Category Science

          Rime frost forms on leaves, branches and other solid objects when an icy wind freezes water droplets over them. It forms a solid white crust on the windward side of objects and can cause damage to buildings and other structures if it is allowed to build up. Rime is usually found in very cold, exposed areas.

          Rime ice forms when supercooled water liquid droplets freeze onto surfaces. Meteorologists distinguish between three basic types of ice forming on vertical and horizontal surfaces by deposition of supercooled water droplets. There are also intermediate formations.

• Soft rime is less dense than hard rime and is milky and crystalline, like sugar. Soft rime appears similar to hoar frost.
• Hard rime is somewhat less milky, especially if it is not heavy.
• Clear ice is transparent and homogeneous and resembles ice-cube ice in appearance. Its amorphous, dense structure helps it cling tenaciously to any surface on which it forms.

          Both rime types are less dense than clear ice and cling less tenaciously, therefore damage due to rime is generally minor compared to clear ice. Glaze ice is similar in appearance to clear ice but it is the result of a completely different process, occurring during freezing rain or drizzle.

          These three types occur also when ice forms on the surface of an aircraft, particularly on the leading edges and control surfaces, when it flies through a cloud made of supercooled water liquid droplets. Rime ice is the least dense, milky ice is intermediate and clear ice is the most dense. All forms of ice can spoil lift and may have a catastrophic effect on an airborne aircraft. Ice is hazardous to flight as it disrupts airflow, increases weight, and adds drag. Ice forming on propellors and/or engine inlets can cause severe vibration and/or damage if ingested.

Rime is also a weather form.

          Hard rime is a white ice that forms when the water droplets in fog freeze to the outer surfaces of objects. It is often seen on trees atop mountains and ridges in winter, when low-hanging clouds cause freezing fog. This fog freezes to the windward (wind-facing) side of tree branches, buildings, or any other solid objects, usually with high wind velocities and air temperatures between ?2 and ?8 °C (28.4 and 17.6 °F).

          Soft rime is a white ice deposition that forms when the water droplets in light freezing fog or mist freeze to the outer surfaces of objects, with calm or light wind. The fog freezes usually to the windward side of tree branches, wires, or any other solid objects.

          Soft rime is similar in appearance to hoar frost; but whereas rime is formed by vapour first condensing to liquid droplets (of fog, mist or cloud) and then attaching to a surface, hoar frost is formed by direct deposition from water vapour to solid ice. A heavy coating of hoar frost, called white frost, is very similar in appearance to soft rime, but the formation process is different: it happens when there is no fog, but very high levels of air relative humidity (above 90%) and temperatures below ?8 °C (17.6 °F).

          Soft rime formations have the appearance of white ice needles and scales; they are fragile and can be easily shaken off objects. Factors that favour soft rime are small drop size, slow accretion of liquid water, high degree of supercooling, and fast dissipation of latent heat of fusion. The opposite conditions favour ice with higher densities, such as hard rime or clear ice.

Picture Credit : Google

          Hoar frost is the most common type of frost. It covers every exposed surface with layers of crunchy ice crystals. In certain conditions, the ground may be covered with a very thick layer of white hoar frost, which looks like snow.

          Hoar frost refers to white ice crystals deposited on the ground or loosely attached to exposed objects, such as wires or leaves. They form on cold, clear nights when conditions are such that heat radiates out to the open air faster than it can be replaced from nearby sources, such as wind or warm objects. Under suitable circumstances, objects cool to below the frost point of the surrounding air, well below the freezing point of water. Such freezing may be promoted by effects such as flood frost or frost pocket. These occur when ground-level radiation losses cool air until it flows downhill and accumulates in pockets of very cold air in valleys and hollows. Hoar frost may freeze in such low-lying cold air even when the air temperature a few feet above ground is well above freezing.

          The word hoar comes from an Old English adjective that means “showing signs of old age”. In this context, it refers to the frost that makes trees and bushes look like white hair.

Hoar frost may have different names depending on where it forms:

• Air hoar is a deposit of hoar frost on objects above the surface, such as tree branches, plant stems, and wires.
• Surface hoar refers to fern-like ice crystals directly deposited on snow, ice or already frozen surfaces.
• Crevasse hoar consists of crystals that form in glacial crevasses where water vapour can accumulate under calm weather conditions.
• Depth hoar refers to faceted crystals that have slowly grown large within cavities beneath the surface of banks of dry snow. Depth hoar crystals grow continuously at the expense of neighbouring smaller crystals, so typically are visibly stepped and have faceted hollows.

