Differences Between Weather and Climate

Weather 

• Weather is the overall state of the atmosphere on a times-scale of minutes to months, wit particular emphasis on those atmospheric phenomena that affects human activity.
• It is the condition of the atmosphere at a particular instance.
• It takes into consideration the conditions over a few days to a week.
• All the short-term day-to-day condition of the atmosphere involving a description of rainfall, humidity, pressure, temperature, cloud cover and winds.
• Weather may change from day to day and is usually expressed with descriptive data.

Climate

• The long-term atmospheric characteristics of a specified area. The characteristics are usually represented using numerical data on meteorological elements such as rainfall, temperature, wind, pressure and humidity. 
•  Refers to the average as well as the highest and lowest rate of temperature, pressure, rainfall, cloud cover, etc. over a period of at least thirty years.
• The climate of a place is a description of the average weather conditions that occur there, measured over a long period of time, usually thirty years. 

Factors Influencing Weather and Climate

Relief and Altitude

Relief refers to the variations in elevations and slope of a particular area on the earth’s surface. For example, when we say an area is flat, gently sloping or mountainous (hilly) we are describing the relief of the land. The relief of an area can influence the weather and climate of that area.  Generally, air temperature decreases with altitude because air at higher altitudes is less dense and cooler. Temperature decreases with height on average 10^oC/km.

Latitude

Latitude is one of the most influential factors of temperature. Latitudes are imaginary lines drawn around the earth parallel to the equator.  Two factors affect the temperature: the angle of the overhead sun and the thickness of the atmosphere. At the equator the over-head sun is high in the sky, as result, high intensity of insolation is received.  On the other hand, at the poles, the sun is low in the sky so less energy or heat is received resulting in cooler temperatures.

Secondly, the thickness of the atmosphere affects temperature. At higher latitudes reduced sunrays strike the earth’s surface. The heat is spread over a larger area and is diffused resulting in lower temperatures in these areas. 

Distance from the Sea

Land heats and cools more quickly than water; this affects the temperatures of coastal and inland areas. Places nearer to the sea will have fewer variations in temperature than places further inland. The sea moderates the temperatures near the coastal areas.  Water takes up heat and emits it much slower than land. This is known as maritime influence. The sea has little influence on the interior of continents. Here the temperatures are more extreme. These areas are under continental influence.

Land and Sea Breeze

Land and sea does not heat up at the same rate. The earth’s surface heats up faster than the ocean as a result; there is a lower pressure over the land than the sea. Wind blows from high pressure to low pressure. Therefore during the day, the wind blows from the sea to the land lowering the temperature. On the other hand, during the night, the land cools and the seas stay warm. The low pressure is now over the sea causing the breeze to blow from the land towards the sea.

Tropical Marine Climates

Several islands and coastal areas located close to the equator experience a a tropical marine climate. These areas such as most of the Caribbean islands and Guyana are located within 10 to 20 degrees north or south of the equator.

Location

Tropical marine climates can be found within the tropics. Specifically 231/2⁰ N and 231/2⁰ S of the Equator. Places located in this region include the West Indies, Central America, the Caribbean coast of South America, the East coast of Brazil, the Philippines.

Climate & Temperature

The prevailing temperature throughout the year is high and uniform but varies slightly as the distance from the equator increases and the position of the overhead sun.

The average temperature during the year is 28 degrees Celsius with an annual range of 3 to 4 degrees Celsius.

In some instances the temperature may be modified by the Trade Winds and land and se breeze in countries such as Barbados. The northeast Trade Winds blows across the Atlantic to the islands bringing moisture from the Atlantic Ocean and influencing the temperature.  

These regions are often affected by tropical waves (regions of low pressure) that are associated with heavier rainfall. These tropical waves sometimes develop into a tropical depression or a hurricane. 

Precipitation

This climatic region is marked by seasonal rainfall. There is a dry season and wet season, however, there is rainfall throughout the year with the wettest period being from late summer to early autumn.  Annually, the total rainfall ranges approximately from 1200mm to 2000mm. 

