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Dewpoint_CloudBase_Calculation.php   7596 Bytes    10-03-2016 07:34:01

## A short introduction ...

With only few (and now widely available) sensors, a lot of interesting meterological data can be calculated. We show here what can easily be obtained with only three sensors. We use the following ones :

 Name Type Accuracy Remarks MCP9808 Temperature ±0.25°C from -40°C to +125°C CC2D35 Humidity ±2.0 %RH - MPL115A2 Barometer ±1 kPa from -40°C to +105°C

## ✈ QFE, QNH & QFF .:. Barometric Pressure

All of them describe the barometric pressure. Whilst QFE is the pressure at a given height, QNH (which is used in aviation) is normalised to a height of 0 m (sea-level). QFF is used by meterologists and is normalised with respect to actual atmospheric situation. QNH to QFE conversion can easily be done with the barometric height formula.

 p(h) = p(0) * (1 - (0.0065 K/m * h ) / (288.15 K) ) ^ 5.255    K: Kelvin, h: height in m, p in hPa, see Wikipedia for details.

As our height with respect to sea-level remains constant, h = 535 m, we may replace the term with the brackets with a constant term k = 0.93818. Replacing further p(h=535) with QFE and p(h=0) with QNH, we arrive at : QNH = QFE / 0.93818
With our measured value of p = 955 hPa (QFE), we calcualte a QNH of 1018 hPa. The nearby airport indicates 1017 hPa.

## ✈ Dew Point

Air is a mixture of several gases. One of these is moisture. The amount of moisture, which may be present in air is limited. The higher the temperature, the higher is the amount of moisture which may be in the air. The relative humidity is a measure, how much of the maximum value is currently in the air. As the maximum value depends on the temperature, the relative humidity decreases with rising temperature (and vice versa). The dew point temperature is defined as the temperature, at which the moisture is maximum (100% relative humidity). Therefore the dew point temperature is independant of the actual temperature. There is no exact formula, but the Magnus formula yields quite accurate values.

Magnus Formula : Saturation Vapor Pressure in [hPa]

 SVP(T) = 6.1078 * 10^((a*T)/(b+T))

T = Temperatur in °C
a = 7.5, b = 237.3 for T >= 0
a = 7.6, b = 240.7 for T < 0 over Water (Dewpoint)
a = 9.5, b = 265.5 for T < 0 over Ice (Frostpoint)

With this result, we may calculate the Vapor Pressure in [hPa]

 VP(R,T) = R/100 * SVP(T)

with R = Relative Humidity

And then, we can finally calculate the Temperature Dewpoint in °C

 TD(R,T) = b*v/(a-v)  with v(R,T) = log10( VP(R,T)/6.1078 )

# Credits • Links The Visualisation uses https://www.mathjax.org/ The Dewpoint mayst also be calculated using the javascript of Stefan Ochs.

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