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Air pressure and wind

Sinking air makes high pressure and clear skies, rising air low pressure and storms; wind blows from high to low, as strong as the pressure gradient is steep.

Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.

1. What you will learn

By the end of this lesson you will be able to read isobars, find a pressure gradient, convert pressure units, and predict wind and weather from pressure.

2. What you already have

You can read a climate graph and you know the water cycle: air rises, cools and forms clouds. This lesson explains what moves the air: differences in pressure, which drive the wind and decide whether the sky is clear or stormy.

3. Words for this lesson

4. Air flows downhill in pressure

Air has weight, and the weight of the air above pressing down is air pressure.

1. Where air sinks, pressure is high; sinking air warms and dries, so skies are usually clear.
2. Where air rises, pressure is low; rising air cools, its water condenses, and clouds and rain form.
3. Wind blows from high toward low pressure, turned by the Earth's spin.
4. The pressure gradient, the difference divided by the distance, sets the wind's strength: crowded isobars mean strong winds.

A falling barometer warns of an approaching low and its weather.

5. Energy drives circulation on several scales

Earth receives shortwave solar energy and emits longwave infrared energy. Reflection by clouds, ice and other surfaces reduces the absorbed share. Greenhouse gases absorb and emit infrared radiation; increasing them changes the balance until the surface and atmosphere adjust. Energy balance refers to incoming and outgoing energy, not equal temperatures everywhere.

Uneven solar heating helps create pressure differences and circulation. Warm air can rise where heating produces buoyancy; air aloft cools and moves, while surface flows replenish it. Rotation deflects moving air, and friction and landforms modify winds near the ground. The three-cell circulation model is a broad average, not a rule that predicts tomorrow's local wind at every site.

For an invented global budget, incoming sunlight averages 100 units, 30 are reflected, and 70 are absorbed. If only 68 leave as infrared during the interval, stored energy increases by 2. A local station can still experience a cold front during that global gain. Weather describes short-lived conditions; climate summarizes distributions over many years. Use pressure maps for an event and repeated observations for climate, linking them through energy and circulation without confusing their scales.

6. Work from energy to a circulation explanation

Use the whole Earth-atmosphere boundary for a global radiation budget. In the worked 100-unit example, 30 units of reflected sunlight and 68 of outgoing infrared are different losses: 100 - 30 - 68 = +2. Do not subtract reflection again from the already absorbed 70. Energy exchanged between surface and atmosphere stays inside that boundary; counting it as another loss to space would count the same transfer twice. These are invented interval totals, not a measurement of today's Earth.

Try a changed budget: 100 arrive, 35 are reflected and 67 leave as infrared. Pause to calculate before reading on. Absorption is 65 and storage change is -2. This interval loses energy even though some places may warm during it. Neither a gain nor a loss specifies the change in degrees without information about heat capacity and where the energy is stored.

Now consider an idealized tropical circulation. Strong heating near the equator supports buoyant rising air and a surface low-pressure belt. Moist rising air expands and cools; condensation can form clouds and rain. Air moves poleward aloft and sinks around 30 degrees latitude, warming as it descends and tending to suppress clouds. Near-surface air returns toward the tropical low, with rotation deflecting it right in the Northern Hemisphere and left in the Southern. This is the Hadley circulation; the middle-latitude and polar cells complete the broad three-cell pattern. It describes average transfers, not three sealed boxes or exact winds at every location.

Practice interpreting a shift: suppose the strongest heating and rising belt move south of the equator in a seasonal model. Which moves with it: the belt favoring moist ascent or Earth's rotation direction? The moist ascent belt moves; rotation does not reverse. Land-ocean heating differences and terrain can modify this pattern, so compare seasonal pressure and rainfall maps before forecasting a place. A rainy spell is weather; a repeated seasonal rainfall distribution over many years describes climate. The global energy budget does not by itself tell us where that rain will fall.

7. High and low pressure

High and low pressure bring different weather.

Weather maps mark a high with an H and a low with an L, and draw isobars around them like contours around a hill or a hollow.

8. Isobars

Isobars join places with equal pressure, usually drawn every $4$ hPa. They work like contour lines: where they crowd together, pressure changes fast over a short distance, and the wind is strong; where they spread out, winds are light.

Reported pressures are adjusted to sea level, so that stations at different heights can be compared on one map.

9. The pressure gradient

The pressure gradient is the difference in pressure divided by the distance. Two stations $300$ km apart that differ by $12$ hPa have a gradient of $12 \div 3 = 4$ hPa per $100$ km.

Double the difference over the same distance and the gradient doubles, and the wind blows harder. The strongest winds on Earth, in hurricanes and tornadoes, blow around the steepest gradients.

10. Why winds curve

If the Earth did not spin, wind would blow straight from high to low. But the Earth's rotation turns moving air: to the right in the Northern Hemisphere and to the left in the Southern. This is the Coriolis effect.

So in the Northern Hemisphere, winds circle a low counterclockwise, spiraling inward, and circle a high clockwise, spiraling outward. That is why hurricanes over the United States spin counterclockwise.

