Wick weather watcher explains why moist air rises
Weather Watch by Keith Banks
A common misunderstanding is that moist air is heavier than dry air. The premise for this erroneous belief is that dry air has the potential to hold water.
Air does not capture and hold water. It does not have any fixed structure like porous materials such as sponges or fabrics possess.
Crucially, air is made up of different gases. There is a classical gas law, called Avogadro’s Law, that controls the ways that gases behave. Avogadro’s Law states that for a known volume of gas at a constant pressure and temperature, the total number of molecules never changes, regardless of the type of gas or gases that are present.
So if a molecule of water vapour is added to a given volume of gas, another molecule of gas is lost. When the atomic weights of different gas molecules that are present in the air are considered, it explains what happens to moist air.
Air in the troposphere consists of appropriately 21 per cent oxygen and 78 per cent nitrogen. Other gases present, including water vapour, account for the remainder. The atomic weights of a molecule of nitrogen (N2), and a molecule of oxygen (O2), are 28 and 32 respectively. A molecule of water (H2O) has an atomic weight of 18.
If six molecules of water (atomic weight 108) are added then four molecules of nitrogen (atomic weight 112) and 2 molecules of oxygen (atomic weight 64), must be removed. Hence, the atomic weight gained is 108, and the atomic weight lost is 176. The overall change in atomic weight is minus 68.
Therefore, in line with Avogadro’s Law, a parcel of humid air at the same temperature and pressure as a parcel of dry air will be less dense, and rise above the dry parcel.
October 2024 was Wick’s mildest since that of 2022, and is currently the 20th most mild since 1910.
In terms of precipitation, October 2024 was the driest since that of 2016. Closer analysis of the town’s rainfall archive for a series of Octobers commencing from 1910 showed that it is presently the 38th equal least wet. The rainfall total was identical in October 1937.
Wick’s mean air temperature for October 2024 was 10.20C (50.36F).
The long-term average in terms of the current 30-year averaging period is 9.16C (48.49F).
Wick’s average maximum temperature for the month was 12.60C (54.68F). The long-term average in terms of current 30-year averaging period is 11.79C (53.22F).
Highest maximum was 17.1C (62.8F), recorded on October 17. Lowest maximum was 8.2C (46.8F), noted on October 10.
Wick’s average overnight minimum air temperature was 7.79C (46.02F). Highest minimum was 11.8C (53.2F), witnessed on October 5 and 24 respectively.
Lowest minimum was 1.6C (34.9F), logged on October 13.
The temperature fell to 0C (32F) or lower at 5cm over the grass on two dates. Lowest temperature over the grass was minus 2.9C (26.8F) observed on October 14.
Precipitation was measurable on 28 dates. The total amount recorded for the month was 63.8mm (2.51 inches), and 68.7 per cent of the current long-term average for October.
Wettest day was October 20. The amount recorded for the 24 hours commencing 9am (GMT) was 12.8mm (0.50 of an inch).
“Days of gale” were experienced on two dates. However, wind velocities reached or surpassed gale force 8 (39.0mph/33.9knots) on six other dates.
Windiest day was October 21 when a severe gale force 9 west-north-westerly wind gusted up to 63.3mph/55.0knots (storm 10 on the Beaufort scale) during the hour ending midnight.
