If you are interested in getting even more from your home weather station then have you considered dew point? If your station measures humidity and air temperature then you already have all that you need.
What is Dew Point Temperature?
Water droplets condensing out of the air as it cools
The dew point is the temperature at which moisture in the air will condense out into water droplets.
Moisture in the air is of course invisible but when the dew point temperature is reached and the moisture forms water droplets, a mist or fog will form.
The warmer the air the more moisture the air can hold without it condensing out.
This is why you can see your breath on a cold day but not on a warm one.
On a cold day you breathe out warm air which holds lots of moisture, it then hits the cold air and is immediately chilled, this reduces the amount of moisture that can be held and the rest condenses out in a cloud of water droplets.
The dew point is the temperature at which moisture in the air changes to water droplets.
What Use is Dew Point?
Dew point and humidity are closely linked (barometric pressure has some influence on both). The closer the actual air temperature is to the dew point temperature the more moist the air and therefore the greater the humidity.
At 100% humidity the air is saturated with water, that is to say the air cannot hold any more moisture, this happens when the air temperature is equal to the dew point. Below the dew point all the moisture that cannot be held by the air condenses out as water droplets.
This also explains why high humidity in the tropics is so uncomfortable, the high temperature causes the body to sweat, usually the heat from the body would be used to evaporate that sweat causing cooling of that body, but in these conditions the sweat cannot evaporate because the air is already saturated with moisture so no cooling occurs, just more sweat and more heat.
Away from the tropics the air is generally colder; therefore the dew point is lower meaning that the saturation point is reached at a much colder temperature.
When therefore, the air is warm enough for the human body to sweat the air holds a relatively small amount of moisture and so evaporation of the sweat and consequent cooling are not a problem.
The Most Important Application for Dew Point
Perhaps the most important application for dew point is aviation.
It is useful in predicting where and when visibility problems associated with fog and mist might arise as well as predicting where and at what altitudes problems such as icing might occur.
How Is Dew Point Measured and Calculated?
Most commonly, dew point is derived from measurements of air temperature and relative humidity.
The calculation for dew point from these values is a little fearsome to say the least (see below)
But much more commonly a dew point table is used (an example is shown below):
Cross reference the air temperature with humidity to find the dew point
In the example above an air temperature of 30 degrees Celsius and relative humidity of 70% means a dew point of 23.9 degrees Celsius.
The calculation is a little more complicated:
Dew Point (Calculated from dry air temperature and relative humidity):
Dew Point (°C) = (239x)/(17.38-x)
Where:
x = ln(Vp/6.107)
and
Vp = RH/100 (6.107 exp(17.38Td/(239.0+Td))) for Td (dry air temperature) > or = 0
Vp = RH/100 (6.107 exp(22.44Td/(272.4+Td))) for Td < 0.
Ln= natural log and Vp= vapour pressure).
As you can see, unless you are a glutton for mathematical punishment the easiest way to find the dew point is to take the readings of air temperature and relative humidity from your home weather station and use a dew point lookup table.
