Sound on the Water Affected by Temperature, Molecular Density

by Jordanna Sheermohamed of Weather Forecast Solutions

Before the invention of GPS, radio or radar, maritime navigation depended on different means of communication to relay messages between vessels. To compensate during times of limited visibility, sound was and is still often used to relay a boat’s position or intention.

While most boats are now fitted with modern communication techniques, general regulation still require that boats carry equipment capable of producing sound, such as whistles, horns or bells. Although these requirements can slightly vary dependent on the boat’s length or location, one to two sound-producing devices should be audible at a minimum of half a mile, and sometimes up to a mile. 

Some conditions make it easier for sound to travel faster and farther than others – consider the deafening silence during a snowy night; the distant, deep horn blast emanating from a vessel in port; or the infamous “In space, no one can hear you scream.” Technically (and scientifically), they weren’t lying. 

Sound is a form of energy that is released during the vibration of matter in any of its phases: solids, liquids or gasses. The distance and speed at which sound travels can be affected by the density of molecules in the phase state physical properties, such as temperature.

Density, defined as the amount of molecules in a given space, provides a pathway for the energy to move. Sound travels farthest in solids, then liquids, then gasses, because of the proximity of neighboring particles. Closer particles allow the energy to propagate farther down the path. Inversely, less dense matter houses more gaps between molecules, thereby absorbing that energy and essentially attenuating the sound. The timeless hearing hack of holding a glass up to a wall or door actually capitalizes on providing a denser medium (glass) for that sound to transmit through, compared with the less dense medium of air.

When it comes to air, temperature and moisture content can affect the speed of sound. Because temperature is a measurement of molecular motion, higher temps indicate more molecular motion, which means slightly faster-traveling sound waves. Humidity can enhance or decrease the speed due to its additional influence on air density; moist air is lighter than dry air because water vapor is lighter than oxygen and nitrogen atoms. This is why sound emitting devices can be especially important on a foggy night, since the excess of moisture in the air can weaken the quality and distance of sound. 

While colder air has less molecular motion, the higher density of molecules helps the sound to travel a farther distance. The coldest Canadian day on record, a bitter -81 F (-63 C) during the winter of 1947, allowed locals to hear dogs barking over a distance of 3.5 miles (6 kilometers).

The distance at which sound travels over open water is subjected to the temperature differences between the cooler maritime layer and the warmer air above it. This difference bends the sound waves, allowing the sound to travel a farther distance.

So next time you hear the horn or bell of a boat on the water, consider not only the safety aspects in place, but all of the specific scientific processes that make that sweet audible sound possible.