Barometric pressure (Pb) from elevation (EL):
|
|
Eq. A3.1 |
Latent heat of vaporization (l) from air temperature (T):
|
|
Eq. A3.2 |
Saturation vapour pressure over water is the vapour pressure of the air when the number of water molecules condensing equals the number evaporating from a flat surface of water with both the air and water at some temperature (T). An equation for the saturation vapour pressure (es) over water at temperature (T) is given by:
|
|
Eq. A3.3 |
When the number of water molecules sublimating equals the number depositing onto a flat surface of ice with both the air and ice at some temperature (T), the saturation vapour pressure (es) over ice at temperature (T) is given by:
|
|
Eq. A3.4 |
Dew-point temperature (Td) from air temperature (T) and relative humidity (%):
|
|
Eq. A3.5 |
Ice-point temperature (Ti) from air temperature (T) and relative humidity (%):
|
|
Eq. A3.6 |
Note that the actual vapour pressure (e) is equal to the saturation vapour pressure (ed) at the dew-point temperature (Td) and, for subzero temperatures, the saturation vapour pressure (ei) at the ice point temperature (Ti).
Dew-point temperature (Td) from vapour pressure (e = ed) over water:
|
|
Eq. A3.7 |
Ice-point temperature (Ti) from vapour pressure (e = ei) over ice:
|
|
Eq. A3.8 |
Slope of Saturation Vapour Pressure over liquid water at temperature T:
|
|
Eq. A3.9 |
Psychrometric Constant (g) as a function of barometric pressure (Pb) and wet-bulb temperature (Tw), from Fritschen and Gay (1979):
|
g = 0.000660 (1 + 0.00115 Tw) Pb (g in kPa, Td in C) |
Eq. 3.10 |
Saturation vapour pressure (ea) at the air temperature (Ta):
|
|
Eq. A3.11 |
Vapour pressure (e = ed) at the dew-point temperature (Td):
|
|
Eq. A3.11 |
Vapour pressure (e = ei) at the subzero ice point temperature (Ti):
|
|
Eq. A3.12 |
Equivalent temperature (Te) from temperature T, vapour pressure e, and the psychrometric constant g:
|
|
Eq. A3.13 |
Absolute humidity (c) from vapour pressure (e) and temperature (T):
|
|
Eq. A3.14 |
Vapour pressure (e) from dry (T) and wet-bulb (Tw) temperature (°C) and barometric pressure (Pb):
|
e = ew 0.000660 (1 + 0.00115 Tw) (T - Tw) Pb (e in kPa, Pb in Kpa) |
Eq. 3.15 |
where ew is the saturation vapour pressure at the wet-bulb temperature. It is calculated by substituting Tw for Tin Equation A3.3.
Vapour pressure (e) from dry (T) and frost-bulb (Tf) temperature (°C) and barometric pressure (Pb):
|
e = ef - 0.000582 (1 + 0.00115 Tf) (T - Tf) Pb (e in kPa, Pb in Kpa) |
Eq. 3.16 |
where ef is the saturation vapour pressure at the frost-bulb temperature. It is calculated by substituting Tffor Tin Equation A3.4.
