The constantly changing condition of Earth’s atmosphere creates our weather—clear skies, wind, cloud, rain, and snow. The amount of sunshine we get and how strong it is determines the temperature and pressure in the atmosphere. The amount of moisture it contains determines how high up clouds form and whether they produce mist, rain, or snow, as well as when storms occur. When we study the weather, we can see predictable seasonal patterns around the world, known as climates.
WEATHER SYSTEMS
Patterns of weather depend on the nature of the local air mass and pressure system, which can change over the course of a year. For example, in the summer, continental land surfaces heat up, making warm, dry air rise.
This produces a low pressure weather system, which draws in more warm air from the surroundings and can cause storms. In winter, continental land surfaces cool, and colder, dense air sinks down from the atmosphere above.
How does the Sun cause the seasons?
Seasons are caused by the Earth’s revolution around the Sun, as well as the tilt of the Earth on its axis.
The hemisphere receiving the most direct sunlight experiences spring and summer,while the other experiences autumn and winter. During the warmer months, the Sun is higher in the sky, stays above the horizon for longer, and its rays are more direct.
During the cooler half, the Sun’s rays aren’t as strong and it’s lower in the sky. The tilt causes these dramatic differences, so while those in the northern hemisphere are wrapping up for snow, those in the southern hemisphere may be sunbathing on the beach.
Monsoon winds are massive seasonal winds that bring heavy summer rain to subtropical regions, such as Southeast Asia and India. In winter, they bring dry, cooler weather. Monsoon winds are strongest in Asia, but they also occur in West Africa,northern Australia, and parts of North and South America.
Monsoon winds change direction between summer and winter.SummerIn summer, the South Asian monsoon blows from the Indian Ocean across India, bringing the torrential rains of the wet season, which are essential for the growth of the continent’s staple food crops.WinterIn winter, the South Asian monsoon reverses, bringing the warm dry winds and fine weather of the dry season across the Indian continent and out into the Indian Ocean.Cloud formationWhen the Sun shines on ponds and lakes, some of the water they hold evaporates into the warm air. This warm water vapor then rises up and away from Earth’s surface.
As the air rises, it cools.Because cold air cannot hold as much moisture as warm air, the water vapor condenses, and forms clouds. Clouds that form very high in the atmosphere,above 16,500 ft (5,000 m), are made of ice crystals rather than water vapor.
1 MOISTURE RISESHot sunlight makes moisture from the ground or sea surface rise into the air as water vapor.2VAPOR CONDENSESAs the vapor rises and cools, it condenses to form visible clouds of tiny water droplets.3 CLOUDS RISEAs water droplets form clouds, they release heat into the surrounding air, lifting the cloud up.HurricanesA hurricane is a huge, rotating tropical storm with high winds and very heavy rain. These storms start from a cluster of thunderstorms, which develop over warm tropical seawater in late summer, then merge together into a larger, spiral hurricane. Their intense low pressure draws in warm, moist winds, which spiral upward as they spin faster. The rapidly rising air then cools, forming towering storm clouds and torrential rains.When they reach land, hurricanes cause flooding and are highly destructive.Naming the storm have the same structure and evolve in the same way wherever they are. However, how they are named depends on the part of the world where they occur. This map shows what they are called in different parts of the world.Water on the moveAll water on Earth is included in the constant circulation of the water cycle. Even the snow on mountain peaks or in the ice sheets of the Antarctic is a part of the cycle—eventually, it will melt and be on the move again.Underground water is also involved—it flows in a similar way to rivers, despite being hidden out of sight.
What causes hurricanes?Depending on where they start, hurricanes may also be known as tropical cyclones or typhoons. They always form over oceans around the equator, fuelled by the warm,moist air. As that air rises and forms clouds,more warm, moist air moves into the area of lower pressure below. As the cycle continues,winds begin rotating and pick up speed.Once it hits 119 kilometres (74 miles) per hour,the storm is officially a hurricane. When hurricanes reach land, they weaken and die without the warm ocean air. Unfortunately they can move far inland, bringing a vast amount of rain and destructive winds.People sometimes cite ‘the butterfly effect’ in relation to hurricanes. This simply means something as small as the beat of a butterfly’s wing can cause big changes in the long term.
Is it possible to stop a hurricane?We can’t control the weather… or can we?
Some scientists are trying to influence the weather through cloud seeding, or altering the clouds’ processes by introducing chemicals like solid carbon dioxide (aka dryice), calcium chloride and silver iodide. It has been used to induce rainfall during times of drought as well as to prevent storms.
