Читать книгу Polar Exploration - Dixie Dansercoer - Страница 10
Оглавление3 UNDERSTANDING THE COLD
Leads, areas of open water, can turn overnight into a fragile Arctic flowerbed
All about Ice and Snow
Ice is frozen water. (OK, so you already knew that.) But just as you gradually learn that there is more than one species of bird, you will discover that there are many types of ice. Ice is an extremely interesting subject (especially for the bored polar explorer who has nothing else to do but look for slight changes in the ice).
Ice can appear in the form of hail, frozen rain, snowflakes, icicles, glaciers, sea ice (7 per cent of the world's oceans) and polar ice-caps and approximately 30 million km2 of Earth are ice-covered.
Calculating ice density in the Antarctic
The colour of ice may vary, depending on:
age
density
possible impurities
light intensity
presence or absence of air.
Fresh water starts to freeze at 0°C (Celsius) or 32°F (Fahrenheit). This standard process may be impaired by atmospheric pressure and/or movements of the water. On a fresh-water lake, when temperatures plummet, the density of the water increases, but levels off at 4°C. As the cooling process continues, colder water at the surface becomes more dense. As soon as the temperatures reach 0°C, a physical transformation of the molecular structure of the fresh water produces ice.
This process, however, is different in salt-water bodies and oceans. Cooling of the ocean surface causes the top layers of water to increase in density and therefore sink, a process which continues until the water reaches freezing point – which itself is lowered by the addition of salt (about −1.8°C for typical sea water).
As soon as salt water solidifies, the main types of ice formed are the following:
Frazil First signs of freezing sea water, with an ‘oily’ look.
Nilas Merging frazil, at first transparent, but soon grey and eventually white.
First-year ice Nilas that has grown in one season up to 1.5–2m deep.
Ice flowers decorating newly born nilas
The impact of the wind over the ice and the currents under the ice makes the Arctic ice mass split open and creates ‘leads’ that represent endless hurdles for expeditioners
By the look of the ice, the following categories can be distinguished:
Pack ice Newer ice.
Ice floes Also known as pan ice.
Hummocks Big piles of ice, part of a pressure ridge.
Pressure ridge Line in the pack ice, where pressure within the ice has pushed the surface layer both upward (‘sail’ −2–3m high) and downward (‘keel’ – below the surface).
Fast ice Old and solid ice, attached to land.
Pancake ice Circular ice, randomly floating on agitated sea water.
Consolidated pancake ice Pancake ice ‘glued’ together in calmer water by frazil.
It is important to know that saline ice will take more than one year to lose the salty flavour that can make your soup inedible if used for cooking. New ocean ice has approximately 10 parts per thousand of salt, while multi-year ice only has 1–3 parts. Older ice is at least a metre thick, and you can see that overturned blocks of ice have ‘lived’. If in doubt, just taste it.
In order to understand the dynamics of sea ice it is important to know that pack ice is constantly in motion, driven by the wind, and that this produces many important changes to its appearance and development. The two most obvious features created are leads and pressure ridges. The impact of the wind results in ice sheets moving by frictional drag; extended masses of ice can move over long distances. It has been estimated that in concentrated pack ice a piece of sea ice can move over 400km upwind. Once in motion, the ice will continue to move under its own momentum long after the wind has died.
Multi-year ice
Ice which has survived one or more summer seasons of partial melt is called ‘multi-year ice’ (more than half the ice in the Arctic falls into this category). Growth continues from year to year until the ice thickness reaches a maximum of about 3m, at which point summer melt matches winter growth and the thickness of the ice oscillates through an annual cycle. This old, multi-year ice is much fresher than first-year ice; it has a lower conductivity and a rougher surface. The low salinity of multi-year ice makes it much stronger than first-year ice.
Multi-year ice pushed up to release some of the pressure on the Arctic ice pack
Snow
Snow is generally white because it reflects the full spectrum of light, which we see as white. However, snow can also be red, green, blue or black. This occurs because of the presence of beautifully coloured (if rare) fungi, as well as bacteria, mosses and algae. These can survive in both the Arctic and Antarctic, especially in humid areas: evidence of life in a habitat where it is least expected.
Most people think of snow simply as frozen water, but it's far more complicated than that. Snow is actually a form of precipitation as ice crystals, hexagonal prisms that form when water freezes up. Prisms are formed because of the molecular structure of water. As these ice crystals are formed, they develop into one of the following:
DON'T EAT SNOW!
If you're wondering how to survive in the cold without any means of thawing ice or snow – it's important to know that it takes more energy to eat snow than it is worth. You use more energy eating it than you gain through hydrating your body with it.
snow crystals Individual, single ice crystals, often with six-fold symmetrical shapes. These grow directly from condensing water vapour in the air, usually around a nucleus of dust or some other foreign material. Typical sizes range from microscopic to at most a few millimetres in diameter.Measuring snow thickness on the Arctic Ocean
snowflakes Collections of snow crystals, loosely bound together into a puffball. These can grow to large sizes (up to about 10cm across) when the snow is especially wet and sticky. A snowflake consists of up to a hundred snow crystals clumped together.Ice crystals in the polar regions deserve a closer look
rime Super-cooled tiny water droplets (typically in a fog) that quickly freeze onto whatever they hit (for example, the small droplets of rime on large snow crystals).
graupel Loose collections of frozen water droplets, sometimes called ‘soft hail’.
hail Large, solid chunks of ice.
