Energy Transfer and Extreme Weather
On a hot July afternoon in Houston, the sky goes from blue to dark gray in under an hour. Then comes thunder, a downpour, and flooded streets. Where does all that energy come from, and why do some storms stay ordinary while others become hurricanes or tornadoes? By the end of this page you will be able to answer that.
Three ways heat moves
Thermal energy always flows from warmer places to cooler ones. In the atmosphere and at Earth's surface it moves in three ways.
| Process | How it works | Example |
|---|---|---|
| Conduction | Energy passes between molecules that touch | A hot parking lot heats the thin layer of air touching it |
| Convection | Fluid itself moves, carrying energy | A warm air parcel rises, cooler air sinks |
| Radiation | Energy travels as electromagnetic waves, no matter needed | Sunlight crossing space to Earth |
Air is a poor conductor, so conduction only heats the first few millimeters of air above the ground. Convection then carries that heat upward. Warm air is less dense than the cooler air around it, so it rises. Together, these transfers move energy among the hydrosphere, lithosphere, and atmosphere, and that exchange drives weather. Ocean water, land, and air warm at different rates, which sets up winds, storms, and patterns such as El Niño. This is why afternoon thunderstorms are common in summer: the Sun (radiation) heats the ground, the ground heats the air (conduction), and the air rises (convection).
Name the three ways thermal energy is transferred. Give one example of each in the atmosphere or at Earth's surface (for example, from a hot parking lot, from the Sun,…
Name the three ways thermal energy is transferred. Give one example of each in the atmosphere or at Earth's surface (for example, from a hot parking lot, from the Sun, or in a rising warm air parcel).
Latent heat and hurricanes
When water evaporates, it absorbs energy from its surroundings. That energy is stored in the water vapor. When the vapor condenses back into droplets, the energy is released into the air. This is latent heat.
For condensation, about $2.26 \times 10^{6}\ \text{J}$ is released per kilogram of water.
A hurricane is a giant engine for latent heat. Warm ocean water at about $26.5\,^\circ\text{C}$ or higher evaporates quickly. The moist air rises, the vapor condenses, and the released energy warms the air, which makes it rise even more. This pulls in more moist air at the surface, and the storm strengthens. Over land or cooler water the supply of vapor drops, so a hurricane weakens.
Water vapor releases energy when it condenses into liquid droplets. Name this energy, and explain why it is important for the strength of hurricanes, which need warm…
Water vapor releases energy when it condenses into liquid droplets. Name this energy, and explain why it is important for the strength of hurricanes, which need warm ocean water of at least about $26.5\,^\circ\text{C}$.
Condensation of water vapor releases about $2.26 \times 10^{6}\ \text{J}$ of energy per kilogram. A large thunderstorm condenses about $5.0 \times 10^{8}\ \text{kg}$ of…
Condensation of water vapor releases about $2.26 \times 10^{6}\ \text{J}$ of energy per kilogram. A large thunderstorm condenses about $5.0 \times 10^{8}\ \text{kg}$ of water (illustrative value).
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Calculate the total energy released.
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One kiloton of TNT is $4.184 \times 10^{12}\ \text{J}$. Express your result in kilotons of TNT.
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Explain where this energy originally came from, tracing it through the hydrosphere and atmosphere.
Severe storms and Tornado Alley
Severe thunderstorms need four ingredients:
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Moisture: a high dew point (about $15\,^\circ\text{C}$ or more) means lots of vapor and lots of latent heat.
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Instability: warm moist air near the ground with colder air above it, so rising parcels keep rising.
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Lift: something that starts the air upward, such as a front, a dry line, or heated ground.
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Wind shear: wind speed or direction that changes with height. This can make a storm rotate.
In spring, warm, moist air from the Gulf of Mexico flows north over the Great Plains. Cool, dry air from Canada and the Rockies moves in from the north and west. The warm air is less dense, so the cold air lifts it along the boundary. Strong winds aloft add shear. The warm, moist air is a maritime tropical (mT) air mass, and the cool, dry air is a continental polar (cP) air mass. This combination makes Tornado Alley (Texas, Oklahoma, Kansas, Nebraska) the most active tornado region in the country.
The central United States is often called Tornado Alley. Name the two types of air masses that commonly meet there in spring and explain in one or two sentences why…
The central United States is often called Tornado Alley. Name the two types of air masses that commonly meet there in spring and explain in one or two sentences why their meeting leads to severe thunderstorms.
