All this attention on blight favouring conditions – warm, wet, humid has made me think it’s time to stick a toe in the airy puddle of humidity.
What is humidity?
Water vapour held in the air. Given we can feel but not see it vapour in the air is measured by hygrometers and calculated by hygrometric tables.
The quantity of water vapour that air can hold depends on the temperature of that air.
- The lower the temperature of the air – the less water vapour it can hold
- The higher the temperature of the air – the more water it can hold
When the maximum amount of water vapour for a given air temperature is reached then that air has reached its Saturation Point.
If air reaches Saturation Point and then the air temperature subsequently drops, the ‘excess’ water vapour it can no longer accomodate has to be dumped. This is achieved by condensing it into water – the liquid version !
In an interior situation this creates that irritating phenomenon – condensation. In micro, on the inside lid of a propagator – or on a larger domestic scale, on the inside window panes.
Outdoors; clouds, fog and mist are the result of water vapour ‘converting’ to liquid.
And Relative Humidity – well it’s all relative, see……relative to the air temperature……
Relative humidity is the % of water vapour that the air can hold at a certain temperature. The water vapour in the air is measure by grams of water vapour per kilograms of air. But then it expressed as a % of what the maximum amount of water vapour air of that particular temperature can hold. With warm air being able to hold a greater maximum amount of water vapour than cold.
And to go back to blight and its relationship to humidity – it would seem it doesn’t get out of bed for anything less than 10C and 90% humidity over the period of two consecutive days. So when us tomato lovers feel sticky and out of sorts for a couple of days on the trot - it’s because we sense something nasty is stirring and coming to life……
