A large winter storm formed over the Texas Panhandle and moved northeast through Kansas and crossed eastern Iowa on its way to the upper Great Lakes. The low center exited Iowa near Dubuque after midnight on February 24, 2019. The pressure decreased as the storm approached and began rising after the storm center moved away.
A barograph traces changes in air pressure. The graph above shows the pressure beginning to fall at Noon on Friday, February 22nd. It reached its low point at Cedar Falls, Iowa around 11 p.m. CST on the 23rd. As the pressure began to fall cirrus clouds began increasing from the southwest. The cloud bases lowered throughout the day and night as cloud types changed from cirrus to altostratus. At the same time, temperatures warmed into the low to mid 30s.
As the pressured reached its minimum, the low center was passing southeast of Cedar Falls. With its passage colder air began drifting in from the north and snow began to fall. Winds also increased from the northwest. During the night winds increased and colder air lowered temperatures into the teens. Blowing and falling snow made travel hazardous with travel not advised and some roads in central and north central Iowa became impassible.
Air pressure is determined by many factors. The factors include the total mass of air above our heads, temperature, the amount of water vapor in the air, and whether air is rising or sinking. Those are topics for another occasion. In the meantime, a barograph, like the one in the photo above, is a useful tool for understanding how pressure changes with time how those changes are related to changes in our weather. Barometers were initially used to forecast the approach of storms. Falling pressure meant that a storm was approaching. The rate of fall and how far it fell was related to the intensity and speed of the storm. We have more reliable ways of forecasting the weather today but barometric pressure is still used to monitor storms. The barograph shown above is very useful for anticipating changes in the weather.