JC-Learn Geography Notes Atmospheric Conditions 1 P a g e
Atmospheric Conditions The atmosphere is a thin layer of gases surrounding the earth. It is usually referred to as air and it is constantly changing. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide and other gases. The earth s atmosphere is about 480 km thick, but most of the atmosphere is within 16 km of surface of the earth. The layers of the atmosphere: Thermosphere Mesosphere Stratosphere Troposphere The troposphere is the lower layer of the atmosphere and weather conditions change constantly within this layer. The Earth Insolation Insolation gets its name from the following words: Incoming, solar and radiation The two major factors that affect the amount of insolation actually absorbed by the earth s surface are: 1. The angle of the sun s rays 2. The content (clouds, dust and water vapour) of the atmosphere The Angle of the Sun s Rays The angle at which sunlight strikes the earth varies with: Location (sun s rays are most direct at equator) Time of day (earth revolves on axis over course of the day) Season (different parts of earth tilted towards sun at different times of year) The Content of the Atmosphere The combination of losses from reflection (30%) and absorption (20%) leaves only 50% of solar radiation to directly heat the earth. 2 P a g e
Reflection - solar radiation can be reflected from a combination of clouds, dust and the earth s surface (ice). Absorption - as energy passes through the atmosphere on its way to earth, ozone, dust, clouds and carbon dioxide absorb about 20% of it. Energy Transfers The atmosphere is always in motion because of the unequal heating of the earth s surface. The energy transfers take the form of global winds and ocean currents. Global Winds Wind is moving air. Difference in temperature causes wind movement. Wind is air moving from a high pressure area to a low pressure area. High pressure is the result of descending air, which is cold. Cold air falls, as it is heavy. Low pressure is the result of rising air, which is warm. Warm air rises, as it is light. Latitude gives the location of a place on earth north or south of the equator. It is the angular distance from the equator and is measured in degrees. Latitude ranges from 0 at the equator to 90 at the Poles. Global winds system: Winds blow from high pressure to low pressure areas. This forms a global pattern of air movement. At the equator, the warm air rises, creating a zone of low pressure. This warm air spreads outwards towards the poles. As it nears the poles the air cools, becoming heavier, and sinks earthwards. This occurs at about 30 north and south of the equator. The sinking air produces areas of higher pressure. The air blows out from these areas of high pressure. Some of it blows back towards the equator, pulled there by vacating warm air, while some of it travels pole wards. The air moving in from the lower latitudes is generally warmer. At about 60 north and south the warm air travelling polewards meets the cold polar air. The areas where these air masses meet form polar fronts and are areas of low pressure. 3 P a g e
At the poles (90 ) the extremely cold air presses down to create a zone of high pressure. Wind belts: The surface winds of each hemisphere are divided into three wind belts - tropical easterlies (trade winds), prevailing westerlies (Westerlies) and polar easterlies (Polar). The Coriolis Effect: Global winds are deflected when the earth rotates. The earth rotates on its axis from west to east. The result is that the winds are deflected to the right of its direction in motion in the Northern Hemisphere and to the left in the Southern Hemisphere. Zones separating the three major wind belts: Doldrums - area near equator where trade winds die out. Horse latitudes - areas of still, dry weather which occur at 30 north and south. Polar fronts - lie at about 60 north and south due to tilt of the earth and influence of sun s rays. 4 P a g e
Ocean Currents Ocean currents can be described as rivers; of water which flow through the oceans. The movement of the ocean currents is the result of a number of factors: Rotation of earth Influence of winds Difference in temperatures in oceans The currents can be hot or cold depending on whether they flow from the equator or from the poles. Warm currents: Flow from areas of low to high latitudes. An example of a warm current is the North Atlantic Drift (Gulf Stream). This flows past the west coast of Ireland, increasing the temperature of the water. Cold currents: They flow from areas of high to low latitudes. An example is the Labrador Current. Effects of Ocean Currents Positive: Negative: Ice-free coasts Creation of icebergs Warmer winds Colder winds Good fishing grounds Difficult sailing conditions Air Masses and Fronts Air masses An air mass is a large body of air that has similar temperature, pressure and moisture levels throughout. Air masses are defined according to their region of origin and course travelled. Masses originating over land are continental (c) and over oceans are maritime (m). They can also travel over Polar (P), tropical (T) and artic (A) regions. 5 P a g e