Much of the rainfall is attributed to the trade winds that blow onshore forming relief or orographic type rainfall. However, a considerable amount of convectional type rainfall also results from the moist tropical air masses of the Inter Tropical Convergence Zone (ITCZ), which moves with the seasons.

Natural Vegetation

A number of vegetation types can be identified in the tropical marine climate (See table below). In the Caribbean region, the vegetation varies and zoned with increasing altitude. The dominant type will depend on various factors.

                                                     Table: Types of vegetation found in Tropical Marine Climates

Equatorial Climates

The equatorial climate lies in regions close to the equator. An abundance of plants and animals thrive in these regions.

Location

The equatorial regions are located within 0⁰ to 10⁰ north and south of the equator. Places in this region includes, the Amazon Basin in Brazil, South America, the Zaire Basin in West Central Africa, Indonesia and East Indies, specifically Borneo, New Guinea, Java and the Guinea Coast. Equatorial regions blend with wet and dry climates.

Climate & Temperature

Throughout the year, the climate is hot and wet generally, averaging 28⁰C. The diurnal range (the difference between maximum and the minimum temperature for the day) or daily temperature range is about 2 degrees Celsius. Rainfall and humidity is above normal particularly during the months of April and October during the equinoxes. Rainfall is generally convectional accompanied by thunderstorms.

Precipitation

The precipitation is heavy, often and well distributed through the year. There is no dry season with average annual rainfall being at 2000 to 4000 mm with daily averages of 100mm. There is a high humidity, over 80%.

Natural Vegetation

The natural vegetation in the equatorial regions is mainly equatorial or tropical rain forest.

The vegetation in the forest is dense, broad-leaved evergreen forest that is called selvas.

There are a wide variety of species of trees in a single location. They do not grow in extensive stands but in mixed varieties. The heterogeneous variety makes is difficulty for lumbering.  

The trees form layers according to their height called stratification. There are three distinct layers:
            A layer, the tallest, at 20 to 40 metres high

            B layer, 15 to 40 meters high

            C layer, the shortest, at 5 to 15 meters high

Other layers include, the (i) canopy and emergent layer and (ii) the forest floor.

The canopy or emergent layer is the tallest layer comprising of dense evergreen trees. These trees receives the most sunlight and forms a dense interlocking canopy of vines, leaves and branches that prevents the sun from reaching the lower layers.

The emergent layer grows through the canopy. These are a few taller trees that grow above the canopy and are called dipterocarps. 

The tree structures of the forest are straight, tall and often thick. Large spreading buttress roots support them. The roots spread out over three meters above the ground and burrow into the soil to support the large trees. The leaves are evergreen and dark green. Several types of hardwood trees such mahogany, ebony, rosewood, greenheart, and ironwood exists in the equatorial forests.

The under storey and forest floor consist of trees 5 to 15 meters tall. The lower levels receive very little rainfall. Only 10 per cent of rainfall reaches this layer.

The vegetation at this level consists of large leaves shrubs, fungi (exists without sunlight); mushrooms (a type of fungi).

Some of these ground level plants are saprophytes that live on decayed vegetation.

Ferns, mosses and orchids called ephiphytes anchor on trees and branches instead of the soil.

Woody creepers called lianas grow upwards on the trunk of trees.  

Mangrove forests exist along tidal estuaries and coastal areas of equatorial forests. The trees here are usually stilt or prop root trees.

Clearing forest areas for farming generates a secondary forest with mixed crops after it is abandoned. This type of forest is usually described as a jungle.

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Caribbean Weather Systems

Tropical Waves

A tropical wave is a belt of relatively low pressure that travels across the Atlantic Ocean along the edge of ITCZ (Inter Tropical Convergence Zone). They are also called Easterly waves, since they develop in the easterly airflows in the lower troposphere. With it comes periods of clouds and heavy rains. The lower pressure weather system develops more frequently in the rainy season, from May to November. Tropical waves have a lower moisture layer and a drier upper layer. If they lower air is lifted up, it cools causing condensation and rain. The waves move slowly from east to west. As they move west, they acquire moisture. They develop in the oceans at latitudes between 5 degrees Celsius and 30 degrees Celsius.  They do not extend across equatorial regions.

