Unit 10: Weather
Why the sky does what it does day to day: humidity condensing into clouds, air masses colliding at fronts, and storms forming when conditions turn violent.
9 topics · 30 questions
Each topic is a path, not a menu: work down it in order and every question builds on the one above. Click any of them to watch the answer draw itself.
Humidity and Cloud Formation
- How it worksHow does rising air cool, reach its dew point, and condense water vapor into visible cloud droplets?
- Worked exampleFind the relative humidity of air holding 9 grams of water vapor per cubic meter when it can hold 12 grams max.
- Common mistakeWhy isn't humidity just about how much water vapor is in the air, with nothing to do with temperature?
Your turn1 of 2
Air holds 12 grams of water vapor per cubic meter, while it could hold up to 21 grams per cubic meter at that temperature before becoming saturated. What is the relative humidity, as a percentage?Types of Clouds and What They Predict
- How it worksHow does a cloud's altitude and shape sort it into the cumulus, stratus, and cirrus families?
- Common mistakeWhy isn't a wispy cirrus cloud made of the same liquid water droplets as a low stratus cloud?
- Ties togetherHow does a towering cumulonimbus cloud connect a cloud's shape directly to severe weather?
Your turn1 of 2
A puffy cumulus cloud often means a fair afternoon, while a flat gray stratus layer often means steady drizzle is on the way. Why do some clouds mean a sunny afternoon while others mean you should grab an umbrella?Air Masses
- How it worksHow does an air mass take on the temperature and moisture of whatever surface it forms over?
- How it worksWalk me through a cold, dry continental polar air mass forming over northern Canada before it moves south.
- Common mistakeWhy doesn't an air mass change its properties the instant it drifts over new ground?
Your turn1 of 2
A cold, dry air mass forms over frozen northern Canada and keeps that same cold, dry character for days as it drifts south over the United States. Why does a single air mass keep its temperature and humidity for days as it drifts across a whole continent?Warm Fronts and Cold Fronts
- How it worksHow does warm air get forced to ride up and over cold air along a gently sloped warm front?
- How it worksWalk me through a fast-moving cold front wedging underneath warm air and shoving it violently upward.
- Common mistakeWhy doesn't warm air ever slide underneath cold air the way cold air slides under warm air?
- Going furtherWhat happens to the weather pattern when a fast cold front catches up to a slower warm front ahead of it?
Your turn1 of 2
A cold front sweeps through and triggers a brief, violent thunderstorm, while a warm front arriving the next day brings only hours of steady drizzle. Why does a cold front bring a sudden, violent thunderstorm while a warm front just brings hours of drizzle?Occluded and Stationary Fronts
- How it worksHow does a fast cold front overtaking a warm front lift the warm air entirely off the ground to form an occluded front?
- Common mistakeWhy isn't a stationary front just a front that has stopped producing any weather at all?
- Ties togetherHow does an occluded front combine the properties of the two fronts that collided to form it?
Your turn1 of 2
A stationary front parks itself over the same city for four straight days, dumping rain the entire time without ever advancing. Why can a storm system just stall out and dump rain on the same city for days without ever moving?High and Low Pressure Systems
- How it worksHow does rising air in a low-pressure system cool and condense into the clouds and storms it's known for?
- How it worksWalk me through why air spirals counterclockwise into a low-pressure system in the Northern Hemisphere.
- Common mistakeWhy doesn't high pressure spin in the same direction as low pressure in the same hemisphere?
Your turn1 of 2
Two weather stations, 360 km apart, record a pressure difference of 15 millibars between them. What is the pressure gradient, in millibars per kilometer?Thunderstorms
- How it worksHow does a cumulonimbus cloud build through the cumulus, mature, and dissipating stages of a storm's life cycle?
- How it worksWalk me through colliding ice crystals building up the electric charge that finally discharges as lightning.
- Worked exampleFind how far away a lightning strike is if you count 6 seconds between the flash and the thunder.
- Common mistakeWhy doesn't thunder happen after the lightning that caused it, even though you hear it later?
Your turn1 of 2
Sound travels at roughly one kilometer every 3 seconds. If you count 22 seconds between seeing a lightning flash and hearing the thunder, about how many kilometers away did the lightning strike?Tornadoes
- How it worksHow does wind shear tip a horizontal, spinning tube of air vertical inside a supercell thunderstorm?
- How it worksWalk me through a rotating mesocyclone tightening and speeding up into a visible tornado funnel.
- Common mistakeWhy isn't a tornado just a funnel-shaped cloud, rather than the visible part of a much larger rotating column of air?
Your turn1 of 2
Thousands of severe thunderstorms occur every year, yet only a small number of them ever produce an actual tornado. Why do only a small fraction of severe thunderstorms ever spin up an actual tornado?Hurricanes
- How it worksHow does warm ocean water evaporating and rising release the heat that powers a hurricane's spiral?
- How it worksWalk me through a hurricane's eye, eyewall, and spiral rain bands, from the calm center outward.
- Worked exampleFind which Saffir-Simpson category a hurricane falls into with sustained winds of 130 mph.
- Common mistakeWhy isn't the calm eye of a hurricane a sign that the storm is finally weakening?
Your turn1 of 2
A hurricane's sustained winds reach 148 miles per hour. The Saffir-Simpson category 4 threshold is 130 miles per hour. By how many miles per hour does this hurricane's wind speed exceed the category 4 threshold?