A note on the title before we get started. My Granddad’s favorite exclamation was “Well, hot dog!” so just roll (your eyes if need be) with my little nod to gramps. Now on to some hot salt!
Heat (temperature) and salt (salinity) are primary causes of density differences between bodies of water. Combinations of hot or cold, salty or fresh water create “pockets” of water that can be thought of as heavier (denser) or lighter (less dense) than the surrounding water. These water pockets will move until they reach an area with surrounding water of the same density, driving major ocean currents (like the Gulf Stream) through thermohaline circulation. Where the prefix thermo- means heat and haline – means salt.
Let’s take a step back and define what exactly density is. Scientifically speaking, the density of a substance is its mass per unit volume. Simply put, density is a measure of how much of something takes up a certain amount of space. You can put more grains of sand in one jar than gravel right? So the sand would be denser than the gravel. Common density units are grams per milliliter (g/mL), kilograms per cubic meter (kg/m3), and you can always convert units (ie: kg/L vs. g/mL).
Okay… so how does temperature affect density, or how much space a substance takes up?
Temperature is the measure of how much energy an object’s molecules hold. Molecules with more energy have a higher temperature. Lower temperatures mean the molecules have less energy. Now, imagine a group of kids on a nice spring day. They’ve just come outside for recess and the sun starts warming them up. Do they all sit nicely in one area? No! They scatter! Running around all over the place and every whichaway. Molecules do the same thing. So a hot object has molecules that are essentially bouncing off the walls because they have so much energy. This doesn’t make it easy to keep those molecules in the same place, or volume. So there are just a few molecules of a substance in a particular area = low density.
High temperature = more energy = less dense
Compare that warm, sunny day with sitting around a campfire on a chilly fall evening. You grab a blanket, skootch closer to the people next to you and closer to the fire. Pretty soon we’re all squished in a tight ring around the fire and don’t really feel much like moving away. If molecules don’t have much energy (heat), it’s pretty easy to pack them into the same area = high density.
Low temperature = less energy = more dense
Of course water is special and always has to be the exception. Cold water is more dense than warm water, but as soon as the water gets cold enough to freeze, it floats. Ice floats because it is less dense than liquid water due to changes in the structure of liquid vs. solid water. As ice forms, water molecules spread out and create rigid bonds. You can only fit so many people around that campfire if we hold hands with our arms out like we’re about to do connected jumping jacks (because that sounds like a great idea). So the density scale of water goes something like this….
The most dense, cold water sinks and warm water rises. Meaning that ocean surface layers are typically warmer than the layers of water below. Of course there are areas of exaggerated warming or cooling, which causes water to rise or to sink accordingly and gets our currents moving!
Now that we’ve taken care of the hot stuff, how does salinity affect density?
Salinity is the amount of salt(s) dissolved in a body of water. If you have ever accidentally gotten a mouthful of seawater you know it’s salty. The average “saltiness” – salinity – of the ocean is 35 ppt (parts per thousand). So if you have 1,000 grams/ounces/buckets/whatever of ocean water, 35 of the 1,000 grams will be salt. Keep in mind this is an average and there are areas that are more or less salty. In fact, the salinity of the Red Sea is 41 parts per thousand (not to be confused with the Dead Sea [actually a lake], which is 9 times more salty than the ocean – between 300 and 400 ppt).
Changes in ocean salinity are caused by many factors. Precipitation (rain, snow, etc.), rivers entering the ocean, melting glaciers and ice caps (BIG issue here we’ll get into later) all decrease salinity. These processes add fresh water that dilutes the surrounding ocean water. Evaporation and seawater freezing both increase salinity. These processes essentially take fresh water out of the ocean and leave the salt, so what’s left behind is more concentrated.
Since density is the measure of how much “stuff” is in a unit, more salt (“stuff”) dissolved in one unit of water would make that unit more dense than the same amount of fresh water. Fresh water is basically salt water without the salt and is less dense.
Temperature and salinity interact to create unique density differences throughout the oceans. Warm salty water is less dense than cold water of the same salinity. Fresh water is less dense than saltwater at the same temperature. Warm fresh water is much less dense than cold salt water. And as far as warm salt water vs. cold fresh(er) water goes, you may have to do some calculations to figure out which is more dense.
I mentioned earlier that there were areas around the globe where water is warmed or cooled and gets the circulation going (want more details? click here). There are also areas with higher levels of evaporation (increasing salinity) or precipitation (decreasing salinity) that also drives thermohaline circulation. One of the major concerns with climate change is that these areas will be disrupted, potentially altering or even stopping our major ocean currents. And with that teaser I will bid you all adieu and encourage you to stay tuned!
Side note: I didn’t realize there was a real product called “hot salt” but there is!