Learn About The Weather

Fine dust from the Sahara desert crosses the Atlantic to South America on a frequent basis. This is the result of the Northeasterly Trade Winds at this latitude. This sand is eventually deposited over the Amazon Basin. This dust is extremely rich in nutrients (mainly phosphorus) which are essential for plant growth ed-hrvatski.com. The excessive rainfall of the Amazon Basin washes away large amounts of nutrients. Without the consistent nutrient input from across the Atlantic Ocean, the Amazon Basin would end up becoming depleted of them. Some 27.7 million tons of fine sand reaches the Amazon Basin annually. It is baffling how the hot and barren Sahara is paramount for the survival of a wet, green and wildlife-rich place like the Amazon! Nature is truly awesome.

The sea has recently breached the shoreline in a number of coastal towns and villages in Malta. This was not caused by the tide but is a phenomenon referred to by experts as an “atmospheric tsunami”. Technically known as seiche, or milgħuba in Maltese, the phenomenon is not uncommon and could take place both in good or bad weather. If the conditions are ideal, the open sea oscillates, creating special signals known as long period waves that are very similar to a tsunami wave. Seiche waves are much smaller than tsunami waves, and they are caused by completely different mechanisms. Seiche waves are caused by fluctuating atmospheric pressure that makes open sea waves rise and fall. Two atmospheric pressure patterns can cause this. An area of high pressure out at sea could push the sea, causing it to rise elsewhere. Conversely, an area of low pressure at the coast could cause the sea to rise. It can also be caused when the wind blows steadily from the same direction for a prolonged period of time. In our case, it was caused by the latter. The intense low pressure (996hPa) over the Maltese Islands caused a temporary rise in sea-level. This is because there is less atmosphere above that particular area of sea, allowing it to rise higher. Winds then pile up the water and push it towards the shore. In our case, the wind was from the South Southwest. Hence, flooding occurred along the south coast. The temporarily elevated sea-level floods low lying coastal areas, similar to an astronomical tide. Due to the bay being enclosed, the water oscillates, like water in a bath tub sloshing from one side to the other, and as a result waves could be seen moving out and back into the bay. Very little can be done to prevent this from happening.

We’ve mentioned the terms ‘Mediterranean Tropical-Like Cyclone’ and ‘Medicane’ multiple times in the past few hours or so. What is it? Mediterranean cyclones are a very rare meteorological phenomenon. The majority form over two regions: the first encompasses an area of the western Mediterranean bordered by the Balearic Islands, southern France, and the shorelines of Corsica and Sardinia; the second is in the Ionian Sea encompassing the region between Sicily and Greece and as far south as Libya. The Maltese Islands are situated within the latter. The mountainous topography of the region, its closed nature and predominantly dry air are great limiting factors. However, these are occasionally overcome. Although they form at any given time of the year they are more common from autumn through to early spring schweiz-libido.com. Unlike their tropical counterparts, Mediterranean cyclones require somewhat different factors to form such as sea surface temperatures below 26°C and an influx of colder air to induce the necessary atmospheric instability and a warm and moist air mass. The atmosphere above the Mediterranean is baroclinic (large areas with significant temperature and pressure differences) unlike in lower latitudes where the atmosphere is barotropic (lack of significant temperature and pressure differences). Instability in the Mediterranean is therefore initiated by the coming together of different air masses while in tropical regions instability depends on the rising of an air mass as a result of a warm sea surface. However, the Mediterranean sea is warm when compared to other regions in the world and this helps in fuelling this region’s cyclones through convection. Similar to tropical cyclones, Mediterranean cyclones require minimal wind shear (a difference in wind speed and direction over a short atmospheric distance) and abundant levels of moisture and vorticity (rotation of a part of the atmosphere). Since the Mediterranean is a dry environment; the required abundant moisture required is often transferred to the region by a low pressure system, jet streams or Rossby waves. Therefore, Mediterranean cyclones often develop from an existing low pressure system. The minimum wind shear is often achieved by a vertical shift of air in the troposphere. This results in a rapid decrease in its temperature. This saturates the air, as cooler air has a lower moisture capacity. Moist air prevents vertical movement of air in the atmosphere and in turn leads to minimal wind shear. Mediterranean cyclones shear are generally small in size and may last from just a few hours to not more than a week. Most develop an ‘eye’ for a few hours and generally feature maximum sustained wind speeds of up to 150 km/h. On a satellite image they show as asymmetric systems with a distinct round eye encircled by atmospheric convection. The weak rotation of the storm is also seen on satellite images.

