Im borrowing the method from the Weiss Nevermore feat so this may not be perfect but...
Volume of rectangular prism = l*w*h = 8m * 1m * 3m = 24m^3
- Density of frozen nitrogen: 1026.5 kg/m^3
- Mass of ice trap: 24636 kg
- Specific heat of nitrogen: 1040 j/kg
- Freezing point of nitrogen: -210 C
- Melting point of nitrogen: -196 C
- Temperature (outdoors): ~48+ C (She can use this ability in Oasis, located in Iraq, where the average temperature in the summer is around 120F, backed up Reinhardt stating that he is cooking alive inside his armor)
Now for the latent heat required to freeze that much nitrogen. The latent heat of nitrogen is listed here.
Q1 = MC(Delta T) = (24636 kg)(1040j/kg)(-196-(-210)) = 358700160 joules
Fusion energy nitrogen = (Mass of nitrogen)(Latent heat fusion) = (24636 kg)(25702j/kg) = 633194472 joules
Q2 = (24636 kg)(1040j/kg)(48-(-196)) = 6251631360 joules
Vaporization energy nitrogen = (24636 kg)(199190j/kg) = 4907244840 joules
Q1 + Fusion + Q2 + Vaporization = 358700160 + 633194472 + 6251631360 + 4907244840 = 12150770832 J, 1.21e+10 Joules, 2.9 Tons of TNT
Large Building level
An actual calc to back up High 8-C Overwatch??
