Science 10
Weather Dynamics

Energy Transfer Weather is all about the Sun and its effects on Earth. Most of the energy needed to support life on Earth and cause our weather is supplied by the Sun. Energy is transferred four ways: radiation, conduction, convection and advection.

Radiation is the motion of energy in the form of waves. We are familiar with waves moving through water but perhaps not with radiation waves that can move through the vacuum of space. It is this type of wave that brings the Sun's energy to us. The types of energy that can move in the form of radiation waves are part of the electromagnetic spectrum. Here are some examples of this type of energy: heat, light, X-rays, radio waves, microwaves and TV waves. Are these energies also able to move through matter? Yes - these types of energy are all around us, moving through the air, through our homes, through us. Are all these types of energy made by the Sun? All except TV signals.

Conduction occurs when heat moves through matter. Hold a warm object and the heat travels from it into your hand. During conduction, adjacent particles bump into each other, transferring their heat energy at the moment of collision. Imagine passing textbooks along a row of people swaying to and fro. As the people bump shoulders, they reach for a book with one hand and pass one on with the other hand.

Convection and advection are similar to conduction but occur when particles in fluids (liquids and gases) bump together to transfer energy.

Here's a "thought experiment". Imagine your hand is next to a lightbulb. Touch it and conduction moves the heat into your skin. Place your hand above it and feel the heat carried up in the rising air due to convection. Hold your hand to the side and feel the heat reaching your hand due to radiation.

Energy Absorption and Reflection Light colors reflect, dark ones absorb energy - that's no surprise but it is true for the Earth just as for clothes. Clouds and snow are light colored while forests and grasslands are darker. What happens to the Sun's energy when it gets to Earth? Say that the total amount of solar energy is given the value of 100%. Clouds reflect 27 %, light colored surfaces on the Earth reflect back 3 %, clouds also absorb 20 % and the surface absorbs the remaining 50 %. So, the clouds and Earth both reflect and absorb. See that a total of 30 % is reflected and 70 % is absorbed. The Sun is far away and the Earth is tiny so we get just a very small amount of the Sun's total energy output but the 70 % absorbed by the Earth is enough to create an average worldwide temperature of 15 C.

Seasons The Earth's rotational axis is tilted at about 23.5⁰. From spring to fall, the Earth's tilt points the Northern Hemisphere toward the Sun. This means the Sun's path carries it high above the horizon as it travels across the sky. This means longer days and the Sun's rays striking the ground more directly. Direct rays mean the Sun's energy is more concentrated where ever it hits the Earth. These factors create long, warm summer days. From fall through winter to spring, the Earth's tilt causes the Northern Hemisphere to lean "away" from the Sun. Now, the Sun's path carries it closer to the horizon as it travels across the sky. This means shorter days and the Sun's rays striking the ground at a slant. Slanted rays mean the Sun's energy is less concentrated where ever it hits the Earth. These factors create short, colder winter days.

Wind Patterns Air moves because it is unevenly heated. When it gains heat, it becomes less dense and rises. As it ascends, it cools and, at some point begins to move sideways. Then, cooler still, it descends back toward the surface. When it reaches the surface, it moves across the ground toward the location where it ascended in the first place. As it moves over the surface, it is warmed and, when it is hot enough, up it goes again. This up - over - down - across - up motion is called a convection current. A low pressure area is formed where the air rises and a high pressure area forms where the air falls back to the surface.

Once the air is in motion, another factor takes over. The spin of the Earth creates an effect called the Coriolis Effect. This effect creates the variety of wind pathways that exist around the globe. In general, the Coriolis Effect causes winds flowing away from the equator to turn to the right, i.e., to flow to the East. And, it causes winds flowing toward the equator to turn to the left, i.e., to flow to the West. So, depending where you are in the world, there will be a prevailing (most common) wind direction. In the age of sail, captains had to know these prevailing wind patterns to get from one port to another. Certain winds would push their ships East, other wind directions carried them West.

Convection currents, low pressure and high pressure areas are basic, important features of air circulation in the atmosphere. Another aspect of prevailing wind patterns are the jet streams. Streams? Yes, there are four. The convection currents we described above form at well defined locations in the atmosphere. Air between adjacent convection currents can become squished into fast flowing "tubes" of air called jet streams. They tend to flow West to East, occur at about an altitude of 7 000 m, and are a major factor in the motion of weather systems.

