Weather: the temporary state of the atmosphere in terms of temperature, humidity, precipitation (snow, rain and hail), wind speed and direction, sunshine hours, visibility and cloud cover.
Climate: the average weather conditions in a given period of time (usually over a period of 3 decades)
A Stevenson’s screen is a box-like structure that houses weather instruments such as a barometer, a Six’s thermometer and a hygrometer. A Stevenson’s screen is adapted to its function as it is:
- Raised on poles at least 1m above the ground to reduce heat radiation
- Located on grass, not concrete as heat radiation from concrete is artificially high
- Coated in white paint to reduce the absorption of light and heat from the sun
- Louvred/slatted to allow free flow of air
- Located away from buildings, as these may radiated heat and block free flow of air
- Fenced to avoid tampering
Maximum-minimum thermometer or Six’s thermometer measures current temperature and the temperature range in a period of time.
How the apparatus works:
If the temperature rises, the alcohol in the right tube evaporates and fills the air space. This allows the mercury to move upwards. The index is pushed upwards and its bottom marker marks the maximum temperature. If the temperature falls, the mercury retreats but the index stays in place. The antagonistic movement can be noticed in the left tube.
Measurement is taken every hour by reading the height of the mercury at eye level off the temperature scales. The thermometer is reset every 24 hours using a magnet.
A barometer is used to measure pressure (in hectopascal or millibar). Low pressure is indicative of a high chance of rainfall, whereas high pressure often leads to sunny weather.
Wet and dry bulb thermometer
Humidity is measured by recording the temperature of a ‘wet’ bulb with that of a ‘dry’ bulb and using a humidity chart. A wet bulb means that the tube is held in water, whereas a dry bulb is just the normal thermometer bulb.
Usually the wet bulb shows a lower temperature than a dry bulb, but if the wet bulb is coated in ice (ie. temperatures below 0°C the wet bulb shows a higher temperature.
A rain gauge is used to record precipitation. The gauge is anchored firmly in the ground and funnel and jar are placed inside. Rain collects in the rain gauge. Every 24 hours water from the jar is poured into the measuring cylinder and precipitation is recorded. The jar is placed back into the rain gauge after it has been emptied.
Wind vane records wind direction, as the arrow points to where the wind is coming from. Below the arrow, directions (North, South, East and West) have been attached so wind direction can be recorded without the use of a compass.
An anemometer measures wind speed. Three cups turn in the wind and the speed of their movement is calculated and can be read off an electronic display.
Sunshine recorders measure the number of sunshine hours . They have a glass sphere which concentrates the sun’s rays on one point and thus scorch marks on the record time sheet. At the end of the day, the number of hours of sunlight can be viewed from the record time sheet.
Measuring cloud cover
A cloud is defined as a collection of ice or water droplets in the air.
Cloud cover is measured in octas, by placing a mirror onto a flat area so the whole sky is visible. The mirror is then ‘divided’ into 8 parts to estimate the extent of cloud cover (by observation).
Cloud type is also recorded by observation. 4 main types of clouds are:
- Cirrus clouds: high in the atmosphere (above 5500 metres), thin, wispy, quick
- Stratus clouds: low level (below 2000 metres), gray, fast moving, produce light rain and snow
- Cumulonimbus clouds (“giant cauliflower”): up to 10km high and wide, produce rain, thunder and lightning in spring and summer
- Cumulus cloud: low (60-1200 metres) and look like cotton wool, produce light rain
Factors influencing climate
Latitude: Temperature tends to be hotter between the Tropics of Cancer and Capricorn than at the poles, as the curving of the Earth causes light and heat rays to scatter over a much larger surface area near the poles, whereas the equator receives direct overhead rays from the sun.
Altitude: Temperature tends to fall with increasing altitude at an average of 1 °C per 100 m.
Ocean currents: Hot ocean currents tend to result in warmer winters, cold ocean currents usually lead to colder summers.
Prevailing wind direction: Prevailing winds from the poles will lead to colder temperatures than those from the equator.
Proximity to the sea: Areas closer to the sea tend to have colder temperatures in spring and warmer temperatures in autumn, as the sea needs longer to warm up or cool down than the surrounding land.
Aspect: Land facing south (on the Northern hemisphere) will be warmer than north-facing slopes as the sun warms south-facing slopes throughout the day.
There are many different types of precipitation including rain, hail, sleet, ice and snow. Type of precipitation relies heavily on the temperatures of the different atmospheric levels.
Relief rainfall (or orographic rainfall) is when air meets higher land and is forced to rise over it. The air begins to cool and becomes oversaturated and water vapour condenses, leading to rainfall.
Convectional rainfall is caused by atmospheric convection currents. This means that hot air rises and cools, so it becomes more saturated (its moisture-holding capacity decreases). This causes the water vapour to condense and usually leads to rainfall. Convectional rainfall is most common, though not limited to, tropical areas.
Frontal rainfall is caused by weather fronts (ie. when cold and warm areas of air meet). Warm air is less dense so it is forced over the cold air and begins to cool. This means it can hold less water and it begins to rain.
Air Pressure and Winds
Air pressure is the force of the weight of the air on the Earth’s surface. Warm air that rises creates low pressure, as rising relieves the force off of the Earth. Cold air, however, will create high pressure as it sinks. Air pressure is measured in millibar or hectopascal.
