Meteorology - Clouds and Fog Study Notes
Module 3 – Nautical Science
Section 1 – Clouds and Fog
Chapter 8 - Clouds and Fog
Unit 3 – Meteorology
What You Will Learn to Do
Demonstrate a working knowledge of meteorology and how it affects us.
Objectives
Describe the factors associated with cloud formation.
Describe cloud classifications as they relate to cloud types, altitudes, classes, and overall appearance.
Describe the three types of low clouds and their characteristics.
Describe the two types of middle clouds.
Describe the three types of high clouds found in our atmosphere.
Describe the factors associated when clouds are at sea.
Explain the formation of rain.
Describe modern rainmaking techniques.
Describe the cloud factors associated with the formation of snow, sleet, hail, frost, and dew.
Describe the process of how fog is formed on the Earth’s surface as well as hazards in relation to fog formation.
Key Terms
Hydroscopic nuclei: Tiny particles that absorb or attract moisture from the air.
Fog: A cloudlike mass or layer of minute water droplets or ice crystals near the surface of the Earth, appreciably reducing visibility.
Cirrus: A cloud of a class characterized by thin white filaments or narrow bands and a composition of ice crystals; of high altitude, about 20,000 – 40,000 feet.
Nimbus: A large grey rain cloud.
Cumulonimbus: A cloud of a class indicative of thunderstorm conditions, characterized by large, dense towers that may reach altitudes of 75,000 feet, uniform except for the tops, which appear fibrous due to the presence of ice crystals.
Nimbostratus: A cloud of a class characterized by a formless layer that is almost uniformly dark gray, a rain cloud of the larger type, of low altitude, usually below 8,000 feet.
Thunderhead: The upper portion of a cumulus cloud characterized by dense, sharply defined cauliflower-like upper parts and sometimes of great verticality.
Hail: Showery precipitation in the form of irregular pellets or balls of ice more than 1/5 inch in diameter.
Hailstone: A pellet of hail.
Sleet: Precipitation in the form of ice pellets created by the freezing of rain as it falls.
Frost: A covering of minute ice needles, formed from the atmosphere at night upon the ground and other exposed objects when they have cooled by radiation below the dew point, and when the dew point is below the freezing point.
Dew: Moisture condensed from the atmosphere and deposited in the form of small drops upon any cool surface.
Cloud seeding: Sowing or scattering clouds with crystals or particles of silver iodide or solid carbon dioxide to induce precipitation.
Silver-iodide crystals: A pale yellow, water-insoluble solid, which darkens on exposure to light; used chiefly in medicine, photography, and artificial rainmaking.
Advection fog: Fog that is formed when warm, moist air comes into contact with colder air.
“Sea smoke”: Fog caused by cold air flowing over a body of comparatively warm water, with vapor condensing in small convective columns near the water surface, giving the appearance of smoke or steam.
Opening Questions
List 2-3 reasons why accurate weather forecasting is important.
Introduction
Water is always present in the air in greater or smaller amounts. It can be present in three states:
Solid
Liquid
Vapor
We will be discussing how water vapor is formed into clouds and what kind of weather the various kinds of clouds bring. This information is vital to meteorologists, but it is also helpful and interesting to the average person.
Definition of a Cloud
Water vapor from evaporation and transpiration, once in the atmosphere, can be moved from place to place or one altitude to another by natural convection and wind.
Tiny particles of dust, sand, pollen, smoke, and salt particles from oceans are always present in the air. These particles are called hygroscopic or condensation nuclei, which are particles that readily absorb moisture.
A cloud is a mass of hydroscopic nuclei that have soaked up moisture from the water vapor in the air. Fog is formed in the same way; it is just a cloud very close to the ground.
Three scenarios may occur to the water droplets:
Re-evaporate
Rise and freeze into ice crystals.
Collide with other nuclei and form larger drops that become rain or snow.
Changes in atmospheric conditions account for the many different shapes of clouds and their presence at various altitudes. Meteorologists keep records of clouds and must account for cloud cover in their periodic weather reports, which is important for forecasting.
Cloud Classifications
There are three basic cloud types:
Cirrus
Cumulus
Stratus
Other types of clouds have names with combinations of the following:
Nimbus – rain
Alto – high
Fracto – fragmented or windblown
Clouds are sometimes classified by the altitudes at which they mostly occur:
Low clouds: surface - 7,000 feet
Middle clouds: 7,000 – 20,000 feet
High clouds: above 20,000 feet
Towering clouds: exceptionally high clouds with their base in low altitude.
