Earth and Space Sciences · 20 min read
Hydrosphere and Atmosphere
This chapter covers Content Category I, Topic B: Earth's oceans and fresh water, the water cycle, the composition and structure of the atmosphere, the causes of weather, weather instruments, climate and its controls, and natural resources.
Distribution of Earth's Water
About 71 percent of Earth's surface is covered by water, but very little of that water is fresh and easy to reach. The distribution of water is a favorite exam topic because the numbers surprise most people.
Where Earth's Water Is Found
| Reservoir | Approximate Share |
|---|---|
| Oceans (salt water) | About 97% of all water |
| Fresh water, total | About 3% of all water |
| Ice caps and glaciers | About 69% of fresh water |
| Groundwater | About 30% of fresh water |
| Lakes, rivers, soil moisture, atmosphere, living things | About 1% of fresh water |
The key ranking to remember is that most fresh water is frozen, the next largest share is underground, and only a tiny fraction is in lakes and rivers. People who picture rivers and lakes as the main fresh water supply have the order backward.
Worked Example: Suppose all of Earth's water were represented by 100 liters. About how much would be liquid fresh water in lakes and rivers?
Fresh water is about 3% of the total: 3% of 100 L = 3 L.
Lakes, rivers and other surface or atmospheric water make up about 1% of fresh water: 1% of 3 L = 0.03 L.
Result: About 0.03 liters, or 30 milliliters, roughly two tablespoons out of 100 liters.
This kind of scaled model is effective in an elementary classroom because students can see the tiny volume of accessible fresh water next to a large container representing the oceans.
Ocean Characteristics
Salinity is the amount of dissolved salt in water. Average ocean salinity is about 35 parts per thousand (3.5 percent), and the most abundant dissolved ions are sodium and chloride. Salts come mainly from the weathering of rocks on land, carried to the sea by rivers, and from volcanic and seafloor vents. When ocean water evaporates, the salt stays behind, so salinity rises where evaporation is high and rainfall is low (such as subtropical seas and enclosed seas in hot, dry regions). Salinity falls where rivers empty into the sea, where rainfall is heavy, and where ice melts.
Salt water is denser than fresh water, and cold water is denser than warm water. Together, temperature and salinity control the density of seawater, which controls how water layers and sinks.
The ocean is layered by temperature:
The surface (mixed) layer is warmed by sunlight and stirred by wind.
The thermocline is a zone where temperature drops rapidly with depth.
The deep zone is cold, dark and nearly uniform in temperature, just a few degrees above freezing.
Ocean currents are of two main kinds. Surface currents are driven by prevailing winds and turned by the Coriolis effect and the shapes of continents. They form large circular gyres that flow clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Warm currents (such as the Gulf Stream) carry heat from the tropics toward the poles along the western sides of ocean basins, and cold currents flow toward the equator along the eastern sides. Deep currents are driven by density differences (thermohaline circulation): cold, salty water near the poles sinks and spreads slowly along the ocean floor. Upwelling brings cold, nutrient-rich deep water to the surface, supporting rich fishing grounds.
Ocean waves are mostly caused by wind. In a wave, energy moves forward, but the water itself moves in small circles and stays roughly in place, which is why a floating object bobs up and down instead of traveling with the wave. As a wave enters shallow water, friction with the bottom slows the base, the wave grows taller, and it breaks. Tsunamis are long-wavelength waves caused by sudden displacement of the seafloor, usually during an undersea earthquake.
Common Trap: Tides are not waves caused by wind. Tides are caused by the gravitational pull of the Moon and Sun and are covered in the astronomy chapter.
The Water Cycle
The water cycle is the continuous movement of water among the oceans, atmosphere, land and living things. The Sun's energy drives evaporation, and gravity brings water back down as precipitation and runoff.
Water Cycle Processes
| Process | What Happens |
|---|---|
| Evaporation | Liquid water becomes water vapor, mainly from oceans |
| Transpiration | Plants release water vapor through leaf openings (stomata) |
| Condensation | Water vapor cools and becomes liquid droplets that form clouds, fog or dew |
| Precipitation | Water falls as rain, snow, sleet or hail when droplets or crystals grow heavy |
| Infiltration | Water soaks into the ground and may recharge groundwater |
| Runoff | Water flows over the surface into streams, lakes and oceans |
| Sublimation | Ice changes directly to vapor without melting |
Evaporation absorbs energy from the surroundings, which is why sweating cools the body. Condensation releases that energy, which is a major source of power for thunderstorms and hurricanes. The water cycle therefore moves energy as well as water.
