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Driven by solar radiation, Earth, ocean, and weather systems work together, transferring energy and matter by convection across Earth’s surface. Convection transfers thermal energy through circular motions caused by heating and cooling in fluids (liquids and gases).

Radiant energy from the Sun heats Earth’s surface. Some of that thermal energy is then transferred to gases in the atmosphere by conduction. The heated air molecules that absorb thermal energy tend to spread out and rise away from Earth’s surface, distributing the heat energy through convection currents.

Wind is caused in part by differential heating (thermal energy absorption) in the atmosphere both locally and at more extremes at Earth’s poles and equator. Warmer, less dense air has less air pressure than cooler, denser air. Differences in air pressure result in movement of air, or wind. High pressure air masses push low pressure air masses.

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Ocean: the entire body of saltwater that covers more than 70 percent of Earth’s surface.

Sun: a star that is the basis of the solar system and that sustains life on Earth, being the source of heat and light.

Energy: capacity of a physical system to perform work. Convection: heat transfer caused by the rising of hotter, less dense fluids and the falling of cooler, more dense fluids

Atmosphere: the whole mass of air surrounding Earth made up of 78nitrogen, 21oxygen, and other trace gases

Winds: a natural movement of air, sometimes with considerable force, from an area of high density and pressure to an area of low density and pressure

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Predictable weather characteristics such as air pressure masses and fronts as well as weather patterns are shown on a weather map. These weather patterns result from the convective transfer of thermal energy and the resulting phase changes of water in the atmosphere. Global patterns of atmospheric movement influence local interaction of air pressures, temperatures and moisture content resulting in local weather conditions.

Air pressure is a measure of the weight of the air over a given area, as indicated by a barometer.

Air masses, which develop characteristic temperatures (warm or cold), pressures (high or low), and moisture content (humid or dry) move across the land retaining their basic characteristics. They are labeled on weather maps by their relative pressures (high or low). High pressure air masses contain winds that flows outward from the center in a clockwise direction. Low pressure air masses contain winds that flow counter-clockwise toward the center where they lift away from Earth’s surface.

High pressures bring clear skies and sunny weather conditions. Low pressures are associated with stormy weather events.

Fronts occur in association with the air circulation around low pressure air masses where portions of air with different temperatures and pressures are drawn toward the low. The interaction between air masses with different properties forms a front. The front is the line of contact at ground level marking the boundary between the two air masses.

A warm front is the leading edge of a warm, humid air mass, which pushes into an area occupied by denser colder air. Such conditions often result in periods of overcast skies and gentle rains. Warm fronts are represented by lines of red half circles on a weather map.

A cold front is the leading edge of a cool, dry air mass moving into an area, displacing the warmer air, and contributing to thunderstorm formation. On a weather map, lines of blue triangles represent cold fronts.

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Earth: The third planet outward from the Sun.

Ocean: The entire body of salt water that covers more than 70 percent of Earth’s surface.

Weather Systems: A specific set of weather conditions, reflecting the configuration of air movement in the atmosphere, that affects a region for a period of time.

Global Pattern: A consistent trend found world-wide. Weather Map: A map or chart depicting the meteorological conditions over a specific geographic area at a specific time.

Front: A boundary between two air masses with differing characteristics (e.g., air temperature or humidity).

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Driven by solar radiation the evaporation of seawater transfers thermal energy to the atmosphere. This energy drives weather systems. Global ocean currents affect ocean and atmospheric temperatures and therefore influence the rate of evaporation in different regions of the world.

As water molecules evaporate from ocean surfaces they transfer thermal energy to the atmosphere. Rising water molecules cool and condense to form clouds. When this process is rapid and wide-spread due to very warm ocean water large storms like hurricanes can form.

Hurricanes use warm, moist, tropical air as their fuel; they form when rapid evaporation of seawater molecules lowers the atmospheric pressure of the local air mass. Low pressure air rises and cools resulting in water vapor condensation and cloud formation. Thus, a storm system is formed.

The warmer the ocean and the greater amount of evaporation, the bigger the storm will be. As the clouds build and rotate around the low pressure area, the system develops the characteristics of a hurricane. The ocean currents and atmospheric winds steer the hurricane across the ocean and onto land.

Ocean currents are important in regulating weather patterns around the globe. As warm water moves into an area, it raises the humidity and temperature because more evaporation takes place. As cold water moves into an area, it can lower the temperature by absorbing more heat from the surrounding area, leading to colder and drier conditions on land.

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Earth: The third planet outward from the Sun.

