Rainfall - poles versus hadley cells

Question

I am attempting to build an earth-like planet using this blog post (mentioned to me by @MonicaCellio).

The post is great and I've been able to quickly and easily create a world. I've had a slight problem with assigning climates though. I'm using the Whittaker Diagram found here, and I've made a table based off of it, listing which climate is created by what amount of rainfall and temperature (mainly so there's no overlap).

In that post, the author states that, "The upper right half is mostly empty, since these climate combinations do not exist on Earth — remember that cold air cannot hold moisture." Unfortunately, my world has exactly that combination of factors, centered around the 60/-60 degree latitudes.

I know why this is: Hadley Cells. In the process described by the blog, we create Hadley cells bringing rain. The rain is centered on the equator, and at 60/-60 degrees latitude - right about where I have my cold/wet environments. And yet according to the temperature, there can't be rain there because it is cold, despite the presence of the hadley cell.

Which is it? Is it warmer where it rains? Does the cool air from the pole negate the rain from the hadley cell? What am I doing wrong?

Answer 1

The lack of water in the cold air is due to psychrometrics. This means that as the article says, you can't get that condition; wet air and cold, on Earth (or an Earthlike planet).

A psychrometric chart shows how much water air mixtures can hold at different temperatures:

The water in air is in a gaseous state, as the temperature decreases the air will hold less water in the vapor phase because as it gets colder the water vapor will condense into water, i.e. rain. This will happen with most of the water before it gets near the freezing temperature of water, notice how little water the air can hold at freezing temps vs. warmer temperatures.

So in terms of climate on your planet if you have a region where it is showing average yearly temperatures below freezing and lots of moisture, that just won't happen. What will happen is that most of that moisture will condense out as rain before it get to the coldest region, so your areas around the cold region will have more rain. Note that the +-60 degree rain bands for Earth are based on temperatures and are a huge approximation.

For a great example look at the precipitation figures for central North America at 60 degrees (marked in red line). There is less rain there in central Canada because the mountain ranges and altitude make those areas cold, hence less moisture, while areas adjacent to them, i.e. the Pacific Northwest get more rain than latitude alone would predict.