SHAPING THE LAND
The landscape may look unchangeable, but it has been shaped by the forces of wind and water over millions of years. Together, they break down, or erode, rocks into tiny fragments called sediment, then carry them away. This is usually a very slow process, but extreme events, such as floods and hurricanes, can speed it up.WINDYou’ve probably had sand blown in your face on the beach, so you’ll know that winds can be strong enough to pick up dust, grit,sand, and soil particles. These sediment particles can be carried over huge distances. Wind erosion and deposition (the laying down of sediment) typically happen in dry places with little vegetation to protect the rocks. Characterized by low rainfall, these environments are known as deserts and often contain sand dunes.
COAST
Coasts are constantly being shaped by nature. Where coasts are exposed, the action of powerful ocean currents and waves wears away the landscape to form cliffs and headlands. On more sheltered coasts, sediments build up to form sandy beaches, dunes, mudflats,and salt marshes. Rivers also affect coasts as they lose energy and leave behind the sediments they carry when they approach the sea.Sand dunesThe more powerful a wind is, the further it can carry sediment particles before dropping them to the ground.As they roll and bounce on the ground,these particles create small, wave-shaped ripples. These ripples sometimes build-up into larger called dunes. With persistent winds, sand dunes can grow to many feet high and several miles long.Wind erosionOver time, the constant lashing of winds can wear away at exposed rock surfaces. This wind erosion produces weird-looking and unstable formations, which eventually collapse in most cases.Even very hard rocks may be slowly shaped and polished by sand blasting. Barren landscapes can be created if all the soil in an area is blown away by the wind.Rocky archSand-blasting can create strange shapes from rocks, such as this natural arch in the Arches National Park in Utah.Transverse dunesConstant winds carrying lots of sand form rows of dunes with crests that lie at right angles to the wind direction.Crescent (barchan) dunesWinds of varying strength and sand content form crescent-shaped dunes with “horns” pointing downwind.Star dunesThese dunes form when the wind direction constantly changes.
They may grow to a considerable height.
WATER
The salty waters of the oceans cover about three-quarters of the Earth’s entire surface. Heat from the Sun evaporates ocean water, which rises into the atmosphere and forms clouds. This airborne moisture eventually falls to the ground as rain. Some rainwater soaks into the soil or becomes stored in rocks underground. The rest of the water runs off the land to form rivers,which return the water to the sea,completing a process that is known as the water cycle.What would happen to our weather without the Moon?It’s difficult to know exactly what would happen to our weather if the Moon were destroyed, but it wouldn’t be good. The Moon powers Earth’s tides, which in turn influence our weather systems. In addition, the loss of the Moon would affect the Earth’s rotation – how it spins on its axis. The presence of the Moon creates a sort of drag, so its loss would probably speed up the rotation,changing the length of day and night. In addition it would alter the tilt of the Earth too, which causes the changes in our seasons. Some places would be much colder while others would become much hotter.
Let’s not neglect the impact of the actual destruction, either; that much debris would block out the Sun and rain down on Earth, causing massive loss of life. Huge chunks that hit the ocean could cause great tidal waves, for instance.
WHAT ARE KATABATIC WINDS?
From the Greek for ‘going downhill’, a katabatic wind is also known as a drainage
The constantly changing condition of Earth’s atmosphere creates our weather—clear skies, wind, cloud, rain, and snow. The amount of sunshine we get and how strong it is determines the temperature and pressure in the atmosphere. The amount of moisture it contains determines how high up clouds form and whether they produce mist, rain, or snow, as well as when storms occur. When we study the weather, we can see predictable seasonal patterns around the world, known as climates.
Monsoons
Monsoon winds are massive seasonal winds that bring heavy summer rain to subtropical regions, such as Southeast Asia and India. In winter, they bring dry, cooler weather. Monsoon winds are strongest in Asia, but they also occur in West Africa,northern Australia, and parts of North and South America.
Monsoon winds change direction between summer and winter.
Summer
In summer, the South Asian monsoon blows from the Indian Ocean across India, bringing the torrential rains of the wet season, which are essential for the growth of the continent’s staple food crops.
Winter
In winter, the South Asian monsoon reverses, bringing the warm dry winds and fine weather of the dry season across the Indian continent and out into the Indian Ocean.
