We hear words to describe weather every day but do
we really know what they mean and how they occur. This page seeks to
explain the various forms of precipitation and also answer some of the
common questions relating to our weather.
Ever wondered exactly what the weather forecasters mean?
Rain is liquid precipitation. Without clouds, it would not rain. Clouds are made up of water droplets formed when warm, moist air rises high into the sky and cools. That water vapour in it condenses and forms what will become raindrops. Precipitation forms via collision with other rain drops or ice crystals within a cloud. Rain drops range in size from oblate, pancake-like shapes for larger drops, to small spheres for smaller drops. (Please note this is a very simplified explanation of why it rains. The science behind rain is far more complex and detailed).
Snowflakes, the basic unit of snow, originate as tiny ice crystals within "cold clouds." Cold clouds are clouds that exist within air that is at, or below, the freezing point. As an ice crystal is blown back and forth between the top and bottom of the cloud, it grows in two ways: by coalescence and by deposition. In coalescence, the ice crystal collides, and sticks to the cold water droplets it encounters in the cloud. In deposition, water vapor molecules (particles made by the combination of two or more atoms) within the cloud freeze directly onto the ice crystal. As the ice crystal grows, it bonds with other ice crystals and takes on the six-sided shape of a snowflake. When the snowflake becomes heavy enough, it falls to the ground.
Wind is moving air and is caused by differences in air pressure within our atmosphere. Air under high pressure moves toward areas of low pressure. The greater the difference in pressure, the faster the air flows.
Please see 'Definitions of precipitation'.
Snow forms when the atmospheric temperature is at or below freezing (0 degrees Celsius or 32 degrees Fahrenheit) and there is a minimum amount of moisture in the air. If the ground temperature is at or below freezing, of course the snow will reach the ground. However, the snow can still reach the ground when the ground temperature is above freezing if the conditions are just right. In this case, snowflakes will begin to melt as they reach this warmer temperature layer; the melting creates evaporative cooling which cools the air immediately around the snow flake. This cooling retards melting. As a general rule, though, snow will not form if the ground temperature is 5 degrees Celsius (41 degrees Fahrenheit).
No, it can snow even at incredibly cold temperatures as long as there is some source of moisture and some way to lift or cool the air. It is true, however, that most heavy snowfalls occur with relatively warm air temperatures near the ground—typically -9 degrees Celsius (15 degrees Fahrenheit) or warmer—since air can hold more water vapor at warmer temperatures.
The fact that snow piles up year after year in Arctic regions illustrates that it is never too cold to snow.
Snow forms in the atmosphere, not at the surface. So snow can fall when surface temperatures are above freezing as long as atmospheric temperatures are below freezing and the air contains a minimum moisture level (the exact level varies according to temperature).
A layer of snow is made up of ice grains with air in between the ice grains. Because the snow layer is mostly empty air space, when you step on a layer of snow you compress that layer a little or a lot, depending on how old the snow is. As the snow compresses, the ice grains rub against each other. This creates friction or resistance; the colder the temperature, the greater the friction between the grains of ice. The sudden squashing of the snow at lower temperatures produces the familiar creaking or crunching sound. At warmer temperatures, closer to melting, this friction is reduced to the point where the sliding of the grains against each other produces little or no noise. It's difficult to say at what temperature the snow starts to crunch, but the colder the snow, the louder the crunch.
Snowflakes are agglomerates of many snow crystals. Most snowflakes are less than one-half inch across. Under certain conditions, usually requiring near-freezing temperatures, light winds, and unstable, convective atmospheric conditions, much larger and irregular flakes close to two inches across in the longest dimension can form.
Generally, snow and ice present us with a uniformly white face. This is because most all of the visible light striking the snow or ice surface is reflected back without any particular preference for a single color within the visible spectrum. The situation is different for that portion of the light which is not reflected but penetrates or is transmitted into the snow. As this light travels into the snow or ice, the ice grains scatter a large amount of light. If the light is to travel over any distance it must survive many such scattering events, that is it must keep scattering and not be absorbed. The observer sees the light coming back from the near surface layers (mm to cm) after it has been scattered or bounced off other snow grains only a few times and it still appears white. However, the absorption is preferential. More red light is absorbed compared to blue. Not much more, but enough that over a considerable distance, say a meter or more, photons emerging from the snow layer tend to be made up of more blue light than red light. Typical examples are poking a hole in the snow and looking down into the hole to see blue light or the blue color associated with the depths of crevasses in glaciers. In each case the blue light is the product of a relatively long travel path through the snow or ice. So the spectral selection is related to absorption, and not reflection as is sometimes thought. In simplest of terms, think of the ice or snow layer as a filter. If it is only a centimeter thick, all the light makes it through, but if it is a meter thick, mostly blue light makes it through.
On average, 10 inches (25 centimeters) of snow is equal to inch (2.5 centimeters) of water. Heavy, wet snow may contain as much as 16 percent meltwater by volume. A dry, powdery snow may be as little as1/13meltwater by volume.
Frost is a crystalline deposit of small, thin ice crystals. This deposit forms on objects when the air directly above those objects reaches the frost point. The frost point is the temperature at which a given volume of air becomes saturated and thus can no longer hold water in the vapor state—provided that the air temperature is at or below freezing. In the formation of frost, a layer of water initially freezes onto the surface. The layer of frost grows as water vapor from the air directly solidifies into ice without going through the liquid phase; this process is called deposition. Usually frost forms on clear, calm nights, especially during early autumn when the air above the Earth is quite moist. A light frost generally damages only the most tender plants and vines, whereas a heavy frost (a heavy deposit of crystallized water) may kill even hearty (nonwoody) plants.