Middle clouds seldom attain heights greater than 13,000 feet in polar regions. However, they may reach 23,000 – 45,000 feet in temperate and tropical regions.
There are ten general types of clouds grouped into low, middle, and high categories.
Classification of Low Clouds
The five types of low clouds include:
Stratus
Nimbostratus
Stratocumulus
Cumulus
Cumulonimbus
Stratus
Lowest cloud type.
Gray layer with a uniform base.
May cause drizzle but never rain.
Fog becomes stratus when it lifts.
Nimbostratus
Dark, shapeless, rain-laden.
Often have streaks of rain extending to the ground.
Often seen in summer at the base of thunderheads.
Brings steady, heavy snow in winter.
Stratocumulus
Irregular, rounded masses spread out in puffy or rolling layers.
Usually gray with darker spots.
Do not produce rain.
Usually precede bad weather.
Cumulus
Dense, puffy clouds with a beautiful, cauliflower-like appearance.
They rise by day and disappear at night.
Fleecy cumulus clouds usually mean fair weather ahead.
Cumulonimbus (Thunderheads)
Can reach heights of 75,000 feet.
Very dense clouds of the towering variety.
The base is a dark nimbus rain cloud.
May produce severe thunderstorms and tornadoes.
Classification of Middle Clouds
Middle clouds with the base beginning at about 10,000 feet include:
Altocumulus
Altostratus
Altocumulus
Gray or whitish layers, puffy, fleecy.
Made up of water droplets.
Sometimes produce a pale blue or yellow corona.
Presence means rain is probable within the next 24 hours.
Altostratus
Dense sheets of gray or blue.
Sun or Moon will show through but without corona.
Light rain is likely within 24 hours.
Classification of High Clouds
High clouds are composed almost entirely of tiny ice crystals. The three basic types are:
Cirrus
Cirrostratus
Cirrocumulus
Cirrus
Thin, wispy; made up of ice crystals.
Called “mare’s tails.”
If scattered, indicate clear, cold weather.
If in parallel lines, indicate a violent change in weather within 36 hours.
Cirrostratus
May nearly cover the sky with a filmy cloud.
Curly appearance at their edges.
Form large halos around the Sun and Moon.
Indicate clear and cold weather.
Cirrocumulus
Thin, patchy clouds that sometimes form in wavelike patterns.
Do not leave shadows.
Precipitation usually follows within 24 hours.
Clouds at Sea
Clouds have been leading lost seamen, navigators, and explorers to land since the days of the earliest hardy sea voyagers.
Stationary clouds on the horizon usually indicate an island is close by.
In the tropics, clouds often reflect the colors of the sandy beaches or coral reefs below.
Precipitation
Precipitation (rain, snow, sleet, and hail) cannot occur without clouds.
Temperature and presence of hygroscopic nuclei or ice crystals determine whether there will be precipitation and in what form it will take.
Raindrops form when moist air is cooled to the point where the moisture condenses into heavy drops.
Cloud moisture droplets are (0.002) inch in diameter and too light to fall to the earth. If the droplet grows to (0.02) inch or larger, it will fall from the cloud.
The combining of moisture droplets is called coalescence.
Coalescence Process
Coalescence occurs in two known ways:
Bigger droplets move about slowly, bumping into other droplets and combining with them (common in low clouds).
Ice crystals and water droplets form near each other. Droplets evaporate, and the resulting vapor collides with ice crystals, condensing into snow or ice pellets. They melt into rain as they pass through warmer air at lower altitudes.
Rainmaking Techniques
Rainmaking has been a concern of humans since ancient times. Rain dances, sacrifices, drums, cannons, and smoke have all been used in attempts to produce rain, particularly during droughts.
Seeding a large cumulus cloud with one pound of artificial nuclei made of dry-ice or silver-iodide crystals can start a shower.
Effects of Seeding
Seeding can cause:
Rain to fall sooner.
More rain to fall.
Rain to fall from a cloud that normally would not produce rain.
Seeding cannot cause:
Rain to fall from fair skies or fair-weather cumulus clouds.
Rain to fall over a large area.
Snow, Sleet, Hail, and Frost
Sleet occurs when rain or snow formed in relatively warm air falls through a layer of freezing air. When falling precipitation does not completely freeze, it may become a supercooled liquid or mush that freezes on contact when it hits the cold ground. This freezing rain may result in an ice storm, causing havoc in cities and at sea.