Common Trap: The water cycle has no fixed starting point and no single path. A water molecule may evaporate from the ocean and fall back into it within days, or it may stay frozen in a glacier for thousands of years.
Groundwater, Aquifers and Watersheds
Groundwater is water that fills the spaces between sediment grains and the cracks in rock below the surface. The zone where every pore is filled is the saturated zone; its upper surface is the water table. Above it lies the unsaturated zone, where pores contain both air and water.
Two properties determine how well a material holds and transmits water. Porosity is the percentage of open space in a material. Permeability is how easily water flows through it, which depends on whether the pores are connected and large enough. Clay can have high porosity but very low permeability, because its tiny pores hold water tightly. Sand and gravel have high porosity and high permeability.
An aquifer is a body of permeable rock or sediment that stores and transmits enough groundwater to supply wells. A layer of low-permeability material, such as clay or shale, that blocks water flow is an aquitard or confining layer. When wells pump water out faster than precipitation recharges the aquifer, the water table drops, wells may go dry, the land surface may sink, and near coasts salt water may move into the aquifer.
A watershed (drainage basin) is the entire area of land that drains into a particular river, lake or other body of water. High ground called a divide separates one watershed from the next. Everything that happens on the land in a watershed, such as fertilizer use, construction or a chemical spill, affects water downstream in that watershed. Small streams join larger ones, and large river systems may drain a large part of a continent.
Exam Tip: If a question describes a pollutant spilled upstream on a tributary, predict effects downstream of the spill in the same watershed, not upstream and not in a neighboring watershed across a divide.
Composition and Layers of the Atmosphere
The atmosphere is a mixture of gases held by gravity. Dry air is about 78 percent nitrogen, 21 percent oxygen, and just under 1 percent argon, with about 0.04 percent carbon dioxide and traces of other gases. Water vapor varies from nearly 0 to about 4 percent depending on place and conditions. Carbon dioxide and water vapor are small in amount but important as greenhouse gases that absorb and re-emit infrared energy.
The atmosphere is divided into layers by the way temperature changes with altitude.
Layers of the Atmosphere
| Layer | Approximate Altitude | Temperature Trend | Key Features |
|---|---|---|---|
| Troposphere | Surface to about 12 km | Decreases with height | Nearly all weather and water vapor; most of the air's mass |
| Stratosphere | About 12 to 50 km | Increases with height | Ozone layer absorbs ultraviolet light |
| Mesosphere | About 50 to 85 km | Decreases with height | Coldest layer; meteors burn up here |
| Thermosphere | Above about 85 km | Increases sharply | Very thin air; auroras; many satellites orbit here |
The temperature rises in the stratosphere because ozone absorbs ultraviolet radiation from the Sun and converts it to heat. The ozone layer protects living things from harmful ultraviolet rays. The troposphere is warmed mostly from below by the ground, which is why it gets colder with height, typically about 6.5 °C per kilometer.
Memory Aid: "The Strong Man Throws" gives troposphere, stratosphere, mesosphere, thermosphere from the ground upward.
Worked Example: The air temperature at the base of a mountain is 20 °C. Using the average cooling rate of 6.5 °C per kilometer, estimate the temperature at a summit 2 km higher.
Temperature drop = 6.5 °C/km × 2 km = 13 °C.
Summit temperature = 20 °C − 13 °C = 7 °C.
Result: About 7 °C at the summit.
Air Pressure
Air pressure is the force per unit area exerted by the weight of the air above a point. At sea level, average pressure is about 1,013 millibars (29.92 inches of mercury). Pressure decreases with altitude because there is less air above; at about 5.5 km, pressure is roughly half of the sea-level value. This is why climbers on very high peaks need supplemental oxygen, and why sealed bags of snacks puff up when carried to a mountain town.
Pressure also changes with weather. Warm air expands and is less dense; rising air at the surface lowers pressure. Cool, sinking air raises surface pressure. Humid air is actually slightly less dense than dry air at the same temperature, because water molecules are lighter than the nitrogen and oxygen molecules they replace.
