Earth and Space Science - Weather Systems Weather Components WEATHER COMPONENTS If you were to describe the weather outside right now, what would you say? What information would you include in your description of the weather? Saying the weather is simply nice or not nice, only provides your opinion of the weather and does not give a detailed description of the precise conditions that exist. Weather reporters must be cautious when describing current or long range conditions. Reporting that the weather is simply nice will take on different meanings for different people. Someone heading off on a overnight camping holiday may wish for nice, warm and dry weather, whereas a farmer may prefer nice, cooler, wet weather for their crops to grow. In weather reporting, current conditions tend to include information or data on the following weather measurements: Temperature wind speed/direction Precipitation cloud cover/sunshine Pressure Humidity It is ultimately the information given for each of these measurements that will then allow us to determine and judge, whether the weather be nice. Temperature Temperature is described as a measure of the average energy of motion for molecules or the degree of hot or coldness of an object. Temperature is measured in degrees Celsius ( C) using a thermometer. Galileo Galilei was one of the first scientists to invent a device for measuring temperature. Galileo s thermometer operated using the principle of fluid density changing with temperature. http://www.youtube.com/watch?v=917uc 2MZOGU His thermometer consisted of a glass cylinder filled with fluid. Suspended in this cylinder were several sealed glass balls containing different types of liquids. As the temperature of the fluid increased the density of the liquids in the balls would change. The glass balls would either rise or fall depending upon the liquid density and therefore temperature. The temperature is recorded from the lowest floating ball. 1
There are various types of thermometers and one of most common used in science classes, is an alcohol filled glass tube. This type of thermometer is far safer to use compared to the older style mercury thermometers, as alcohol is far less toxic. No matter what type of thermometer, they all work by means of the same principle observed by Galileo. As the temperature increases, the liquid will expand and rise up the tube. It is this expansion and contraction of liquid that will display the temperature that may be read from the graduated markings on the side of the tube http://en.wikipedia.org/wiki/thermometer http://www.energyquest.ca.gov/how_it_w orks/thermometer.html http://home.howstuffworks.com/therm.ht m 1. Everyone knows what is meant by the description nice weather. 2. In Canada, temperature is measure in degrees Fahrenheit ( F). 3. Thermometers work based on the principle that as liquids are heated they will expand and rise. WIND SPEED Wind speed is determined by the rate at which air moves relative to a fixed point on earth. Wind speed is measured in units of kilometres/hour (km/h) or for nautical purposes, in knots (nautical miles/hour). A device called an anemometer can either measure the pressure or the velocity of the wind. Anemometers have either a series of cups or a propeller that will turn. It is the rate of rotation/turning that will determine wind speed. Anemometers can take a variety of forms, some of which include a weather vane device that is used to keep the instrument aligned into the wind and to determine the wind direction. 2
Canadian meteorologist John Patterson, created the three cup anemometer WIND DIRECTION Wind direction is recorded as the direction from which the wind is blowing. This direction is measured using compass orientations or azimuth degrees that are given in a clockwise direction from a compass north starting point. Instruments used to determine wind direction include: weather vanes, wind vanes, aerovanes, and wind socks. The Beaufort wind scale is a rating system that is used to judge wind intensity/speed based upon observable land and sea conditions. The scale couples Beaufort numbers, wind descriptors, and speeds, with observable land and sea conditions. Terms such as light breeze and gale force wind are descriptors used in the Beaufort scale rating system. http://en.wikipedia.org/wiki/weather_van e http://en.wikipedia.org/wiki/windsock http://www.zetnet.co.uk/sigs/weather/met _Codes/beaufort.htm http://en.wikipedia.org/wiki/beaufort_win d_scale 3
1. An aerovane measures wind speed as well as wind direction. 2. The rate of rotation/turning of the anemometer will determine wind speed. 3. The Beaufort wind scale describes wind intensity by using a series of numbers, descriptors, and visible land/sea observations. PRECIPITATION Precipitation is classified as any form of water that reaches the earth s surface. This may include rain, sleet, freezing rain, hail, or snow. Precipitation is measured in millimetres (mm) using a standard rain or snow gauge. These devices are very simple in form and may consist of an inner and outer overflow tube with gradations on the side as found in the graduated cylinder gauge. There are other types of gauges such as: weighing or tipping bucket gauges and simple buried pit collectors. CLOUD COVER/SUNSHINE Cloud cover refers to the fraction of the sky that is covered by clouds at a specific time and location. Cloud cover is estimated visually based on a fraction or percentage of the sky that is obscured by clouds. The higher the value given, the greater the amount of cloud cover. Estimating cloud cover is difficult on foggy days when visibility is low. Satellite images have made determining cloud cover easier to do and far more accurate. http://commons.wikimedia.org/wiki/cloud _cover#sky_cover_.e2.80.94_bew.c3.b6lk ung http://amsglossary.allenpress.com/glossar y/search?id=sky-cover1 http://www.weather.com/glossary/s.htm PRESSURE Although standard atmospheric air pressure is 101.3 kpa (kilopascals), meteorologists more often report air pressure in the unit of mb (millibar) or bar. Standard atmospheric pressure is 1.013 bar or 1013 mb. A barometer is the instrument that is used to measure air pressure. Simple mercury barometers consist of a tube of mercury in a reservoir pool. As the air pressure rises, a greater force is place on the mercury reservoir resulting in the movement of mercury up the tube. This movement of liquid upward signifies an increase in pressure. http://www.youtube.com/watch?v=vqkka JdarZ8 Today meteorologists use more sophisticated electronic sensors to measure air pressure. These sensors consist of silicon membranes that display a change in electrical resistance under differing pressures. 4
