Dew and Frost

• Today

– Dew

– Frost

– Clouds

Condensation nucleus demonstration

Dew and Frost

• At night the ground cools by radiation.

• The air in a thin layer in contact with the ground cools as it loses heat to the ground

• The cooling air may then become saturated

• Moisture can condense out as droplets on the surface

Dew and Frost

• This process is aided by the surface itself• This provides something for the moisture

to condense onto• Rough surfaces are better

– Condensation likes rough surfaces– Rough surfaces prevent too much air flow

• Vegetation gets more dew as it is a source of moisture (air will have higher humidity)

Dew and Frost Types

Rime

Formation of Dew & FrostFormation of Dew & Frost

As air cools to its saturation, or dew point, vapor molecules slow As air cools to its saturation, or dew point, vapor molecules slow down and can adhere as dew on the ground surface or as frost down and can adhere as dew on the ground surface or as frost when air temperature drops below freezing.when air temperature drops below freezing.

Daily temperature lows often occur by radiational cooling, Daily temperature lows often occur by radiational cooling, forming dew at night or early morning.forming dew at night or early morning.

Figure 6.1Figure 6.1Figure 6.2Figure 6.2

Fog and Dew

• Most common on clear nights when there are no clouds to reflect (or absorb and re-radiate) the energy emitted by the Earth’s surface

• Need a source of moisture which is then cooled

CLOUDS

Cloud Groups & TypesCloud Groups & Types

Clouds are water droplets or ice crystals (or a mixture Clouds are water droplets or ice crystals (or a mixture of the two) suspended in the atmosphere.of the two) suspended in the atmosphere.

Clouds are grouped by their elevation as high, middle, Clouds are grouped by their elevation as high, middle, low, and those that vertically stretch across many low, and those that vertically stretch across many altitudes.altitudes.

There are several cloud types in these 4 groups.There are several cloud types in these 4 groups.

Cloud Classification

Classification is based on appearance and the altitudeof the clouds. The original classification schemeconsisted of four categories:

Sheet-like Stratus “Layer”

Puffy Clouds Cumulus “Heap”

Wispy Clouds Cirrus “Curl of Hair”

Rain Clouds Nimbus “Rain”

High CloudsLate in the 1800’s, the classification system was expandedto include more descriptive terms. Today there are fourmajor cloud groups.

1. High Clouds- Cirrus (Ci)- Cirrostratus (Cs)- Cirrocumulus (Cc)

High clouds are usually above 6000m and consist primarilyof ice crystals. They are blown by the upper level winds andare responsible for haloes and sundogs, etc.

Cirrus CloudsCirrus Clouds

High clouds (above 6000 m in middle High clouds (above 6000 m in middle latitudes) that are thin and wispy latitudes) that are thin and wispy and comprised mostly of ice crystals.and comprised mostly of ice crystals.

Figure 6.9Figure 6.9

Cirrocumulus CloudsCirrocumulus Clouds

Figure 6.10Figure 6.10

High clouds High clouds that are that are rounded rounded puffs, puffs, possibly in possibly in rows, are rows, are less common less common than cirrus.than cirrus.

Cirrostratus CloudsCirrostratus Clouds

High clouds High clouds that thinly that thinly cover the cover the entire sky entire sky with ice with ice crystals.crystals.

Light Light passing passing through through these these crystals may crystals may form a halo.form a halo.

Figure 6.11Figure 6.11

Middle Clouds

2. Middle Clouds- Altostratus (As)- Altocumulus (Ac)

Middle clouds are usually between 2000m and 6000m andconsist primarily of water droplets. The sun can still be seenthrough altostratus although there is no halo.

Altocumulus CloudsAltocumulus Clouds

Figure 6.12Figure 6.12

Middle clouds Middle clouds (between 2000 (between 2000 and 6000m in and 6000m in middle latitudes) middle latitudes) that are puffy that are puffy masses of white masses of white with gray edges.with gray edges.

With your hand With your hand overhead, they overhead, they are about the are about the size of your size of your fingernail.fingernail.

Altostratus CloudsAltostratus Clouds

Middle Middle clouds that clouds that cover the cover the entire sky entire sky and may and may create a create a dimly visible dimly visible or watery or watery sun and sun and diminish diminish formation of formation of shadows.shadows.

Figure 6.13Figure 6.13

Low Clouds

3. Low Clouds- Stratus (St)- Stratocumulus (Sc)- Nimbostatus (Ns)

Low clouds are usually below 2000m and consist primarilyof water droplets. The sun cannot be seen through stratus clouds.

Nimbostratus CloudNimbostratus Cloud

Figure 6.14Figure 6.14

Low clouds Low clouds (below 2000m) (below 2000m) with with precipitation precipitation that reaches the that reaches the ground.ground.

Shredded parts Shredded parts of these clouds of these clouds are called are called stratus fractus stratus fractus or scud.or scud.

Stratocumulus CloudsStratocumulus Clouds

Low clouds with rounded patches that range in color from light to Low clouds with rounded patches that range in color from light to dark gray.dark gray.With your hand extended overhead, they are about the size of your With your hand extended overhead, they are about the size of your palm and cover most of the sky.palm and cover most of the sky.

Figure 6.15Figure 6.15

Stratus CloudsStratus Clouds

Figure 6.16Figure 6.16

Low clouds that resemble a fog, but do not reach the ground, and Low clouds that resemble a fog, but do not reach the ground, and can generate a light mist or drizzle.can generate a light mist or drizzle.

Clouds With Vertical Development

4. Clouds With Vertical Development- Cumulus (Cu)- Cumulonimbus (Cb)

Cumulus Humilis CloudsCumulus Humilis Clouds

Clouds with vertical development that take a variety of shapes, Clouds with vertical development that take a variety of shapes, separated by sinking air and blue sky.separated by sinking air and blue sky.

Shredded sections are called cumulus fractus.Shredded sections are called cumulus fractus.

Figure 6.17Figure 6.17

Cumulus Congestus CloudsCumulus Congestus Clouds

Figure 6.18Figure 6.18

Clouds with vertical development that become larger in height, with Clouds with vertical development that become larger in height, with tops taking a ragged shape similar to cauliflower.tops taking a ragged shape similar to cauliflower.

Cumulonimbus CloudCumulonimbus Cloud

Clouds with vertical development that have grown into a towering Clouds with vertical development that have grown into a towering thunderstorm cloud with a variety of key features, including the thunderstorm cloud with a variety of key features, including the anvil top.anvil top.

Figure 6.18Figure 6.18

Summary of Cloud TypesSummary of Cloud Types

Figure 6.20Figure 6.20