Weather and Climate

WEATHER is the short-term conditions of the atmosphere around us

CLIMATE is the long-term average conditions of a place

Convectional Rainfall

This happens as the sun heats the earth. The earth heats the air above it which causes it to start rising. As it rises it cools, condenses and rains.

On a sunny day clouds often form in the afternoon due to this process and can lead to heavy rain.

Relief Rainfall

This is caused by physical features causing the air to rise.

Mountains cause the air to rise, it cools and condenses. They often have clouds around the tops and have higher rainfall totals.

Air Pressure

Low pressure = rising air. This happens around the equator due to the strength of the sun. It creates a lot of cloud and rain, often unstable weather conditions.

High pressure = descending air. As it descends it warms up. It brings clear skies and sun. This happens around 25 degrees North and South of the equator and creates deserts.

Atmospheric Circulation

Equatorial low: low pressure surrounding the equator. This is where tropical rainforests exist.

Sub-tropical high: high-pressure areas around the tropics of Cancer & Capricorn. This is where the major deserts of the world exist.

These air cells redistribute heat around the globe through moving warmer air from the equator north & south, and colder air from the poles towards the equator.

Solar Radiation Received

The concentration of the suns energy reaching the earth diminishes with distance from the equator. As the diagram shows, the curvature of the earth results in the solar radiation being spread over a much wider surface area at higher latitudes.

The result of this is much warmer temperatures around the equator which results in high levels of evaporation from the oceans.

Distance from Coast

Sunlight warms only the top layer of the earth's surface whereas in the oceans the sunlight penetrates much deeper. Because of this, the land heats up quicker in the Summer month than the oceans. In Winter though, the land loses its heat much faster.

Coastal climates are likely to be warmer in Winter and cooler in Summer than continental locations.

Incoming solar radiation enters the earth atmosphere. Some is reflected by clouds and some more is reflected by atmospheric pollution.  On reaching the earth's surface some of the light is reflected depending on the albedo rate (ice sheets, oceans etc). The rest is used by plants (photosynthesis) or absorbed by the land and oceans. This heats the land and oceans with the heat being released into the atmosphere as longwave infrared radiation. The greenhouse gases trap some of this infrared radiation in the atmosphere. Increasing levels of greenhouse gases in the atmosphere increase the amount being re-emitted back in the atmosphere.

Stevenson’s screen

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

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.

Barometer

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.

Rain gauge

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

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.

 Anemometer

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.

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

Climate

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. The 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.

Vegetation

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.

Climate

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.

Vegetation

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.