SOME INUIT WORDS FOR SNOW
| anniu | falling snow |
| api | ground snow |
| siqoq | smoky, drifting snow |
| upsik | wind-beaten snow |
| kimoaqtruk | snow drift |
| salumaroaq | smooth snowy surface of fine particles |
| natatgonaq | rough snowy surface of large particles |
The story of a snowflake begins with water vapour in the air, caused by evaporation from oceans, lakes and rivers. When a parcel of air cools down, at some point the water vapour it holds will begin to condense; when this happens near the ground, the water may condense as dew on the grass. High in the atmosphere water vapour condenses into countless minute droplets, each one containing at least one dust particle. A cloud is nothing more than a huge collection of these water droplets suspended in the air.
In winter, snow-forming clouds are still mostly made of liquid water droplets, even when the temperature is below freezing. The water is said to be super-cooled, simply meaning that it is cooled below the freezing point. As the clouds get colder, however, the droplets start to freeze, and fall as snowflakes. This begins to happen around −10°C, but it's a gradual process; the droplets don't all freeze at once.
Warm-Blooded Animals and the Cold
Our bodies produce heat at a constant 37°C, with a slight change of 0.5°C lower in the morning and 0.5°C higher in the evening. A body temperature of above 38°C denotes a fever, and hypothermia sets in once the body temperature drops below 35°C. Extra energy is needed when the body is confronted with cold temperatures which draw that heat away. Humans put on clothes to retain that heat, while animals (unless cold-blooded) are protected from the outside temperatures by fur and layers of fat. When animals lose their insulation layer, they are at risk; their survival depends on adequate food intake. The availability of their food source determines whether or not they will survive.
Infrared illustrations of our body's heat radiation
Polar bears can withstand lengthy periods without food. Male bears, for instance, are routinely forced to go without a major meal for three or four months each summer, when melting ice prevents them from hunting seals. Pregnant females apparently go without food for eight months – a record among mammals. Mothers even keep fasting for some weeks after their 1lb cubs, usually twins, are born between late November and March. By the time the cubs have left her care, one to three years later, however, the mother has rebuilt her energy stores and is ready for another litter.
Staying cool and keeping warm
Warm-blooded animals sweat or pant to lose heat through water evaporation. They can also cool off by moving into a shaded area or by getting wet. Only mammals can sweat. Primates, such as humans, apes and monkeys, have sweat glands all over their bodies; dogs and cats have sweat glands only on their feet. Whales have no sweat glands, but since they live in water don't really need them. Large mammals can have difficulty cooling down if they get overheated. This is why elephants, for example, have large, thin ears through which heat is lost quickly.
Mammals have hair, fur or blubber, and birds have feathers to help keep them warm. Mammals with thick coats of fur which keep them warm in winter shed much of this in summer to help them cool off and maintain body temperature. Warm-blooded animals can also shiver to generate more heat when they get too cold. Some warm-blooded animals, especially birds, migrate from colder to warmer regions in the winter.
Thus warm-blooded creatures try to keep their body temperature constant by generating their own heat when in a cooler environment, and by cooling themselves when in a hotter environment. To generate heat, warm-blooded animals convert food into energy; compared to cold-blooded species they have to eat a lot of food to maintain a constant body temperature. Only a small amount of food is converted into body mass; the rest is used to fuel a constant body temperature.
Cold-blooded creatures take on the temperature of their surroundings: they are hot when their environment is hot, and cold when it is cold. In hot environments, the blood of cold-blooded animals can be much warmer than warm-blooded animals. Cold-blooded animals are much more active in warm environments, and very sluggish in cold environments: their muscle activity depends on chemical reactions which work quickly when it is hot and slowly when it is cold. A cold-blooded animal can convert much more of its food into body mass.
Human heat regulation
Over thousands of years we have evolved to a state where the most comfortable outside temperature (for a clothed human being) is 21°C. We need to be surrounded by air that is cooler than our body temperature by just the right amount so that heat flows away from our bodies at the same rate we generate it. We can work and play without being either cold or sweating profusely to maintain our core body temperature at 37°C. This temperature can only be maintained when we provide our bodies with enough energy (food and water) so that the heart can pump our energy-laden blood to our extremities and back.
In a cold environment, we need to counter-attack the cold by ingesting more food and water, but that is clearly not enough. We also need suitable clothing to retain the heat produced by a moving body. A resting body produces a mere 100 watts of heat, whereas an intense workout can produce ten times as much.
Of course, there comes a time when any living creature needs to rest or sleep. And when the cold gets a grip on a cooling human body, that body starts shivering as a reaction (shivering increases the amount of heat produced four or five times). At the same time, less blood will be delivered to both the extremities and to vital organs such as the heart and the brain.