          When surface hoar covers sloping snowbanks, the layer of frost crystals may create an avalanche risk; when heavy layers of new snow cover the frosty surface, furry crystals standing out from the old snow hold off the falling flakes, forming a layer of voids that prevent the new snow layers from bonding strongly to the old snow beneath. Ideal conditions for hoarfrost to form on snow are cold clear nights, with very light, cold air currents conveying humidity at the right rate for growth of frost crystals. Wind that is too strong or warm destroys the furry crystals, and thereby may permit a stronger bond between the old and new snow layers. However, if the winds are strong enough and cold enough to lay the crystals flat and dry, carpeting the snow with cold, loose crystals without removing or destroying them or letting them warm up and become sticky, then the frost interface between the snow layers may still present an avalanche danger, because the texture of the frost crystals differs from the snow texture and the dry crystals will not stick to fresh snow. Such conditions still prevent a strong bond between the snow layers.

Picture Credit : Google

          ICICLES usually form when the water from thawed ice and snow freezes again. This happens during a bright winter’s day when sunny areas are warm, but shaded areas remain below freezing, or when a cold night follows a warmer day. When the melt-water drips over the edge of a surface, the drops will freeze to form an icicle.

          Icicles can form during bright, sunny, but subfreezing weather, when ice or snow melted by sunlight or some other heat source (such as a poorly insulated building), refreezes as it drips off under exposed conditions. Over time continued water runoff will cause the icicle to grow. Another set of conditions is during ice storms, when rain falling in air slightly below freezing slowly accumulates as numerous small icicles hanging from twigs, leaves, wires, etc. Thirdly, icicles can form wherever water seeps out of or drips off vertical surfaces such as road cuts or cliffs. Under some conditions these can slowly form the “frozen waterfalls” favored by ice climbers.

          Icicles form on surfaces which might have a smooth and straight, or irregular shape, which in turn influences the shape of an icicle. Another influence is melting water, which might flow toward the icicle in a straight line or which might flow from several directions. Impurities in the water can lead to ripples on the surface of the icicles.

          Icicles elongate by the growth of ice as a tube into the pendant drop. The wall of this ice tube is about 0.1 mm (0.0039 in) and the width 5 mm (0.20 in). As a result of this growth process, the interior of a growing icicle is liquid water. The growth of an icicle both in length and in width can be calculated and is a complicated function of air temperature, wind speed, and the water flux into the icicle. The growth rate in length typically varies with time, and can in ideal conditions be more than 1 cm (0.39 in) per minute.

          Given the right conditions, icicles may also form in caves (in which case they are also known as ice stalactites). They can also form within salty water (brine) sinking from sea ice. These so-called brinicles can actually kill sea urchins and starfish, which was observed by BBC film crews near Mount Erebus, Antarctica.

Picture Credit : Google

          Rivers and lakes can become frozen over if the temperatures get very low. This will usually depend on the depth of the water — the shallower it is, the more likely it is to freeze over. A layer of ice forms on the surface that is strongest and thickest nearest to the banks. Weaker spots will exist further out, making it very dangerous to walk or skate on the ice – anyone falling in could easily become trapped in the freezing water.

          Ice found on natural bodies of water raises the risk of danger due to natural variables. There is no way to judge the strength and safety of ice by looking at it or by the temperature of the day. Fluctuating temperatures, exposure to sunlight and other factors affect the consistency of the ice. It can be a several inches thick in one spot and only an inch thick 10 feet away. Be especially wary of ice covered with snow. Snow can hide cracks and weaknesses in open ice. Parents should educate their children about the danger of going out onto a frozen body of water, including never following a friend or a pet into these potential danger zones.

Basic tips

• Parents should always supervise children skating or playing on or near ice. Educate them on the risks of playing on ice, and outfit them with lifejackets. Never leave children alone on or near ice covered bodies of water.
• Adults should prepare before going on ice. Wait to walk out onto ice until there is a minimum of four inches of clear, solid ice measured from multiple locations. Start measurements in an area where the water is shallow. If the thickness in the shallow area is less than three inches, do not walk on the ice.
• Take someone with you, wear a life jacket, and bring safety equipment, including a cell phone, in case of an emergency.
• Always keep your pets on a leash near frozen bodies of water. If a pet falls through the ice, do not attempt to rescue your pet, call 9-1-1 or go for help.
• Stay clear of white ice. White ice contains air and snow within it, therefore, making it weaker.
• Stay off river ice. Currents can quickly change the thickness of ice, making it more fragile.

When Someone Falls Through Ice

• If you come across someone who has fallen through the ice, don’t attempt a rescue yourself. Call 9-1-1 or immediately or go for help. Local public safety officers have proper training and equipment to handle ice emergencies.
• If the ice did not support the victim’s weight, it will not support you. Avoid going onto the ice to attempt a rescue, but extend a ladder, rope, jumper cables, or tree branch to the victim along with something that will keep them afloat.
• Once the person is rescued from the cold water, help the victim into dry clothes as soon as possible. If dry clothes aren’t an option, leave the wet clothes on for insulation to trap body heat.
• Transport the victim to get medical attention if necessary.

