There are a few ways to measure the strength of hurricanes. One is by air pressure: the lower the pressure, the stronger the storm. Early Tuesday morning, as it approached Jamaica, Melissa was measuring a minimum pressure of 901 millibars (mb)—lower than Hurricane Katrina’s peak low pressure of 902 mb and the lowest pressure ever recorded in a hurricane this late in the year, according to Colorado State University meteorologist Philip Klotzbach.