On a spring afternoon near Dallas, Texas, the surface temperature is $27\,^\circ\text{C}$ and the dew point is $21\,^\circ\text{C}$, with a southeast wind of…
On a spring afternoon near Dallas, Texas, the surface temperature is $27\,^\circ\text{C}$ and the dew point is $21\,^\circ\text{C}$, with a southeast wind of $5\ \text{m/s}$. At about 5 km altitude, winds blow from the west at $30\ \text{m/s}$. A dry, warm layer sits above the moist surface air, and a cold front is approaching from the northwest.
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Identify the ingredients for severe thunderstorms and tornadoes that are present in this scenario (moisture, instability, lift, and wind shear), and justify each.
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Explain how the Gulf of Mexico, the land surface, and the atmosphere exchange thermal energy to produce these conditions.
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Predict the most likely type of severe weather and explain one human action that reduces risk.
Flooding: rain, runoff, and the land
How much rain turns into a flood depends on the ground. Rainwater either soaks in (infiltrates) or runs off. The runoff coefficient is the fraction of rain that becomes surface runoff: about 0.10 for forest and 0.90 for pavement.
A city like Houston is flat, has clay soils, and has a lot of pavement. Urbanization replaces soil and wetlands with impervious surfaces, so the same rain produces more runoff, faster. Add a slow storm like Hurricane Harvey, and the flood risk rises sharply.
A watershed of $2.0\ \text{km}^2$ receives $50\ \text{mm}$ of rain in one storm. The runoff coefficient (fraction of rain that becomes surface runoff) is 0.10 for forest…
A watershed of $2.0\ \text{km}^2$ receives $50\ \text{mm}$ of rain in one storm. The runoff coefficient (fraction of rain that becomes surface runoff) is 0.10 for forest and 0.90 for pavement.
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Calculate the total volume of rain that falls (in m$^3$).
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Calculate the runoff volume if the watershed is forest and if it is entirely paved.
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Explain how urbanization changes the risk of flash floods in a city such as Houston.
Hurricanes, surge, and a warming ocean
A hurricane's damage comes from wind, rain, and storm surge, a rise in sea level pushed onto land by the storm's winds and low pressure. Surge often causes the most deaths.
Warmer sea-surface temperature gives a hurricane more evaporation and so more latent heat. A rise from $27\,^\circ\text{C}$ to $30\,^\circ\text{C}$ in the Gulf adds fuel, and it also can deepen the warm layer, which helps storms keep their strength.
Higher sea level raises the starting point for the surge. Water depth over land is roughly surge plus sea-level rise minus the land's height.
Natural features reduce damage: marshes and mangroves absorb wave energy and slow the surge. Human choices change it too: filling in wetlands, dense building on low land, and lack of evacuation plans all add damage.
Two terms are easy to mix up:
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Frequency: how many storms occur per year (for example, 12 hurricanes instead of 8).
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Intensity: how strong each storm is (for example, more Category 4 and 5 storms).
The sea-surface temperature in part of the Gulf of Mexico rises from $27\,^\circ\text{C}$ to $30\,^\circ\text{C}$ over the summer. 1. Explain how warmer water increases…
The sea-surface temperature in part of the Gulf of Mexico rises from $27\,^\circ\text{C}$ to $30\,^\circ\text{C}$ over the summer.
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Explain how warmer water increases the energy available to a hurricane.
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Explain how rising sea level can make storm surge more destructive even if wind speeds do not change.
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Distinguish between a change in the frequency and a change in the intensity of hurricanes, and give an example of each.
Two coastal counties on the Gulf Coast each face a hurricane with a $3.0\ \text{m}$ storm surge. County A has intact marshes and mangroves and low-density housing.…
Two coastal counties on the Gulf Coast each face a hurricane with a $3.0\ \text{m}$ storm surge. County A has intact marshes and mangroves and low-density housing. County B has filled in most wetlands, has dense housing on land only $1.5\ \text{m}$ above current sea level, and has had $0.3\ \text{m}$ of sea-level rise in recent decades.
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Calculate how deep the floodwater would be over the lowest land in County B (ignoring waves), and compare it with the situation if sea level had not risen.
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Analyze natural and human factors that affect the severity of damage in each county.
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Write a CER argument stating which human decisions could most reduce the damage from future hurricanes, and which factors people cannot control.
Back to the beginning: the Houston storm gets its energy from the Sun, through warm ground and ocean, evaporation, and then latent heat released in condensation. Whether it becomes a flood depends on how much pavement the rain falls on.