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Weather is the state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloud cover. It differs from climate, which is all weather conditions for a particular location averaged over about 30 years.
Weather is influenced by latitude, altitude, and local and regional geography. It impacts the way people dress each day and the types of structures built. Explore weather and its impacts with this curated collection of classroom resources. Catastrophic weather events include hurricanes, tornadoes, blizzards, and droughts, among others. As these massively destructive and costly events become more frequent, scientific evidence points to climate change as a leading cause.
While they can often be predicted, the loss of life and property take an emotional and economic toll on the community impacted. Explore these resources to teach your students about catastrophic weather events and how they impact every part of the world. Floods are events where water overflows onto land that is typically dry.
This can occur when there is a large amount of rain, rapid snow or ice melt, a blast of water onto a coastline during a storm, or the failure of manmade infrastructures, such as dams or levees.
Floods are among the most expensive and frequent natural disasters in the United States, and as the impacts of climate change are more acutely felt, floods are expected to worsen. In addition to property damage, floods, on average, kill more people than tornadoes, hurricanes, or lightning strikes in the United States each year. Learn more about floods with these resources. Wind is the movement of air caused by the uneven heating of the Earth by the sun.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Summer Monsoon The summer monsoon is associated with heavy rainfall. Winter Monsoon The Indian Oceans winter monsoon, which lasts from October to April, is less well-known than its rainy summer equivalent.
The rice paddies of Southeast Asia depend on the seasonal monsoon. Photograph by James P. Asian-Australian monsoon. Monsoon Cup. Monsoon Zone. Also called a mudflow. North American monsoon. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Related Resources. View Collection. Flooding in Maharashtra killed more than 1, people. On July 26, , the city of Mumbai, Maharashtra, received almost a meter The Indian Oceans winter monsoon, which lasts from October to April, is less well-known than its rainy summer equivalent.
The dry winter monsoon blows from the northeast. These winds start in the air above Mongolia and northwestern China.
Winter monsoons are less powerful than summer monsoons in Southeast Asia, in part because the Himalaya Mountains prevent much of the wind and moisture of the monsoons from reaching the coast.
The Himalayas also prevent much of the cool air from reaching places like southern India and Sri Lanka, keeping them warm all year.
Winter monsoons are sometimes associated with drought s. Not all winter monsoons are dry, however. Unlike the western part of Southeast Asia, the eastern, Pacific coast of Southeast Asia experiences its rainy season in the winter.
Other Monsoons. This huge monsoon wind system then stretches into the Indian Ocean. Finally, it reaches its end on the Indian coast of Africa. Monsoon winds exist in other parts of the world, too. The North American monsoon happens once a year, usually in the middle of summer. Warm, moist air from the Gulf of California blows northeast, while warm, moist air from the Gulf of Mexico blows northwest. These two winds meet over the Sierra Madre Occidental mountains in central Mexico.
The monsoon brings moisture to the mountain ecosystem before continuing north to the U. The North American monsoon can be a natural aid to firefighter s. Summer temperatures in Arizona regularly reach more than degrees Fahrenheit, making wildfire s difficult to contain.
Air warmed in the tropics rises, flows towards the poles, then downward in the subtropics, and back to the equator. Image: UCAR. Sunlight, and the energy it brings to Earth, is the driving force behind the Hadley Circulation. Sunlight heats land and ocean surfaces near the equator. The warmed surface releases energy into the atmosphere, in the form of heat and evaporated water.
As it flows toward the poles, this air cools and drops down toward the surface of the Earth in the subtropics, near 30 degrees latitude north or south of the equator.
As air rises near the equator and then flows poleward, it leaves an area of fewer air molecules at the equator. This is a region of low pressure because there is a smaller mass of air left over the equator. Air from the subtopics, north and south of the equator, flows in to fill the space, completing the loop of Hadley Circulation. Water vapor condenses as air rises and cools in the ITCZ, forming clouds and falling as rain. The ITCZ can be seen from space as a band of clouds around the planet.
This is where monsoon rainfall occurs. If the Earth were not rotating, winds would blow directly towards the Intertropical Convergence Zone from the north and south. But the Earth is rotating — making a full turn on its axis each day — which turns the wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
It also affects the movement of ocean currents and the direction of rotation in hurricanes. Motions that span hundreds to thousands of miles feel the Coriolis force. It does not impact smaller scale phenomena like tornadoes. And contrary to legend, the direction that water spins in a flushing toilet is due to toilet design, as toilets are much too small to feel the Coriolis force. The way the trade winds turned to the west on their way to the equator was of great interest to George Hadley, an 18th Century British lawyer who dabbled in meteorology.
He proposed that it was the spin of the Earth that caused the winds to turn as they blew towards the equator. He produced what was essentially the first global theory of atmospheric circulation. Over the years other scientists have refined and further developed these ideas, but Hadley did get some of the basics correct. Today, the Hadley Circulation in the tropics is named after George Hadley.
The Hadley Circulation doesn't stay in the same place year-round, but varies with the seasons. This is the key to understanding why many tropical regions around the world have patterns of wet monsoon summers and dry winters. During December and January, the Southern Hemisphere is heated more strongly by the sun than the Northern Hemisphere, so the hottest air — the air that rises in the ITCZ — is found a little south of the equator.
As the ITCZ changes location through the year, the winds and rains and the location of monsoon wet weather changes, too. As the Intertropical Convergence Zone ITCZ changes location through the year, the winds, rains, and the location of wet monsoon weather changes, too. Remember that the Coriolis force changes direction on the equator: It turns winds toward the right in the Northern Hemisphere and to the left in the Southern Hemisphere.
So when air crosses the equator as it flows from the cold winter hemisphere toward the ITCZ in the summer hemisphere, it experiences a change in the Coriolis force.
This causes the trade winds to reverse direction and blow toward the west in the winter hemisphere and to the east in the summer hemisphere. This seasonal reversal of the winds was historically very important for trade between Africa and Asia; ships would sail from Asia to Africa in winter and then undertake their return voyage when the summer monsoon changed the wind from westward to eastward.
The animation above shows how the ITCZ, winds, and rain patterns change through the months of the year. Video: UCAR. The summer monsoon is what people often think of as monsoon conditions: large amounts of rain. But the winter monsoon, where dry conditions prevail, is part of the pattern too. During winter, air descends over tropical continents as the part of the Hadley Circulation that is outside of the ITCZ. Descending air causes high pressure, and makes clouds and rain uncommon.
The dry conditions during winter can even lead to drought if they are too intense or persist for too long. Geography affects the amount of rainfall that an area receives as the ITCZ moves through the seasons. Low-level winds blow south towards the ITCZ, picking up moisture as they move over the warm, tropical ocean.
Meanwhile in India, dry air descending over land means there is little precipitation. During Northern Hemisphere summer, the ITCZ is north of the equator and monsoon rains fall in India and other parts of south Asia as winds blow north from the tropical ocean to the land, while northern Australia experiences very dry conditions as air descends. When intense summer sunlight hits land, its energy is absorbed and transferred quickly back into the atmosphere. This means that, in summer, air over land is heated more than air over ocean, which shifts the ITCZ toward land regions.
In regions where continents lie north or south of the equator, as in Asia and Australia, this causes the ITCZ to shift farther off the equator during the summer season. There is year-to-year variation in the amount of monsoon rainfall during summer.
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