Condensation Process
Monday, June 29, 2026 8:46 AM
Dew and Frost
- Dew is condensation on cool surface areas & objects, created when moist air is brought
in contact w/ a cold surface causing the air to reach dew point & condensate. On a cool
night, soil & plants lose heat, cooling rapidly by longwave radiation. If the air that strikes
these cool surfaces are cool enough to reach the dew point, it forms. Beads of water
collect on the cold surface
- If condensation occurs in temps below freezing, water vapor will condense as ice crystals
& frost will appear on exposed surfaces rather than beads of water!
On orchards you find wind machines. Warm air leaving the surface is brought back down to
keep the trees frost-free! They stir up the air so it wont cool below 32F (to prevent frost
damage to crops). Prior to wind machines, iron smudge pots were placed around the
orchard. They would light the pots to generate heat & keep the surface from cooling below
freezing!
- Ever notice dew on your car on a clear afternoon? Maybe on your car in the
morning. This is b/c at night, when the air is cool & the surface is also cool, if air is moist
enough & comes in contact w/ your cool car, it cools to dew point & collects
Fog
- Fog is a cloud at or near the earth's surface. It forms when moist air near the ground is
cooled to its dew point & condenses on nuclei near the surface. Depending on how the
air cools to its dew point, there are different types of fog
1. Radiation
- Fog is formed on cool, clear night when the ground cools & heat from the ground rises
upward. Remember radiational inversion? (where the surface is cool & air higher
above is warm) if the air near the surface cools to its dew point temp, fog forms just
above the cool surface. The thickness of the fog depends on the level of inversion, but
it may be a few inches to a few hundred feet deep. This is a very common type of fog
that covers large expanses of flat surface
§ The reason the fog sits over selected surfaces is b/c it forms above cool enough
surfaces for air to reach its dew point. Fog will evaporate when the surface & the
air above the surface warms w/ the rising sun. The field stays cooler than the
paved parking lot & building & lasts longer over the field
2. Advection fog
- Forms when warm, moist air moves over a cold surface, like snow-covered ground or a
cold ocean current is cooled to its dew point. This is common along the Pacific Coast &
occasionally in New England. In coastal SoCal, when air blows toward the coast in a
sea breeze, warm air reaches the cold current along the coast
3. Orographic
- (Upslope) fog is associated w/ mountain regions where warm, moist air is forced up
mountain slopes & cools to its dew point. Fog clouds are produced over a sloping
ground on the windward sides of mountains
4. Evaporation
- Fog (steam) is formed when colder air blows over a warm body of water. The warm air
above water cools to its dew point & fog forms. It’s common when bodies of water
(lakes, wetlands, etc.) are warmer than the air, which is typical in fall before water
cool down
- Remember, it takes longer for water to heat & cool than air. Since it looks like steam rising
out of water, it’s sometimes called steam fog. The same process that occurs over a hot tub
when the air temperature is cold enough
Precipitation Forms
- Precipitation, a term that covers all the forms of water from the atmosphere, mainly
rain, snow, sleet & hail. Why do some clouds give rain & others don’t? Every cloud
contains water drops, ice particles & crystals, which are so tiny they usually float on air
• To fall to the surface as rain, they grow a 100x larger, this happens when the air rises
& cools so much that it releases more & more water by condensation. When these
droplets merge together, they become so heavy that they fall as precipitation
1. Rain is formed as a result of air that is saturated. Cloud particles collide & combine into
small droplets. Drizzle is about 1/50th of an inch in diameter & merging (coalescence)
increase’s droplet size. Cloud droplets turn to raindrops only when they are heavy enough
to overcome air resistance
• Once droplets are large enough (1/10' 'inch) by colliding & merging together, they
form rain 1/10- 1/4’ of an inch in diameter. When they get larger than 1/4'' in, they fall
& break into smaller drops falling toward the surface
- You can see this by spraying some water onto a window until the droplets have grown
large enough to overcome the surface tension. Keep spraying & the drops will land close
enough to merge & grow heavy enough to roll down
- Most rain is the result of condensation & precipitation in rising air in temps above freezing
within the cloud. In warm clouds of the tropical zone, rain forms directly from liquid
condensation & coalescence
• In the middle & high latitudes, rain is the product of melting snow as it falls to warmer,
lower levels. Within higher latitude clouds, temps are often below freezing (32F or
colder), water vapor turns into ice crystals to form the cloud. These ice crystals grow
when more water vapor condenses onto them
- Supercooled water droplets (ones that remain in liquid form by resisting freezing in
temperatures below 32F) are also parts of these freezing clouds & provide the cloud w/
more moisture
2. Snow is produced in clouds, composed of ice crystals & supercooled water droplets. Snow
appears white b/c snowflakes act like prisms, scattering light in all directions, giving them
the white color.
