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How to Find Mixing Ratio With Dew Point

Weather balloons carry scientific instruments that record conditions in the atmosphere (vertical measurements or soundings) such as temperature, dew point, air pressure, moisture and wind. The soundings are radioed back to a receiver on Earth, and the data is plotted on a graph, either a thermodynamic diagram or a Skew-T Thermodynamic Diagram. The data yields information about weather stability, and the likelihood of severe weather. The ratio of the mass of water vapor to the mass of dry air is called the mixing ratio. This measurement can be found using either a thermodynamic diagram, or by substituting values into a formula.

Things You'll Need

Thermodynamic diagram
Skew-T thermodynamic diagram
Current temperature, dew point and station air pressure readings
Calculator

Instructions

Thermodynamic Diagram
- 1
  Locate the solid lines that slope at a small angle from vertical. These lines indicate the constant saturation mixing ratio, expressed in grams of water vapor per kilogram of dry air, that relate the maximum water vapor level the air can hold. These lines also represent the constant mixing ratio (w).
- 2
  Locate the given air pressure level on the vertical axis. These isobars are lines that run horizontally, starting with 1050 mb (millibars) and decreasing as you move up the vertical axis. Since air pressure decreases exponentially with increasing height, the spacing of the isobars varies.
- 3
  Locate the intersection of the saturation mixing ratio, air pressure and the given dew point. This is the mixing ratio.
Skew-T Thermodynamic Diagram
- 4
  Locate the curvy vertical line to the left of vertical center. This is the mixing ratio (w) line.
- 5
  Locate the dew point (Td) at the given pressure. The dew point appears on a slanted red line near the bottom of the graph. Label the reading in degrees Celsius.
- 6
  Record the reading of the intersection of the mixing ratio line and the dew point to obtain the mixing ratio. Label the reading from the Skew-T diagram in g/kg. This diagram is called a Skew-T because the temperatures are plotted at a 45-degree angle to the right (skewed), instead of vertically.
Formula
- 7
  Gather air temperature (T), dew point (Td) and the air pressure reading at the station (Psta). Convert the Fahrenheit air temperature (Tf) and dew point temperature into degrees Celsius (Tc). Insert the values into the formula Tc = 5/9 (Tf -- 32).
- 8
  Calculate the vapor pressure (e). Insert the values into the formula e = 6.11 x 10 (7.5 x Td/237.7 + Td).
- 9
  Convert the station pressure into millibars (mb) or hectopascals (hPa). Insert the values into the formulas Pmb = PHg x 33. 8637526, or PhPa = PHg x 33.8639.
- 10
  Calculate the mixing ratio. Insert the values into the formula w = 621.97 x e/Psta --e.

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