Water Cycle Water is an important, common part of the atmosphere. Just as air circulates through the atmosphere, so does water. The water cycle shows where and how tons and tons of water moves between the surface and atmosphere. Evaporation carries water up from streams, rivers, ponds, lakes and oceans. Sublimation transfers water from ice (in the polar areas) to the atmosphere. Transpiration moves water from leaves up into the air. As the water accumulates in the atmosphere, clouds form. Eventually, precipitation (rain, snow, hail, sleet) returns the water to the surface and the cycle continues.

Ocean Currents Just as there are prevailing wind patterns in the atmosphere, so are there major current patterns in the world's oceans. In general, water in equatorial regions tends to move in warm surface currents. As these currents move into cooler areas, north and south, they sink and then flow back toward the equatorial zones. As they reach the warmer equatorial region, they warm and rise to the surface again. See that this rising of warm water and sinking of cooler water is just like the convection currents in the atmosphere. The circulation of ocean water tends to even out temperatures around the globe. For example, the Gulf Stream moves huge amounts of warm equatorial waters over to the western coasts of the British Isles, creating unusually warm climates there. Ocean currents also distribute nutrients for ocean life. Fishermen know where currents rising from the depths concentrate food and where fish are therefore likely to be found.

Clouds We mentioned clouds as a part of the water cycle. Clouds form in one of three ways. Convective clouds are created when warm moist air is carried aloft in a convection current. As the air ascends, it cools and condensation of the water vapour creates the cloud. Frontal clouds form when a cold and warm front meet. As the cold air wedges in underneath the warm moist air, it is forced up. Cooling and condensation result in clouds. Orographic clouds form when warm moist air flowing across the surface runs into mountains and is forced upward. Again, condensation of the water vapour in the rising air results in clouds. Fog is just a cloud very close to the surface. It forms when moist air is cooled by breezes from the ocean or when most air loses heat on cold, cloudless evenings. So, anytime moist air becomes cooled enough, clouds form. In winter, your exhalations become temporary clouds. So does the exhaust from cars.

Clouds are classified according to: shape, altitude and rain making ability. Lumpy shapes are called cumulus while flat ones are stratus. Medium altitude clouds are alto and high altitude ones are cirrus. rain making clouds are called nimbus.

Weather Systems A weather system is a set of conditions: moisture, temperature, air pressure and wind speed. A weather system will control the weather in a certain area for a few days. Some weather systems create stable conditions. For example, equatorial weather is pleasant and predictable: twelve hours of light and of dark, hot, humid, often partial clouds. In contrast, weather in the mid-latitudes is complex and changeable because of the effects of coastal and continental air masses. Generally, coastal air masses are moist and continental ones are dry. The latitude determines the temperature of the air masses: polar air masses are cool and tropical ones are warm. (Insta-review: latitude is how far North or South we are; longitude is how far East or West we are.) The prevailing wind patterns over North America tend to move cold air southward and warm air northward. It is the mix of temperature and moisture level that creates the variety of weather across North America.

Pressure Systems Remember we mentioned low and high pressure are associated with convection currents. Low pressure systems tend to bring clouds, storms and warm temperatures while high pressure systems bring clear, calm cooler weather.

When warm and cold air masses run into each other, fronts or boundaries forms. A cold front is the leading edge of the cold air mass; it forms where cold air pushes in underneath warm air. A warm front is the leading edge of the warm air mass; it forms where warm air runs up over cold air. Recall that the rising warm air results in clouds and rain. Where cold air pushes underneath warm air, the warm air is forced rapidly upward and a thunder storm results. In the case of warm air advancing up over a cold air mass, the rate of ascent is more gradual and a wide rain cloud forms.

The ascent of the air is assisted by the jet stream which sucks it up out of the area where the fronts meet and so a low pressure system forms. As the air rises, the Coriolis Effect causes it to swirl counterclockwise direction, in a "cyclonic" direction. Eventually, the ascending air cools, dries and returns to the surface where it forms an area of high pressure. As the air hits the surface and spreads out, the Coriolis Effect causes the currents to swirl in a clockwise (anti-cyclonic) direction.