• At the center of snowflake’s are freezing nucleus where water vapor can condense,
collect, freeze & form crystals. The crystals clot to form snowflakes that become heavy
enough to fall to earth as snow
- When the underlying air layer is freezing, snow reach’s the ground as a solid form of
precipitation; if the underlying air layer is above freezing it will melt the snow & we will
have rain
3. Sleet is a rare form of precipitation, it forms when there is a temp inversion. The surface air
must be below freezing in temp & the temperature higher up is above freezing. Rain falls to
the surface & freezes into ice grains or pellets
4. Glaze is formed when rain freezes on contact w/ a freezing surface after impact. A film of
ice is formed on the surface, b/c the ice is transparent, when it forms on the blacktop
surface of the road, the black surface is still visible
• This is why it is sometimes called black ice. It is also sometimes called freezing
rain. It's one of the most treacherous of winter hazards for the motorist.
5. Hail is another precipitation form which consists of spheres of ice, ranging in size from
small pellets to grapefruit size. Formed by the up & down movement of ice particles in a
large cloud
• These clouds are so tall, lower parts of the cloud lie in temperatures above freezing in
temp & higher parts in temps below freezing. The cloud type most common for
producing hail is the cumulonimbus (thunderstorm) cloud.
• Strong updrafts & downdrafts cause them to move up/down melting on the outer
layer, freezing again creating onion-like rings of ice. They get so heavy that updrafts
can no longer carry them up & they fall as hail.
Monday, June 29, 2026 8:46 AM
Dew and Frost
- Dew is condensation on cool surface areas & objects, created when moist air is brought
in contact w/ a cold surface causing the air to reach dew point & condensate. On a cool
night, soil & plants lose heat, cooling rapidly by longwave radiation. If the air that strikes
these cool surfaces are cool enough to reach the dew point, it forms. Beads of water
collect on the cold surface
- If condensation occurs in temps below freezing, water vapor will condense as ice crystals
& frost will appear on exposed surfaces rather than beads of water!
On orchards you find wind machines. Warm air leaving the surface is brought back down to
keep the trees frost-free! They stir up the air so it wont cool below 32F (to prevent frost
damage to crops). Prior to wind machines, iron smudge pots were placed around the
orchard. They would light the pots to generate heat & keep the surface from cooling below
freezing!