TABLE A3.1
Saturation vapour pressure (kPa) over a flat
surface of liquid water calculated using Tetens' formula (Equation A3.3) for air
temperature between 0.0 °C and -14.9°C
|
|
TEMPERATURE °C |
|||||||||
|
|
-0.0 |
-0.1 |
-0.2 |
-0.3 |
-0.4 |
-0.5 |
-0.6 |
-0.7 |
-0.8 |
-0.9 |
|
-14 |
0.207 |
0.205 |
0.203 |
0.202 |
0.200 |
0.199 |
0.197 |
0.195 |
0.194 |
0.192 |
|
-13 |
0.224 |
0.223 |
0.221 |
0.219 |
0.217 |
0.216 |
0.214 |
0.212 |
0.210 |
0.209 |
|
-12 |
0.243 |
0.241 |
0.240 |
0.238 |
0.236 |
0.234 |
0.232 |
0.230 |
0.228 |
0.226 |
|
-11 |
0.264 |
0.262 |
0.260 |
0.258 |
0.256 |
0.253 |
0.251 |
0.249 |
0.247 |
0.245 |
|
-10 |
0.286 |
0.283 |
0.281 |
0.279 |
0.277 |
0.275 |
0.272 |
0.270 |
0.268 |
0.266 |
|
-9 |
0.309 |
0.307 |
0.304 |
0.302 |
0.300 |
0.297 |
0.295 |
0.293 |
0.290 |
0.288 |
|
-8 |
0.334 |
0.332 |
0.329 |
0.327 |
0.324 |
0.322 |
0.319 |
0.317 |
0.314 |
0.312 |
|
-7 |
0.361 |
0.359 |
0.356 |
0.353 |
0.350 |
0.348 |
0.345 |
0.342 |
0.340 |
0.337 |
|
-6 |
0.390 |
0.387 |
0.384 |
0.381 |
0.378 |
0.376 |
0.373 |
0.370 |
0.367 |
0.364 |
|
-5 |
0.421 |
0.418 |
0.415 |
0.412 |
0.409 |
0.405 |
0.402 |
0.399 |
0.396 |
0.393 |
|
-4 |
0.454 |
0.451 |
0.447 |
0.444 |
0.441 |
0.437 |
0.434 |
0.431 |
0.428 |
0.424 |
|
-3 |
0.490 |
0.486 |
0.482 |
0.479 |
0.475 |
0.472 |
0.468 |
0.465 |
0.461 |
0.458 |
|
-2 |
0.527 |
0.524 |
0.520 |
0.516 |
0.512 |
0.508 |
0.504 |
0.501 |
0.497 |
0.493 |
|
-1 |
0.568 |
0.564 |
0.559 |
0.555 |
0.551 |
0.547 |
0.543 |
0.539 |
0.535 |
0.531 |
|
0 |
0.611 |
0.606 |
0.602 |
0.598 |
0.593 |
0.589 |
0.585 |
0.580 |
0.576 |
0.572 |
TABLE A3.2
Saturation vapour pressure (kPa) over a flat
surface of liquid water calculated using Tetens' formula (Equation A3.3) for air
temperature between0 °C and 14.9 °C
|
|
TEMPER |
|||||||||
|
|
0.0 |
0.1 |
0.2 |
0.3 |
0.4 |
0.5 |
0.6 |
0.7 |
0.8 |
0.9 |
|
0 |
0.611 |
0.615 |
0.620 |
0.624 |
0.629 |
0.633 |
0.638 |
0.643 |
0.647 |
0.652 |
|
1 |
0.657 |
0.661 |
0.666 |
0.671 |
0.676 |
0.681 |
0.686 |
0.691 |
0.696 |
0.701 |
|
2 |
0.706 |
0.711 |
0.716 |
0.721 |
0.726 |
0.731 |
0.737 |
0.742 |
0.747 |
0.752 |
|
3 |
0.758 |
0.763 |
0.769 |
0.774 |
0.780 |
0.785 |
0.791 |
0.796 |
0.802 |
0.808 |
|
4 |
0.813 |
0.819 |
0.825 |
0.831 |
0.836 |
0.842 |
0.848 |
0.854 |
0.860 |
0.866 |
|
5 |
0.872 |
0.878 |
0.885 |
0.891 |
0.897 |
0.903 |
0.910 |
0.916 |
0.922 |
0.929 |