The answer lies in their thickness – and how far we can see through them (ie: the visibility). If we can see less than 1 km through the cloud of water droplets, it is known as fog. If we can see between 1 and 2 km, we call it mist.
Mist and fog usually form at night when the air is too cold to hold all its moisture. Clear skies mean that the ground gets cold and it then cools the air close to it. This cool air causes condensation and water droplets form in the air. Fogs are thickest when the air can hold a lot of moisture.
Although mist is not as thick as fog, they are both formed in this same way. Mist, however, usually stays closer to the ground and you can see over the top of it. Mist is often seen on autumn mornings when nights get longer and cooler again. This is particularly true in valleys, because cold air sinks down and collects in the valley during the night.
Mist and fog also form over areas where there is plenty of moisture, such as river valleys, lakes and harbours. If warm air meets cold seas or waters (or any cooler surface) then condensation will once again occur and fog will form.
In cold weather, the windchill factor (also called the windchill equivalent temperature or windchill index) is included in weather reports. The windchill factor is a measure of how cold the air feels, due to the interaction of wind and temperature. Wind intensifies the effects of low temperature by removing heat from the body more rapidly than usual.
The windchill factor is the temperature at which the body would lose an equivalent amount of heat, if there were no wind. For instance, if it were 10° Fahrenheit (-12° Celsius) and winds were blowing between 29 and 32 miles per hour (46 and 51 kilometers per hour), the windchill factor would be -35° Fahrenheit (-37° Celsius).
Count the number of seconds between seeing a flash of lightning and hearing thunder. Divide that number by 5 to determine how many miles away the lightning is. For instance, if 5 seconds has elapsed between seeing lightning and hearing thunder, the storm is one mile away.
Lightning is produced in thunderstorms when liquid and ice particles above the freezing level collide, and build up large electrical fields in the clouds. Once these electric fields become large enough, a giant "spark" occurs between them, like static electricity, reducing the charge separation. The lightning spark can occur between clouds, between the cloud and air, or between the cloud and ground. The bolts of lightning are very hot, much hotter than the surface of the sun. It is estimated that the bolt has a temperature of 30,000 to 50,000 degrees F (28,000 degrees C). When this high temperature bolt hits the surrounding air, there is an instant expansion of the air, sending out a shock wave or vibration, which we hear as a sound of explosion (see what causes thunder).
Thunder is the sound caused due to lightning. The flash of lightning and the accompanying thunder occur around the same time. However, lightning is seen first, followed by the sound of thunder after few seconds. This is due to the fact that light wave travels much faster than sound waves.
Thunder is created when the lightning passes. As the air is superheated by the lightning, it expands by a large amount - it basically explodes (compression). However after the lightning has passed and the energy dissipated, the area where it passed through is now at a lower pressure. This creates a suction and air rushes in to fill the gap extremely quickly (rarefaction). Together these actions form a sound wave which we hear as the thunder.
Thus, in short, thunder is caused due to rapid heating and cooling of the air, near the stroke of lightning.
Lightning takes on a range of colors, depending on conditions in the clouds and in the air.
Since 1950, meteorologists have been assigning names to all hurricanes and tropical storms that form in the western North Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico. (A tropical storm is weaker than a hurricane and has maximum sustained winds of 39 to 73 miles per hour [63 to 117 kilometers per hour].) They have been naming eastern Pacific storms since 1959.
Names are assigned in advance for six-year cycles. The names are suggested by countries that lie in the path of hurricanes. The names must be approved by the Region 4 Hurricane Committee of the World Meteorological Organization, which is made up of representatives of countries affected by hurricanes. Once a tropical storm develops, staff members at the National Hurricane Center near Miami, Florida, automatically assign it the next name on the list
A rainbow is composed of the entire spectrum of colors of visible light, from the longest wavelength, red, to the shortest wavelength, violet. The order of colors in a rainbow is easiest to remember by the following mnemonic (a formula that helps one remember something): ROY G. BIV. R=red, O=orange, Y=yellow, G=green, B=blue, I = indigo, and V=violet. Red is at the top edge of the rainbow and violet is at the bottom edge, with the other colors in between.
Rainbows are created both by reflection and refraction (bending) of sunlight in raindrops. As sunlight enters a raindrop, it bends and it is separated into its constituent colors (the colors that comprise white light [ROY G. BIV]. Another Way To Remember The Order Of the Rainbow Is.: Richard Of York Gave Battle In Vain.
Barometric pressure, also called air pressure or atmospheric pressure, is the pressure exerted by the weight of air over a given area of Earth's surface. This value is a function of how many molecules of air there are in a specific area, how fast those molecules are moving, and how often they collide. (Molecules are particles made by the chemical combination of two or more atoms.) Barometric pressure is measured by an instrument called a barometer.
At sea level (the level of the ocean's surface used as a standard in determining land elevation and sea depths), where gravity is strongest and attracts the greatest number of molecules, air pressure is greatest. Because gravity weakens as you go up, air pressure is lower at higher altitudes.