Ocean: The entire body of salt water that covers more than 70 percent of Earth’s surface.

Weather System: A specific set of weather conditions occurring in the lowest levels of the atmosphere, reflecting the configuration of air movement.

Hurricane: A large, tropical weather system consisting of an extremely low pressure air mass with heavy rains and wind speeds of at least 119km/h. Also known as a typhoon or cyclone.

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Also: Cyclone or Typhoon

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Wind not only redistributes energy in the atmosphere, but also influences ocean circulation. As wind blows along the surface of the ocean, the surface water moves along with it. These surface ocean currents transport energy from the tropics to the poles. For example, wind is responsible for the Gulf Stream that moves warm, less dense surface water from the tropics, north along the eastern coast of the U.S., all the way to Europe.

Since ocean currents transport energy (temperature) and moisture around the globe, they play an important role in global weather patterns. As ocean currents move warm water into an area, evaporation increases, moving moisture and energy to the atmosphere, hence increasing the temperature and humidity in the area. If cold water is moved into an area by ocean currents, it can lower surface air temperatures, as well as reduce evaporation rates, leading to colder, relatively drier conditions in the areas nearby.

Because oceans play a key role in supplying energy and moisture to the atmosphere, they also influence the formation of weather systems, such as thunderstorms, blizzards, and hurricanes. Along the Texas coast often thunderstorms develop because winds shift direction and blow warm, moist air from the Gulf of Mexico onto the coast.

Hurricanes are more complex weather systems, which generate thunderstorms, rains, high winds, hail, and even tornadoes. Hurricanes have sustained winds of over 75 mph and form in warm, tropical waters in the Atlantic and Pacific Oceans, the Caribbean Sea, and the Gulf of Mexico. They are fueled by warm, moist air. As water evaporates from warm ocean waters, this warm, moist air (less dense) rises in the atmosphere, leaving less air near the surface and forming a low-pressure area. Since winds move from high- to low-pressure areas, surrounding air moves into the low-pressure area, is warmed, and rises. The warm air cools as it rises, eventually reaching an elevation where it condenses and forms clouds.

At the top of the storm, winds flow outwards, allowing the warm air from below to keep rising. In the northern hemisphere hurricanes spin counterclockwise due to the rotation of Earth (the Coriolis Effect). In the northern hemisphere, the Coriolis Effect deflects air to the right. So as surface winds flow toward the center of the storm, they deflect to the right, which creates a counterclockwise rotation.

You can see this if you draw a circle and draw arrows that curve to the right (as they reach the circle) all around the circle’s edge. When you are done, you can see that the arrowheads point around the circle in a counterclockwise direction. As more ocean water evaporates and fuels the hurricane, the low pressure at the surface will get stronger and it will spin faster, leading to higher-sustained wind speeds. Hurricanes need the right conditions in the oceans, warm waters and light winds, in order to form.

Extreme events such as hurricanes, tornadoes, floods, droughts, earthquakes, volcanoes, and fires are classified based on their intensity and extent of their impact on ecosystems. Some catastrophic events have measurable scales. Hurricanes are categorized based on strength, measured by their sustained wind speed and using the Saffir-Simpson Scale; tornadoes are classified based on damage measured by the Fujita Scale; earthquakes are classified by their intensity as measured by the Richter Scale. Impacts on ecosystems have no formal scale.

Meteorologists can forecast weather-related, catastrophic events by tracking atmospheric patterns and understanding what atmospheric conditions lead to these events. They can forecast the probability, intensity, and path of hurricanes and some tornadoes, as well as flooding from heavy rainstorms. This can help people protect their property and lives. The impacts catastrophic events can have on ecosystems can be visible and last for decades.

When hurricanes make landfall they can impact ecosystems and people in a variety of ways. Hurricanes are unique in that during a hurricane, you can have damage from other catastrophic events, such as flooding and tornadoes. Storm surge associated with hurricanes can lead to erosion of shorelines and destruction of coastal habitats, such as wetlands. High winds and tornadoes can knock over trees in forests used for shelter by animals, as well as houses and buildings used by people.

Flooding further inland can happen due to intense rainfall and put entire ecosystems underwater. Hurricane Katrina, which hit New Orleans in 2005, was only a category 3 storm, but was the most expensive (in terms of damage) and third most deadly (1,500 deaths) in recorded history. The Hurricane of 1900 was a category 4 storm that hit Galveston Island and was the most deadly storm, with estimates of over 8,000 deaths.