- Ever notice dew on your car on a clear afternoon? Maybe on your car in the
morning. This is b/c at night, when the air is cool & the surface is also cool, if air is moist
enough & comes in contact w/ your cool car, it cools to dew point & collects
Fog
- Fog is a cloud at or near the earth's surface. It forms when moist air near the ground is
cooled to its dew point & condenses on nuclei near the surface. Depending on how the
air cools to its dew point, there are different types of fog
1. Radiation
- Fog is formed on cool, clear night when the ground cools & heat from the ground rises
upward. Remember radiational inversion? (where the surface is cool & air higher
above is warm) if the air near the surface cools to its dew point temp, fog forms just
above the cool surface. The thickness of the fog depends on the level of inversion, but
it may be a few inches to a few hundred feet deep. This is a very common type of fog
that covers large expanses of flat surface
§ The reason the fog sits over selected surfaces is b/c it forms above cool enough
surfaces for air to reach its dew point. Fog will evaporate when the surface & the
air above the surface warms w/ the rising sun. The field stays cooler than the
paved parking lot & building & lasts longer over the field
2. Advection fog
- Forms when warm, moist air moves over a cold surface, like snow-covered ground or a
cold ocean current is cooled to its dew point. This is common along the Pacific Coast &
occasionally in New England. In coastal SoCal, when air blows toward the coast in a
sea breeze, warm air reaches the cold current along the coast
3. Orographic
- (Upslope) fog is associated w/ mountain regions where warm, moist air is forced up
mountain slopes & cools to its dew point. Fog clouds are produced over a sloping
ground on the windward sides of mountains
4. Evaporation
- Fog (steam) is formed when colder air blows over a warm body of water. The warm air
above water cools to its dew point & fog forms. It’s common when bodies of water
(lakes, wetlands, etc.) are warmer than the air, which is typical in fall before water
cool down
- Remember, it takes longer for water to heat & cool than air. Since it looks like steam rising
out of water, it’s sometimes called steam fog. The same process that occurs over a hot tub
when the air temperature is cold enough
Precipitation Forms
- Precipitation, a term that covers all the forms of water from the atmosphere, mainly
rain, snow, sleet & hail. Why do some clouds give rain & others don’t? Every cloud
contains water drops, ice particles & crystals, which are so tiny they usually float on air
• To fall to the surface as rain, they grow a 100x larger, this happens when the air rises
& cools so much that it releases more & more water by condensation. When these
droplets merge together, they become so heavy that they fall as precipitation
1. Rain is formed as a result of air that is saturated. Cloud particles collide & combine into
small droplets. Drizzle is about 1/50th of an inch in diameter & merging (coalescence)
increase’s droplet size. Cloud droplets turn to raindrops only when they are heavy enough
to overcome air resistance
• Once droplets are large enough (1/10' 'inch) by colliding & merging together, they
form rain 1/10- 1/4’ of an inch in diameter. When they get larger than 1/4'' in, they fall
& break into smaller drops falling toward the surface
- You can see this by spraying some water onto a window until the droplets have grown
large enough to overcome the surface tension. Keep spraying & the drops will land close
enough to merge & grow heavy enough to roll down
- Most rain is the result of condensation & precipitation in rising air in temps above freezing
within the cloud. In warm clouds of the tropical zone, rain forms directly from liquid
condensation & coalescence
• In the middle & high latitudes, rain is the product of melting snow as it falls to warmer,
lower levels. Within higher latitude clouds, temps are often below freezing (32F or
colder), water vapor turns into ice crystals to form the cloud. These ice crystals grow
when more water vapor condenses onto them
- Supercooled water droplets (ones that remain in liquid form by resisting freezing in
temperatures below 32F) are also parts of these freezing clouds & provide the cloud w/
more moisture
2. Snow is produced in clouds, composed of ice crystals & supercooled water droplets. Snow
appears white b/c snowflakes act like prisms, scattering light in all directions, giving them
the white color.
• At the center of snowflake’s are freezing nucleus where water vapor can condense,
collect, freeze & form crystals. The crystals clot to form snowflakes that become heavy
enough to fall to earth as snow
- When the underlying air layer is freezing, snow reach’s the ground as a solid form of
precipitation; if the underlying air layer is above freezing it will melt the snow & we will
have rain
3. Sleet is a rare form of precipitation, it forms when there is a temp inversion. The surface air
must be below freezing in temp & the temperature higher up is above freezing. Rain falls to
the surface & freezes into ice grains or pellets
4. Glaze is formed when rain freezes on contact w/ a freezing surface after impact. A film of
ice is formed on the surface, b/c the ice is transparent, when it forms on the blacktop
surface of the road, the black surface is still visible
• This is why it is sometimes called black ice. It is also sometimes called freezing
rain. It's one of the most treacherous of winter hazards for the motorist.
5. Hail is another precipitation form which consists of spheres of ice, ranging in size from
small pellets to grapefruit size. Formed by the up & down movement of ice particles in a
large cloud
• These clouds are so tall, lower parts of the cloud lie in temperatures above freezing in
temp & higher parts in temps below freezing. The cloud type most common for
producing hail is the cumulonimbus (thunderstorm) cloud.
• Strong updrafts & downdrafts cause them to move up/down melting on the outer
layer, freezing again creating onion-like rings of ice. They get so heavy that updrafts
can no longer carry them up & they fall as hail.