Cloud formation
When the Sun shines on ponds and lakes, some of the water they hold evaporates into the warm air. This warm water vapor then rises up and away from Earth’s surface. As the air rises, it cools.
Because cold air cannot hold as much moisture as warm air, the water vapor condenses, and forms clouds. Clouds that form very high in the atmosphere,above 16,500 ft (5,000 m), are made of ice crystals rather than water vapor.
1 MOISTURE RISES
Hot sunlight makes moisture from the ground or sea surface rise into the air as water vapor.
2VAPOR CONDENSES
As the vapor rises and cools, it condenses to form visible clouds of tiny water droplets.
3 CLOUDS RISE
As water droplets form clouds, they release heat into the surrounding air, lifting the cloud up.
Hurricanes
A hurricane is a huge, rotating tropical storm with high winds and very heavy rain. These storms start from a cluster of thunderstorms, which develop over warm tropical seawater in late summer, then merge together into a larger, spiral hurricane. Their intense low pressure draws in warm, moist winds, which spiral upward as they spin faster. The rapidly rising air then cools, forming towering storm clouds and torrential rains.
When they reach land, hurricanes cause flooding and are highly destructive.
Naming the storm have the same structure and evolve in the same way wherever they are. However, how they are named depends on the part of the world where they occur. This map shows what they are called in different parts of the world.
Water on the move
All water on Earth is included in the constant circulation of the water cycle. Even the snow on mountain peaks or in the ice sheets of the Antarctic is a part of the cycle—eventually, it will melt and be on the move again.
Underground water is also involved—it flows in a similar way to rivers, despite being hidden out of sight.
What causes hurricanes?
Depending on where they start, hurricanes may also be known as tropical cyclones or typhoons. They always form over oceans around the equator, fuelled by the warm,moist air. As that air rises and forms clouds,more warm, moist air moves into the area of lower pressure below. As the cycle continues,winds begin rotating and pick up speed.
Once it hits 119 kilometres (74 miles) per hour,the storm is offi cially a hurricane. When hurricanes reach land, they weaken and die without the warm ocean air. Unfortunately they can move far inland, bringing a vast amount of rain and destructive winds.
People sometimes cite ‘the butterfl y effect’ in relation to hurricanes. This simply means something as small as the beat of a butterfl y’s wing can cause big changes in the long term.
Is it possible to stop a hurricane?
We can’t control the weather… or can we? Some scientists are trying to influence the weather through cloud seeding, or altering the clouds’ processes by introducing chemicals like solid carbon dioxide (aka dryice), calcium chloride and silver iodide. It has been used to induce rainfall during times of drought as well as to prevent storms.
Sand dunes
The more powerful a wind is, the further it can carry sediment particles before dropping them to the ground.
As they roll and bounce on the ground,these particles create small, wave-shaped ripples. These ripples sometimes build-up into larger called dunes. With persistent winds, sand dunes can grow to many feet high and several miles long.
Wind erosion
Over time, the constant lashing of winds can wear away at exposed rock surfaces. This wind erosion produces weird-looking and unstable formations, which eventually collapse in most cases.
Even very hard rocks may be slowly shaped and polished by sand blasting. Barren landscapes can be created if all the soil in an area is blown away by the wind.
Rocky arch
Sand-blasting can create strange shapes from rocks, such as this natural arch in the Arches National Park in Utah.
Transverse dunes
Constant winds carrying lots of sand form rows of dunes with crests that lie at right angles to the wind direction.
Crescent (barchan) dunes
Winds of varying strength and sand content form crescent-shaped dunes with “horns” pointing downwind.
Star dunes
These dunes form when the wind direction constantly changes. They may grow to a considerable height.
The salty waters of the oceans cover about three-quarters of the Earth’s entire surface. Heat from the Sun evaporates ocean water, which rises into the atmosphere and forms clouds. This airborne moisture eventually falls to the ground as rain. Some rainwater soaks into the soil or becomes stored in rocks underground. The rest of the water runs off the land to form rivers,which return the water to the sea,completing a process that is known as the water cycle.
What would happen to our weather without the Moon?
It’s difficult to know exactly what would happen to our weather if the Moon were destroyed, but it wouldn’t be good. The Moon powers Earth’s tides, which in turn influence our weather systems. In addition, the loss of the Moon would affect the Earth’s rotation – how it spins on its axis. The presence of the Moon creates a sort of drag, so its loss would probably speed up the rotation,changing the length of day and night. In addition it would alter the tilt of the Earth too, which causes the changes in our seasons. Some places would be much colder while others would become much hotter. Let’s not neglect the impact of the actual destruction, either; that much debris would block out the Sun and rain down on Earth, causing massive loss of life. Huge chunks that hit the ocean could cause great tidal waves, for instanc.