Low clouds have cloud bases below 2,000 meters so their various shapes and characteristics are clearly evident to us on the ground. They are somewhat more varied and dynamic than higher clouds, subject as they are to the effects of fluctuating ground heating. Low cloud types are cumulus, nimbostratus, stratocumulus and stratus.

NIMBOSTRATUS 

• Nimbostratus begins as the mid-level Altostratus translucidus and Altostartus opacus. Locally, we refer to  them as “sħab tal-Ġimgħa l-Kbira” or “sħab tar-riħ isfel”. The former refers to their ability to trigger a feeling of unhappiness, whilst the latter refers to their North African origin. Altostratus translucidus is a featureless layer of thin, grey-blue cloud that usually spreads to cover most of the sky, give rise to dull, overcast conditions brasil-libido.com. Sunlight becomes diffused and watery. It normally forms as a result of the lifting of an extensive mass of warm air ahead of an incoming warm front or occlusion. If the warm front or occlusion continues to advance, pushing more moist air upwards, altostratus translucidus can thicken into altostratus opacus. If moisture is sufficient, the cloud will thicken in several layers. The sun and moon become increasingly obscured. Even if the cloud layer grows so thick that the sun and moon are fully obscured, the cloud is not yet technically a nimbostratus until rain starts to fall from it. Once rain occurs, the cloud becomes completely featureless. It can appear to connect directly with the sodden landscape below, leaving us feeling not just under the weather, but in it. All three cloud species mentioned here can be very persistent. Over time, the air underneath them can become highly saturated, forming low stratus or mist/fog. Nimbostratus can produce a day of steady rain in the Maltese Islands, and is most common in winter and early spring.

Low clouds have cloud bases below 2,000 meters so their various shapes and characteristics are clearly evident to us on the ground. They are somewhat more varied and dynamic than higher clouds, subject as they are to the effects of fluctuating ground heating. Low cloud types are cumulus, nimbostratus, stratocumulus and stratus.

CUMULUS

• Cumulus Fractus and Cumulus Humilis are the initial product of columns of ascending air. These rise in plumes from the Sun-warmed ground, with the smaller cumulus fractus clouds seen emerging from fragments of haze on warm and calm mornings. Seeding themselves on condensation nuclei (microscopic grains of dust, smoke, pollen or sea salt), the rising pockets of water vapour cool and condense into droplets which begin to coalesce, growing upwards and outwards into a puffy white cloud. When completely formed, these appear as dense, white, detached clouds with wide areas of blue sky between them. They have a clear-cut horizontal base and rounded top. They are usually referred to as ‘fair-weather’ cumulus. The main rule of thumb with cumulus humilis is that they are wider than they are tall. When the warm air begins to cool at sunset, and the convection ceases, these clouds will begin to dissipate, breaking down into even smaller fragments.
• Cumulus Mediocris and Cumulus Congestus are the product of the continued vertical growth of cumulus fractus and cumulus humilis. This continue vertical growth is sustained by the upward convection of columns of warm and moist air on sunlit days. As these thermals rise, they expand and cool until their load of moisture condenses and coalesces into clouds. Condensation releases a great deal of heat. This serves to warm the air in the growing cloud further, leading to stronger convection and thus a further build-up of cumulus clouds https://southafrica-ed.com/kamagra-in-south-africa/. It will carry on growing upwards until the temperature within the cloud is equal to that of its surroundings. These are more common under an unstable atmosphere. The main rule of thumb with cumulus mediocris is that they are as tall as they are wide. Cumulus congestus, on the other hand, are taller than they are wide. They can go on to produce rain showers.