Hurricane strength measured by wind speed does not always directly measure how much damage a storm will do. Storm duration, flooding caused by rainfall, and storm surge can also cause damage to ecosystems and society.

Tornadoes are unique extreme weather events that can develop at the base of severe thunderstorms or hurricane storm clouds. They are intense, rotating columns of air that extend down from a storm cloud in the shape of a funnel, and their rotation is evident at the ground. Tornadoes have extremely strong wind speeds, stronger than those of hurricanes. Luckily, tornadoes do not usually impact wide areas and do not typically stay on the ground for long distances. Anything found in the path of a tornado, including houses, cars, forests, and other ecosystems, is often completely destroyed. It can take ecosystems years to decades to recover from a tornado.

Floods occur when the ground, creeks, streams, and rivers can no longer absorb and hold the amount of water being produced by a rain event. When this happens, water will overflow the banks of a river and spread out across the land or floodplain. Flooding can also occur in cities when sewers and runoff catchments cannot keep up with the amount of rain. Often streets are the first to flood in this situation, and waters will continue to rise until the rains stop and the water is given time to drain. Buildings with basements, tunnels, or houses at low elevations are susceptible to flooding. For example, the tunnel system in downtown Houston, many highways, streets, houses, and buildings flooded during Tropical Storm Allison in 2001.

Often floods consist of moving water, and it can be hard to tell how fast that water is moving. People sometimes try to drive their cars through flooded areas in a road thinking that the water is shallow enough or that it is not flowing, only having to abandon their car and watch it float away down the road. This moving water erodes sediment and can uproot plants and trees and destroy roads and buildings. After the waters subside, all this sediment must be deposited, leading to a reshaping of the area’s topography.

Droughts are the most subtle extreme weather event and occur when an ecosystem has not received rain for days, weeks, or months, which the area needs in order to recharge its groundwater. Droughts can cause plants to get sick and die if it lasts long enough. Droughts can also lead to wildlife dying if they cannot get the water they need to survive, or if all the plants die, the wildlife will no longer have a food source.

When catastrophic events occur, often we focus on the impact they have on people and society. These events can often impact ecosystems as well. Extinctions in an area can occur due to loss of habitat or food supplies. Ecosystem types can change after an event. For example, if a tornado destroys a forest, grassland will likely grow in its place. It is important to understand how different ecosystems are impacted and how to respond to different catastrophic events.

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Global wind patterns are the result of the combination of unequal heating of Earth and the Coriolis Effect. Atmospheric movement is driven by the unequal heating, resulting in warm air rising at the equator and moving toward the cooler air at the poles. The Coriolis Effect is caused by the combination of latitude and the relative speed of the rotation of Earth. The speed of the rotation is faster for objects located near the equator and slower for objects located near the poles. This divides the global atmospheric movement into three cells. Each cell has a predictable pattern of atmospheric movement.

The low-latitude cell near the equator maintains the tropical and sub-tropical climates. This is the area of the trade winds that blow from the east to the west. In this cell the air rises at the equator, flows away from the equator, and falls as it meets the border of the mid-latitude cell. The winds of the mid-latitude cell, know as the westerlies, blow from the west to the east and flow toward the poles. In this cell the air rises at the border with the polar cell and falls at the border with the low-latitude cell. The polar cell has weak easterly winds dominated by a high at the poles. The air rises at the border with the mid-latitude and falls over the pole.

Jet streams, or narrow bands of strong, fast-moving air, blow from west to east and form along the borders of masses of hot and cold air. While the direction of jet streams is dictated by the rotation of Earth, the actual path fluctuates to the north or south depending on the location of hot and cold air masses. There are four jet streams that affect the global patterns of weather, two in the northern hemisphere and two in the southern hemisphere. Polar jet streams are generally located near the border between the mid-latitude cells and the polar cells; while subtropical jet streams are located near the border between the low-latitude cells and the mid-latitude cells. Jet streams play a role in the atmospheric movement of large air masses and can push low-pressure systems, resulting in weather fronts.

Atmospheric movement, or wind, is the result of differences in air pressure. Differences in air pressure create a force (known as the pressure gradient force) that causes air to move. Air moves from areas of high pressure to areas of low pressure. This movement of air masses caused by differences in air pressure (or density) leads to the formation of fronts.

Fronts occur at the boundary between high- and low-pressure air masses. As fronts move into an area, they change the weather conditions. A warm front occurs at the boundary between a mass of warm, moist air and cold air, where the warm air mass is overtaking the cold air mass. Since the warm air is less dense than the air it is replacing, it gently gets pushed up over the colder air mass. Warm fronts have wide areas of cloud cover along the front and gentle precipitation. They are associated with high pressure and nice weather once they have passed.