Before the tropical wave, air is subsiding (sinking). The air is being warmed, relative temperature humidity is dropping and there are dry sunny conditions. During the passing of tropical waves, the air pressure reduces and the air begins to rise strongly. Cumulonimbus clouds and heavy rainfalls occur during the wave. Wind speed is faster and gusty along the front of the wave. Behind the tropical waves, there still is uplift but no so much, resulting in cloudy conditions and light showers. Tropical waves at times grow in intensity and may eventually develop into storms or hurricanes.

Cold Fronts

A cold front is the boundary zone in a depression between dense cold air and the warmer air ahead of it. Occasionally, winds blow from the cold interior plains of North America in the winter. They blow out from a centre of high pressure in continental North America between December and March. For this reason they are called ‘northers’. They are fast moving and associated with a sudden drop in temperature. The margin between the cold air from North America and the warm Caribbean air is known as a cold front.  

At a cold front, the dense cold air lifts warm air up. As the cold air travels south, it pushes the warm air upwards leading to an uplift, cooling and condensation that form clouds. Heavy rain then results. Cold fronts bring with it cold, wet conditions over many days at times, that may lead to damaged crops on north facing slopes.

Ahead of the cold front there may be bright sunshine and light wind approaching from the east.  At the cold front, the cold continental air pushes south, underneath the warm tropical air. There are strong updraughts, which form thick cumulonimbus clouds. After the cold front, a strong northerly wind with continental air blows from North America and is warmed as it passes over the ocean.

Hurricanes

Hurricanes are one of the most natural disasters to the environment. They threaten to impact the region every year during the period of July through November.  The official hurricane season is from June to November. In Asia, hurricanes are known as tropical cyclones. As some tropical waves travel over the Atlantic Ocean, they become unstable and develop into a hurricane. This is the initial sequence of a hurricane. A hurricane is a severe or intense revolving tropical storm with winds reaching 119 km per hour or 74 miles per hour or more. It is a very large circular area of low pressure, driven by evaporation from warm seas. Hurricanes develop in the Mid-Atlantic Ocean between latitude 7 degrees north and 20 degrees north. The hurricanes generally move in a north-westerly direction in the Caribbean.

Structure of a Hurricane

Conditions under which hurricanes form

Even though there is not certain cause of a hurricane, there are certain conditions that cause a hurricane to form. 

Hurricanes generally form over large expanses of warm water. A temperature over 26.5 degrees Celsius is required. This explains why hurricanes occur in the tropics. The consistent temperature ensures that there is a good supply of moisture for the hurricane through evaporation, which is the energy for the hurricane. 

Warm tropical waters with a depth of at least 5o meters are required to fuel the hurricane. As the ocean surface is heated, hot air rises carrying with it water vapour that condenses into storm clouds brining with them thunderstorm as the hurricane gather heat and energy through contact with warm waters.

There must be an updraft of moisture-laden air from the sea to supply or replace the air that is lost in the troposphere resulting in the formation of thick and extensive cloud cover.

The rotation of the earth causes the winds to rotate anti-clockwise in a spiral and upward motion in the centre of the hurricane. This effect is cause due to the Coriolis force. Suitable conditions exits between latitudes 5 degrees to 30 degrees north and south of the Equator.

How hurricanes develop

Air rises rapidly over warm oceans where huge quantities of water evaporate very quickly causing an area of low pressure.  As the air rises, it cools and condenses forming towering cumulus and cumulonimbus clouds.

The air flows from high pressure to low pressure along with the earth’s rotation causes the spinning effect and the storm simulates the spinning characteristics like a Catherine-wheel shape, spinning in an anti-clockwise direction (in the northern hemisphere).  

The initial stage of the disturbed weather is known as a tropical depression and is given a number. If the winds intensify then a tropical storm forms and is named. When the surface wind speeds increases to 75 mph, the storm is officially becomes a named hurricane.

