This booklet belongs to

______

Fall 2011

WEATHER UNIT (20%)

WEATHER UNIT / Outcome Number / Outcome Description
Students should be able to:
Weather: Observations & Measurements / 212-1 / Identify questions to investigate that arise from the operation and findings of the weather station
213-3, 213-6, 213-7 / Use weather instruments effectively and accurately for collecting local weather data and collect and integrate weather data from regional and national weather observational networks
331-5, 214-10 / Analyse meteorological data for a given time span and predict future weather conditions, using appropriate technologies and methodologies, identifying sources of error and uncertainty.
Water Cycle / 331-1, 214-3 / Using scientific theory, identify questions, illustrate and explain heat transfers that occur in the water cycle. Use this information to describe winds.
331-3 / Describe how the atmosphere and the hydrosphere act as heat sinks in the water cycle.
213-2 / Design experiments that can be used to determine the latent heat of fusion and vaporization for water.
Weather Dynamics: Heat and Energy / 331-2 / Use weather data to describe and explain heat transfers in the hydrosphere and atmosphere showing how these affect air and water currents.
215-5 / Illustrate and display how science attempts to explain seasonal changes and variations in weather patterns for a given location.
Global Weather Systems / 331-4, 115-2 / Describe and explain the effects of heat transfer on air and water currents and the consequences to the development, severity and movement of weather systems (e.g. storms).
Understand the formation of lightening storms, hurricanes and tornadoes.
117-10,
115-6 / Describe examples of Canadian contributions and weather forecasting and satellite imaging, showing how scientific knowledge evolves.
STSE & Weather Forecasting / 118-2, 117-6, 114-6 / Identify and report the impact of accurate weather forecasting from the person to the global point of view.
118-7, 214-11, 116-1 / Analyze and report on the risks, benefits, and limitations of society’s responses to weather forecasting.

Weather Instruments & Equipment

Weather forecasts today depend on collecting and analysing data and measurements from around the world. As a variety of atmospheric conditions need to be recorded, a wide range of equipment is needed to obtain that information. Details of some of this specialist meteorological equipment is given below.
/ Thermometer – measures temperature in degrees centigrade (°C) or degrees Fahrenheit (°F) using a liquid such as mercury that expands when it warms up. It then moves up a thin tube marked with a temperature scale, and will fall back down the tube as the temperature falls and the liquid contracts. Thermometers are kept in a white Stevenson screen which allows air to circulate but shields the thermometer from direct sunlight. This ensures the measurements are correct and accurate. Maximum and minimum thermometers record the highest and lowest temperatures reached daily.
/ Anemometer – measures the speed or force of the wind. The speed that the cups rotate shows the wind strength.
/ Barometer – measures air pressure. Pressure falls when it is about to rain and rises when the weather is dry. You can see this as the needle moves.
/ Hygrometer – measures the amount of moisture in the air. It usually incorporates a needle that is made to move by a paper strip which shrinks or stretches depending on the dampness of the air (i.e.: the humidity).
/
Rain Gauge – shows how much precipitation (rain, snow or hail) that falls each day.
/ Wind Sock – shows the speed and direction of the wind. They are most often used at airports, seaports and on other open areas such as mountain roads.
/ Weather Vane – measures wind direction. It is always recorded as the direction from which the winds are blowing, ie: a south-westerly wind is blowing from the south-west.

Global Air Circulation & Winds

General Circulation

The worldwide system of winds, which transports warm air from the equator where solar heating is greatest towards the higher latitudes, is called the general circulation of the atmosphere, and it gives rise to the Earth's climate zones.

The general circulation of air is broken up into a number of cells, the most common of which is called the Hadley cell. Sunlight is strongest nearer the equator. Air heated there rises and spreads out north and south. After cooling the air sinks back to the Earth's surface within the subtropical climate zone between latitudes 25° and 40°. This cool descending air stabilises the atmosphere, preventing much cloud formation and rainfall. Consequently, many of the world's desert climates can be found in the subtropical climate zone. Surface air from subtropical regions returns towards the equator to replace the rising air, so completing the cycle of air circulation within the Hadley cell.