Never the same twice, clouds are the most dynamic features of our natural world. They can vary from benign, fluffy white masses drifting in the summer sky to towering storm clouds that drench the Earth’s surface with rain and hail and illuminate the sky with vivid flashes of lightning. Not matter what shape they take, every cloud is a visible mass of tiny water droplets or ice crystals. They are essential in the production of life-giving rainfall. With only some very rare exceptions, clouds exist entirely within the troposphere, the lowest layer of the atmosphere.

Clouds are formed when moist air is cooled sufficiently for its water vapour content to reach saturation, followed by either condensation to form water droplets, or by deposition to form ice crystals. Usually, the cooling occurs when relatively warm air from near the ground is carried up and expands into colder, lower-pressure levels higher in the atmosphere. The formation of droplets or crystals requires the presence of tiny airborne nuclei in the atmosphere. These serve as sites for condensation or deposition to occur. In the case of blood rain, water vapour condenses onto fine desert sand in atmosphere. The fine desert sand falls with the rain.

When conditions are above freezing, water droplets will form. Over time, the droplets grow in size as more vapour molecules are drawn toward them and are captured. This process continues until the droplets are about 0.02 mm in diameter, then the growth rate slows and the cloud is made up of a stable population of droplets.

Precipitation occurs when some of the droplets or ice crystals that constitute a cloud grow large enough to fall to Earth under the influence of gravity. Two processes can cause this to happen, and they can occur both individually and in combination.

The first process, known as coalescence, is likely to occur in very moist cumuliform clouds and when temperatures are above freezing. The water droplets in clouds are generally so small that they are kept aloft by air resistance and rising air currents, despite the effects of gravity. However, if turbulence within the cloud causes these droplets to collide, they may merge with one another to form larger droplets, and eventually these droplets will become heavy enough to fall from the cloud. As they fall, they collide with more droplets, continuing to grow in size until reaching the ground as rain.

The second process, known as the Bergeron process, requires ice crystals to be present in the cloud. It is most likely to occur in thick clouds at middle or high latitudes, where super-cooled water droplets and ice crystals coexist. Ice crystals and super-cooled water droplets have different saturation points, so when they coexist in a cloud, water molecules will move from the water droplets to the ice crystals. Under these conditions, the ice crystals will grow quickly at the expense of the water droplets until they are large enough to fall. When falling, they will tend to grow further through the process of accretion (growth by the gradual accumulation of ice crystals), and may reach the ground as ice pellets or may melt on the way down to fall as rain.

CLOUD FORMATION

Almost all clouds form by the rising of warm, moist air upward into cooler, less dense levels of the atmosphere, where the water vapour condenses. There are various mechanisms that drive this movement and each gives rise to a distinctive set of cloud types. The strength of the uplift can result in anything from benign cumulus or cirrus clouds to violent cumulonimbus storm clouds. Orographic lifting occurs when an air mass encounters a mountain range. The mountain range forces the air upward, often lifting it to condensation level. Convectional lifting occurs when an air mass, heated by contact with warm ground or surface, becomes buoyant and rises. This lifts it to the condensation level. Frontal lifting occurs when two air masses of different temperatures meet along a front. The warmer air is forced to rise. This lifts it to the condensation level. Convergence occurs when two air masses collide. Some air is forced upwards. This lifts it to the condensation level.

CLOUD CLASSIFICATION

Many different types of clouds are observed in nature and each type of identified by its special name. In 1803, Luke Howard first used Latin words to describe four main types: cirrus (wisp), cumulus (heap), stratus (layer) and nimbus (rain-bearing). With the addition of the word ‘alto’ for mid-level clouds, a combination of these prefixes is used to refer to all main clouds. The main clouds are:

• Cumulus – A small cumulus cloud with little vertical development, usually seen in fair weather.
• Nimbostratus – Dark gray, generally featureless cloud layers that precipitate rain or snow.
• Stratocumulus – A horizontally spreading cumulus cloud.
• Stratus – A layer cloud, thin with considerable horizontal development, Usually featureless.
• Altostratus – A layer cloud that frequently covers the whole sky.
• Altocumulus – An accumulation of small cumulus clouds forming a layer.
• Cirrus – Ice crystal clouds with ‘mare’s tails’ caused by horizontal winds varying with height.
• Cirrostratus – Cirrus clouds that cover a wide area of the sky in an often barely-visible layer.
• Cirrocumulus – Puffy clouds that coalesce in sheets or regularly spaced lines.
• Cumulonimbus – A thunder cloud capable of moderate to heavy precipitation.

