Tropical cyclones form due to the warm, moist air over the ocean near the equator. The warm, moist air rises and creates a low pressure area near the surface. The pressure gradient force causes the surrounding air to move towards the lower pressure area. That air then warms and rises as well. As the warm air rises, it cools and the water vapor in the air forms clouds. This continues to happen until a whole system of clouds spins and grows, and an eye forms in the center, marking a calm, clear, area of low pressure.
Figure 1 below shows what a cyclone would look like if you could slice into it. The red arrows symbolize the warm, moist air rising from the ocean’s surface while the blue arrows symbolize the cooler air being pushed towards the surface. The spinning causes the clouds to form the bands that are shown around the eye.
(Figure 1)On Thursday, September 17th, a low pressure system in the Gulf of Mexico evolved into a well-defined circulation that was called Tropical Cyclone Beta. Tropical cyclones have four different labels that are based on wind speed. From lowest to highest wind speed, they are tropical depression, tropical storm, hurricane, and major hurricane. At this point, Beta was labeled as a tropical depression. Figure 2 below is a visible satellite image taken on the 17th, and you can see the system of clouds that had formed into the circulation.
(Figure 2)On Friday, September 18th, Tropical Depression Beta was upgraded to Tropical Storm Beta. It was expected to grow and intensify, but dry air coming in from the west over Northern Mexico and Texas caused dry air entrainment that significantly weakened the storm. Figure 3 below shows the dry air in the atmosphere on that Saturday.
(Figure 3)
(Figure 4)Figure 4 above is a sounding from Corpus Christi, Texas on Saturday the 19th. The orange dashed lines represent the dry adiabatic lapse rate, and the red solid line is the environmental temperature. By noticing that the environmental temperature follows parallel to the dry adiabatic lapse rate near the bottom of the graph, we know that the atmosphere near the surface that day in Corpus Christi was very dry. We can see from Figure 3 that this was definitely the case.
As Beta slowly approached land, it caused lots of rain and storm surges across the coasts of Northern Mexico, Texas, and Louisiana. Figure 5 below shows a picture taken in Galveston, Texas on Monday, September 21st.
(Figure 5)On Tuesday, September 22nd, Beta made landfall on the coast of Texas between Corpus Christi and Galveston. After making landfall, Beta continued to slow and weaken until denigrating into a post tropical cyclone remnant low.
Sources:
https://spaceplace.nasa.gov/hurricanes/en/
https://www.nhc.noaa.gov/refresh/graphics_at2+shtml/205430.shtml?cone
https://www.nhc.noaa.gov/archive/2020/BETA.shtml?