January 9th’s Big Storm

Welcome back to another exciting read!

About a month ago, all afterschool activities were canceled – even the big Jordan High School vs Durham Academy basketball games! Public schools, such as Riverside, closed completely for the day. But why exactly were activities canceled and schools closed? There was a big, big storm. By the time the climax of the storm was over, many neighborhoods were left without power and multiple flash flood warnings and wind advisories had been issued to central North Carolina.

During this storm, I put out the weather vane for ten minutes every half-an-hour and measured barometric pressure (in Hg), relative humidity, and temperature (ºC). Then, the morning after the storm blew through, I took these measurements once again. During the storm, barometric pressure continued to decrease, relative humidity fluctuated but stayed really high overall, and temperature increased steadily. The morning after, though, the barometric pressure had increased, the relative humidity had decreased significantly, and temperature had also taken a serious dip.

This probably just seems like a lot of random stuff thrown together so I am going to try my best to explain what everything really means. Firstly, barometric pressure, something I am still working to understand, is the same thing as atmospheric pressure or air pressure. It is the total mass of air above a certain point and the density of the air molecules at that certain point (a higher density equals a higher pressure because when there are more particles colliding that means force increases which leads to an increase in pressure)¹. There is a higher density of air molecules closer to the Earth’s surface because gravity pulls them there.

As you can see in this graph, the lower the elevation, the more air particles there are above and the more particles there are to collide with.

This is why you have to calibrate a barometer by telling it what elevation it is at; so that it can take into account the pressure it is recording due to elevation and not the actual pressure of the air specifically. It is like a tare on a scale. 

The thing with pressure meteorologists are more interested in is whether the environment they are recording is experiencing high pressure or low pressure. There are low pressure and high pressure systems that move throughout the world. They tend to last for one to two weeks and, in the US specifically, they move from the West to the East (because they are pushed by the jet stream).³

I noticed a concrete correlation between the weather and the pressure of the system: it is rainy when the low pressure system blows through and it is sunny when the pressure is high. This is because when there is higher pressure, the air and water vapor is being pushed downwards which stops clouds from forming. On the other hand, in low pressure, there is less pressure so the air and water vapor can rise to a high enough point where it gets cold enough for the water to condense and form clouds. 

Secondly, relative humidity is how much water vapor the air can hold in certain conditions. It is a percentage that represents the amount of water vapor in the air vs how much the air can hold at a given time. But, as you may or may not have noticed before, sometimes the relative humidity can reach 100%! But, as I previously thought, this does not mean that water vapor starts condensing, it means that no more water vapor can fit so water just stays liquid. It is like a sponge, if a sponge is 100% saturated, that doesn’t mean it will leak a bunch of water, it just doesn’t take in any more.⁵

Another common misconception that I too believed is that relative humidity increases when it is raining. I mean, it seems obvious that when there is water slicing through the air, it will increase the amount of water vapor in the air! But, this is only true when the rain is warm. When it is warm, water vapor evaporates off of the droplet, adding water vapor to the air. BUT, when the droplet is cold, it actually sucks in the water vapor and the vapor condenses onto the drop. This ends up lowering the relative humidity.

Thirdly, you probably already know about temperature but what is cool to see here is that during the storm, the temperature was increasing but the next morning, the temperature had dropped significantly. This is because the storm that came through was caused by a cold front (more on this coming soon). A cold front happens when a cold air mass collides with a warm air mass, and as the cold air mass scrapes under the warm air mass (under because cold air is denser than warm air) causing the quick formation cumulonimbus clouds that cause shorter, but usually big, storms.

¹ Meteorology Textbook