In upcoming posts we’ll be dealing with the different cloud types separately.

DEW AND FROST

Dew is water in the form of droplets that appears on exposed surfaces in the morning or evening due to the process of condensation. On a cold and clear night, the ground is cooled as it radiates energy into space. The air near the ground also cools and if the dew point is reached, water vapour will condense on to surfaces near the ground. If the temperature is above freezing, dew is formed. If the temperature is below freezing, frost is formed. In regions with considerable dry seasons, adapted plants benefit from dew. A case in point is the Maltese Islands, were dew is the main source of water for local fauna in the dry summer months.

MIST AND FOG

Both are visible aerosols consisting of tiny water droplets or ice crystals suspended in the air at the surface. They are considered a type of low-lying cloud and are heavily influenced by local topography and weather conditions. The only contrast between the two is that fog reduces visibility to less than 1 kilometre, whereas mist causes lesser impairment of visibility.

Mist and fog form when the difference between air temperature and dew point is less than 2.5°C. For mist or fog to form there must be water vapour present in the air. There are several ways by which water vapour is added to the air. These include precipitation, evaporation from the surface of oceans, water bodies or wet land and transpiration from plants. Water vapour begins to condense on condensation nuclei such as dust and salt. Fog normally occurs when the relative humidity reaches 100%. This is reached either by added moisture in the air or falling air temperature.

Fog can form in a number of ways, depending on how the cooling that caused the condensation occurred.

• Radiation fog is formed by the cooling of land after sunset by infrared thermal radiation in calm conditions with a clear sky. The cooling ground then cools adjacent air by conduction, causing the air temperature to fall and reach the dew point, forming fog. Radiation fog occurs at night, and often does not last long after sunrise. Radiation fog is most common in autumn and early winter.
• Advection fog occurs when moist air passes over a cool surface by advection (wind) and is cooled. This causes the moist air to cool down. For this type of fog to form, the wind must be light. The light wind allows air to slide slowly over the cooler area of land or sea increasing the chances of saturation and condensation. Normally, this fog dissipates as it moves inland, away from the sea or water bodies.
• Frontal fog forms when raindrops, falling from relatively warm air above a frontal surface, evaporate into the cooler air below.
• Up slope fog occurs when forms when moist air is pushed up the slope of a mountain or hill, causing it to cool down and condense.

THE WATER SUBSTANCE

Earth is often called the Blue Planet for it is the presence of liquid water that sets our home apart in the solar system. Water is a relatively simple but changeable substance. One molecule of water consists of two atoms of hydrogen and one atom of oxygen. This basic molecular structure gives water some unique chemical and physical properties that assure it an extremely important role in maintaining Earth’s habitable environments. Water exists in three quite different forms: solid, liquid and gas. This depends on the temperature and air pressure to which it is exposed.

THE WATER CYCLE

Approximately 97.5 percent of water found on Earth is in liquid form and is found in the oceans. The comparatively small amount of fresh water on the planet is mostly locked up in surface ice or is underground. Only 2.5 percent of Earth’s water is fresh. It is distributed among ice caps, groundwater channels, lakes, vapour, rivers and inside living organisms. Water vapour in the atmosphere only constitutes about 0.001 percent of the total, but has a key role in driving the weather. The water vapour content varies from near zero over dry deserts to about four percent in air saturated with vapour.

A continuous interchange of moisture between the oceans, land, plants and clouds fuels much of our weather. This process is know as the water cycle. The water cycle is driven by the Sun which causes water to leave the oceans as a result of evaporation. It re-condenses to form small cloud droplets or ice crystals that grow and fall out of clouds, giving us rain and snow. Rivers or groundwater channels carry the water back to the oceans and the cycle continues. Water circulates at a very changeable rate, depending on where it is in the cycle. On average, a water molecule will spend about 10 days in the atmosphere but 10,000 years as deep groundwater polska-ed.com. Residence time in the oceans is longer still, at about 37,000 years.

