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What is fog? Bob Reid from Glouchester, Ontario sent me a very short fax: What is fog?

Fog is simply a cloud sitting on the ground. Like any cloud, fog is composed of millions of tiny droplets of water, or in extremely cold weather, tiny floating ice crystals. For aviation purposes fog is reported whenever it is observed with 10 km or less visibility. Thick fog reduces visibility to 1 km or less. The transparency of fog depends mainly on the concentration of droplets: the more droplets the denser the fog.

Squeeze all the water out of a cubic metre of wet sea fog and you'd get about a gram of water, from a very light fog you'd get as little as .02 grams of water. What keeps these tiny water droplets airborne? Afterall water is 800 times denser than air. Fog droplets are buoyed up by rising air currents or are continuously replenished by new droplets condensing from the water vapour in the air. An important factor in the formation of fog is for the wind to be neither still or strong; otherwise the fog won't amount to much.

A slight breeze mixes the cooled air through a deeper layer and produces fog several hundred metres deep.

What causes steam to rise from a wetted asphalt surface?
Sometimes you can see steam rising from asphalt surfaces when the sun comes out after a rain or a snowfall. The black top soaks up the sun's heat and warms the asphalt and the water on it. The cold air in close contact with the road surface is warmed, presumably to the same temperature of the water on the road. At the same time, some of the rain water or melt water from the wetted asphalt evaporates into the air. Just above the surface, the air is still relatively cold and its moisture content much lower than that of the surface. As the rising vapours from the surface cool, some of it condenses into tiny droplets that we see as wispy streamers that resemble smoke. Eventually, the rising tufts of steam fog encounter drier air and disappear. You see the same thing even more prominently on cold, crisp mornings near lakes or over rivers as cold air drifts across relatively warm open water; or in a cold bathroom when the bath is filled with hot water. Your bath water is often much hotter than the room air and so the cloud can be much denser - but its the same principle.

What are waterspouts? From Kim Votour of Oromocto, New Brunswick and Elizabeth Morin of Deep River Ontario, the same question, what are waterspouts?

A waterspout is a rapidly whirling, funnel-shaped column of vapour and water between a cloud and the earth's surface. They look like tornadoes over water but they're really just first cousins. Waterspouts come in a variety of sizes and strengths and arise from different mechanisms. They can start over land as true tornadoes and drift out to sea or over a lake or large river. Much more common are the fair-weather spouts in which small whirlwinds start from the water surface. They climb skyward, but rarely extend to the cloud level. They are similar to the little dust whirls that form over dry roads and fields. Larger waterspouts may also grow down from above in a manner similar to a tornado with the circulation beginning in the cloud and working its way down to the water.

Spouts formed over water are much smaller, less destructive, live shorter lives and move more slowly than land-based tornadoes. Waterspouts have been known to pick up small fish and frogs and other water creatures and carry them inland, where they are suddenly dropped on startled residents.

Whiteout David Barnaby of Toronto wonders whether the word "whiteout" used for a blizzard that obscures visibility is the correct use of the word?

No, this is not a true whiteout. Real "whiteouts" occur mostly in the Arctic and Antarctic during the spring, when snow is still deep on the ground and there is lots of daylight and surprisingly calm weather and excellent visibility. Polar whiteouts occur when rays of sunlight are bounced in all directions between bright white clouds, especially a thin layer of overcast and bright snow or ice. Clean snow and ice reflects nearly 85% of incoming light. Falling snowflakes, suspended fog droplets or ice particles in the air would make conditions even worse. In a true whiteout, neither shadows, nearby objects, landmarks, nor clouds are discernable. All sense of direction, depth perception and even of balance may be lost. Land and sky seem to blend, and the horizon disappears into a white nothingness. Someone said you feel you are drowning in milk or it's like being inside a Ping-Pong ball. Whiteouts trick pilots into believing down is up and travellers in thinking far is near and confusing everyone in thinking danger is safety.

What are plough winds?
Following very destructive storms in the towns of Oxbow and Pilot Butte, Saskatchewan in 1995, I received several letters asking about plough winds. Two letters came from residents of Oxbow, Gina Helmer and Bonnie Morrow.

Plough winds belong to a family of strong, straight-line downburst winds found in thunderstorms. As for the name, I can only guess one helped to plough a field and was so named. Inside a severe thunderstorm there are both updrafts and downdrafts of air. The downdrafts rush to the ground with great force, maybe 100 to 150 km/h and occasionally even higher. When they strike the ground the air spreads horizontally in a burst of wind, much like water pouring from a tap and striking the sink below. Plough winds can blow continuously but the damage is usually confined to an area less than 3 km across. They are capable of toppling trees, ripping apart buildings and lifting roofs. In fact, plough winds and other downbursts may be responsible for some damage attributed to tornadoes. They are more common than tornadoes and have the power of a twister but the damage pattern looks different - a circular or semi-circular swath left by tornadoes and a starburst or more commonly a straight line pattern from downbursts.

