Lesson 6

Tides are the regular rise and fall of sea level caused by the gravitational pull of the moon and the sun, combined with Earth's rotation. Gravity is the key player. As the moon pulls on Earth's oceans, water closest to the moon feels a stronger pool, so it bulges outward forming high tide, but high tides also occur at the opposite side of the Earth. Why? Well, the moon pulls more strongly on the near side of the Earth. There is less pull on the far side, so water there is "left behind" as Earth moves forming another high tide. The area between bulges experiences low tide. As the Earth spins, coastlines move into and out of these bulges where most places get two high tides and two low tides per day known as a semidiurnal tide. However, some locations can have diurnal tides where there is only one high tide and one low tide. Although when the sun gets involved, it can become concerning as there could be possible coastal flooding as a result. 

The sun is very huge compared to the moon, but much further away. Thus, it's tidal pool is about half as strong as the moon's. When there is a new moon or full moon, their gravity adds together between the sun and the moon, which produces extra high-tides and extra low-tides known as spring tides. When the sun and the moon are at right angles during the first or third quarter moon, their gravity partially cancels, which weakens tidal range and produces neap tides. Essentially, think of the Earth as wearing a loose sweater with the moon tugging on the sweater, which stretches the fabric outward on both sides. If the sun joins in on the party, then that sweater stretches even more if aligned, but if at a right angle with one another, then there's an occasional tug, but they don't seem to bother as much. An important concept to coastal communities as high tides add more to the already dangerous storm surge that hurricanes may bring when making landfall. 

It is well known that the freezing point of water is 32F, but it is important to note that is only for pure, fresh water. Salt water actually freezes at a lower temperature. This is because as salt dissolves in water, made up of sodium and chloride (NaCl), the sodium and chloride ions split. These ions hold an electrical charge from gaining or losing electrons and disrupt water molecules from bonding and if water molecules are unable to bond, then the crystalline structure of ice is unable to form. Therefore, as salinity, concentration of dissolved salts, increases, then the freezing point of water decreases and vice versa. Very similar to when adding salt or a dissolved substance into a cloud droplet from the solute effect, it decreases the relative humidity needed to reach saturation for precipitation to form. This is why typically in the winter; salt is used on roadways to lower the freezing point at which snow sticks to help create better travel. The downside to using salt on roadways is the damage to the environment and corrosion on your car due to an electrolyte solution. It is highly recommended to frequently give your car a car wash in the winter to minimize any corrosion and damage. 

Recall that density is measured as mass divided by volume (g/cubic meter). It is fundamental that the colder the water is, the more dense it becomes, and while that is true, it is true to a certain extent. It turns out that water is actually most dense at 38F. As the temperature cools below 38F, the water starts to become lighter. Why? This may seem backwards, but at 38F, water starts arranging its molecules in an open, hexagonal structure as they approach freezing. The open, hexagonal structure takes up more space (volume), so the density decreases even though the temperature is dropping. This is very typical in the winter where the bottom of a lake is warmer and denser at 38F while the surface waters may be 32F but lighter. This concept reverses in the summer where the coldest, denser waters are at the bottom and the warmest, lighter waters are at the surface. In the summer, daytime heating from the sun will warm the surface of the ocean more efficiently than the deep ocean. Therefore, lakes freeze top-down and not bottom-up or else many fish would die in the winter. Additionally, this is why ice typically floats on water as it is less dense. 

Now, recall that the atmosphere's layers are divided based off of temperature. Well, the ocean also has layers based off of temperature. In a lake or ocean, the warmer waters are typically layered at the surface followed by colder waters in the deep ocean. The rapid temperature decreases between the surface layer, and the deep ocean is called The Thermocline. It acts like a "boundary" separating warm, surface water from cold, deeper water. The surface layer is typically turbulent and forms waves due to the surface winds while the deep ocean is relatively stable. It forms because warm water is less dense and stays at the surface while cold water is deser and sinks. Since the sun only heats the upper ocean and mixing doesn't reach very deep, a sharp temperature gradient naturally forms between the two layers. If the thermocline is shallow, then it is easier for wind-driven surface currents to bring cold, nutrient efficient waters to the surface known as upwelling. However, if the thermocline is deeper, then it suppresses upwelling placing colder, nutrient waters further from the surface. Therefore, the thermocline introduces a temperature gradient, which controls how heat is stored and distributed globally and how surface currents interact with deeper currents.

