Measure of temperature/precipitation as an index for wetness or aridity (climate, averages)

[Stumbler.]

I remember I once sat in on a geography class where climatographs (the typical kind discussed on this forum, with average temperatures and average precipitation shown by month, for a normal year) were talked about.

I remember it being mentioned that if you scale the temperature units and the precipitation units a certain way, it call tell you whether at that point (the month) evaporation exceeds precipitation (indicating aridity) or vice versa (an excess of precipitation, indicating wetness) based on the ratio of temperature to precipitation amount.

For example, I don't know, say an inch of rain for a month where the high is 100F could be an arid month, but the same inch of rain for a month that averages 40F as a high could have an abundance of moisture.

This measure of wetness or aridity is supposed to have implications for plant growth, as it measures if there's a net gain or loss of moisture, and some climates are supposed to have evaporation exceeding rainfall for all the months of the year (ie. true deserts), and some have an excess of moisture for the whole year (I remember my climate was one of them).

Do any of you happen to know what this ratio or at least a rough measure is? It would be interesting to know so we can compare climates (like we usually love to do on this forum!) in their aridness.

[ben86]

Originally Posted by Stumbler.

I remember I once sat in on a geography class where climatographs (the typical kind discussed on this forum, with average temperatures and average precipitation shown by month, for a normal year) were talked about.

I remember it being mentioned that if you scale the temperature units and the precipitation units a certain way, it call tell you whether at that point (the month) evaporation exceeds precipitation (indicating aridity) or vice versa (an excess of precipitation, indicating wetness) based on the ratio of temperature to precipitation amount.

For example, I don't know, say an inch of rain for a month where the high is 100F could be an arid month, but the same inch of rain for a month that averages 40F as a high could have an abundance of moisture.

This measure of wetness or aridity is supposed to have implications for plant growth, as it measures if there's a net gain or loss of moisture, and some climates are supposed to have evaporation exceeding rainfall for all the months of the year (ie. true deserts), and some have an excess of moisture for the whole year (I remember my climate was one of them).

Do any of you happen to know what this ratio or at least a rough measure is? It would be interesting to know so we can compare climates (like we usually love to do on this forum!) in their aridness.

I don't know an actual formula but this nearby unofficial station (the one I use to get sun data from) has an evapotranspiration index saying how much rain we'd need to replace the water used by plants and in evaporation. For example, we'd have needed 61.4mm so far this month to stay level as far as the water table is concerned, and we are 59.4mm down on that so far:

http://www.hillviewweather.co.uk/wxhiloavg.php

[ricardobrazil]

I've never heard anything like this before.

[Stoney63]

I don't know what the actual formula is, but we average about 80 days a year of soil moisture deficit. Wind can be the problem here. Either sea breezes or dry SE winds. Summer rain is variable here. If it looks like is is going to be drought conditions for the summer, we sell off our small flock of sheep.

[ChesterNZ]

I think this is what you're referring to: Aridity index - Wikipedia, the free encyclopedia