Fronts & Storms
Monday, June 29, 2026 9:18 AM
What causes air to rise?
- Precipitation is produced by two basic mechanisms:
1. The spontaneous rise of moist air
2. The forced rise of moist air
○ In both cases, as air moves upward, it results in condensation & precipitation. This means since
air is rising, it creates low pressure at the surface. Low pressure is not exclusively caused by a
warm surface, that's only one cause of low pressure
Stability & Instability
- Temperature is the KEY to stability. The stability/instability of air is important to the atmosphere b/c
it determines when a parcel of air will rise w/o any other forces involved. Air is unstable if warmer
than surrounding air. Air is stable when resisting vertical motion & will only rise if forced to rise. A
parcel of air will be stable if the same temp or cooler than the surrounding air.
- This means the parcel of air is as heavy as the surrounding air or heavier. This doesn't mean it will stay
stable, it's possible for a stable parcel of air to become unstable once it reaches the lifting condensation
level b/c of the release of latent heat, which slows its cooling; this is called “conditional instability.”
There are several ways precipitation is formed (whether it rises on its own or it’s forced to rise). Let’s
look at the basic precipitation processes:
1. Convection
- The spontaneous rise of moist air caused by surface heating, like a warm patch of earth, causes the
air to rise & cool. The air rises in strong updrafts b/c it’s warmer & less dense than surrounding air.
Differential heating of surfaces affects the rate of convection since certain types of surfaces heat
quicker than other types. Slope angle, availability of water for evaporation, variations in albedo
(reflectivity) all affect the differential heating.
I.E. bare ground & woodlands heat rapidly, while a forested area would receive less insolation &
thus be cooler.
- Once the air molecules heat, they spread out in all directions & the parcel of air becomes lighter, w/
fewer air molecules in a small space, causing the air to become buoyant. In other circumstances, if the
parcel maintains its instability, convection may continue & the cumulus cloud may grow into a dense
cumulonimbus/thunderstorm. Unstable air parcels often do maintain instability b/c once condensation
begins, the parcel cools off even slower (cooling at a slowed saturated adiabatic lapse rate due to the
release of latent heat in the condensation process)
- Thunder Storms (START WRITING HERE)
○ The thunderstorm is the world's most common storm type, at any given moment it's estimated
1800 thunderstorms are in progress, there are about 16 million thunderstorms a year, often
caused by convection. Most thunderstorms consist of several individual convection cells, in these
cells, warm air rises in heavy updrafts (winds may reach 40+ mph) & as we have already learned, as
warm, moist air rises, when it cools to its dew point, latent heat is released which helps maintain
the warmth of the air. Air continues to rise b/c it stays warmer than the surrounding air.
○ A single cumulonimbus will develop & grow to high levels (9 miles high), a thick cloud means plenty
of water particles are colliding & coalescing to produce heavy rainfall. Since these clouds are so
tall, hail may also result. In the mature stage of a thunderstorm, there is still the prevailing intense
updraft of air, but it’s joined by a cold downdraft generated by precipitation from high levels in the
cloud. The rainfall is often accompanied by thunder & lightning.
- Lightning and Thunder
○ Lightning is created as cloud particles become electrically charged, as they move up & down
inside the cloud. The positively charged particles collect at the top & the negatively charged
particles stay at the bottom. Since the ground is positively charged, lightning discharges toward
the positively charged ground (also from one cloud to another in the sky). A lightning rod
(Invented by Benjamin Franklin) is an iron rod mounted onto the rooftop that protects
buildings by providing a route for lightning to discharge harmlessly
○ Bolts hit somewhere on earth 100x each second, traveling w/ thunderstorms. Your chances of
getting struck by lightning are 1 million to one, some places greater than others. The voltage of
each bolt varies, most have so much electrical power (about billion volts) that they can heat the
surrounding air to as much as 54,000 F!
○ This causes air molecules to pass through, expand & make the crackling sound of thunder. So,
thunder is the sound made by lightning as it scorches through the air.
○ Thunder & lightning occur at the same instant, light reaches you almost instantly, while sound
travels more slowly. Sound travels 5 seconds/ mile, so the distance in miles to a thunderstorm can
be estimated by counting the number of seconds between seeing the lightning flash & hearing the
thunder, then dividing that number by 5
The 3 remaining precipitation mechanisms all involve the FORCED rise of stable air:
1) Orographic precipitation
○ This is the forced rise of large masses of air up a mountain slope. If we consider the adiabatic
processes: both cooling & heating results from orographic precipitation as air rises, sometimes
descends up & over the mountains.
2) Frontal Precipitation
○ This happens when a warm mass of air encounters cold air. Air masses aka "surrounding air” are in
the atmosphere, large sections of air about a thousand miles wide & several miles high. They form
in the high & low latitudes where they sit over the land, water, become uniform in temp &
moisture characteristics. The westerly wind belt pulls these air masses into the middle latitudes
○ When cold air meets up with warmer air, the warmer air mass is less dense than the cold air mass,
and the two do not readily mix. The warmer, less dense, air is forced to rise over the colder, denser
air. The interface where they meet is called a "front," named after WWI battlefronts. The rising
warm air will hence cool, condensate, and precipitate
3) Convergent
○ Precipitation is associated w/ major belts of surface winds blowing together from opposite
directions. This includes the convergence of the NE & SE Trades, the Polar Easterlies w/ the
Westerlies. Air converges, is forced to rise, clouds & precipitation result. The Intertropical
Convergence Zone (hence its name), & the Subpolar Low pressure belts are the 2 primary areas
where convergence occurs.