Other circulation cells exist in the mid-latitudes and polar regions. The general circulation serves to transport heat energy from warm equatorial regions to colder temperate and polar regions. Without such latitudinal redistribution of heat, the equator would be much hotter than it is and the poles would be much colder.

Without the Earth's rotation, air would flow north and south directly across the temperature difference between low and high latitudes. The effect of the Coriolis force as a consequence of the Earth's rotation however, is to cause winds to swing to their right in the Northern Hemisphere, and to their left in the Southern Hemisphere. Thus the movement of air towards the equator swings to form the northeast and southeast trade winds of tropical regions. Air flowing towards the poles forms the westerlies associated with the belt of cyclonic low pressure systems at about 50 to 60° north and south. In general, where air is found to descend, high pressure develops, for example at the subtropical latitudes and again near the poles. Where air is rising, atmospheric pressure is low, as at the equator and in the mid-latitudes where storms or frontal systems develop.

Prevailing Winds

The direction of wind is measured in terms of where the air is coming from. A northerly wind blows air from north to south. A southwesterly wind blows air from the southwest to the northeast.

The prevailing wind is the wind that blows most frequently across a particularly region. Different regions on Earth have different prevailing wind directions which are dependent upon the nature of the general circulation of the atmosphere and the latitudinal wind zones.

In general, the following prevailing winds across the Earth may be identified, although variations arise due to the positions and differential heating rates of the continents and oceans.

Latitude / Direction / Common Name
90-60°N / NE / Polar Easterlies
60-30°N / SW / Southwest Antitrades
30-0°N / NE / Northeast Trades
0-30°S / SE / Southeast Trades
30-60°S / NW / Roaring Forties
90-60°S / SE / Polar Easterlies

Wind generally blows from high pressure to low pressure.

The British Isles lie at the convergence zone between the warm southwest antitrades and the cold polar easterlies. Convergence of warm and cold air masses at this latitude produces cyclonic low pressure and the development of rain-bearing depressions, which sweep across the Atlantic to influence the British climate. The prevailing winds are the mild southwesterlies, but for much of the time, the British Isles are influenced by polar air masses with a northwesterly or northerly airflow, that bring with them colder showery weather.

Prevailing winds in the Indian Oceans are northeasterly. During the summer months however, a larger low-pressure system develops over southern Asia due to continental heating. Winds in this region now reverse to form the Southwest Monsoons, which bring a prolonged wet season to Southeast Asia and the subcontinent of India.

Sea Breeze

Nature and Causes
Sea breezes occur during hot, summer days and are caused by unequal heating of adjacent land and water. During the day, solar radiation causes the land surface to heat up more quickly than the water surface. Therefore, the air above the land is warmer than the air above the ocean. Because of the fact that warmer air is lighter than cooler air the warm air above the land surface is rising. At the same time the cool air above the ocean is flowing over the land as to replace the rising warm air. This is the sea breeze. As the sea breeze moves inland, the cool sea air advances like a cold front characterized by a wind change, a temperature drop and a rise in humidity. A drop in temperature of 2 to 10 C degrees within 15-30 minutes is usual as the sea breeze advances.

Development of Sea Breezes
With weak general wind circulations, a sea breeze will occur over the coastline soon after the temperature of the land surface begins to become higher than the water temperature (late morning to early afternoon).

The more the difference increases, the stronger the sea breeze will become and the farther will extend inland. The highest wind speed will occur mid to late afternoon. A weak sea breeze will die away soon after sunset, but a stronger sea breeze will remain at the coast until 8 to 10 p.m.