|
6 |
0.935 |
0.942 |
0.948 |
0.955 |
0.961 |
0.968 |
0.975 |
0.981 |
0.988 |
0.995 |
|
7 |
1.002 |
1.009 |
1.016 |
1.023 |
1.030 |
1.037 |
1.044 |
1.051 |
1.058 |
1.065 |
|
8 |
1.073 |
1.080 |
1.087 |
1.095 |
1.102 |
1.110 |
1.117 |
1.125 |
1.133 |
1.140 |
|
9 |
1.148 |
1.156 |
1.164 |
1.172 |
1.179 |
1.187 |
1.195 |
1.203 |
1.212 |
1.220 |
|
10 |
1.228 |
1.236 |
1.245 |
1.253 |
1.261 |
1.270 |
1.278 |
1.287 |
1.295 |
1.304 |
|
11 |
1.313 |
1.321 |
1.330 |
1.339 |
1.348 |
1.357 |
1.366 |
1.375 |
1.384 |
1.393 |
|
12 |
1.403 |
1.412 |
1.421 |
1.431 |
1.440 |
1.449 |
1.459 |
1.469 |
1.478 |
1.488 |
|
13 |
1.498 |
1.508 |
1.517 |
1.527 |
1.537 |
1.547 |
1.558 |
1.568 |
1.578 |
1.588 |
|
14 |
1.599 |
1.609 |
1.619 |
1.630 |
1.641 |
1.651 |
1.662 |
1.673 |
1.684 |
1.694 |
TABLE A3.3
Saturation vapour pressure (kPa) over a flat
surface of ice calculated using Tetens' formula (Equation A3.4) for air
temperature between 0.0 °C and -14.9 °C
|
|
TEMPERATURE °C |
|||||||||
|
|
-0.0 |
-0.1 |
-0.2 |
-0.3 |
-0.4 |
-0.5 |
-0.6 |
-0.7 |
-0.8 |
-0.9 |
|
-14 |
0.181 |
0.179 |
0.177 |
0.176 |
0.174 |
0.173 |
0.171 |
0.169 |
0.168 |
0.166 |
|
-13 |
0.198 |
0.196 |
0.194 |
0.193 |
0.191 |
0.189 |
0.187 |
0.186 |
0.184 |
0.182 |
|
-12 |
0.217 |
0.215 |
0.213 |
0.211 |
0.209 |
0.207 |
0.205 |
0.204 |
0.202 |
0.200 |
|
-11 |
0.237 |
0.235 |
0.233 |
0.231 |
0.229 |
0.227 |
0.225 |
0.223 |
0.221 |
0.219 |
|
-10 |
0.259 |
0.257 |
0.255 |
0.253 |
0.250 |
0.248 |
0.246 |
0.244 |
0.242 |
0.239 |
|
-9 |
0.284 |
0.281 |
0.279 |
0.276 |
0.274 |
0.271 |
0.269 |
0.266 |
0.264 |
0.262 |
|
-8 |
0.310 |
0.307 |
0.304 |
0.302 |
0.299 |
0.296 |
0.294 |
0.291 |
0.289 |
0.286 |
|
-7 |
0.338 |
0.335 |
0.332 |
0.329 |
0.326 |
0.323 |
0.321 |
0.318 |
0.315 |
0.312 |
|
-6 |
0.368 |
0.365 |
0.362 |
0.359 |
0.356 |
0.353 |
0.350 |
0.347 |
0.344 |
0.341 |
|
-5 |
0.401 |
0.398 |
0.395 |
0.391 |
0.388 |
0.385 |
0.381 |
0.378 |
0.375 |
0.372 |
|
-4 |
0.437 |
0.433 |
0.430 |
0.426 |
0.422 |
0.419 |
0.415 |
0.412 |
0.408 |
0.405 |
|
-3 |
0.476 |
0.472 |
0.468 |
0.464 |
0.460 |
0.456 |
0.452 |
0.448 |
0.445 |
0.441 |
|
-2 |
0.517 |
0.513 |
0.509 |
0.505 |
0.500 |
0.496 |
0.492 |
0.488 |
0.484 |
0.480 |
|
-1 |
0.562 |
0.558 |
0.553 |
0.548 |
0.544 |
0.539 |
0.535 |
0.530 |
0.526 |
0.522 |
|
0 |
0.611 |
0.606 |
0.601 |
0.596 |
0.591 |
0.586 |
0.581 |
0.576 |
0.572 |
0.567 |
