Why is Little Lin's nose bleeding? 还不习惯北方的气候，估计是天气太干。 This snippet is from 标准教程HSK4上 chapter 7 (my translation):

小李：你的鼻子怎么流血了？快用纸擦擦。
Little Li: Your nose is [how] bleeding? Quickly get a tissue and wipe [it].
小林：我还不习惯北方的气候，估计是天气太干。今天天气不是很冷，你怎么穿这么多？
Little Lin: I am still not accustomed to the north's climate, [my] estimate is the weather is too dry.  Today the weather is not very cold, how come you wear so much?

I don't understand what caused 小林's nose to bleed.  The logic seems to be something like this:

小林 is now in the north [of China].
小林 is not accustomed to the north's climate.
小林 estimated the weather is &quot;too dry&quot; [估计是天气太干], but this was incorrect.
???
小林's nose is bleeding.

I'm going wrong somewhere here.
Question: Why is 小林's nose bleeding?
It seems like 小林 has predicted the north's weather is drier than it actually is.  But I would expect an underestimation of the dryness to lead to 小林's nose bleeding, not an overestimation.
The weather caused the problem of his nose. Specifically, he suspected it's too dry. 

It seems that you misunderstand the word 估计 in the context. It's not an estimation actually. It's his suspection instead. I'd interpret 估计 as reckon, suspect, feel, and etc in this context. 

So, 


  我还不习惯北方的气候，估计是天气太干 // I am still not accustomed to the north's climate, I reckon (it's because) the weather is too dry.