In my head it’s the amount of moisture in the air relative to the temperature as hot air can hold more. If it gets cold then the moisture can’t be held in the air so it forms into water. They speak of dew points and other things but that’s when my brain melts and I lose it.
That's pretty close. It's actually relative to the most it can hold at that temperature (saturation). The moisture content - that is kg of water vapour per kg of air - is lower when air is cold. So in winter, the air outside is cold and has less moisture content, but it can still be 100% RH - saturated (usually means it's raining or thick fog). When you heat that winter air up with dry heat - a radiator or whatever - the moisture content stays the same and the temperature goes up. Therefore the air COULD hold more moisture, and so the RELATIVE humidity starts to fall. 22degrees in summer at 35 or 50%RH is comfortable. But 22degrees in winter could be an RH way down at 10 or 20% RH, which will make your eyes feel itchy. Which is why lots of people find central heating - especially turned up high - quite uncomfortable.
Dew point is the temperature at which air with a given moisture content will reach 100% RH. If the air in your house is at 21C and 50% RH, the dew point of the air will be about 10 to 11C. If the wall of your old house has a surface temperature around or below the dew point temperature, condensation (and then mould) will form on it, because it's too cold for the air next to the wall to 'hold' the moisture.
In a bedroom with the door shut and no ventilation, overnight you add a lot of moisture to the air (breathing) so the RH starts to rise. The risk is that with the heating off, the air temperature is dropping while the moisture content goes up, so the dew point is falling and the risk of condensation is increasing. Opening the window promotes air movement so the air next to the wall is kept moving, and doesn't get as cold as the wall. Therefore the risk of condensation is lower. Putting a big stack of boxes, or a wardrobe, or even a picture on the wall, traps the air close to the surface, so that air gets colder and then condensation and mould is more likely.
Gardening on the edge of Exmoor, in Devon
“It's still magic even if you know how it's done.”
All that the Met Office forecast is showing you is that there's no precipitation expected, so the moisture content of the air is constant but the temperature is falling, so the RH increases. Inside your loft at night, if the temperature doesn't drop, then the relative humidity won't increase either. Nothing is adding moisture (unless your shower is there or you're boiling pasta) so the dampness isn't actually increasing. It's just the temperature falling. Our forecast here gets to 96% RH at about 3am. Probably means we'll get a heavy dew fall and/or mist in the river valley below us, because either the ground or the water in the river (or both) will be colder than the dew point of the air.
Gardening on the edge of Exmoor, in Devon
“It's still magic even if you know how it's done.”
Fire said: yes, that's the bit that does my head in.
Think of it as fake weather news, from your house's point of view. Relative humidity doesn't really tell you much about how your house feels, at least up to the point you get to 100%. It's a human response indicator, which is why it gets talked about.
Gardening on the edge of Exmoor, in Devon
“It's still magic even if you know how it's done.”
Posts
Dew point is the temperature at which air with a given moisture content will reach 100% RH. If the air in your house is at 21C and 50% RH, the dew point of the air will be about 10 to 11C. If the wall of your old house has a surface temperature around or below the dew point temperature, condensation (and then mould) will form on it, because it's too cold for the air next to the wall to 'hold' the moisture.
In a bedroom with the door shut and no ventilation, overnight you add a lot of moisture to the air (breathing) so the RH starts to rise. The risk is that with the heating off, the air temperature is dropping while the moisture content goes up, so the dew point is falling and the risk of condensation is increasing. Opening the window promotes air movement so the air next to the wall is kept moving, and doesn't get as cold as the wall. Therefore the risk of condensation is lower. Putting a big stack of boxes, or a wardrobe, or even a picture on the wall, traps the air close to the surface, so that air gets colder and then condensation and mould is more likely.
“It's still magic even if you know how it's done.”
I finally get it. 🙏🏻👍🏼❤️
Our forecast here gets to 96% RH at about 3am. Probably means we'll get a heavy dew fall and/or mist in the river valley below us, because either the ground or the water in the river (or both) will be colder than the dew point of the air.
“It's still magic even if you know how it's done.”
“It's still magic even if you know how it's done.”