Regional Weather Local conditions may create specific (regional) weather conditions. When the sun heats the ground, convection currents form. The thermal updrafts keep birds and gliders aloft for hours. Sometimes, the thermals result in afternoon thunderstorms.

Sea breezes form during the day when air rises above the warm land, drawing in cooler air from the ocean. Land breezes form at night when the water is warmer so the air above the ocean rises, drawing out breezes from the land. Cold air blowing across an open lake will become warmer and pick up moisture from the lake. When the moist air reaches shore, the cold land causes it to cool and condensation creates a local lake-effect snowstorm, sometimes intense. When wind passing up over a mountain range descends on the other side, it compresses as it flows onto the ground at the foot of the mountain. The compression creates friction among the air particles which causes the temperature of the air to increase by several degrees. As the warm air flows out from the foot of the mountain, it is called a chinook and brings short lived, unusually mild conditions to the towns in its path.

Weather Forecasting Short and long term forecasts are the most difficult to make. Instruments aboard satellites, aircraft, balloons and rockets provide the "big picture". But, local, onsite data collection provides the detail needed to increase accuracy. Even inexpensive weather stations gather the necessary data: temperature, rainfall, wind, humidity and air pressure.

Weather records have been kept in one form or another for hundreds of years but only in the last couple of hundred have they become "scientific". Today, calendars and books abound with weather folklore and records. Weather forecasting has become a business not just a service.

Weather Phenomena Two dramatic examples of short duration weather phenomena are thunderstorms and tornados. We know thunderstorms require a sudden uplift of tons of moist air. Condensation due to cooling of the rising creates rain, ice and hail. Once the air has cooled, it descends. The strong up- and downdrafts keep the ice and hail in motion, rubbing against each other, creating huge static charges. When they build to a sufficient intensity, a discharge occurs, within the cloud or between it and the ground. Thunderstorms die out when the volume of descending air overpowers ascending air currents.

Tornados are associated with severe thunderstorms because the huge mass of ascending air can be set swirling counterclockwise by the Coriolis Effect. The rotational motion shapes the cloud into a funnel. As air flows into the base of the swirling mass, the rate of rotation increases, making the funnel more distinct. The air pressure inside the funnel is much reduced so, when the swirling winds tear apart things, the pieces get pulled up into the funnel.

Two longer lived weather phenomena are El Nino and La Nina. Normally, equatorial surface currents in the Pacific flow westward, away from the coast of Peru. This allows cold water to well up along the coast, bringing up nutrients for fish. But, in certain years, the waters off the Peruvian coast become hotter than usual. The temperature increase causes the water to expand and form a dome. Water flowing away from the dome reverses the equatorial surface currents so they move toward the Peruvian coast instead away from it. The warm water pushing in against the coast acts like a cap, blocking the upwelling. This phenomenon is known as El Nino. No upwelling - no nutrients - no fish and so the complex food chain, and the huge Peruvian fishing industry, both suffer. Because El Nino is a large scale phenomenon, it affects weather over a wide area. North American winters are milder, especially along the western coast, and precipitation in the southern parts of the United States is increased.

A La Nina is the opposite phenomenon - a cooling of eastern Pacific waters. A result is cooler winters and less precipitation.

What's Ahead? Emissions accumulating in the air due to from the combustion of fossil fuels has increased greenhouse gases, resulting in an increase in the Earth's average temperature. Water levels are on the rise, the ice caps appear to be receding and food chains are being affected. What does the future hold? No one has a clue (plenty of guesses!) but weather research and attempts to reduce the greenhouse effect are efforts in the right direction.

With so much at stake and with so little known, are there experiments to purposely manipulate the weather? A couple of examples. Some coastlines, e.g., Peru, Equador and Chile, are subject to frequent fogs but are otherwise quite dry. Plastic mesh fences are placed in the path of the coastal fog banks driven inland by breezes. The moisture in the fog collects on the mesh and becomes water droplets. These are collected and can be used for drinking and washing. Another example of getting water in dry areas is the practice of releasing silver iodide crystals into clouds. The crystals act as moisture magnets, encouraging the formation of rain droplets. This same technique also can be used to reduce the formation of hailstones in thunderclouds. If the crystals result in more rain droplets being formed, less moisture is available for the formation the hail. So, less hail and less damage to property.