11. Pressure and height

Pressure falls as you climb, because there is less air above. Near sea level it falls about $1$ hPa for every $8$ m. At Denver, a mile above sea level, typical pressure is only about $840$ hPa.

At the top of Mount Everest, it is about a third of sea-level pressure, which is why most climbers carry bottled oxygen.

12. Units: hectopascals and inches

Scientists and most weather services use hectopascals, also called millibars. American TV weather reports and home barometers use inches of mercury, from the height of a column of mercury the air can hold up.

One inch of mercury is $33.86$ hPa, so average sea-level pressure, $1013$ hPa, is about $29.92$ inches.

13. Local winds

Pressure differences also drive local winds. On a sunny day, land heats faster than the sea; air rises over the land, pressure falls there, and a cool sea breeze blows in from the water. At night the pattern can reverse.

Mountains make valley breezes by day and mountain breezes by night, for the same reason: uneven heating makes uneven pressure.

14. Reading a barometer

A barometer's reading matters less than its trend. A steady or rising barometer means fair weather is likely to continue. A falling barometer means a low is approaching, with clouds, wind and perhaps rain.

A fall of several hundredths of an inch per hour, or several hectopascals in three hours, warns of a strengthening storm.

15. The method, step by step, and how to check it

1. High and low: find them on the map.
2. Isobars: read their values, stepping by the interval.
3. Difference: higher minus lower pressure.
4. Gradient: difference over distance in hundreds of kilometers.
5. Wind: from high toward low, curving right, stronger where the gradient is steep.

Checking an answer. Isobars must fall steadily toward a low. A gradient should be a few hectopascals per hundred kilometers in ordinary weather, more in storms.

16. Why each step is allowed

Dividing a difference by a distance is allowed because it measures how fast pressure changes across the ground, which is what pushes air. Stepping isobars by a fixed interval is allowed because weather maps draw them that way, just as contour maps do.

Subtracting height over eight is an approximation, allowed only near the ground, where the fall is nearly steady.

17. Pressure and forecasting

The National Weather Service combines pressure readings from thousands of stations, ships, weather balloons and satellites into maps drawn several times a day. Computer models use them to predict where highs and lows will move and how they will change.

Before modern models, sailors and farmers forecast with a barometer alone, watching for a fall that meant a storm was coming.

18. Global winds

Pressure differences also drive the planet's great wind belts. Near the equator, strong sunshine heats the air, which rises and leaves a belt of low pressure with towering clouds and heavy rain. Around thirty degrees north and south, that air sinks again, making belts of high pressure where many of the world's great deserts lie, including the deserts of the American Southwest and northern Mexico.

Between those belts, winds blow from high toward low pressure, turned by the Earth's spin: the trade winds blow toward the equator from the east, and the westerlies blow across most of the United States from the west. That is why weather systems usually cross the country from west to east, and why a forecaster watching the Pacific coast can see tomorrow's weather for the Rockies and the Plains.

19. Common slips

The most common slip is thinking wind blows from low to high pressure. Another is thinking high pressure brings storms; it usually brings calm, clear weather.

A third is forgetting the distance when comparing pressure differences. A fourth is reading a barometer's number without its trend.

20. In the world: the Great Lakes' November gales

In November, strong lows often cross the Great Lakes, and the pressure difference between them and highs to the west can be large. The isobars crowd together, and gales sweep across the open water, raising waves many meters high.

On November 10, 1975, the freighter Edmund Fitzgerald sank in Lake Superior during such a storm, with all twenty-nine of its crew. The low that caused it deepened quickly as it crossed the lakes, and winds on the lake gusted to hurricane force.

Today the National Weather Service issues gale and storm warnings for the lakes when it forecasts a steep pressure gradient, and ships watch their barometers and forecasts closely, especially in the stormy weeks of late autumn.

21. In the world: Denver's thin air

Denver, Colorado, sits about a mile above sea level, and its air pressure is typically only about 840 hectopascals, roughly 25 inches of mercury on an uncorrected barometer. There is about seventeen percent less air in each breath than at sea level.

Visitors notice. Water boils at about 95 °C instead of 100, so pasta and beans take longer to cook, and baking recipes often print special high-altitude instructions. Athletes train there because working in thin air builds endurance.

Denver's weather reports still give pressure near 30 inches, because every station's reading is adjusted to what it would be at sea level. That adjustment lets forecasters draw isobars across mountains and plains alike, and compare Denver with Kansas City fairly.

22. Wind blows from high to low

It is easy to get the direction backward, or to think high pressure means stormy weather because high sounds strong. But air flows from where it is pressed hardest toward where it is pressed least, like water flowing downhill, and high pressure comes from sinking air that brings clear, calm skies.

Low pressure is where storms form, because rising air makes clouds. A falling barometer is the warning to watch for.

23. A pressure gradient

1. Two stations $400$ km apart differ by $20$ hPa. Write the distance in hundreds.

   $\dfrac{400}{100} = 4$

   Hundreds of kilometers.

2. Divide the difference.

   $\dfrac{20}{4} = 5$

   Hectopascals per hundred kilometers.