What are winter rainbows?
I have received many letters about seeing winter rainbows on very cold days. This one is from Mrs. Ethel St. Jean of New Liskeard, ON. Others such as Celine Trifts of Moncton and Bob Admans of Stratford, ON ask about the appearance of rainbows or snowbows in blizzards.

If there are no water droplets, there can be no rainbows. In the winter, the sky is filled with brilliantly coloured optical phenomena including haloes, light pillars, arcs, spots, patches, and coloured circles. In winter, these luminous arcs in the sky, and rainbow-tinted halos and sundogs are often confused with rainbows. A rainbow is due to the action on light of water droplets. A halo results from the refraction of sunlight by ice crystals. You will not confuse rainbows with halos if you remember that rainbows are always centred in the sky opposite the sun or other source of light, while halos, except for a few rare varieties, are seen surrounding or otherwise near the source of light whether it be the sun, moon, streetlight or something else. The winter skies are constantly changing. Coloured optics abound, but we rarely take the time to admire them. Treat yourself, look up!

What is Hoar Frost?
Ernie Bayer of Surrey BC wrote to tell me that as a child in Saskatchewan he often heard the term hoar frost. We don't often hear hoar frost any more. Instead, it's just frost, white frost, silver frost - a familiar sight on roofs, pavements, vegetation and inside windows, in the early morning after a cold night. The word hoar comes from the Old English meaning grey or greyish white, referring to the colour of hair in old age. The word accurately describes ordinary frost - deposits of small, feathery ice crystals sometimes so thick and white it is mistaken for snow.

Frost forms under a clear sky on calm nights when ground objects cool rapidly. If the shallow layer of air near the ground contains a considerable amount of water vapour, some of it condenses on the surface of cold objects, either as water or ice. If it is below freezing the condensation will be hoar frost.

Sometimes, the temperature may drop below freezing, but the air is too dry to produce visible frost. The freezing though causes cell damage in plants, often blackening the vegetation. That's why it is termed black frost or simply freeze.

A "Hoars frost" forms when the dew point of the air below 0 C and the humidity is 100%. Water in our natural environment can be found in 3 states: gas, liquid and solid. This type of frost forms when saturated water vapour in the air changes directly from a gas (moist air) into a solid (ice) by coming in contact with cold objects. It skips going through a liquid state.

Hoars frost forms on all kinds of objects with really neat looking ice-crystal patterns.

What are double rainbows?
On a tour of the Cabot Trail, Guy and Marie Atkins of Saint John, New Brunswick saw two striking rainbows almost side by side. They wanted to know how unusual was their sighting. Double rainbows are not unusual, in fact, they occur with every rainbow. It's just that they are rarely bright enough to be seen. Dale Little of Sidney BC also wanted to know how double rainbows form. When white sunlight enters each raindrop, it bends. The bending breaks light into its rainbow colours. The light then reflects off the back of the raindrop. On leaving the drops the light bends again. [Each raindrop scatters the full spectrum of colours, but you will see only a single colour from each drop]. Sometimes light reflects twice inside the raindrop. This action will form two rainbows: a primary rainbow that is produced by the first reflection of light and a larger secondary bow above the first, produced by the second reflection.

Colours in the secondary bow are in reverse order to those in the primary rainbow, with red on the inside and violet on the outside. Since only a small number of rays experience this double reflection, the secondary bow is always fainter than the primary.

What are sundogs?
Lionel Richard of Fredericton wanted to learn something about sundogs. Sundogs are bright spots on either or both sides of the sun. They are also called mock suns, but the technical name for them is parhelia. They also occur (less brightly) around the moon. Of course, they are called moon dogs. Sundogs are images of the sun formed as a result of light bending through tiny, floating ice crystals in the air or high clouds. The ice crystals are shaped like plates falling in such a way that the flat surfaces face parallel to the Earth. Sunlight enters at one side, bends and bends again as it exits. The light refraction creates an image of the sun 22° to the left and/or right of the sun. Sundogs are not rare - perhaps visible 10 times a year. The most brilliant ones occur on a cold sunny morning or evening, when the sun is near the horizon and the air is loaded with ice crystals. Spotting sundogs and other sky phenomena like haloes and rings often means rain or snow will arrive within 18 to 36 hours.

What are Chinooks?
A Chinook is a warm and dry wind which can lead to a significant temperature change over a region affected by such a wind.

Consider air moving eastward from the Pacific Ocean onto the North American continent. As the as the air encounters the mountain ranges, it is forced upward, a process which cools the air. If the air cools enough, condensation can occur and clouds and precipitation can develop. This is why one observes relatively high amounts of precipitation on the western side of the coastal B.C. mountain ranges.