If You Fall Through Ice

• Remain calm, and try not to panic. The body will undergo cold water shock when suddenly immersed in cold water, and you will experience an increase in heart rate and blood pressure.
• Face the direction you came from and spread your arms out on the unbroken ice. Kick your feet and try to pull yourself onto the ice.
• Once out of the cold water, remain lying on the ice (do not attempt to stand) to keep your weight distributed and avoid breaking through the ice. Roll away from the hole and crawl back to solid land. This keeps your weight distributed.
• Treat yourself for hypothermia and seek medical attention.

Picture Credit : Google

          If you were to cut through a hailstone, you would see that it is made up of several layers of ice and frost. Each layer is produced by one journey up and down the height of the cloud. The greater the amount of turbulence inside the cloud, the greater the number of layers and the bigger the hailstones.

          If you’ve ever seen a tree that has been cut down, you’re probably familiar with the many rings that you will find inside. If you slice through a hailstone, you would find that the cross-section has rings similar to the inside of a tree.

          This layered appearance inside the hailstone is due to the coatings of ice that were added over time while the hailstone was suspended aloft in the thunderstorm. The series of photos above, some more vivid that others, illustrate the layers that can be found in hailstones.

          So, why are some hailstones so much larger than others?  The answer lies in how many ice layers are formed on a hailstone.

          Thunderstorms have an inflow of warm, moist air that rises into the storm. This rising unstable air is referred to as the updraft.

          The updrafts suspend some initial small ice kernels aloft, where water droplets collide with the small hail and make it grow larger with each additional ice coating.

          While the majority of thunderstorms contain at least some hail, many times it does not reach the ground because the hailstones are small and melt in the above-freezing layer in the lower atmosphere. These storms likely have a weak updraft.

          For very large hail to reach the ground, a strong and long-lasting updraft is needed to allow the hailstone to stay aloft and add more layers of ice. Sometimes the updraft in the storm can exceed 100 mph!

          Eventually, the hailstone will reach a point where it becomes too heavy and the updraft can no longer support it aloft. This hail then falls to the ground below in various sizes and shapes.

          Every year we see thunderstorms with strong updrafts that produce hail the size of baseballs and softballs in the nation’s heartland. The largest hailstone ever measured in the United States fell from the sky.

Picture Credit : Google

          Frost most often forms over the land when the air remains clear after a cold winter’s day. The ground loses its heat quicker than the air above it, and if the temperature falls below freezing, any moisture in the air will freeze, covering almost any surface with frost. Different conditions can produce different types of frost.

          Morning condensation (dew) is very common in some regions and can easily be forecasted. The favorable weather elements for dew include clear skies, light wind, decent soil moisture, and low night-time dew point depressions.

          Dew forms when the temperature becomes equal to the dew point. This often happens first at ground level for two reasons. First, longwave emission causes the earth’s surface to cool at night. Condensation requires the temperature to decrease to the dew point. Second, the soil is often the moisture source for the dew. Warm and moist soils will help with the formation of dew as the soil cools overnight.

          The cooling of warm and moist soil during the night will cause condensation especially on clear nights. Clear skies allow for the maximum release of longwave radiation to space. Cloudy skies will reflect and absorb while re-emitting longwave radiation back to the surface and that prevents as much cooling from occurring. Light wind prevents the mixing of air right at the surface with drier air aloft. Heavier dew will tend to occur when the wind is light as opposed to when the wind is strong. Especially when soils are moist, the moisture concentration will be higher near the earth’s surface than higher above the earth’s surface. As the air with higher moisture concentration cools, this air will produce condensation first.

          Soil moisture is EXTREMELY critical to producing dew (especially heavy dew). Dry regions that have not received rain in over a week or two are much less likely to have morning dew (especially a heavy dew). Once the soil gets a good soaking from a rain, it takes several days for the soil to lose the moisture through evaporation. If nights are clear after a good rain, dew can be expected every morning for the next few days (especially in regions with abundant vegetation, clear skies and light wind). The dew point depression is important because it determines how much the air will need to cool to reach saturation. With a large dew point depression (greater than 25 units of F), quite a bit of night-time cooling will need to take place in order to produce dew. A low dew point depression with the other factors favorable for dew is more likely to produce heavy dew.

          Dew is important to forecast since it impacts people. Dew can produce a thick film of water all over the car in the morning (can be especially annoying for people that don’t have a garage). Time has to be spent wiping the water off the windows in order to see on-coming traffic. Dew is also important to agriculture. Dew recharges the soil moisture and limits evaporation from the soil during the time the dew is forming. Dew can make the mowing of the lawn more difficult. It is much easier to mow the lawn in the late afternoon when the dew has evaporated than it is in the morning. Wet grass clumps together and sticks to everything. Also, you are more prone to getting a dirty shoe when walking on dew covered grass as compared to dry grass.

Picture Credit : Google