3. Compare with a gentler day of $2$.

   $5 > 2$

   Stronger wind today.

4. Say which way it blows.

   $\text{toward the lower station}$

   Curving right.

24. Isobars toward a low

1. The isobar near a high reads $1024$ hPa, drawn every $4$. Find the next.

   $1024 - 4 = 1020$

   One interval.

2. Find the next two.

   $1016, 1012$

   Stepping down.

3. The low's center reads $996$. Count the intervals from $1024$.

   $\dfrac{1024 - 996}{4} = 7$

   Isobars between.

4. Say where the wind is strongest.

   $\text{where the isobars crowd}$

   The steepest gradient.

5. Say the weather at the low.

   $\text{clouds and rain}$

   Rising air.

25. A falling barometer

1. A barometer reads $30.10$ inches, and $12$ hours later $29.50$. Find the fall.

   $30.10 - 29.50 = 0.60$

   Inches of mercury.

2. Find the fall per hour.

   $\dfrac{0.60}{12} = 0.05$

   Inches per hour.

3. Convert the whole fall to hectopascals.

   $0.60 \times 33.86 \approx 20$

   Hectopascals.

4. Convert the final reading.

   $29.50 \times 33.86 \approx 999$

   Below average.

5. Read the warning.

   $\text{a storm approaching}$

   A steady, sizable fall.

6. Say what to expect.

   $\text{wind, clouds, rain or snow}$

   A low's weather.

26. Your turn: stations $200$ km apart differ by $16$ hPa. What is the gradient per hundred kilometers?

1. Write the distance in hundreds.

   $2$

   Hundreds of kilometers.

2. Divide the difference.

   $\dfrac{16}{2} = 8$

   Per hundred kilometers.

3. Your turn: work this step out. Its working is at the end of the packet.

   Judge the wind.

27. Guided practice

Two weather stations $450$ km apart differ in pressure by $9$ hPa. What is the pressure gradient between them, in hectopascals per hundred kilometers?

• $2$ hPa per 100 km
• $0.02$ hPa per 100 km
• $50$ hPa per 100 km
• $9$ hPa per 100 km

28. Guided practice

Complete the worked solution: one weather station reads $1024$ hPa and another, $200$ km away, reads $1008$ hPa. Find the pressure difference and the gradient in hectopascals per hundred kilometers.

1. Find the difference.

   $\text{higher} - \text{lower} =$ _d

   Hectopascals.

2. Find the gradient.

   $\dfrac{\text{difference}}{\text{distance} \div \text{a hundred}} =$ _g

   Per hundred kilometers.

3. Say which way the wind blows.

   $\text{from the higher toward the lower}$

   Turned right in the Northern Hemisphere.

4. Say what a bigger gradient brings.

   $\text{stronger wind}$

   Steeper pressure slope.

29. Guided practice

Match each feature of a weather map to the weather it brings.

30. Practice

On a weather map, the isobar nearest a high-pressure center reads $1004$ hPa, and isobars are drawn every $4$ hPa. Fill in the next three isobars toward the low.

31. Practice

Sea-level pressure today is $1013$ hPa. Near the ground, pressure falls about $1$ hPa for every $8$ m of height. Write the pressure, in hectopascals, as a function of the height $h$ in meters, for the first few hundred meters.

Answer:

32. Practice

American weather reports give pressure in inches of mercury, and one inch of mercury is $33.86$ hPa. What is $1020$ hPa in inches of mercury, to two decimal places?

Answer: inHg

33. Somewhere new

Suppose a barometer in Buffalo, New York reads $30.15$ inches of mercury, and $20$ hours later it reads $29.55$. How fast did the pressure fall, in inches of mercury per hour?

Answer: inHg per hour

34. Somewhere new

A hypothetical global budget receives 100 energy units, reflects 25 and emits 72 as infrared over the same interval. In a seasonal circulation model, strongest heating and a moist rising low-pressure belt have shifted to 10 degrees north. Air sinks in a high-pressure belt at 30 degrees north. A station has 8 cool days. Which interpretation correctly connects the budget, surface return flow and weather, allowing for Earth's rotation?

• The global gain is 3 units; surface air returns from 30 toward 10 degrees north with rightward deflection, and moist rising air favors cloud formation. A few cool local days do not contradict that budget.
• The global gain is 28 units; surface air returns from 30 toward 10 degrees north with rightward deflection. Cool local days can still occur.
• The global gain is 3 units; surface air flows from the rising 10-degree low toward the sinking 30-degree high, while the return flow toward the low occurs aloft.
• The global gain is 3 units and surface air returns toward the 10-degree low; therefore every station must warm each day and receive rain.

35. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

36. Test question

Sea-level pressure today is $1013$ hPa. Near the ground, pressure falls about $1$ hPa for every $8$ m of height. Write the pressure, in hectopascals, as a function of the height $h$ in meters, for the first few hundred meters.

Answer:

37. What you can do now

You can read pressure and wind. Explain why crowded isobars on a weather map mean strong winds.

Working for the steps left to you

26. Your turn: stations $200$ km apart differ by $16$ hPa. What is the gradient per hundred kilometers?, step 3