Category: convection

An astronaut captured this towering cloud over…

An astronaut captured this towering cloud over Andros Island from orbit aboard the ISS. This is a cumulus castellanus cloud, named for the castle-like crenelations at its top. Castellanus clouds form in areas with strong vertical updrafts, often due to cloud-level atmospheric instabilities rather than heating at the Earth’s surface. These clouds frequently proceed rain or even thunderstorms. What distinguishes castellanus from other types of cumulonimbus clouds is their shape: castellanus clouds have protrusions that are taller than they are wide – like the castles for which they are named. (Image credit: NASA / Expedition 48; via NASA Earth Observatory)

Latte lovers may be familiar with the layered …

Latte lovers may be familiar with the layered latte, a beverage with distinctive horizontal layers mixing espresso and milk, but you may not have taken the time to wonder how these layers form. Like many layering phenomena in our oceans, the layered latte is the result of double-diffusive convection. This means that there are two variables that both affect density in the fluid mixture and that they act at different rates.

In the latte, those factors are 1) the different densities of the milk and espresso and 2) density changes caused as the latte cools to room temperature. A layered latte forms when the lighter espresso is poured into denser milk. If it’s poured quickly enough, the momentum of the pour forces some of the espresso down into the milk, despite the buoyant force that tries to keep the espresso on top. So that initial pour sets up a density gradient that runs from pure espresso at the top to pure milk on the bottom, with varying mixtures of the two in between.

The distinct layers won’t form until the latte begins cooling off. Along the walls of the container, heat is lost more quickly, causing fluid to cool and start sinking. But a specific bit of fluid can only sink until the fluid surrounding it is the same density. That can carry a cooler bit of latte to the bottom of a layer, but not into the denser layer below. At this point, our bit of latte moves inward, starts to warm up, and circulates up through the center of its layer. As when it sank, the fluid can only move up until it encounters a layer with equal or lesser density, at which point it must move horizontally instead. This thermal convection, combined with the density gradient formed by the initial pour, sets up the distinctive layers of the latte. The layers are quite stable – neither gentle stirring nor taking a sip will disrupt them for long – provided the drink remains warmer than the surrounding air. (Image credits: kopeattugu/Instagram, N. Xue et al.; research credit: N. Xue et al.; via NYTimes; submitted by Kam-Yung Soh)

When a droplet falls on a pool, we expect it t…

When a droplet falls on a pool, we expect it to coalesce. There are exceptions, like bouncing droplets, but in general a droplet only sticks around for a split second before being engulfed. And yet, from morning coffee (top image) to walks in the woods, we frequently see millimeter-sized droplets sticking around for far longer than it seems like they should. New research offers a clue as to why: it’s thanks to a temperature difference. 

When there’s an appreciable temperature difference between the drop and the pool, it causes rotating convective vortices (bottom image) in both the drop and the pool. When the temperature difference is large, the vortices are strong enough that their motion recirculates air inside the tiny gap between the drop and the pool. This supports the weight of the drop and keeps the two liquids separate. But the convection also redistributes heat, and eventually the drop and pool become similar enough in temperature that the circulation dies out, the air gap drains, and the two coalesce. (Image and research credit: M. Geri et al.; via MIT News; submitted by Antony B.)

Looking at convective cells, it’s easy to think that they are…

Looking at convective cells, it’s easy to think that they are still and unmoving. But when you add particles, their inner flow becomes obvious. Warm, light fluid moves up through the center of each cell, skims along the surface, and then sinks at the edges of the cell after losing its heat at the cooling surface. Below, the fluid moves back toward the cell center, getting warmer as it’s heated by the lower surface. Once it reaches the middle of the cell, it’s light enough to rise up and start the process again. Convective cells like these are typical in cooking – watch for them forming in your miso soup or hot chocolate – but they can also be found on the sun and even in situations without heating! (Image credit: G. Kelemen, source)

Looking at convective cells, it’s easy to think that they are…

Looking at convective cells, it’s easy to think that they are still and unmoving. But when you add particles, their inner flow becomes obvious. Warm, light fluid moves up through the center of each cell, skims along the surface, and then sinks at the edges of the cell after losing its heat at the cooling surface. Below, the fluid moves back toward the cell center, getting warmer as it’s heated by the lower surface. Once it reaches the middle of the cell, it’s light enough to rise up and start the process again. Convective cells like these are typical in cooking – watch for them forming in your miso soup or hot chocolate – but they can also be found on the sun and even in situations without heating! (Image credit: G. Kelemen, source)

Blue paint in alcohol forms an array of polygonal convection…

Blue paint in alcohol forms an array of polygonal convection cells. We’re accustomed to associating convection with temperature differences; patterns like the one above are seen in hot cooking oil, cocoa, and even on Pluto. In all of those cases, temperature differences are a defining feature, but they are not the fundamental driver of the fluid behavior. The most important factors – both in those cases and the present one – are density and surface tension variations. Changing temperature affects both of these factors, which is why its so often seen in Benard-Marangoni convection

For the paint-in-alcohol, density and surface tension differences are inherent to the two fluids. Because alcohol is volatile and evaporates quickly, its concentration is constantly changing, which in turn changes the local surface tension. Areas of higher surface tension pull on those of lower surface tension; this draws fluid from the center of each cell toward the perimeter. At the same time, alcohol evaporating at the surface changes the density of the fluid. As it loses alcohol and becomes denser, it sinks at the edges of the cell. Below the surface, it will absorb more alcohol, become lighter, and eventually rise at the cell center, continuing the convective process. (Image credit: Beauty of Science, source)

Without our magnetic field, life as we know it could not exist…

Without our magnetic field, life as we know it could not exist on Earth. Instead, our atmosphere would be stripped away and the surface would be bombarded by charged particles in the solar wind. Relatively little is known about the dynamo process that governs our magnetic field, though it’s thought to be the result of liquid iron moving in the Earth’s outer core. The video above shows a slice of a recent 3D simulation of this liquid iron segment of our core. The colors show variations in the temperature, revealing vigorous convection in the core. This motion, combined with the spinning of the Earth, is the likely source of our magnetic field. Researchers hope that simulations like these can help us understand features we observe in our magnetic field – like local variations in field strength and the pole reversals in our geological record. (Video credit: N. Schaeffer et al.; CNRS via Gizmodo)