> Vapor pressure deficit (VPD), which describes the difference between the water vapor pressure at saturation and the actual water vapor pressure for a given temperature, is an important driver of atmospheric water demand for plants (1). Rising air temperature increases saturated water vapor pressure at a rate of approximately 7%/K according to the Clauius-Clapeyron relationship, which will drive an increase in VPD if the actual atmospheric water vapor content does not increase by exactly the same amount as saturated vapor pressure (SVP).

https://www.science.org/doi/10.1126/sciadv.aax1396

This is a commonly overlooked factor in gardening and agriculture especially in beginners.

As a beginner gardener, I'm curious. How is this applied in practical terms? (eli5)

They're just saying that hotter air can hold more water, so if the temperature rises, you need to add water to the air, to keep a constant water vapor pressure, and presumably plants have a preferred vapor pressure more than a preferred humidity %, as the same humidity % at different temperatures has a different vapor pressure, is how I read it.

Doesn't humidity already take this into account? It's measured in relative humidity which is the percent saturation of the air. So RH goes down as temperature goes up, unless there's more evaporation happening.

Apparently it doesn't. From what I read, relative humidity is maximum possible humidity divided by actual humidity (probably not the correct terms), whereas vapor pressure deficit is maximum possible minus actual. Fraction versus difference. So the same vapor pressure deficit could correspond to 80% relative humidity at one temperature, and 85% relative humidity at a higher temperature.

Depending on where you live and what plants you are growing and how picky they are, you may need to water less in hot weather (which is counterintuitive to most people). Unfortunately that can either be really good or really bad advice depending on your climate, the time of year, the type of plants you grow, etc so it's not a hard and fast rule, but especially if you have plants that struggle with molds, mildews, and/or fungi, keeping the top of the plant dry and letting the top part of the soil dry out regularly can be the difference between a healthy robust plant and a sickly one. The safest solution is an in-ground drip system, but that's also one of the hardest.

Honestly the best advice is to find a nice, elderly/retired person (usually women, but not always) who is into gardening in your area and have discussions with them. Bring pictures of your plants to the nursery and/or gardening store, or join a Facebook group or something and you can usually find people. Don't get discouraged if the first few people don't work out. Particularly if you are young, you'll get a lot better results if you are overly respectful and polite, at least until you get to know each other better. Give them the benefit of the doubt if they say something you find offensive and just observe/listen rather than lecture. Many older generations can be hesitant to engage with younger people because the social customs are just so different and it can make them uncomfortable. Ideally try to watch them do what they do (planting, pruning, tending, mulching, etc), because if you are on HN you may be more scientifically minded, but most of these people just do what they do and don't even think about it or even notice sometimes. It's very much an art to them. If you ask them things like "why do you mulch there and why do you use grass clipping" be prepared for them to have a terrible answer that doesn't make sense, but know that they are probably doing it because it works, but the reason why it works might be an exercise left for you to find out. A lot of them have generations worth of muscle memory and convention that is all good stuff, so don't discard it just because they can't explain why they do it.

Yes, the amount of vapor increases, which also contributes to global warming. But at the same time, the relative humidity might go down if the capacity of the air to hold water increases faster than the actual vapor amount. The consequence is: while there is more water in the air, there is less accessible for the plants.

an article just about your question: https://www.carbonbrief.org/guest-post-investigating-climate...

But where does it get wetter? It's not helpful if the air over the oceans gets wetter while the air over dry regions gets dryer.

> But where does it get wetter? It's not helpful if the air over the oceans gets wetter while the air over dry regions gets dryer.

On average, global land areas have seen more precipitation since 1950. But even as much of the world has become wetter, some regions have become drier.

- https://www.nytimes.com/interactive/2021/08/24/climate/warme...

Absolutely or relatively? If the dry areas get 2 less inches of rain per year and the wet areas get 2 inches more, it's a much bigger impact on vegetation going from 12 to 10 inches of rain per year than going from 120 to 122 inches per year.