A barometer measures air pressure. A falling barometer usually signals approaching clouds and precipitation, and a rising barometer usually signals clearing, fair weather.
Wind and Uneven Heating
Wind is the horizontal movement of air from areas of higher pressure to areas of lower pressure. Pressure differences arise from uneven heating of Earth's surface. The tropics receive more direct sunlight than the poles; land heats and cools faster than water; dark surfaces absorb more energy than light ones. Wherever air is warmed, it tends to rise and lower the surface pressure; wherever it is cooled, it tends to sink and raise the pressure. The greater the pressure difference over a given distance, the stronger the wind.
Sea and land breezes are a small-scale example that the exam often uses. Water has a much higher specific heat than land, so it warms and cools more slowly.
Sea Breeze and Land Breeze
| Feature | Sea Breeze | Land Breeze |
|---|---|---|
| Time | Sunny afternoon | Clear night |
| Warmer surface | Land | Water |
| Low pressure over | Land | Water |
| Wind blows from | Sea toward land | Land toward sea |
Winds are named for the direction they come from. A sea breeze blows from the sea, and a north wind blows from the north. A wind vane points into the wind, toward the direction the wind is coming from.
The Coriolis effect is the apparent deflection of moving air and water caused by Earth's rotation. In the Northern Hemisphere, moving objects are deflected to the right; in the Southern Hemisphere, to the left. The effect is zero at the equator and strongest near the poles. Because of it, air flowing into a Northern Hemisphere low-pressure center spirals counterclockwise, and air flowing out of a high-pressure center spirals clockwise. The pattern is reversed in the Southern Hemisphere. The Coriolis effect also produces the global wind belts: the trade winds blow from the east in the tropics, the westerlies blow from the west in the middle latitudes, and the polar easterlies blow near the poles.
Common Trap: The Coriolis effect is far too weak to control the direction water swirls down a sink or toilet. It matters only over large distances and long times, such as in storms and ocean currents.
Humidity, Dew Point and Clouds
Humidity is the amount of water vapor in the air. Relative humidity compares the amount of water vapor present with the maximum the air could hold at that temperature, expressed as a percentage. Warm air can hold more water vapor than cold air, so if the amount of vapor stays the same while the temperature drops, relative humidity rises.
Worked Example: At 30 °C, a cubic meter of air can hold about 30 g of water vapor. A sample contains 12 g. What is its relative humidity?
Relative humidity = actual vapor ÷ maximum vapor × 100. 12 g ÷ 30 g × 100 = 40%.
Result: The relative humidity is 40%.
The dew point is the temperature to which air must cool, with no change in its water vapor content, to become saturated (100 percent relative humidity). When air cools to its dew point, water vapor condenses. Below freezing, the same process forms frost. The closer the air temperature is to the dew point, the more humid the air feels and the more likely fog or clouds become. A hygrometer or psychrometer measures humidity. A psychrometer uses a wet-bulb and a dry-bulb thermometer; the larger the difference between them, the drier the air, because more evaporation cools the wet bulb.
Clouds form when air rises and cools to its dew point. Rising air expands because pressure is lower aloft, and expansion cools it. Air rises for four main reasons: heating of the ground (convection), being forced over mountains (orographic lifting), being pushed up along a front, and converging into a low-pressure area. Condensation also needs tiny particles of dust, salt or smoke, called condensation nuclei, for droplets to form on.
Clouds are made of tiny liquid water droplets or ice crystals, not water vapor. Water vapor is an invisible gas.
Basic Cloud Types
| Cloud Type | Appearance | Typical Weather |
|---|---|---|
| Cirrus | High, thin, wispy, made of ice crystals | Fair now; may signal an approaching warm front |
| Cumulus | Puffy with flat bases | Fair weather when small |
| Cumulonimbus | Towering, anvil-topped | Thunderstorms, heavy rain, hail, possible tornadoes |
| Stratus | Low, flat, gray layers | Overcast, drizzle |
| Nimbostratus | Thick, dark, low layer | Steady, prolonged rain or snow |
Memory Aid: "Nimbo" or "nimbus" in a cloud name means it produces precipitation.