A cold front occurs when a cold, dry air mass overtakes a warm air mass. This cold air mass is denser than the warm air mass already in the location. As the cold air mass moves in, it pushes underneath the warm air already there, pushing this warm air up quickly, leading to intense precipitation and, sometimes, thunderstorms. After the cold front passes, the area experiences colder temperatures.

Atmospheric circulation, the atmospheric movement of gases and water, is caused by differences in solar energy absorption on Earth’s surface. This energy-driven atmospheric movement causes the weather we see day to day. Atmospheric circulation or movement leads to changes in atmospheric conditions, such as wind speed and direction, temperature, humidity, and precipitation, which can be tracked over time. Weather stations exist all over the world and measure these conditions daily, hourly, or even every minute. By knowing the atmospheric conditions today and for the past week, month, year, or decade, meteorologists can use this information to predict what the weather will do tomorrow and next week.

Meteorologists use weather maps to keep track of atmospheric conditions. This allows them to understand the large-scale global patterns of atmospheric movement, such as fronts or storms. These weather maps can be used to forecast or predict the weather at smaller scales, such as an individual city, like Houston. Weather maps are created by plotting data such as temperature, wind direction and speed, and atmospheric pressure for weather stations at different locations on the map.

By observing changes in these conditions at weather stations over time, or differences between weather stations, we can track weather systems, such as storms and fronts, and predict what they will do next. For example, since we know a cold front occurs when a cold, dry air mass pushes out a warm air mass, we know higher temperatures occur ahead of the front and lower temperatures behind. The cold air pushing out the warm air leads to storms, so stations where there is precipitation can indicate where the cold front is located.

Meteorologists represent cold fronts on weather maps with lines of blue triangles. Similar indicators (temperature, wind speed and direction, pressure, and precipitation changes) are used by meteorologists to determine where warm fronts occur, and they represent warm fronts on weather maps with lines of red circles.

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The Sun’s radiation provides Earth with energy. This energy from the Sun heats up Earth’s surface and all things on it. Even though the Sun emits a constant amount of energy, Earth does not receive equal amounts of energy during the year as it revolves around the Sun. In addition, due to the tilt of Earth’s axis, not all areas of Earth receive equal amounts of energy from the Sun. For example, the equator receives more direct radiation than the poles. The process of convection redistributes this energy north or south from the equator to the poles.

Convection is the process that transfers heat, either in vapor or water, by mass movement. When you boil a pot of water on the stove, the water on the bottom heats first, since it is closest to the heat source. As this water warms, it expands, becomes less dense, and rises to the surface. Cooler (more dense) water sinks to the bottom to replace it, and this circulation pattern continues. The same occurs in the atmosphere.

As Earth’s surface is warmed by the Sun, the air mass closest to the surface is warmed, expands, rises, and is replaced at the surface by cooler air from above. Air pressure and winds are critical to the convection process in the atmosphere.

Air masses in the atmosphere are made up of millions of gas molecules that are being pulled toward Earth’s surface by its gravity. The force that these air masses exert on Earth’s surface is known as surface pressure. Air pressure decreases as you go up in elevation because at higher elevations, there are fewer air molecules above you.

In other words, air pressure is the weight of the air above a given level. Air pressure varies with temperature. Warmer air masses are less dense and have less pressure than colder air masses, which are denser and therefore have more pressure. When meteorologists talk about weather systems, they talk in terms of high or low pressure. High-pressure systems, known as anticyclones, have winds moving away from the high-pressure center toward areas of lower pressure, in a clockwise direction in the northern hemisphere.

These systems are associated with cool, dry air masses and clear skies. Low-pressure systems, known as cyclones, have winds moving toward the low-pressure center in a counterclockwise direction in the northern hemisphere. These systems are associated with warm, moist air masses and stormy weather. Extreme examples of this would be hurricanes.

Meteorologists measure air pressure using an instrument called a barometer. A barometer consists of a tube with a liquid in it (for example, water or mercury). This tube is set up vertically, closed at the top, and open at the bottom. The bottom of the tube sits in a tiny cup filled with the same liquid in the tube. The liquid in the cup not covered by the tube is exposed to the atmosphere, so air pressure pushes on this liquid’s surface. If air pressure is high, it pushes this liquid up the tube to a higher level. When the tube is marked in units of pressure, we can measure changes in pressure.