Land Breeze

Nature and Causes
Land breezes occur at night. The land surface cools down more quickly than the water surface. Therefore, the warmer air above the ocean is lighter and is rising. The heavier cool air over the land is flowing towards the water in order to replace the rising warm air. This flow is called land breeze. It can expand for about 10 km seaward.

Climatology of the Sea Breeze
Along coasts with steep shorelines or volcanic island coasts, however, it is a frequent phenomenon sometimes reaching speeds up to 32 km/h. in the temperate zones the land breeze occurs in the cold seasons, especially when there is a warm sea current flowing along the coast.

The Affect of Land & Water on Climate

Climates around the world are shaped by the transfer of heat energy via the general circulation of the atmosphere. The idealised model of climate zones, pressure patterns and global wind belts that can be sketched however, is complicated by the position of continents and oceans. Land surfaces react quickly to heat gain and loss, becoming warm in summer, cold in winter. The oceans react far more slowly and during the summer they are cooler than the adjoining land, whilst in winter they are warmer.

The moderating influence of the oceans helps to restrict extremes in climate in coastal areas of the world. The typical annual average temperature range in the UK is only about 10 to 15°C, whereas in central Siberia or central Canada it can be over 40°C. For this reason maritime climates influenced by airflow from the oceans are usually more pleasant than the continental climates of landmass interiors, although they are frequently wetter.

Lightning Storms

Step by step

Lightning is one of the most unpredictable forces of nature. It can strike from minor or major storms and can hit a target 10 or even 25 miles away from the parent cloud. There's no way to see it coming, because it happens so fast, and scientists are still trying to understand how it works.

The most commonly accepted theory of how lightning forms is that, when ice and water particles collide in a cloud, they become charged. Lighter particles tend to be positively charged and end up near the top of the cloud, while negatively charged particles collect near the bottom of the cloud. (Sometimes storms have inverted polarity, and some research has found there can be several layers with different electric charges in a cloud, but all that's too complicated to get into here!)

Simplified lightning diagram
In a typical storm, rain and ice particles collide, separating charge in the cloud. As a negative stepped leader descends from the cloud, positive charge is enhanced at the ground. / Ground streamer and descending bolt connect and "complete the circuit." Return stroke, from ground to cloud in less than 100 microseconds, is the bright bolt seen by the eye.

So ... in your typical storm cloud, the negative charge at the bottom of the cloud forces the negative charges on the ground to repel away from the clouds and the positive charges to collect along the ground and the surfaces of all buildings. The cloud wants to "complete the circuit" and sends out a stepped leader toward the ground. That further enhances the positive charge near the ground, and objects may form a streamer of positive charge that reaches up, trying to make the connection. In less than a second, the process is complete: The stepped leader from the cloud makes the connection, and a return stroke from the ground, which we see with the naked eye, flashes brightly as it channels the charge to the cloud. Multiple discharges through the same channel make the lightning seem to flicker. Woe to the object that serves as a focal point for this lightning discharge, which moves at 60,000 miles per second!

Leaders from the cloud can take many steps to reach the ground - as many as 10,000. Each bolt has the potential to be as strong as a billion volts with temperatures as high as 54,000 degrees Fahrenheit, the National Aeronautics and Space Administration says.

Because a lightning bolt is so hot, it superheats the air around it. The air particles quickly expand and contract, so fast that they break the speed of sound and create sound waves - that is, thunder. If you want to know how far away a storm is, count the seconds between a lightning flash and the sound of thunder (light travels faster than sound). If you count to 5 between flash and thunderclap, the storm is a mile away. Add a mile for every 5 seconds after that.

Tornadoes

Tornadoes are short lived, but their extremely strong and fast moving winds do incredible damage. Tornadoes result from conditions that arise due to thunderstorms so you must understand thunderstorms to understand tornadoes.

THUNDERSTORMS

In order for a thunderstorm to occur you need two things:

A)  A lot of moisture to form clouds and rainfall;

B)  A lot of quickly rising air (uplift) to push the tops of clouds high up into the air.