At which location would you expect air to have the highest density on a typical winter day?

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To know, at which location would you expect air to have the highest density on a typical winter day, we have to know what is air density.  The density of air is the mass per unit volume of atmospheric gases. The density or how heavy or light the air is dependent on both the temperature and pressure of the air. STP ( standard temperature and pressure) is considered as the value for the density of air.

Ideal Gas Law is used to calculate the density of air. The law indicates density as function temperature and pressure. This law is ideal for low pr ordinary pressure and temperature but increasing temperature and pressure also increases the chances of errors in the calculations. The equations are mentioned below: 

ρ = p / RT

where:

ρ is the air density in kg/m3

p is the absolute pressure in Pa

T is the absolute temperature in K

R is the specific gas constant for dry air in J/(kg·K) or is 287.058 J/(kg·K).

Which location has the highest air density on a typical winter day? 

If you have a question in mind like "where is the highest air density on a typical winter day?", let me answer you now. The density of air is dependent on pressure, temperature, and humidity. So, what is air pressure!

  1. Low air pressure 

In the lower pressure system, there is lower pressure at its center than the areas around it. Due to the effect of Earth's spin and The Coriolis effect, the winds of the low-pressure system swirl clockwise in the south of the equator and anticlockwise in the north of the equator. This flow of wind is often called the Cyclonic flow of wind. RED L is used to denote a Low-pressure system on weather maps.

  1. High air pressure  

High pressure in the air means the air has higher pressure in its center than the area surrounding it. The air pressure rotates clockwise in northwards direction towards the north of the equator and counterclockwise to the south. This process of air pressure is called anticyclonic flow.

During winters, the temperature factor becomes an important aspect that decides the location of the air. Atmospheric pressure plays an important role during winter which is caused due to the cold condition of the atmosphere, experienced in the particular region.  The cold air is heavier and has a tendency to sink than warm air. This is the reason why the pressure is relatively higher and it's easier than warm air. The cold air during winter tends to stay low at ground level. This is the exact reason why winter months gave higher atmospheric pressure than summers. That's it, this is the answer to "at which location would you expect air to have the highest density on a typical winter day?".

Factors responsible for the highest air density on a typical day

high and low pressure
Image credit: Meteoblue.com

Factors affecting air density are: 

  1. As the air pressure increases the air becomes denser. More mass in a given volume is the result of pressure forcing the air molecules together. Decreasing altitudes result in increased air pressure.
  2. The density of air is affected even by temperature or it is a determining factor of air pressure. As the temperature is increased, the air molecules travel rapidly and expand further apart on collision among themselves. When the air is dense, it creates drag and moves objects through it. As in, on a hot summer day, a ball hit will go further than on a winter day. Lower temperature and higher air pressure found at higher altitudes combine to higher air density.
  3. Humidity is also a determining factor in air density. The amount of moisture in the air is called humidity. Less moisture in the air, higher will be the density of air. 

How does humidity affect air density?

Air is a mixture of 78% Nitrogen, 21% oxygen, and 1% other gasses. The water vapor amount varies in the air composition but it can even contribute to 1% in the air mixture. Nitrogen in the atmosphere usually exists as an N2 molecule typically means two Nitrogen are bound together which measures up to 14 units for one molecule together and 7 units for single Nitrogen. Again Oxygen present in the atmosphere usually exists as an O2 molecule, which typically means two Oxygen molecules are bound together which measures up to 16 units as one Oxygen molecule weight is 8 units.

Two Hydrogen atoms weight up to 2 units and one Oxygen atom weigh 8 units. These two Hydrogen and one Oxygen atom make H2O which weighs up to 10 units. This is clear from the above that water vapor is the lightest molecule among the three. As water vapor weights less than Nitrogen and Oxygen molecules, it makes up the rest of the air in the atmosphere. So when you have less water vapor the air, the air has more mass which means it is denser and if you have more water vapor in the air, the air has less mass which means it is less dense.

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