HUMIDITY

Humidity is the measure of the water vapour content in the atmosphere. Absolute humidity is the mass of water vapour in a given volume of air measured in grams per cubic metre. Specific humidity is similar but is expressed in grams of water per kilogram of air. Relative humidity, on the other hand, is the amount of water vapour in the air at a given temperature expressed as a percentage of the maximum amount of vapour that the air can hold at that temperature. In forecast, reference is made to relative humidity. If the relative humidity is 100 per cent, the air is saturated. If it lies between 80 and 99 per cent, the air is said to be moist and the weather is humid or clammy. When relative humidity drops to 50 per cent, the air is dry. Figures as low as 10 per cent have been recorded over hot deserts.

Humidity depends upon the temperature of the air. At any given temperature, there is a limit to the amount of moisture that the air can hold. When this limit is reached, the air is said to be saturated. Cold air can hold only relatively small quantities of vapour before becoming saturated but this amount increases rapidly as temperatures rise. This means that the amount of precipitation obtained from warm air is generally greater than that from cold air.

EARTH AS A WATER WORLD

Earth is the only planet in the solar system that has large volumes of surface water. Oceans cover 71% of Earth’s surface and contain 97.5% of all water found on Earth. Most water that falls as precipitation over land evaporates from ocean surfaces, hence the oceans, although salty, are also the main source of fresh water. Oceans absorb more solar energy than land and form a huge heat reservoir. This heat is transported around the world by ocean currents, affecting climate and local weather conditions.

WATER CYCLES

Oceans determine Earth’s climate but also have a direct influence on weather. It is water’s high heat capacity (the energy needed to raise water’s temperature by 1°C). that is particularly important. For the same amount of heat exchange, the temperature change in water is only one-fifth of the temperature change for a similar mass of sand. This means that oceans heat and cool much slower than land, creating daily and annual temperature cycles that cause sea breezes and monsoons.

Heat is stored mainly in the turbulent, shallow upper layer that lies above the cold, deeper ocean. These warm and cold ocean layers are separated by a zone of rapid temperature change. The mean temperature of the world ocean is only 3.8°C.

Oceans are also important for the balance of atmospheric gases. About one-third of the man-made carbon dioxide is absorbed and stored in the oceans. Since solubility of gases depends on both temperature and pressure, the gas accumulates in the deep, cold waters. If the ocean waters warm, the gas is released back into the atmosphere.

OCEAN CLIMATE

The oceans’ ability to store heat has a strong impact on world climate, and leads to difference between maritime and continental climates. Areas farther away from the ocean have bigger annual temperature ranges and drier climates than coastal regions. Sea surface temperatures, determined by ocean currents, lead to climatic differences along the same latitude.

Case-Study 1: The Strait of Gibraltar connects the Mediterranean Sea to the rest of the world’s oceans. Being adjacent to the Sahara Desert, the Mediterranean Sea receives a lot of solar energy and little rainfall. Its salinity is higher than average, mainly due to evaporation.

Case-Study 2: The presence of the surrounding water mass greatly influences the climate of the Maltese Islands. The general weather is often cooler and more humid than what is experiences in inland areas of larger land masses at the same latitude. The high heat capacity of the sea also reduces large fluctuations in the temperature across the Maltese Islands. The presence of surrounding warm waters during the end of the summer season is a source of major weather instability when colder air migrates into the Central Mediterranean, thus creating areas with heavy thunderstorms and intense precipitation.

CURRENTS

An ocean current is a continuous, directed movement of sea water generated by a number of forces acting upon the water, including wind, waves, and temperature and salinity differences. Ocean currents flow for great distances, and together, create the thermohaline conveyor which plays a dominant role in determining the climate of many of the Earth’s regions. More specifically, ocean currents influence the temperature of the regions through which they travel. For example, warm currents travelling along more temperate coasts increase the temperature of the area by warming the sea breezes that blow over them. Perhaps the most striking example is the Gulf Stream, which makes northwest Europe more mild than other locations at the same latitude.

