Tag Archives: water

Want to keep cool on hot summer days? Here’s how

The Conversation

By Nigel Taylor, University of Wollongong

Are neck, hand or forearm cooling, ice-cube sucking or cold showers effective ways to lose heat on those dog days of summer? Can sports clothing keep you cool by wicking away sweat? When the heat is on, some of us are prepared to entertain even snake-oil solutions for the sake of personal comfort, but do such cooling strategies really work?

Let’s first consider heat loss from a physical perspective, putting aside physiological heat-loss mechanisms, such as sweating and skin blood flow. Cooling down is more easily understood when reduced to this level because the physical properties of heat exchange are well known.

Understanding heat loss

The first of these properties is the temperature gradient; the bigger the temperature difference between two things, the more rapidly heat (thermal energy) flows towards the cooler one.

Substances conduct heat at different rates (thermal conductivity). Water, for instance, is 24 times more conductive than air at the same temperature. Consider walking into the cold room of a bottle shop, which is usually a cool five degrees Celsius, versus swimming in water of the same temperature. The latter is excruciating, with death from hypothermia just around the corner.

We also need to keep in mind the heat retained within various substances, which is known as mass-specific heat. This tells us how heat is required to increase the heat of two objects that weigh the same by one degree Celsius. Water has a specific heat four times that of air, for instance, so a kilogram of water can remove four times as much heat as the equivalent mass of air.

Density is important too, because it determines the mass of a substance that can be contained within a fixed space. Since water is 800 times denser than air, a bath filled with water is many times heavier than one that contains air.

Together, specific heat and mass define the volume-specific capacity of substances to store heat. Going back to our example with water: it has a heat capacity more than 3,000 times that of air because of the combined effects of its mass-specific heat and density.

An object’s mass and surface area are important as well, because heat is stored in its mass and lost through its surface. Spheres have the largest mass for a given surface area, while wafers have the opposite characteristic. In other words, an object’s surface-area-to-mass ratio dictates its heat-exchange potential, with flatter and thinner surfaces (such as hands and feet) losing heat more rapidly.

Cooling solutions

So, to cool an object, maximise the temperature gradient, choose a coolant with a high thermal conductivity and heat capacity (liquids), and modify the shape of your object to resemble a wafer. Without question, water is ideal for cooling non-living objects.

But does it work as well for living bodies? And how is it influenced by the physiological responses that we all experience when exposed to heat?

So far, we have only considered heat conduction, or heat exchanged between objects in direct contact – touching a hot stove for example. But conduction speed is influenced by the distance heat must travel.

Animals enhance cooling by delivering heat closer to the skin surface. This convective mechanism, which involves delivering hotter central-body blood to the cooler skin, shortens the conductive pathway and promotes heat loss.

Natural selection has ensured that naked human skin is ideally suited for evaporative cooling.
Obi/Flickr, CC BY

But this mechanism relies on increasing and sustaining skin blood flow, which is dictated by the separate and combined effects of deep-body and local skin temperatures. Maximal skin blood flow occurs only when both the deep-body and local skin tissues are heated, but not if only one region is hot.

When a hot person is placed in very cold water (say of about five degrees Celsius), skin blood flow is dramatically reduced, so heat loss is compromised. Paradoxically, submerging that same person in temperate water (25 degrees Celsius) increases heat dissipation by preventing this blood-flow suppression.

Clothing and comfort

Natural selection has ensured that naked human skin is ideally suited for evaporative cooling, and anything placed on the skin interferes with that process.

The average person has some 110 sweat glands per square centimetre of skin (although this varies with location). When heated, these glands secrete sweat that wets the skin. The ensuing evaporation transfers heat to water molecules, which change from a liquid to a gas, leaving the sweating person cooler.

But, in still conditions, the characteristics of the air in direct contact with the skin change; it rapidly becomes warmer and more humid. This warmer air is less dense and spontaneously rises, taking with it heat (natural convection) and water vapour, and permitting the incoming air to be warmed and humidified.

When we move, or when air moves across the skin (forced convection), convective and evaporative cooling are magnified. Clothing reduces these benefits.

So these are the principles that dictate human heat loss. But we must now distinguish between thermal strain and comfort.

Strain is the physiological impact of heating the body, as quantified through deep-body and skin temperatures; comfort relates to the pleasure derived from different thermal states. We now need to consider whether we wish to feel more comfortable or to reduce thermal strain.

Since comfort follows reduced thermal strain, our energies should be directed accordingly. The first strategy should be to resist counter-evolutionary practices designed to minimise strain (heat avoidance, for instance, and air conditioning), and allow our bodies to adapt to seasonal variations.

So, use natural ventilation whenever possible, dress appropriately and experience the climate. With adaptation, you can improve both physiological heat loss and thermal comfort.

The second strategy is for desperate times, like those dog days of summer: water immersion. Showers help, but are very wasteful. Hand and forearm immersion are good, but time consuming. Neck cooling and ice-cube sucking suck!

Instead, bathe in enough temperate water to just cover yourself, and stay there until you feel cool-cold. Natural water sources are ideal. And as for sports clothing, there is no clothing that can improve the heat-loss capability of your skin; donate your money to a worthy charity!