Precipitation forms when cloud droplets or ice crystals grow large enough to fall. Rain is liquid. Snow forms when ice crystals grow in clouds below freezing and fall through cold air. Sleet forms when snow melts in a warm layer and refreezes into ice pellets before reaching the ground. Freezing rain falls as liquid and freezes on contact with cold surfaces. Hail forms when strong updrafts in cumulonimbus clouds carry ice pellets up and down repeatedly, adding layers of ice.
Air Masses, Fronts and Pressure Systems
An air mass is a large body of air with fairly uniform temperature and humidity, taking on the character of the region where it formed. Continental air masses are dry; maritime air masses are moist. Polar air masses are cold; tropical air masses are warm.
A front is the boundary between two air masses of different density.
Weather Fronts Compared
| Front | What Happens | Typical Weather | Map Symbol |
|---|---|---|---|
| Cold front | Cold air wedges under warm air and forces it up steeply | Narrow band of cumulonimbus, thunderstorms, gusty winds, then cooler, drier, clearer air | Blue line with triangles |
| Warm front | Warm air slides gradually up over cold air | Wide area of layered clouds (cirrus, then stratus and nimbostratus), long periods of light rain, then warmer, humid air | Red line with half circles |
| Stationary front | Neither air mass advances | Days of clouds and precipitation | Alternating red and blue symbols |
| Occluded front | A cold front overtakes a warm front | Complex clouds and precipitation | Purple line with both symbols |
Pressure systems shape day-to-day weather. In a low-pressure system (cyclone), air converges at the surface and rises, cools and forms clouds, so lows bring cloudy, stormy weather. In a high-pressure system (anticyclone), air sinks and warms as it descends, which evaporates clouds, so highs bring clear, calm, dry weather. On weather maps, lows are labeled L and highs are labeled H. In much of the United States, weather systems generally move from west to east, carried by the prevailing westerlies and the jet stream.
Exam Tip: Link the barometer to the forecast. Falling pressure means a low is approaching and conditions are likely to worsen. Rising pressure, often after a cold front passes, means clearing.
Severe Weather
Thunderstorms form from cumulonimbus clouds when warm, moist air rises rapidly. Lightning is a giant electric discharge; thunder is the sound of air expanding explosively from lightning's heat. Because light travels much faster than sound, you can estimate the distance to lightning by counting the seconds between the flash and the thunder: about 3 seconds per kilometer, or about 5 seconds per mile.
Tornadoes are narrow, violently rotating columns of air extending from a thunderstorm to the ground. They usually form from rotating thunderstorms (supercells) where warm, moist air meets cooler, drier air and winds change direction and speed with height.
Hurricanes (also called typhoons or tropical cyclones in other regions) are large low-pressure systems that form over warm tropical ocean water, generally at least about 27 °C. They draw energy from the heat released when water vapor condenses. They weaken over land or cold water because they lose their source of warm, moist air. In the Northern Hemisphere they rotate counterclockwise. The calm eye at the center is surrounded by the eyewall, the zone of the strongest winds. Storm surge, a rise in sea level pushed ashore by the winds, often causes the most deaths.
A watch means conditions are favorable for severe weather to develop. A warning means severe weather is happening or about to happen in the area, and people should take shelter.
Worked Example: A student sees lightning and hears thunder 9 seconds later. About how far away was the strike?
Sound travels about 1 km in 3 seconds.
9 seconds ÷ 3 seconds per km = 3 km.
Result: The strike was about 3 km away.
Weather Instruments
Weather Instruments and What They Measure
| Instrument | Measures |
|---|---|
| Thermometer | Air temperature |
| Barometer | Air pressure |
| Anemometer | Wind speed |
| Wind vane | Wind direction |
| Hygrometer or psychrometer | Humidity |
| Rain gauge | Amount of precipitation |
Meteorologists also use weather balloons with instrument packages (radiosondes) to measure conditions aloft, Doppler radar to detect precipitation and wind motion inside storms, and satellites to track clouds and storms from space.
Weather Versus Climate
Weather is the condition of the atmosphere at a particular place and time: today's temperature, cloud cover, wind and precipitation. Climate is the long-term pattern of weather in a region, usually averaged over 30 years or more. A useful phrase is "Climate is what you expect; weather is what you get." A single cold week does not disprove a warming climate, and a single hot summer does not prove one. Climate statements describe averages, ranges and typical patterns.