The prevailing winds then carry the storm across the ocean by the prevailing winds from the east to west. As the storm approaches land the supply of warm water stops and the storm begins to weaken.

Weather conditions associated with Hurricanes

As a hurricane approaches, temperatures and pressure begins to fall. The winds are gentle and from the north-west. Clouds begin to form and a light shower develops. The winds now begin to gust.  

During the storm, pressure falls rapidly, wind speeds increases up to 62 miles per hour. Extensive cloud cover by cumulonimbus clouds with thunder and severe rainfall.  At the eye of the storm, there is a period of calm. Low pressure and temperatures rises as the sun may appear briefly.  As the eye passes, hurricane force winds and torrential rainfall begins. Wind direction now in south-east. The temperature drops again and the pressure begins to rise. 

When the storm passes, pressure and temperatures rise. Rainfall decreases to light showers and temperatures increases with the sunny intervals. Wind speeds decreases.

Measuring Hurricanes 

Hurricanes are measured using the Saffir-Simpson scale. This scale has units that reflect the wind speed and its effects. The scale ranges from 1 to 5, with 5 being the level the greatest damage is caused and 1 having the least or minimal damage.

Effects of Hurricanes

Hurricanes can have several effects on coastal areas as well as inland areas.

Flat coastal areas are most vulnerable to hurricane damage. Hurricane effects come about in three main ways, strong winds, heavy rainfall and storm surge.

Strong winds, with wind speeds in excess of 75 mph (120 km/p) and gust of over 125 mph (200 km/p) hurricanes can causes extensive damage to buildings, power lines and crops and trees. Roofs may be blown off, power lines down and trees and crops may be levelled.

Heavy rainfall, over 200 mm of torrential rainfall may be associated with heavy hurricanes, resulting in widespread flooding and landslides that may damage infrastructure such as roads, bridges and buildings. Crops are also destroyed as a result of flooding.

Storm surge, large waves that a blown onshore by the strong winds. Storm surges often 3-5 meters high surge over the land in low-lying areas and up river valleys.  Hurricane storm surge are the biggest killers and would flatten everything in its path similar to a tidal waves. Crops will be destroyed and large quantities of salt water will be left in the areas. 

Reducing Hurricane Hazard

With the aid of satellites and radar technology, scientists and meteorologist are able to detect hurricanes and tract their paths. Warnings may then be issued to the areas at risk. 

Persons are made aware of Hurricanes through, hurricane watch and hurricane warning.

Hurricane Watch, Be prepared, hurricane conditions are likely in the next 36 hours. Secure and protect windows and doors, collect water and emergency supplies. 

Hurricane Warning, Time to act, Hurricane conditions are expected in 12-24 hours. Evacuate low-lying areas or seek shelter.

Intertropical Convergence Zone (ITCZ)

The ITCZ is a broad zone of very unstable weather that forms at the convergence of winds at two Hadley Cells.  At ground level, moist air converges and is forced to rise. Resulting in rapid cooling, condensation and the formation of towering cumulonimbus thunderstorm clouds. The ITCZ brings heavy rainfall to the southern Caribbean countries such as St. Lucia, Trinidad and Barbados. This weather system is responsible for the unsettled wet weather between the months of June and November.  

In summer, in the northern hemisphere, the sun moves north of the equator, close to the Tropic of Cancer. The ITCZ affects much of the southern Caribbean then. During winter in the northern hemisphere, the ITCZ moves further south to lie close to the equator. The weather in the Caribbean is now affected by a large subtropical belt of high pressure to the north and generally more settled with sunshine, drier conditions and less rain.

Anticyclones

These are large areas of relatively high atmospheric pressure that brings with it settled weather conditions. The weather associated with anticyclones is often with relatively calm winds, clear skies, high temperatures and low rainfall.  Anticyclones are mainly caused as a result of the global circulation of air. At about 25—30 degrees north at the edge of the Hadley Cell, air descends. The sinking air causes a high pressure on the ground which gives rise to dry conditions experienced in the Caribbean during winter and spring.   Hurricanes can have several effects on coastal areas as well as inland areas.

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