy fog and dewy grass galore. Now, a dew point in the 50s at night with temps in the 50s will feel damp with high relative humidity in this scenario. Dews above 65 are downright muggy and even tropical when they reach the 70s. The general rule of thumb is that dew points in the 50s or lower is comfortable during the warm months. The farther apart they are the lower the relative humidity. We can see from these examples that the closer the temp is to the dew point, the higher the relative humidity. In fact, the heat index temperature – what it actually feels like outside – would be 98 degrees! In this scenario, it would feel quite humid or muggy and even uncomfortable despite the relative humidity being fairly low. Hold that dew point at 65 with the air temp rising to 95. Relative humidity in this situation is 100%. Take the example where the dew point is 65 degrees and the air temp is 65 degrees. Dew point will be a more reliable in determing comfort level during warm summer days.Dew point measures how much moisture is in the air.Relative humidity is temperature dependent while dew point is not. There is a difference between dew point and relative humidity.For air near room temperature and atmospheric pressure, the water vapour enhancement factor affects the result by approximately 0.5 % of value. The accuracy of these calculations depends slightly on the pressure and temperature of the gas concerned. The uncertainties associated with these equations are: (Formulae due to Sonntag, 1990, updated from formulae given by Wexler, 19.) This is a more accurate but complex alternative formula for vapour pressure (in pascals) from dew point (in kelvin) for water: Ln e i(t) = ln 611.2 + (22.46 t)/(272.62+ t) (Equation 3)įor the range -65 ☌ to +0.01 ☌, values given by this equation have an uncertainty of < ☑.0 % of value, at the 95% confidence level. Ln e w(t) = ln 611.2 + (17.62 t)/(243.12+ t) (Equation 2)įor information, 100 Pa = 1 millibar (mbar)įor the range -45 ☌ to +60 ☌, values given by this equation have an uncertainty of < ☐.6 % of value, at the 95% confidence level. Vapour pressure can be calculated using the Magnus formula. This states that at a temperature t (in ☌), the saturation vapour pressure e w(t), in pascals (Pa), over liquid water, is Use Equation 2 or 3 below (or Equation 4 or 5) to calculate dew or frost point temperature from vapour pressure (requires iteration if using Equations 4 or 5).Use Equation 1 to calculate water vapour pressure from saturation vapour pressure and known relative humidity.Use Equation 2 or 3 below (or Equation 4 or 5 for greater accuracy) to find the saturation vapour pressure from ambient temperature.To convert from relative humidity and ambient temperature to dew point Use these values of vapour pressure in Equation 1 to find relative humidity.Convert dew-point temperature and ambient temperature into water vapour pressures using Equation 2 or 3 below (or Equation 4 or 5 for greater accuracy).To convert from dew point or frost point to relative humidity Conversions between these two parameters must be carried out via the intermediate step of evaluating both the actual vapour pressure of water and the saturation vapour pressure at the prevailing temperature. Unfortunately, there is no simple, direct formula for converting between dew point and relative humidity. Relative humidity (in %) = e/ e s × 100 (Equation 1) The relative humidity is the ratio of the amount of water vapour, e, in the air to the amount of water vapour, e s, that would be in the air if saturated at the same temperature and pressure. Where the condensate is ice, this is known as the frost point. The dew point, or dew-point temperature, is the temperature at which dew, or condensation, forms as you cool a gas.
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