The precipitation removes much of the moisture which was originally in the air impinging on the mountains. East of the mountains, the air again descends, and this process warms the air. Therefore, this is warm and dry air and is referred to as a chinook. The same process can occur in other mountain ranges, although the term "chinook" is local to the Rockies.

I just want to understand why I never hear thunder during the winter?
Although it is more rare than in summer, it is possible to experience thunderstorms during the winter as well. The development of thunderstorms depends largely on how quickly the temperature decreases as you go up in atmosphere. Because surface temperatures in the summer are much warmer than in the winter, there is usually a greater difference in temperatures at the surface and aloft in the summer. However, if a similar vertical temperature profile develops in the winter, thunderstorms can develop.

What are cold air funnels?
Cold air funnel clouds are similar to regular funnel clouds in many ways. A funnel cloud is a rotating area of cloudiness which generally extends from the base of a large thunderstorm cloud. If a cloud touches the ground, it is known as a tornado. Typically, these thunderstorms form in relatively warm air masses. However, under certain circumstances, funnel clouds can form in relatively cold air masses, for example following the passage of a cold front. This occurs most frequently in the fall or in the spring. These funnel clouds rarely reach the ground so they don't generally become tornadoes.

Why does the sky on the horizon turn red or pink in the evenings?
As the sun begins to set at the end of the day, the sun's rays which reach our eyes are farther away and must pass through a thicker layer of the atmosphere than when sun is directly overhead. As it does so, molecules in the earth's atmosphere absorb more of the radiation, but it gets absorbed preferentially. The shorter lightwaves of the visible light spectrum, the greens and blues, get absorbed, while the longer red rays are able to penetrate. Therefore, red is the colour we see!

Why is the sky blue?
White light is composed of a spectrum of colours. Particles in the atmosphere scatter light, especially the shorter wavelengths such as violet, blue and green. Since our eyes are more sensitive to blue light these waves viewed together appear as a blue sky.

I understand that jet streams form about 2000 feet below the tropopause and in the warm section of an advancing cold front. Why does the jet stream map shown on television usually extend throughout all of North America even though most of the time no fronts are depicted in the area?
The jet stream you are referring to in the warm section of an advancing cold front is called the level jet. It is found at a height of between 3000 to 3500 meters above the ground. Generally, it is a good indication of middle level moisture.

The jet stream shown on television is a zone of winds with very high speeds existing at between 200 to 500 mb which is at least above 5500 m high. Since we know that the atmospheric circulation in the upper atmosphere is a strong westerly, the jet stream shown on television represents the strongest speeds inside the westerlies. The jet stream is a good indication of the warm and cold trends according to thermowind relations. It also tends to steer surface weather systems, so meteorologists like to talk about the jet stream before they talk about the national weather.

Is there such a thing as the right time to see the northern lights? Over the last 15 years at our cottage in northern Ontario we have only seen them once. Other than a clear sky what conditions are best for viewing this beautiful sight?
The northern lights, also called aurora borealis, are a result of energy particles from a disturbance of the earth's protective magnetosphere reacting with gas particles in the upper atmosphere. To see these reactions, we need a clear night sky with little water vapour and/or pollution blocking our view. Many winter nights are clear and free of haze and that is why winter is often suggested at the best viewing time.
Can you tell me what is sea smoke and how it forms?
Sea smoke (fog) is formed when colder air covers warmer waters and winds are calm. As the fog forms, it becomes so dense that it looks like smoke as it hangs in the air.

We had the good fortune to witness the formation of a small water spout while canoeing on a lake on a warm sunny day with a slight breeze. It started off with a splashing sound near the shore. After a short while it developed into a tornado like funnel about 8 feet tall and 8-10 inches in diameter, scurried across the water in front of us leaving a trail of bubbles on the surface. It travelled in an arch for about 100 ft and then dispersed. Quite an experience!
You were very lucky to have been able to experience first-hand such a wonder of nature. Fair weather water spouts are formed on warm sunny days, when there is unstable air above the lake and slight breeze above the land. As the air warms and rises, air rushes in from all directions to fill the slightly lowered pressure formed over that spot, but rapidly rising air is not enough to initiate a waterspout. Shore line shape and topography, and local breezes from the shore to the lake all play a role in the development of these fair weather mini waterspouts as well.

What does a cross, similar to rings around the moon mean?
The cross you saw was probably parts of a halo breaking up (halos are produced by the refraction of sunlight through ice crystals). You need a solid deck of high cirrostratus clouds to get a complete halo. If there are breaks, you will see some arcs of the halo. Chances are that when you looked up, there happened to be some breaks at just the right spot for you to see a cross. In any case, a halo indicates a weather change because high clouds are usually the first to make an appearance from approaching storms.

