When temperatures climb into the 90s with high humidity levels (dew points in the 70s), one is likely to hear someone say: 'The air is really heavy today.' Or, maybe, given today's low humidity, one may think that air seems particularly light.
But, assumption that humid air is heavier than dry air is wrong.
In very hot, humid weather, one may feel that air is heavier because moving around seems to take more energy on dry days. Sticky days feel 'heavier' because heat and high humidity slows evaporation of perspiration, body grows hotter and this saps one's strength.
One may also say: 'Water is heavier than air.' True, a glass of liquid water weighs more than a glass filled only with air. But, humidity is water vapour, not liquid water and water vapour molecules are lighter then molecules of nitrogen and oxygen which make up about 99% of the atmosphere.
At this stage, one could argue: 'It does not make any difference that water molecules are lighter than those of oxygen and nitrogen. Water molecules are being added to oxygen and hydrogen in air, which make it heavier. One would be wrong. To see why, we need to go back to 1811 and work of Italian scientist Count Lorenzo Avogadro, who hypothesized that equal volumes of gases at same temperature and pressure contain equal numbers of molecules.
By 1860, Avogadro's law was a bedrock of chemistry as it still is.
In other words, when water molecules evaporate into a cubic foot of air, an equal number of other molecules will leave that particular cubic feet of air.
Each molecule of water which evaporates into a particular parcel of air as vapour will replace a molecule of either nitrogen or oxygen, which accounts for roughly 99% of air's gases. Nitrogen in air has molecular weight of 28% and oxygen molecule has molecular weight of 32%.
Water vapour molecules, which are one oxygen atom with a weight of 16 and two hydrogen atoms each with a weight of 1, add up to a molecular weight of 18, which is much lighter than the nitrogen and oxygen they displace when they evaporate into air.
For example, at both 3:52 and 4:52 p.m. on July 19th 2013, temperature at National Airport was 94 degrees and dew point 74 degrees. This combination made air's density 0.0708 pounds per cubic foot or in metric system 1.134 kilograms per cubic meter.
On a very dry day, with same 94-degree temperature and atmospheric pressure as on 19th but a dew point of only 50 degrees, air's density would have been 0.0712 pounds per cubic foot or 1.141 kilograms per cubic meter.
There are three ways of turning moist air into cloud so that it rains or snows.
Rain and other forms of precipitation occur when warm moist air cools and condensation occurs.
Since warm air can hold more water than cool air, when warmer air is cooled, moisture condenses to liquid and it rains.
Frontal rain occurs when two air masses meet. When a warm air mass meets a cold air mass, they don't mix as they have different densities. Instead, the warm less dense air is pushed up over cold dense air creating the 'front'. As a result, much like when air is forced up over mountains, warm less dense air cools and water vapour condenses into water and falls as raindrops. This type of rain can happen anywhere in the UK.
Orographic rain is rainfall produced as a result of clouds formed from topography or shape of the land. Where there is high ground, moist air is forced upwards producing cloud and precipitation.
Mountainous areas close to prevailing westerly winds are most likely to experience this type of rainfall. Geography of the UK means that this type of rainfall is most common in the north and west of the UK where warm moist air from Atlantic cools as it is forced upwards over high altitudes.
Convective rain is produced by convective cloud. Convective cloud is formed in vertical motions which result from instability of atmosphere. One way that atmosphere can become unstable is by heating from the sun, ground warms up, causing moisture in the ground to evaporate and rise and hot ground too heats the air above it. As water vapour rises, it cools and condenses into clouds and eventually rains.
When you heat the air from below, much like in boiling kettle, you tend to get 'bubbles' of rising air, known as updraughts. These are smaller than large-scale lifting of air which occurs at fronts and over mountain ranges. This tends to give us smaller areas of rain, with clear spells in between, referred to in the UK as 'sunshine and showers'.
This type of rainfall is common in the south and east of the UK, where it is warmer. This area is also prone to very heavy showers because warmer air can hold more water.
Sometimes, one can get all three types of rain at one time and this can lead to severe flooding.