Factors That Control Climate
Inland locations lack the moderating effect of water, so their temperature range is larger.
City A's range is 18 − 4 = 14 °C. City B should have a colder winter and a hotter summer.
Result: A pair such as −8 °C and 26 °C (a range of 34 °C) fits City B.
Seasonal Weather Patterns
Seasonal patterns in weather follow the changing angle and duration of sunlight, which are explained in the astronomy chapter. In the middle latitudes, summer brings higher sun angles, longer days, more convective thunderstorms and warmer air masses, while winter brings lower sun angles, shorter days and more frequent cold air masses. Some tropical regions have wet and dry seasons instead of warm and cold ones; monsoons are seasonal wind reversals caused by the different heating of land and ocean over a season, a sea-breeze effect on a continental scale.
Natural Resources and Human Use
A renewable resource is replaced by natural processes on a human time scale. Sunlight, wind, flowing water, geothermal heat, timber and crops are renewable if managed well. A nonrenewable resource forms far more slowly than people use it, so its supply is limited. Coal, oil, natural gas, uranium and most minerals and metals are nonrenewable. Fossil fuels formed from the remains of ancient organisms buried and transformed over millions of years.
Some resources can shift categories depending on use. Groundwater in a slowly recharging aquifer acts as a nonrenewable resource if it is pumped faster than it refills. Soil is renewable in principle but forms so slowly that soil lost to erosion is effectively lost for generations. Forests are renewable only if harvest does not outpace regrowth.
Energy Resources Compared
| Resource | Category | Advantage | Drawback |
|---|---|---|---|
| Coal, oil, natural gas | Nonrenewable | Reliable, energy-dense | Carbon dioxide and air pollution |
| Nuclear (uranium) | Nonrenewable | No carbon dioxide during operation | Radioactive waste |
| Solar | Renewable | No fuel, no emissions in use | Varies with weather and time of day |
| Wind | Renewable | No fuel, no emissions in use | Varies with wind; land use |
| Hydroelectric | Renewable | Steady supply | Changes river habitats |
| Geothermal | Renewable | Constant output | Limited to certain locations |
Conservation strategies follow a simple order that students can learn: reduce, reuse, recycle. Reducing use avoids the energy and materials needed to make a product at all.
Teaching Notes: Common Student Misconceptions About Water and Weather
"Clouds are made of water vapor" or "steam is the white cloud above a kettle." The visible cloud is liquid droplets; the clear gap just above the spout is the actual vapor. Make a cloud in a jar with warm water, ice on the lid and a bit of smoke for nuclei.
"The water on the outside of a cold glass leaked through." Add food coloring to the water inside; the droplets outside stay clear. Weigh a sealed cold container before and after condensation forms to show mass came from the air.
"Evaporated water disappears." Cover a cup of water with plastic wrap and watch droplets form under the wrap.
"Air has no weight" or "air is nothing." Balance two inflated balloons on a stick and pop one.
"Cold air comes in through the window" or "cold flows." Energy moves from warmer to cooler places; it is the heat that flows out.
"Rain falls from clouds because clouds get too heavy and burst" or "clouds are cotton." Explain that droplets combine until gravity overcomes the updraft.
"Weather and climate are the same." Have students compare one day's weather with a month of class weather data and with decades of averages.
Strong weather investigations ask students to collect data over time with simple instruments, record it in tables, graph it, and look for patterns, such as falling pressure before rain. That work supports the practices of analyzing and interpreting data and constructing explanations from evidence.
Key Takeaways
About 97 percent of Earth's water is salty; most fresh water is in ice caps and glaciers, then groundwater, with only a tiny share in lakes and rivers.
The atmosphere is about 78 percent nitrogen and 21 percent oxygen; temperature falls with height in the troposphere and rises in the ozone-rich stratosphere.
Uneven heating creates pressure differences, and wind blows from high to low pressure; the Coriolis effect deflects it right in the Northern Hemisphere.
Air cooled to its dew point condenses into clouds of droplets; cold fronts bring short, intense storms and warm fronts bring long, gentle precipitation.
Climate is the long-term pattern of weather and is controlled by latitude, altitude, nearness to water, ocean currents and mountains.
Renewable resources are replaced on a human time scale; fossil fuels, uranium and overpumped groundwater behave as nonrenewable resources.
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