No it really will be that cold

Yes you will have to get knitting because it will be -70 C ish plus or minus a bit.

I won't say that I found that out here

http://sunshine.chpc.utah.edu/labs/atmosphere/atm_measure2.html

And that its all the balloon flying I've ever done ;-)

Perhaps a more scientific list would look like this

Height Temperature Pressure Density
(m) (C) (hPa) (kg/m3)
0000 15.0 1013 1.2
1000 8.5 900 1.1
2000 2.0 800 1.0
3000 -4.5 700 0.91
4000 -11.0 620 0.82
5000 -17.5 540 0.74
6000 -24.0 470 0.66
7000 -30.5 410 0.59
8000 -37.0 360 0.53
9000 -43.5 310 0.47
10000 -50.0 260 0.41
11000 -56.5 230 0.36
12000 -56.5 190 0.31
13000 -56.5 170 0.27
14000 -56.5 140 0.23
15000 -56.5 120 0.19
16000 -56.5 100 0.17
17000 -56.5 90 0.14
18000 -56.5 75 0.12
19000 -56.5 65 0.10
20000 -56.5 55 0.088
21000 -55.5 47 0.075
22000 -54.5 40 0.064
23000 -53.5 34 0.054
24000 -52.5 29 0.046
25000 -51.5 25 0.039
26000 -50.5 22 0.034
27000 -49.5 18 0.029
28000 -48.5 16 0.025
29000 -47.5 14 0.021
30000 -46.5 12 0.018
31000 -45.5 10 0.015
32000 -44.5 8.7 0.013
33000 -41.7 7.5 0.011
34000 -38.9 6.5 0.0096
35000 -36.1 5.6 0.0082

Now where I picked up the -70 I'm not sure but it got you attention ;-)

G