Many different air masses meet around Ireland and so weather is very changeable: Continental polar (cp) - dry and cold, frosty nights and heavy snowfall. Maritime polar (mp) - cold wet air and showery conditions. Continental tropical (ct) - dry sunny weather. Maritime tropical (mt) - thunderstorms in summer and heavy rainfall in winter. Artic (A) - brings cold weather conditions. Fronts When one air mass meets another, a boundary or front forms between the two air masses. Cold fronts A cold front forms where a cold air mass moves into a warm air mass. Cold air is heavier than warm air, so the cold air pushes underneath the warm air. This forces the warm air mass upward where it becomes cooled and condensation takes place. Cumulus clouds develop and heavy showers may occur. Warm fronts A warm front forms when a warm air mass moves into a cold air mass. Because the warm air is less dense, it slides up and over the colder air. This air cools and condenses. At first, cumulus clouds may appear, and precipitation may follow. 6 P a g e
Occluded fronts When a cold front catches up with a warm front, it forms an occluded front. The warm air is totally lifted off the ground by the cold air in front and behind. Occluded fronts usually have heavy rainfall and strong wind speeds. High Pressure (Anticyclones) High pressure is the result of cool air descending on the earth s surface. When the air descends, it gets warmer and condensation is unlikely, so louds don t form. Characteristics of a high pressure system: Winds blow in clockwise direction Made up of a single air mass The air descends and warms Very slow moving Weather associated: clear cloudless skies, calm conditions and dry weather 7 P a g e
Low Pressure Low pressure systems develop when warm air rises. Winds tend to blow into low pressure areas because air moves from areas of higher pressure into areas of lower pressure. Characteristics of a low pressure system: Winds blow in an anticlockwise direction The air rises and the temperature falls Cold air forces warm air to rise The rising air condenses and the water vapour forms cloud Weather associated: cloudy skies, windy conditions and wet weather Depressions A depression is an area of low pressure that develops when a warm front meets a faster moving cold front. They form where polar and tropical air masses meet. 8 P a g e
Weather Weather is the day-to-day state of the atmosphere over any place on the earth s surface. Weather is made up of a number of different elements: Temperature Humidity Atmospheric pressure Precipitation Wind speed and direction Sunshine Cloud amounts A meteorologist is a person who studies and predicts weather. Meteorology is the study of weather. A weather station is a place with land-based instruments and equipment where different aspects of the weather are monitored and measured. An example of a weather station in Ireland is the one in Mullingar. Weather maps (synoptic charts) - They summarise the weather over a wide area using symbols. Isobars are lines on the map, which connect places of equal atmosphere pressure. Isotherms join places of equal temperature. Isohyets join places with equal rainfall. Isohels are lines showing equal sunshine. Measuring Weather The instruments used to measure the elements of weather are as follows: Instrument Thermometer Hygrometer Barometer Rain gauge Wind Vane Anemometer Campbell Stokes recorder 9 P a g e Measures Temperature Humidity Atmospheric pressure Precipitation Wind direction Wind force (speed) Sunshine
Temperature Temperature measures how hot or cold the air is. A thermometer is an instrument used to measure temperature. It is a small glass tube, which contains mercury/alcohol. With an increase in temperature, the liquid expands and rises. A fall in temperature causes the liquid to contract and fall. Temperature is measured in degrees Celsius ( C). The maximum thermometer measures the highest temperature for the day. The minimum thermometer measures the lowest temperature for the day. Adding the minimum and maximum temperature for that day and dividing it by two calculate the mean daily temperature. The temperature range is the difference between the high and low temperatures recorded. An isotherm is a line on a weather map, which joins places having equal temperature for a time period. Humidity Humidity refers to the amount of water vapour or moisture in the air. Relative humidity is the amount of water vapour in the air compared to the amount it would contain if saturated. It can be measured with a hygrometer. The difference in temperatures between two thermometers is used to find the relative humidity. Pressure Pressure is the weight of the atmosphere. An instrument called a barometer measures atmospheric pressure. The unit of measurements for atmospheric pressure is millibars (mb). Isobars are lines on a weather map, which show areas of equal atmospheric pressure. There are three kinds of barometer: mercury, aneroid and barograph. Mercury barometers are glass tubes containing mercury and are marked in millibars. Aneroid barometer is a metal box partly emptied of air, which forms a partial vacuum. When air pressure rises the box contracts. When it falls the box expands. As the sides of the box 10 P a g e
