Most flight occurs in the Troposphere, which is the layer of the atmosphere immediately surrounding the earth, and is the only layer where weather occurs. This is because this layer contains water vapour, which is the gaseous form of water, totally invisible, and is the cause of all weather. The amount of water vapour which the air contains is known as relative humidity, and is expressed as a percentage. Relative humidity, and dew point are important factors to you as a pilot, as it can help you determine what possible weather you may encounter en-route, and where the dangers lurk.

When air becomes saturated, it cannot hold any more water vapour, and condensation will occur. The dew point is the point where water vapour condenses, and becomes visible in the form of cloud. It is also the point where you will find the cloud base. In order for air to become saturated, the temperature needs to decrease progressively. Warm moist air will naturally start to rise from the surface of the earth, and as it rises, it generally cools as the temperature of the atmosphere decreases with height. An exception to this, would be if there is an inversion, where the temperature is increasing with height (for various reasons associated with inversion formation), and in which case cloud formation is inhibited. The principle behind all of this, is that the colder the temperature, the less water vapour it can hold, as the air molecules are denser, and the volume decreases, and therefore it will reach saturation, or 100% relative humidity quicker than in warmer air.

The dew point, or saturation point is particularly important to the VFR pilot, who, by law needs to stay clear of cloud:

There are two important calculations based on the temperature and dew point, that can help you determine not only what weather to expect, but also to choose the appropriate altitude to fly at, that will keep you at the legal distance from clouds.

Your starting point will be the METAR (remember to look at all the METARs appropriate to your route), where the temperature and dew point are easily identifiable – the dew point will always be lower than the temperature:

Example: METAR FAOR 020630Z 31011KT 9999 SCT008 15/12 Q1026 NOSIG=

Next, the relative humidity can tell us if we can expect cloud (100% RH), Mist (<95%) or Fog (95%+):
Formula: RH = 100 – 5 x (temperature – DP)
Temperature: 15°C
Dew Point: 12°C
RH = 100 – 5 x (15 – 12)
RH = 100 – 5(3)
RH = 85%

So in this case we are unlikely to experience mist or fog, but because the RH is quite high, we can expect saturation (and cloud base) to occur at a relatively low altitude.

How to calculate cloud base:
1. Determine the difference between the Temperature and the Dew point.
2. Divide this number by 2.5 (for degrees celsius)
3. Multiply by 1000
4. Add this to the airport elevation.
15 – 12 = 3
3/2.5 = 1.2 x 1000 = 1200
If the airport elevation is 5300ft + 1200ft = 6500ft (Cloud Base)

The dangers of flying into clouds or very close to them?
• Disorientation (Especially if you are not instrument rated)
• Turbulence (Depending on the cloud type)
• Icing (Depending on the cloud type, altitude and temperature)