The ConversationThis article was originally published on The Conversation. (Reblogged with permission). Read the original article.

Leave a comment

Filed under Reblogs

The Rosetta lander detects organic matter: the seeds of life?

The Conversation

By Jonti Horner, University of Southern Queensland

Scientists working with data sent back by the now-slumbering Philae lander have announced the discovery of organic molecules on the comet 67P/Churyumov-Gerasimenko.

Finding organic compounds on 67P’s surface is not actually particularly surprising. Organic compounds have been detected in material shed by comets before, and have been observed throughout interstellar space.

But we have never before been able to measure them in-situ, and this is where Philae offers something new and exciting.

While the results are preliminary, with researchers still working on a more detailed analysis, they are a tantalising reminder of the role comets played in the origin of life on Earth.

A rough but rewarding ride

Last week, the world watched in awe as the Philae lander departed from the Rosetta spacecraft then hopped, skipped and jumped across the surface of the comet.

European Space Agency’s mission of landing on the surface of a comet – a dirty snowball left over from the solar system’s birth – is surely one of the greatest technological achievements in the history of mankind.

Unfortunately, Philae’s landing wasn’t quite as smooth as was hoped, and the lander bounced to a stop leaning against a rock, in a shadowy region of the comet’s surface.

The result – Philae currently receives too little sunlight to stay awake, and after a couple of days of frantic activity on the surface, has now gone into hibernation.

Hopefully, as the comet swings towards perihelion (its closest approach to the sun) next August, the amount of light Philae receives will increase and the lander will awake from its slumber – but we can’t know for sure.

For now, Philae’s work is done, and the baton has been passed to the teams who are now furiously studying the hard-earned data sent back to Earth before Philae fell asleep.

That data was squirted back to the Earth shortly before the lander entered sleep mode – and it is likely that exciting results will continue to appear over the next weeks and months.

The first such results have already been made public, with the scientists confirming the detection of organic molecules on the comet’s surface. Not much is currently known – the scientists are still trying to fully disentangle the story of what has been observed – but the result is a tantalising glimpse of what is to come.

Comets and the origin of life

The reason that these results are particularly exciting goes back to two of the great unanswered scientific questions:

  1. what was the origin of life?
  2. how common is it throughout the universe?

Current theories of planet formation suggest that the Earth should have formed dry – this close to the sun in the proto-planetary nebula that birthed our planet, temperatures would have been too high for water to freeze out.

As a result, Earth required hydration, and it is thought that comets such as 67P would have been one of the main sources of the Earth’s water, delivering it in countless comet collisions during the final stages of planet formation.

Beyond the question of the origin of water, though, the origin of complex chemistry, the precursor to life, has long puzzled scientists. Where did the chemical building blocks that make up life as we know it come from?

Were those compounds “cooked” in the early oceans, or in the vast tidal zones that fringed the continents following the formation of the moon?

Or did they come from beyond the Earth, delivered in the collisions that dominated the process of planet formation?

Organics from space and home

As time goes by, it is seeming ever more likely that the origin of complex organic compounds on Earth is two-fold. Some was almost certainly cooked on our planet’s surface, with the rest delivered by comets and asteroids, smashing into our planet.

It is in this context that the Philae observations are so exciting – further evidence that organic compounds are common in the universe.

Could comets hold the seeds of life?

The result is an important confirmation that such compounds must be abundant. To have detected them after just a “sniff” of the comet suggests that they’re everywhere on its surface.

And given that we know comets have crashed into the Earth in vast numbers throughout our planet’s history, we must have been repeatedly doused in the kind of compounds that are the direct precursors to life itself.

Panspermia?

Interestingly, the idea that life could have been delivered to Earth by comets has another, more speculative side – a theory known as “panspermia”. What if life didn’t start on Earth at all, but rather began elsewhere, and was delivered to our planet by rocks (or snowballs) from space?

The idea isn’t actually as far-fetched as it sounds. Experiments have shown that bacteria can survive the kind of forces that would be experienced in the collision of a comet or asteroid on a planet such as Earth.

And we know that impacts can eject solid, complete rocks from the surfaces of planets intact – we have meteorites on Earth that were definitely ejected from Mars. Still other experiments show that bacteria can survive, dormant, in the vacuum of space.

Following all of these results, it is quite possible, and perhaps even likely, that life in our solar system has been scattered back and forth between the planets over the billions of years since the planets formed. So if we do find life on Mars, then perhaps it will share a common origin with life on Earth, thanks to the countless collisions that have wracked both planets since they formed.

Some scientists go further, though, noting that life could be carried in comets from one planetary system to another. We know that they carry a rich organic budget – as demonstrated by Philae’s latest exciting result – but what if they carry more than just the precursors to life? Perhaps comets are actually an inter-stellar delivery mechanism, by which youthful planets are seeded with life as they form.

The more extreme versions of panspermia remain both speculative and controversial. Despite this, it is becoming more apparent that comets are, at the very least, a prime source of the precursors to life. They delivered the water on which life thrives, as well as the compounds upon which it is built.

Without comets, it seems, we may well not be here.

The ConversationJonti Horner does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.

This article was originally published on The Conversation. (Republished with permission). Read the original article.

Leave a comment

Filed under Reblogs