People in cold climates are used to signs warning that “bridge freezes before road.” Water on surface will freeze once the surface becomes cold enough, but why does the road cool faster?

Warming and cooling result from the net energy flow: gains minus losses. As you face an evening bonfire, your front warms because energy gain outweighs energy loss. For opposite reasons, your back cools, because it loses more energy to the cold night air than it gains.

The fundamental reason why a bridge freezes first is that it loses energy from the top, sides and bottom, while the roadway loses energy mainly from the top. With more surface area to exchange energy with the atmosphere, the bridge cools down to air temperature more quickly.

Also, steel bridges are excellent heat conductors, and so when cold air contacts the steel, rapid heat transfer to the air produces even faster cooling.

Although a roadway also loses heat to the cold air above, it gains energy from the ground, which slows the cooling process. This energy gain is another reason why a road cools and ices more slowly than a bridge.

Lake ice formation is dynamic. Even when a lake is completely frozen, the ice is not stagnant. It expands and contracts as it warms and cools. Differences in day and night temperatures can be large enough to cause the ice to crack. As the air temperature drops at night, lake ice cools and contracts. Since the ice is stuck to the shoreline, the entire sheet cannot contract as a whole, so cracks develop in the ice.

When the ice warms during the day, it expands. This expansion can cause a collision between both sides of the crack, which can cause the ice to buckle up at that pressure point. Cracking, collisions and buckling can cause loud noises.

This expansion can even push the ice up on shore. This can happen because, at night, the ice contracts and cracks develop which can fill with water. This water freezes. In the day, as the ice warms, it expands and pushes some ice up on shore.

A layer of snow on the ice acts like a blanket, insulating the ice. This keeps the ice from warming or cooling too much. Snow-free conditions can lead to the greatest temperature changes in the ice, particularly in early spring when the temperature can change so much during one day. When a spell of cold weather is followed by weather with temperatures in the 60s F, lake ice will warm, expand, and buckle anew.

It is highly unlikely that you’ll find two identical snowflakes. Feel free to try to prove us wrong, but your search may involve some cold feet! If you compared 1 million snowflakes (a minuscule fraction of the flakes in a single snowstorm), performing two comparisons per second, you’d need about 100,000 years!

A simple snowflake is a single ice crystal, but most flakes are an aggregation of a group of ice crystals. Ice crystals are six-sided and come in four basic shapes: column, needle, hexagonal plate and dendrite. The shape in which an ice crystal grows depends on the air temperature around it. As a crystal moves through a cloud it continually encounters different temperatures, which continually modify its shape. Crystals also hit other crystals and break, chip or crack, further altering their shape.

You might be able to find two similar shapes of the initial crystals, but they will be hard to locate in the cloud. Maybe it’s better to just stand in a snowfall and enjoy their beauty rather than look for twins! Or view wonderful photos of ice crystals at The Bentley Collection.

The term ‘black ice’ refers to either a new layer of transparent ice on water, which allows us to see the deep water below, or a layer of clear ice on a roadway, which makes for hazardous driving. In both cases, the ice is transparent, not black, and so it shows the color of the underlying surface.

Photo: AdamKR

Black ice in Halifax

The ice is clear because no air bubbles are trapped in it. Lots of trapped air turns ice white; snow is white because of air trapped between crystals.

Driving on a road covered with black ice is particularly dangerous because the roadway can appear to be merely wet, so drivers may not recognize the slippery conditions until too late. Similarly, a sidewalk covered with clear ice will look dark gray – like a wet sidewalk. This ‘grey ice’ can be hazardous to walking!

Because bridges span the open air, they cool faster than other roadways, so black ice often occurs first on bridges. That’s why we see those warnings that “Bridge May Freeze Before Road Surface.”

Lakes freeze from the top down. Ice is less dense than water, which is why ice floats. The density of liquid water is determined by its temperature, and water is most dense at about 40 F.

Why is that important? As winter sets in, lakes lose energy to the atmosphere, and water near the surface cools, becomes more dense, and sinks. Warmer, less dense water under the surface will rise to replace this surface water. When the entire lake reaches 40 F, the surface water cools further, dropping below 40 F. Because this water is now less dense than the surrounding water, it will stay on the top and continue to cool.