Some of my students and I are debating whether lighting comes DOWN from the SKY or UP from the ground to the sky? Please could you give us the definitive answer! Also what causes sheet lightning as opposed to fork lightning?
To put an end to your debate, lightning actually does both.

An invisible streamer of ionized air (called a step ladder) from the cloud travels downward following a jagged path will try to meet up with a ground streamer going upwards towards the cloud. Generally, there area few of these upward streamers, and when the first one that connects with the main downward step stream - Wham! You can see the discharge but it happens so fast that you just see the flash.

Scientists using high speed cameras, observed that the 'visible' lightning discharge starts from the connection point of the streamers downward to the ground then all the way up into the cloud. Once again, it is way too fast for our eyes to see. Lightning travels up and down that visible channel many times, but all this happens in a 100th of a second! In brief, both streamers are visible until they connect, then it is visible going down and back up into the cloud.

In answer to your second question, there are many factors that cause different types of lightning: wind, precipitation, the severity of the storm, dust, etc. Which one it is depends on the situation.

I have a couple of questions on the jet stream. — We always see the Jet Stream for North America. What does it look like for the other side of the world, i.e. Europe or Asia? Is there a web site where I can see this? — Is there a Jet Stream in the Southern Hemisphere?
Yes, the Jet Stream follows a similar cycle over Europe and Asia. As for a web site, try looking at http://www.met.reading.ac.uk/~brugge/index.html. And yes again, there is a Jet Stream for the Southern Hemisphere. Occasionally, we show the southern branch of the Jet Stream, but usually we show the northern branch.

How do we predict tides? How do we predict when tides will be high or low and by how much?
Tides are caused by the influence of the moon and sun on the earth's surface and on oceans in particular. The moon accounts for approximately 2/3 of the influence; the sun accounts for 1/3. The daily cycle of tides is about 25 hours, or 12.5 hours between two high tides. This means that about 6.25 hours after a high tide, there is a low tide.

The moon's distance from the earth is one factor that effects the tides. The tides are highest and lowest (have their greatest range) when the moon is nearest the earth in its orbit (called Perigee). The tides have their smallest range when the moon is farthest from the earth in its orbit (called Apogee).

The phases of the moon also have an influence on tides. The tides are highest and lowest (have their greatest range) when the moon is new or full. At this time the moon, sun, and earth are aligned and the sun and moon experience their strongest gravitational attraction towards the earth. These tides are called Spring tides (nothing to do with the season). The tides have their lowest range when the moon is in first or last quarter. This is because at these phases the sun, moon, and earth form a right angle and therefore the gravitational attraction is weakest. These tides are known as Neap tides.

Factoring in the sun's distance, we find that the tides are highest when the earth is nearest to the sun, which occurs about January 4 (Perihelion). The tides are lowest when the earth is farthest from the sun. This occurs about July 4 (Aphelion).

Taking all of these factors into account we can determine that the greatest range of tides (e.g., the highest high tides and the lowest low tides) occur when the moon is new or full, at its nearest point to the earth in its monthly orbit (at Perigee), and when the earth is nearest to the sun (at Perihelion).

What is that odd blue ring around the moon at night?
The bluish ring around the moon at night is caused by ice crystals high up in the atmosphere (about 7 km or 22,000 feet above the surface) which refract the moonlight in such a way that they create a halo.

The effect is much like that which occurs when sunlight passes through an area of rain and creates a rainbow, or when a light passes through a prism and is split into the colours of the spectrum.

A halo often precedes the onset of unsettled weather within the next 24 hours or so. This is because the ice crystals are part of cirrus clouds, which are those thin wispy high clouds that often run ahead of an approaching storm system.

What is a "snow roller"?
A snow roller is formed when the wind blows wet snow into a roll. As it rolls along, it picks up more snow and grows larger. If the wind is strong enough, the moving clump of snow becomes cylindrical, often with a hole through it lengthwise. Their size may range from that of eggs to small barrels. The tracks that they make are usually several meters long and less than 1 cm deep. Snow rollers in populated areas may be mistaken as being made by children.

What was meant by the "Perfect Storm"?
The perfect storm was a combination of Hurricane Grace, which formed near the Bermudas, being forced northeastward into the Maritimes by a very strong Arctic cold front descending from Canada. This added push from the front helped to accelerate the speed at which the hurricane tracked towards the Maritimes. As a result, the hurricane didn't have time to lose any of its tropical characteristics and dissipate in strength before it hit the Andrea Gail, the fishing boat featured in the movie, The Perfect Storm.

What causes lightning to sometimes appear green in colour?
On rare occasions especially in strong supercells, the sky will appear to have a greenish tinge during thunderstorms. That is in most, if not all cases, an excellent sign that the storm contains hail, and probably large hail. The greenish color is caused by light (either sunlight, or in some instances the light of a lightning flash), refracting off the large ice particles suspended in the storm.

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