The intensity of a tropical cyclone is often measured by wind speed, but meteorologists rely on barometric pressure. Barometric pressure, or atmospheric pressure, is the weight of the column of air pressing down on the Earth’s surface. For Hurricane Katrina, this measurement is important because the storm’s pressure varied significantly, marking its peak intensity over the Gulf of Mexico and its diminished strength at landfall. The standard atmospheric pressure at sea level is approximately 1013 millibars (mb); a dramatic plunge from this baseline denotes a powerful hurricane.
Defining the Measurement: Pressure and Intensity
Lower barometric pressure directly correlates with higher wind speeds and a more intense hurricane. This relationship is governed by the pressure gradient force, which drives air from high-pressure areas toward low-pressure areas. A hurricane is a massive, low-pressure engine; the greater the difference between the low pressure in the storm’s center and the higher pressure outside, the stronger the force pushing air inward. This rush of air creates the ferocious winds that define a major tropical cyclone.
The Saffir-Simpson Hurricane Wind Scale (SSHS) classifies storms based on sustained wind speed, but barometric pressure provides an internal measure of a storm’s structural depth. For example, a Category 4 hurricane typically corresponds to a central pressure range of 920 to 944 millibars. The continuous drop in central pressure is the most reliable sign of rapid intensification, indicating a deepening core and strengthening winds.
The Record Lowest Pressure Aboard Katrina
Hurricane Katrina reached its maximum intensity over the central Gulf of Mexico, achieving an extremely low central barometric pressure. This peak was recorded on Sunday, August 28, 2005, when the storm became a Category 5 hurricane. At approximately 1:00 PM Central Daylight Time, a minimum central pressure of 902 millibars (26.64 inches of mercury) was measured by a reconnaissance aircraft. This data was captured by the National Oceanic and Atmospheric Administration (NOAA) Hurricane Hunters.
The 902 mb measurement marked Katrina’s most powerful state, confirming it as one of the most intense Atlantic hurricanes recorded at the time. This record-low pressure resulted from the storm passing over the exceptionally warm waters of the Loop Current. This warm water provided the massive energy needed to fuel the storm and dramatically lower the pressure in its core.
Pressure Readings at Landfall
Although Katrina reached its peak intensity as a Category 5 hurricane over open water, its central pressure had risen significantly by the time it reached the coast. The storm made its first landfall on the morning of Monday, August 29, 2005, near Buras-Triumph, Louisiana. At this time, the barometric pressure in the eye was measured at 920 millibars. This pressure increase from 902 mb to 920 mb reflected a weakening of the storm, which was classified as a strong Category 3 hurricane.
This weakening was due to an eyewall replacement cycle, a natural process where a new outer eyewall forms and causes the inner eyewall to dissipate. The storm also began to feel the effects of land interaction and slightly cooler shelf waters as it approached the coast, which contributed to the pressure rise. Later that morning, Katrina made a second landfall near the Louisiana-Mississippi border, where its central pressure was recorded as 928 millibars. Despite the rise in pressure, the storm remained a Category 3 hurricane, and its immense size meant that hurricane-force winds and a devastating storm surge extended far from the center.
How Katrina Compares to Other Major Hurricanes
Katrina’s peak central pressure of 902 millibars places it among the most intense Atlantic hurricanes recorded. While this number is exceptionally low, it was not the lowest in the Atlantic basin’s history. The 1935 Labor Day Hurricane, which struck the Florida Keys, holds the record for the lowest pressure at U.S. landfall, measuring 892 millibars.
In the same 2005 Atlantic hurricane season, two other storms surpassed Katrina’s peak intensity based on this metric. Hurricane Rita reached a minimum central pressure of 897 millibars shortly after Katrina, and Hurricane Wilma later that season achieved a record-setting 882 millibars. The comparison with other cyclones demonstrates that pressure measures instantaneous intensity, not necessarily overall impact, which is influenced by a storm’s size and path.