Monday, June 29, 2026 9:18 AM
What causes air to rise?
- Precipitation is produced by two basic mechanisms:
1. The spontaneous rise of moist air
2. The forced rise of moist air
○ In both cases, as air moves upward, it results in condensation & precipitation. This means since
air is rising, it creates low pressure at the surface. Low pressure is not exclusively caused by a
warm surface, that's only one cause of low pressure
Stability & Instability
- Temperature is the KEY to stability. The stability/instability of air is important to the atmosphere b/c
it determines when a parcel of air will rise w/o any other forces involved. Air is unstable if warmer
than surrounding air. Air is stable when resisting vertical motion & will only rise if forced to rise. A
parcel of air will be stable if the same temp or cooler than the surrounding air.
- This means the parcel of air is as heavy as the surrounding air or heavier. This doesn't mean it will stay
stable, it's possible for a stable parcel of air to become unstable once it reaches the lifting condensation
level b/c of the release of latent heat, which slows its cooling; this is called “conditional instability.”
There are several ways precipitation is formed (whether it rises on its own or it’s forced to rise). Let’s
look at the basic precipitation processes:
1. Convection
- The spontaneous rise of moist air caused by surface heating, like a warm patch of earth, causes the
air to rise & cool. The air rises in strong updrafts b/c it’s warmer & less dense than surrounding air.
Differential heating of surfaces affects the rate of convection since certain types of surfaces heat
quicker than other types. Slope angle, availability of water for evaporation, variations in albedo
(reflectivity) all affect the differential heating.
I.E. bare ground & woodlands heat rapidly, while a forested area would receive less insolation &
thus be cooler.
- Once the air molecules heat, they spread out in all directions & the parcel of air becomes lighter, w/
fewer air molecules in a small space, causing the air to become buoyant. In other circumstances, if the
parcel maintains its instability, convection may continue & the cumulus cloud may grow into a dense
cumulonimbus/thunderstorm. Unstable air parcels often do maintain instability b/c once condensation
begins, the parcel cools off even slower (cooling at a slowed saturated adiabatic lapse rate due to the
release of latent heat in the condensation process)
- Thunder Storms (START WRITING HERE)
○ The thunderstorm is the world's most common storm type, at any given moment it's estimated
1800 thunderstorms are in progress, there are about 16 million thunderstorms a year, often
caused by convection. Most thunderstorms consist of several individual convection cells, in these
cells, warm air rises in heavy updrafts (winds may reach 40+ mph) & as we have already learned, as
warm, moist air rises, when it cools to its dew point, latent heat is released which helps maintain
the warmth of the air. Air continues to rise b/c it stays warmer than the surrounding air.
○ A single cumulonimbus will develop & grow to high levels (9 miles high), a thick cloud means plenty
of water particles are colliding & coalescing to produce heavy rainfall. Since these clouds are so
tall, hail may also result. In the mature stage of a thunderstorm, there is still the prevailing intense
updraft of air, but it’s joined by a cold downdraft generated by precipitation from high levels in the
cloud. The rainfall is often accompanied by thunder & lightning.
- Lightning and Thunder
○ Lightning is created as cloud particles become electrically charged, as they move up & down
inside the cloud. The positively charged particles collect at the top & the negatively charged
particles stay at the bottom. Since the ground is positively charged, lightning discharges toward
the positively charged ground (also from one cloud to another in the sky). A lightning rod
(Invented by Benjamin Franklin) is an iron rod mounted onto the rooftop that protects
buildings by providing a route for lightning to discharge harmlessly
○ Bolts hit somewhere on earth 100x each second, traveling w/ thunderstorms. Your chances of
getting struck by lightning are 1 million to one, some places greater than others. The voltage of
each bolt varies, most have so much electrical power (about billion volts) that they can heat the
surrounding air to as much as 54,000 F!
○ This causes air molecules to pass through, expand & make the crackling sound of thunder. So,
thunder is the sound made by lightning as it scorches through the air.
○ Thunder & lightning occur at the same instant, light reaches you almost instantly, while sound
travels more slowly. Sound travels 5 seconds/ mile, so the distance in miles to a thunderstorm can
be estimated by counting the number of seconds between seeing the lightning flash & hearing the
thunder, then dividing that number by 5
The 3 remaining precipitation mechanisms all involve the FORCED rise of stable air:
1) Orographic precipitation
○ This is the forced rise of large masses of air up a mountain slope. If we consider the adiabatic
processes: both cooling & heating results from orographic precipitation as air rises, sometimes
descends up & over the mountains.
2) Frontal Precipitation
○ This happens when a warm mass of air encounters cold air. Air masses aka "surrounding air” are in
the atmosphere, large sections of air about a thousand miles wide & several miles high. They form
in the high & low latitudes where they sit over the land, water, become uniform in temp &
moisture characteristics. The westerly wind belt pulls these air masses into the middle latitudes
○ When cold air meets up with warmer air, the warmer air mass is less dense than the cold air mass,
and the two do not readily mix. The warmer, less dense, air is forced to rise over the colder, denser
air. The interface where they meet is called a "front," named after WWI battlefronts. The rising
warm air will hence cool, condensate, and precipitate
3) Convergent
○ Precipitation is associated w/ major belts of surface winds blowing together from opposite
directions. This includes the convergence of the NE & SE Trades, the Polar Easterlies w/ the
Westerlies. Air converges, is forced to rise, clouds & precipitation result. The Intertropical
Convergence Zone (hence its name), & the Subpolar Low pressure belts are the 2 primary areas
where convergence occurs.