On a much smaller scale, the ocean influences the atmosphere from the dispersion of salt aerosols. Aerosols are tiny solid or liquid particles suspended in the atmosphere, ranging from a few nanometers to tens of micrometers, and they play major roles in visibility, air quality, cloud formation, and climate. Let's say you're at the beach and as always, the waves are breaking upon the shoreline. It makes the air smell like salt. These waves that break disperse salt aerosols into the air, which can act as a particle for cloud droplets to form. Since salt reduces the freezing point, sea ice forms at 28F, which rejects the salt called brine rejection. This increases the salinity of nearby water. Sea ice has gaps or holes when it forms, so when wind blows through them, it forms what we like to call sea spray that disperses more salt particles in the air. Sea ice is purely fresh water, which forms ice crystals first and thus rejecting the salt and flushing away. 

Sea ice is important due to climate regulation much like the thermohaline circulation. It has a high albedo, meaning it reflects a lot of the incoming solar energy. When sea ice melts, darker ocean water absorbs more heat from the sunlight, which amplifies warming. When the sea ice disperses salt to the surrounding water, the dense water sinks, aiding the drive of the thermohaline circulation. It is especially important because if sea ice disappeared, Earth's climate would warm dramatically, ocean circulation would weaken, sea levels would rise faster, and global weather patterns would become more extreme. 

One of the most powerful climate regulators, which also aids in the thermohaline global circulation, is latent heat. Latent heat is the "hidden energy" stored or released when water changes phase. It quietly moves vast amounts of energy between the ocean and atmosphere, shaping global temperature patterns, storm intensity and long-term climate stability. In other words, latent heat is the energy absorbed or released when water changes phase without changing temperature. For example, when the sun heats surface of the ocean, it warms and eventually evaporation (from liquid to vapor) occurs. In order for evaporation to occur, energy is absorbed from the environment. In this case, the sun's energy. As the parcel rises, it cools and condenses in the atmosphere and once reaching saturation at 100% relative humidity, clouds form through condensation (from water vapor to liquid). In order for water vapor to change to a liquid, the internal system needs to cool down, so in return, it releases that stored energy into the atmosphere. This is why the moist adiabatic lapse rate cools at a rate slower than the dry adiabatic lapse rate is because of heat that is released through condensation. This can be very explosive in stronger thunderstorms. Let's say it is winter, and after the cloud droplets grow through collision and coalescence and falls as precipitation, raindrops freeze through a deep surface layer after a shallow warm layer forming sleet. Thus, heat is released through freezing as energy needs to be released transitioning from a liquid to a solid. Melting is the opposite, in which energy is absorbed from the surrounding atmosphere in order for the ice to melt. Sometimes, a solid would go directly from a solid to a gas known as sublimation, like dry ice, so heat from the surrounding environment is absorbed in order to change phase. The opposite from a gas to a solid occurs through deposition where a gas goes straight to a solid, like frost on a bitterly cold, clear morning. 

Essentially, latent heat moves heat around the planet. When water evaporates from warm oceans, it stores energy as latent heat. Winds transport that vapor thousands of miles. When it condenses in clouds, the energy is released into the atmosphere. This process redistributes heat from the tropics toward the poles, drives major circulation patterns, and moderates temperature extremes. Condensation releases huge amounts of heat into the atmosphere, fueling thunderstorms, monsoons, and tropical cyclones. The released energy warms the surrounding air, causing it to rise faster and intensify storms. Since water can absorb so much energy during evaporation without warming, latent heat acts like a planetary thermostat. These effects include cooler ocean surfaces during strong evaporation, milder coastal climates, and reduced day-night and seasonal temperature swings. This buffering is a key reason Earth's climate is more stable than that of planets with little water like Mars. Furthermore, land surfaces also exchange latent heat through evaporation. When soils dry out, less latent heat flux occurs, and more energy goes into sensible heat, essentially heat energy that changes the temperature of an object, causing temperatures to spike. Climate models show that soil moisture and latent heat coupling is crucial for predicting heat waves and drought behavior. Therefore, the Earth as a whole is all one feedback loop.