WAVES

Winds blowing across a stretch of sea or ocean produce waves. The frictional forces between the moving air and the water transfer wind energy to the water molecules, deforming the surface and creating waves. Depending on the speed of the wind, waves range from little ripples to rogues. The distance between one wave crest and the next is the wavelength. The functions of a wave are many. They create air bubbles which help oxygenate water. They also shape shorelines. Breaking waves release sea salt and other particles into the air. As the wave approaches shore, the water continues to arrive at the same speed and the wind remains constant. As a result, the wavelength shortens and the waves become steeper. Waves slow down as they travel over shallow water because friction along the seafloor acts like a brake. This friction slows the base of the wave. When the wave becomes too steep, the crest breaks over the base. The wave energy is released and the water carries sand, pebbles and other material back and forth. Waves can be divided into two separate groups; destructive and constructive. Destructive waves are created in storm conditions. They tend to erode the coast. Many sandy beaches tend to be at their narrowest in spring, after the previous winter’s destructive waves. On the other hand, constructive waves are created by fair weather. They break smoothly on the coast and deposit material, building up beaches. Sandy beaches tend to be at their widest at the start of autumn, after the previous summer’s more constructive waves. Another interesting feature of waves is the sea spray. These are droplets of water that blow in the wind after a wave breaks. Have you noticed how your cars and windows are covered in a white film? That is the salt left behind from sea spray. Sea spray can travel some many kilometres inland. As a result, nowhere in the Maltese Islands is far enough inland away from the influence of sea spray.

LAND AND SEA BREEZES

Have you ever noticed how during the day in summer, a fresh breeze blows over a beach and then at nights it stops?

Land warms faster during the day than water and it also cools faster at night creating a daily cycle of difference in air pressure that drives the development of local winds called land and sea breezes. They develop anywhere there is land adjacent to a large body of water, even beside a lake. During the day, the breeze blows from the cooler water onto the land. The reverse occurs at night.

Land and sea breezes form when coastal land heats up more quickly than the adjacent water (most effective on a sunny day with a light or no wind at all). Land has a lower heat capacity than water, so the same amount of solar energy causes different surface temperatures over land and sea. This accounts for the variation in temperature between coastal areas and inland areas in the Maltese Islands. This variation is small as sea breezes may affect areas up to 7 km inland and given that our islands are 14 km at their widest, very few areas remain unaffected by this.

Day Breeze (top image)

A closed circulation forms as cooled air from over the water rushes to the shore to replace rising warm air, which then descends back toward the sea.

Night Breeze (bottom image)

Overnight, land cools quickly, but sea-surface temperatures remain much unchanged. Cooler air drains toward the sea.

EFFECTS OF LAND AND SEA BREEZES

A sea-breeze front is a front created by a sea breeze. The cold air from the sea meets the warmer air from the land and creates a boundary like a cold front. When powerful this front may lead to the formation of cumulus clouds, and if the air is humid and unstable enough, the front can sometimes trigger thunderstorms. If the flow aloft is aligned with the direction of the sea breeze, places experiencing the sea breeze frontal passage will have benign, or fair, weather for the remainder of the day. At the front warm air continues to flow upward and cold air continually moves in to replace it and so the front moves progressively inland. Its speed depends on whether it is assisted or hampered by the prevailing wind, and the strength of the thermal contrast between land and sea.

MOUNTAIN AND VALLEY BREEZES

A mountain and valley breeze form through a process similar to land and sea breezes. During the day, the sun heats up mountain air rapidly while valleys remain relatively cooler. Convection causes it to rise, causing a valley breeze. At night, the process is reversed. During the night the slopes get cooled and the dense air descends into the valley as the mountain breeze. Like land and sea breezes, these breezes occur mostly during calm and clear weather. Mountain and valley breezes are other examples of local winds caused by an area’s geography. Campers in mountainous areas may feel a warm afternoon quickly change into a cold night soon after the sun sets.