DOES IT EVER SNOW IN AFRICA?
Several countries in Africa see snow – indeed, there are ski resorts in Morocco and regular snowfall in Tunisia. Algeria and South Africa also experience snowfall on occasion. It once snowed in the Sahara, but it was gone within 30 minutes. There’s even snowfall around the equator if you count the snow-topped peaks of mountains.
WHAT COLOUR IS LIGHTNING?
Usually lightning is white, but it can be every colour of the rainbow. There are a lot of factors that go into what shade the lightning will appear, including the amount of water vapour in the atmosphere, whether it’s raining and the amount of pollution in the air. A high concentration of ozone, for example, can make lightning look blue.
WHY DO SOME CITIES HAVE THEIR OWNMICROCLIMATE?
Some large metropolises have microclimates – that is, their own small climates that differ from the local environment. Often these are due to the massive amounts of concrete, asphalt and steel; these materials retain and reflect heat and do not absorb water, which keeps a city warmer at night. This phenomenon specifically is often known as an urban heat island. The extreme energy usage in large cities may also contribute to this.Cut off from the Sun, rain and wind that we experience on the surface,you might assume meteorological conditions in caves never change.However, the reality is that their climates do vary significantly – not only from location to location, but within individual caves over time.Indeed, some examples, like the Er Wang Dong cave system in Chongqing Province, China (main picture), even host their own weather. Ultimately this is because very few caves are 100 per cent cut off from their surroundings. In the case of Er Wang Dong, it all comes down to an imbalance in the local topology.There are several tunnels around the cave system’s perimeter where wind can blow in.Once trapped underground air from outside gains moisture, pooling into huge chambers like Cloud Ladder Hall – the second-biggest natural cavern in the world with a volume of 6 million cubic metres (211.9 million cubic feet).Once in an open chamber this humid air rises.While there are numerous entrances into this subterranean complex, exits are few and far between. In Cloud Ladder Hall’s case, it’s a hole in the roof some 250 metres (820 feet) above the floor, leading to a bottleneck effect.As the damp air hits a cooler band near the exit, tiny water droplets condense out to create wispy mist and fog. In other chambers plants and underground waterways can also contribute to underground weather.Even caves without any direct contact with the outside world can still experience climatic variations, as they are subject to fl uctuations in atmospheric pressure and geothermal activity,where the heat from Earth’s core emanates through the rocky fl oor. However, in such caves, changes are more evenly distributed so take place over longer time frames.The simple fact of the matter is that weather is unpredictable. So how is it that we can gather information and make predictions about what conditions on Earth will be like?Most weather phenomena occur as a result of the movement of warm and cold air masses. The border between these bodies of air are known as ‘fronts’, and it’s here that the most exciting weather,including precipitation and wind, occurs.As a body of air passes across different types of terrain – such as over the oceans, low-lying areas or even mountainous regions – air temperature and moisture levels can change dramatically. When two air masses at different temperatures meet, the less dense, warmer of the two masses rises up and over the colder. Rising warm air creates an area of low pressure (a depression), which is associated with unsettled conditions like wind and rain.We know how a frontal weather system will behave and which conditions it will produce down on the ground. The man who fi rst brought the idea of frontal weather systems to the fore in the early 20th century was a Norwegian meteorologist called Vilhelm Bjerknes. Through his constant observation of the weather conditions at frontal boundaries, he discovered that numerical calculations could be used to predict the weather. This model of weather prediction is still used today.Since the introduction of frontal system weather forecasting, the technology to crunch the numbers involved has advanced immeasurably, enabling far more detailed analysis and prediction. In order to forecast the weather with the greatest accuracy,meteorologists require vast quantities of weather data – including temperature, precipitation, cloud coverage, wind speed and wind direction – collected from weather stations located all over the world.Readings are taken constantly and fed via computer to a central location.Technology is essential to both gathering and processing the statistical data about the conditions down on Earth and in the upper atmosphere. The massive computational power inside a supercomputer, for example, is capable of predicting the path and actions of hurricanes and issuing life-saving warnings. After taking the information collected by various monitors and sensors, a supercomputer can complete billions of calculations per second to produce imagery that can reveal how the hurricane is expected to develop.