Mean sea level pressure is 1013 millibar, so areas with less than that are usually low pressure systems whereas those with more than 1013 millibar are usually high pressure systems. However, an area may be of high pressure even if it is below mean sea level pressure, provided its pressure is high compared to the surrounding pressure systems.
Wind is created by air moving from areas of high pressure to areas of low pressure. Wind direction and wind speed influence characteristics of temperature and moisture that will strike an area, so wind is an important factor in weather forecasting.
The prevailing wind direction is the compass direction from which winds blow most of the time.
The dominant wind direction is the compass direction from which the strongest winds blow.
Presenting weather and climatic information
Synoptic charts are often shown in weather forecasting, particularly on TV and in newspapers. They can depict temperature, fronts of alternating pressure, wind and rainfall all on one map.
Here is an overview of the most common symbols used on synoptic charts.
A tropical cyclone is a rapidly rotating storm system that forms:
- in areas with a warm sea temperature (over 26.5 °C)
- at least 5°N or 5°S from the equator, so Coriolis force can cause rotation in the inward flowing air
A tropical storm produces strong winds, torrential rainfall, and a storm surge (an elevation of the sea surface of up to 6 metres above the norm. Consequently, cyclones often lead to flooding of agricultural land and urban areas, destroying crops, killing livestock and damaging housing property (besides obviously loss of life). Flooding may also pollute water supplies and thereby increase the risk of waterborne diseases such as cholera. Furthermore, tropical storms may disrupt services such as electricity supply by tearing apart power lines. They may also damage the natural vegetation, particularly in coastal areas where erosion is common due to heavy storm surges.
Tropical rainforest ecosystem
The tropical rainforest is the most diverse ecosystem on the planet. While covering only 6% of the land, it accounts for more than 50% of world’s species.
Tropical rainforests have a very high rate of rainfall (especially convectional) and a high humidity, which often leads to radiation fog. The annual temperature is high – around 26-27°C. Diurnal temperature fluctuates a lot but there is little seasonal variation.
Rainfall is high as tropical rainforests are located in a low pressure belt, where concentrated sun rays increase evaporation and thereby result in convectional rainfall. A saturated air mass leads to the formation of cumulonimbus clouds due to condensation.
The tropical rainforest is a multi-layered ecosystem that consists of emergents, canopy, undercanopy, and shrub layer. Emergents are the tallest trees that have grown straight and branch off only at the top to compete for sunlight. Emergents have thick buttress roots to provide anchorage. The canopy is the most densely populated layer of the rainforest, and its trees have shallow roots to collect nutrients from the fertile top soil. The shrub layer consists of ferns and plants with dark, pigmented leaves to absorb more light from photosynthesis. Many leaves have a drip tip, a deep central vein and a waxy cuticle to remove excess water from the surface (to sustain transpiration). The waxy cuticle and sharp-edged sides of leaves also protect against parasites.
Originally, 15% of the world’s land was covered in rainforest, but that number has fallen to 6% due to deforestation. Large scale deforestation is taking place due to logging for industrial uses such as furniture-making. Also, land is being cleared to allow mining or to extend commercial cultivation and livestock. Deforestation may also take place to allow for the construction of settlements and transport routes such as roads.
Deforestation is disrupting food chains and resulting in a loss of biodiversity, as plant species are removed from the ecosystem. Herbivores may not have enough food, and as their population decreases due to starvation, carnivores have nothing to prey on. Also, clearance of land exposes the soil to heavy rainfall and winds, allowing for soil erosion from heavy rainfall. Compaction of topsoil occurs as a result of too little protection by vegetation.
Deforestation of tropical rainforest will increase the levels of CO2 in the atmosphere, as the trees are no longer a carbon sink, but may even emit carbon during combustion for energy. Deforestation can contribute to up to 20% of global greenhouse gas emissions and is therefore considered responsible for poor air quality.
Deforestation is also a problem for the local community, as people rely on forests for subsistence agriculture, hunting and gathering and harvesting forest products.
Hot desert ecosystem
Hot deserts have a high daily temperature and a large diurnal temperature range. They are dry, but receive irregular precipitation, often in the form of torrential rain. Hot deserts have low relative humidity and high pressure.
Tropical deserts are hot and dry for the following reasons:
- The latitude of 15° and 30° north or south of the equator, where the curving of the Earth is low, so light and heat entering the atmosphere is concentrated.
- Convection currents cause air to sink at latitudes of 15° to 30°, so it warms and can hold more moisture.
- Hot deserts are located close to cold ocean currents, so onshore winds blow cold air over the land. Air warms and can hold more moisture.
- Tropical deserts are found in areas of rain shadow, as they are often on the leeward side of a mountain, where there is no relief rainfall, due to prevailing winds from the other side.
- Hot deserts receive little frontal rainfall, as they are located mainly in areas of high pressure, which are formed by stable fronts.
Natural vegetation and wildlife
The tropical desert ecosystem has only sparse vegetation that is specifically adapted to the hot and dry climate. The vegetation has spines rather than leaves to reduce water loss from evapotranspiration and as a protection against predators. Succulents have a trick stem to store water, eg. cacti. Plants have long tap roots to reach the ground water supply. Many plants have a short life cycle, thus they reproduce during wet spells only and their seeds either grow or remain dormant till the next wet spell.
Wildlife consists of mainly nocturnal species, as daytime temperatures are too high to supply sufficient water and energy. However, camels store fat in their humps, as it can be metabolised (respiration) to release water and energy.