Does altitude affect wifi signals?
It's difficult (almost impossible) to answer this question.

Why? Because the range (vertial or not) depends a lot on factors that you did not specify.

First you should have a look at notions such as "Link budget" Wikipedia

$$ P_{RX} = P_{TX} + G_{TX} - L_{TX} - L_{FS} - L_M + G_{RX} - L_{RX} \,$$

where:

\$P_{RX}\$ = received power (dBm)

\$P_{TX}\$ = transmitter output power (dBm) 

\$G_{TX}\$ = transmitter antenna gain (dBi)

\$L_{TX}\$ = transmitter losses (coax, connectors...) (dB) 

\$L_{FS}\$ = free space loss or path loss (dB)

\$L_M\$  = miscellaneous losses (fading margin, body loss, polarization mismatch, other losses...) (dB)

\$G_{RX}\$ = receiver antenna gain (dBi)

\$L_{RX}\$ = receiver losses (coax, connectors...)  (dB)

You can see that the antenna gain is part of the equation. Using the stock "omni" antenna sold with the WiFi access point will not achieve the same distance that using a very directional antenna such as a Yagi Uda or a parabolic one.
Without specifying this, it's impossible to give you a number for the range.
Then it comes to the transmitter power and receiver sensitivity. Each access point and WiFi card is different.

But, there is also another factor not related to the strength of the signal: the time delay.

The WiFi protocol (which one? this has to be defined) defines the maximum delay between a sent packet and the related acknowledge. If the distance is too big, a timeout will occur and this will not work. But without knowing the specific WiFi norm you plan to use, it's impossible to give you a number.

According to Wikipedia page about long range WiFi using modified firmware in order to increase the timeout value and dedicated antennas, ranges up to 304 km is possible without using additional amplifiers.