Humidity lowers air pressure by displacing nitrogen and oxygen molecules with lighter water molecules. Differences in humidity can cause storms to form. Humid air also presents challenges to pilots.
Water molecules contain one heavy oxygen atom and two light hydrogen atoms, so it weighs less than atmospheric oxygen, which contains two heavy oxygen atoms, and atmospheric nitrogen, which contains two heavy nitrogen atoms. These water molecules force heavier molecules to spread out, which reduces the air pressure.
Because they are so light, water molecules tend to rise in the atmosphere, which is why clouds typically form high in the sky. These differences in air pressure cause air to move between different layers, which can eventually cause storms. Most storms form over the ocean where the hot sun heats the water and causes these types of disturbances.
Since cold, dry air is denser than warm, humid air, airplane wings have more molecules to displace, which allows them to generate more lift. As a result, pilots need to exercise caution when traveling through areas of high humidity and high heat. Air pressure is even lower at high altitudes, so part of Mexico and other locations with these conditions requires extra caution when pilots take off and land.