1. Standard atmospheric pressure is measured in pounds per square inch (PSI). 2. As air pressure increases the height of the mercury column in the barometer will increase. 3. An aneroid barometer operates as a result of the expansion and contraction of the aneroid cell. HUMIDITY Humidity reflects the amount of water vapour in the air. Absolute humidity is a measurement of the precise amount of water vapour present in a specific volume of air. It is expressed in units of grams of water vapour per cubic cm of air (g/cm 3 ). Relative humidity is a ratio or percentage of the actual amount of water vapour in a specific volume of air, compared to the maximum amount of water vapour that could be contained in that same air, at a specific temperature. Humidity is measured using an instrument called a hygrometer. A simple hygrometer, or psychrometer contains two thermometers (one exposed to the air and the other surrounded by a wet wick). The difference in recorded temperature from the dry and wet thermometers is used to help determine relative humidity. Another type of hygrometer uses lengths of hair. The relative humidity is indicated by the change in length of the hair strands, as they absorb differing amounts of water vapour. The humidex is a Canadian innovation, first used in 1965. It was devised by Canadian meteorologists to describe how hot, humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. http://www.smc.ec.gc.ca/cd/humidity_e.cf m http://en.wikipedia.org/wiki/psychrometer http://www.physicalgeography.net/funda mentals/8c.html http://science.howstuffworks.com/questio n651.htm 1. Relative humidity is a measure of the amount of water vapour held in the air. 2. When the relative humidity is high, so is the chance of experiencing a bad hair day! 3. Humidex combines temperature and humidity into one reading to reflect the perceived temperature. 5
DEW POINT When air becomes saturated, water droplets are able to form more easily when the dew point is reached. Dew point is the temperature at which dew forms (water vapour turns into liquid water). Water droplets will form when air reaches the saturation temperature and the relative humidity is 100%. http://www.youtube.com/watch?v=zbltzk pmioq FRONTAL SYSTEMS We looked at what happens when two large masses of different temperature air are formed and located in various areas over the continent. The interaction of these air masses creates significant weather dynamics and systems. When air masses of differing temperature meet, a front is formed. Fronts clip COLD FRONTS As warm air is pushed quickly over a cold air mass cumulus clouds are formed just ahead of a cold front. These clouds often result in short periods of heavy precipitation that can possibly lead to severe storms if the frontal system is moving quickly. Cold fronts bring a drop in temperature producing cool dry air. Warm Fronts When a warm front overtakes a cold front there is a gradual transition in weather conditions. Stratus and cirrus clouds are evident with the approach of this type of frontal system. Warm fronts move more slowly in comparison to cold fronts and tend to consist of more stable and calm air. Warm fronts produce lighter levels of precipitation that will last for a longer period of time. Warm fronts bring a rise in air temperature. OCCLUDED FRONTS Occluded fronts are formed when a cold front catches up and overtakes a warm front. This type of frontal system is usually associated with a low pressure cell and brings longer periods of steady precipitation. STATIONARY FRONTS A stationary frontal system occurs between warm and cold air masses that are not changing position. This type of frontal system will persist for several days and tend to have periods of precipitation with longer duration. This type of frontal system is common in summer months. Frontal systems are given specific symbols on weather maps to designate the type of front found and the location/direction of movement. 6
The frontal system symbols are numbered as follows: 1. cold front 2. warm front 3. stationary front 4. occluded front 5. surface trough 6. squall line 7. dry line 8. tropical wave HIGH PRESSURE SYSTEMS High pressure systems (sometimes called high pressure cells, areas, or anticyclones) are associated with air that is moving and pushing vertically down and outward in a clockwise direction. This air contains little water vapour as it is cooler. These pressure systems are associated with calm weather and clear skies that persist for several days. LOW PRESSURE SYSTEMS Low pressure systems (sometimes called low pressure cells, areas, or cyclones) are associated with air that is moving vertically upward. The warmer rising air contains a greater amount of water vapour and therefore upon rising, creates a great deal of cloud. These systems bring unstable, changing weather with stronger surface winds. Low pressure systems bring overcast heavy skies and precipitation. These pressure systems can often create severe weather events and storms. http://www.qc.ec.gc.ca/meteo/documenta tion/front_e.html http://en.wikipedia.org/wiki/low_pressure _area http://en.wikipedia.org/wiki/high_pressur e_area http://en.wikipedia.org/wiki/weather_fron t 1. Frontal systems form between different temperature air masses. 2. Warm fronts move faster than cold fronts. 3. A low pressure system has upward moving air that rotates in a counterclockwise direction to the right. 7