Once the surface water falls to 32 F, it freezes. The freezing then spreads downward into the lake and the ice thickens. Unless the lake is very shallow, you will find liquid water below the ice. This deeper water is about 40 F; fortunately fish can live in this cold temperature.

Freezing first occurs along the shoreline, where the water is shallow. Before ice can form on the surface, the entire water column must first reach 40 F, which is likely to first occur along the shoreline.

Heavy frost on Burr Oak tree in Wisconsin

Frost on objects is just water vapor in the air that has condensed as ice onto a surface. Frost forms on objects close to the ground, such as blades of grass. At night, a blade of grass loses energy by emitting a non-lethal kind of radiation, but it absorbs energy emitted by surrounding objects. Under clear nighttime skies, objects near the ground emit more radiation than they receive from the sky, and so a blade of grass cools due to the net energy loss. Once a grass blade gets cold enough, frost will form on it.

Overnight cooling of air near the ground causes morning frost on grass and car windshields. Frost will only form on a surface that is at or below freezing temperature. The observed air temperature may be above 32 F, since observations are taken at about four feet above the ground, where it can be warmer.

If you get outside early in the morning, you may notice that frost forms in open fields but not under trees. Trees emit more radiation toward the ground than does the clear sky, and so grass under a tree loses less energy than grass in an open field. Grass in an open field cools faster and reaches the frost point first.

Photograph of Cumulus clouds in fair weather taken by Michael Jastremsk

Clouds are made of water and clean water is clear. So why are clouds white? Because clouds are made of billions of small water droplets and ice crystals.

When light beams interact with particles suspended in air, some of the energy is scattered, which means the light beam changes direction, and usually color as well. The amount of light scattered is a function of the size of the particle relative to the wavelength of light falling on it. Cloud particles are large enough to scatter any color of light that falls on them. The repeated scattering of light, called multiple scattering, causes whitish light because enough light of all colors is scattered to your eye, and those colors combine to make white light.

You can demonstrate this with a glass bottle. Without the labels, the glass will look clear. Wrap the bottle in a rag, smash it, and pour the small pieces into a pile. The pile will be whitish, even though each tiny piece is clear.

The bottoms of even the whitest clouds appear gray, sometimes ominously so. Why? Because multiple scattering above the cloud base redirects the incoming sunlight out the top and the sides of the cloud, leaving very little light to emerge from the cloud base, which is dark.

62 degrees below zero! From Wikimedia

Snow can make both ‘squeaky’ and ‘crunchy’ sounds. Snow is a mixture of ice crystals, liquid water and air, and the sound it makes when you walk on it depends on the proportions of this mixture.

When you walk on snow, your boots apply pressure that can break its ice crystals. When the snow is warmer than about minus 35 degrees F, its ice crystals are surrounded by a film of liquid water that lubricates them so they can slide past one another without breaking.

The more water around the crystals, the less likely the breakage, so if the snow is colder than 14 degrees F, your boot will crush the ice crystals, making that squeaking, or creaking, sound. Snow above approximately 14 degrees F contains enough liquid water for the crystals to “flow” silently under your boot. You’ll never get a squeak striding through slush!

The crunchy sound, on the other hand, is made by cold, packed snow. Snowpack is made of ice grains. Where they touch, they bond or weld together like a matrix. These bonds are weak in fluffy snow and strongest in cold, dense snow. The crunchy sound occurs when the bonds between the ice grains in the snowpack break apart when you step on them.

You’ll never hear the crunch walking in fluffy snow!

Saturn in eclipse

Yes, this is a real picture. More accurately, it’s 165 pictures pasted together from NASA’s Cassini spacecraft’s flyby of Saturn as the planet between the probe and the sun. From this unique vantage point, the contrast of light and shadow enabled astronomers to discern new bands of ice and dust — perhaps the remnants of a shattered moon — between the inner and outermost edges of the ring system. This panorama reveals Saturn casting a massive shadow some 75,000 miles long over Cassini’s camera. Though the brightness in this image has been enhanced to reveal detail, the photo’s sharp contrasts owe foremost to the millions of square miles of orbiting ice crystals that ring the planet. The dark gaps between the bands are thought to be the result of gravitational forces exerted by the planet’s many moons, but these forces are not the only cause. The Encke Gap, among the largest of these vacuous rings, is maintained by Saturn’s innermost moon and “ring shepard,” Pan, which plows through the orbiting field of ice and dust.

Image courtesy CICLOPS team.