move in and out, a needle or pointer moves on a dial marked in millibars. A barograph is an instrument, which connects a barometer to an inked pen resting on a chart on a drum, which rotates. Precipitation Precipitation is all the products of condensation, which fall from clouds: drizzle, rain, snow, sleet or hail. Lines of equal rainfall shown on a weather map are called isohyets. A rain gauge is used to measure the amount of rainfall. The amount of rainfall is measured in millimetres, using a graduated cylinder. This measures the amount of rainfall. 11 P a g e
Wind Wind is the movement of air caused by difference in temperature and pressure. A wind vane measures wind direction. It has a horizontal arm with an arrow at one end and a tail at the other. The wind blows the tail round and the arrow points to the direction from which the wind is coming. 12 P a g e
An anemometer measures wind speed or force. It is made up of three cups, which pivot and rotate according to the force of the wind acting upon them. A dines tube measures the direction and speed of the wind. The Dines tube contains both a wind vane and an anemometer. The Beaufort scale measures the force of the wind. Force is recorded on a scale of 0-12. Sunshine Sunshine is measured using a Campbell-Stokes sunshine recorder. It records the number of hours of bright sunshine each day. Isohels are lines on a weather map showing equal sunshine. 13 P a g e
Stevenson Screen This is a meteorological screen to shield instruments against forces of wind, precipitation and direct heat from the sun. It is a storage box for weather instruments and not a weather instrument itself. Clouds Clouds are a collection of tiny water droplets and/or ice crystals held in the atmosphere. Clouds form when water vapour turns back into liquid, creating water droplets. This happens when rising air cools becoming saturated. The water vapour begins to condense as tiny water droplets, forming a cloud. Types of clouds Stratus clouds (layer): These clouds stretch across the sky like a blanket and block out the sun. Cumulus clouds (heap): These clouds look like puffs of cotton in the sky. When these clouds are woolly, they can cause heavy showers. Cirrus clouds (curl): Cirrus clouds are wispy and are high up in the sky. They are associated with fair weather. 14 P a g e
Naming clouds - Nimbus is another word associated with clouds. Adding nimbus means precipitation is falling from the cloud. Cloud names with the prefix alto- are found between 2,000 to 6,000 metres. Cloud names containing the prefix cirr-, are located above 6,000 metres. Role of clouds: To provide precipitation To provide shade from sun To keep heat within the atmosphere Precipitation There are three basic types of rainfall: convectional rainfall, cyclonic/frontal rainfall and relief rainfall. Convectional Rainfall This is caused by the sun heating the earth s surface. Warm air rises as it is less dense. The rising warm air, which contains water vapour, cools and condenses to form clouds. If the air is hot enough, it rises quickly and can cause thunderstorms. 15 P a g e
Cyclonic/ Frontal Rainfall This happens when two air masses of warm and cold air meet, causing a front. This occurs when lighter warm air is forced to rise over heavier cold air. The moisture in the warm air condenses as it cools, which causes clouds and rain. Relief Rainfall This occurs when warm, water-bearing air is forced to rise when it reaches a mountain range. The air is then forced up over an area of high land. This causes the air to cool and the moisture in the air condenses resulting in the production of clouds and rainfall. Rain falls on the windward side of the mountain. As the air moves over the mountain, it descends on its leeward slope (rain shadow). 16 P a g e
The Water Cycle The water/hydrological cycle is the continuous movement of water around the earth s oceans, land and atmosphere. The hydrosphere is the collective mass of water found on, under and over the surface of the earth. The circulation of water happens through the processes of evaporation, condensation, precipitation and run-off. 1. Evaporation occurs when the sun heats up water and changes it from liquid to gas (water vapour) in the air. 2. The light water vapour in the air rises and cools. It then condenses and changes back into liquid in the form of tiny droplets, which then form clouds. 3. The cooling and condensing continues until the cloud becomes saturated (cannot hold any more water at that temperature). Eventually, the droplets are too heavy to be suspended and precipitation occurs. 4. Run off - when the precipitation returns to the earth s surface it flows to the seas and lakes via the rivers and stream, or through the soil and rocks. 17 P a g e