Hail usually occurs in the summertime. It begins as raindrops that are taken to greater heights by updrafts. They are coated with water from lower clouds, lifted again and again until too heavy to be lifted. Most hailstones are smaller than marbles, but hailstones as large as baseballs have killed people and animals.
Sleet and hail both fall to the ground as ice pellets. Sleet is formed in winter as it falls and freezes from the outside in, while hail is formed during summer in clouds and freezes from the inside out.
In winter, when the upper air is very cold, water vapor will condense into ice crystals. Frozen droplets grow into flakes as they acquire more available water droplets and vapor. The result is snow, with snowflakes sometimes combining into clumps to form aggregate flakes several centimeters or larger in size.
Dew is water vapor that condenses on objects that have cooled below the condensation point of the air around them; Frost is similar to dew but forms at temperatures below freezing. Neither dew nor frost falls from the skies.
Fog
Fog is really a low-lying cloud that is near or touching the surface of the Earth. Its formation requires the presence of:
Moisture
A gentle breeze
A combination of warm and cold temperatures
Hazards of Fog
Fog is hazardous on land, air, and sea because it limits visibility. Larger aircraft and vessels typically have radar to assist them in foggy conditions. The nautical “Rules of the Road” require that extra lookouts be stationed aboard ships in foggy conditions.
Vessels in fog must reduce speed, sound appropriate fog signals, and turn on navigational lights. Ships at anchor must sound fog signals and turn on anchor lights.
Aircraft flights are often delayed when visibility is poor due to fog; in some instances, fog has caused even well-equipped aircraft to crash. On land, fog frequently causes vehicle crashes on roads and highways, sometimes involving whole strings of cars and trucks.
Types of Fog
Advection fog: Air-mass fog produced by air in motion, or fog formed in one place and transported by wind to another. It will generally dissipate when the sun rises.
Steam fog: A type of advection fog formed by air saturation, occurring when cold air moves over warm water. In the far north, “sea smoke” can often be seen in late fall or winter when a river or pond “steams” as frigid air cools the water, causing it to form a coating of ice.
Radiation fog: It forms only at night and over land. The Sun usually burns the fog away. The heat that the Earth radiates causes radiation fog.
Leader Questions
Discuss two techniques used in modern rain-making.
Closing Questions
(Q) Which of these belongs to the middle clouds?
A. Stratocumulus
B. Cirrocumulus
C. Altocumulus
D. Altostratus
E. Cirrostratus
(Q) When rain formed in relatively warm air falls through a layer of freezing air, what occurs?
A. Rain
B. Sleet
C. Snow
D. Hail
(Q) For fog to occur, the air temperature must cool until it drops below the:
A. Relative humidity
B. Advection point
C. Dew point
D. Temperature of the land or water
Index Questions
Tiny particles that absorb or attract moisture from the air: Hydroscopic nuclei
A cloudlike mass or layer of minute water droplets or ice crystals near the surface of the Earth, appreciably reducing visibility: Fog
A cloud characterized by thin white filaments or narrow bands composed of ice crystals; of high altitude, about 20,000 to 40,000 feet: Cirrus
A large grey cloud: Nimbus
A cloud indicative of thunderstorm conditions, characterized by large, dense towers that may reach altitudes of 75,000 feet: Cumulonimbus
A cloud characterized by a formless layer that is almost uniformly dark gray, a rain cloud of the larger type, of low altitude, usually below 8,000 feet: Nimbostratus
The upper portion of a cumulus cloud characterized by dense, sharply defined cauliflower-like upper parts and sometimes of great verticality: Thunderhead
Showery precipitation in the form of irregular pellets or balls of ice more than 1/5 inch in diameter: Hail
A pellet of hail: Hailstone
Precipitation in the form of ice pellets created by the freezing of rain as it falls: Sleet
A covering of minute ice needles, formed from the atmosphere at night upon the ground and other objects when they have cooled by radiation below the dew point, and when the dew point is below the freezing point: Frost
Moisture condensed from the atmosphere and deposited in the form of small drops upon any cool surface: Dew
Sowing or scattering clouds with crystals or particles of silver iodide or solid carbon dioxide to induce precipitation: Cloud seeding
A pale yellow, water-insoluble solid, which darkens on exposure to light; used chiefly in medicine, photography, and artificial rainmaking: Silver-iodide crystals
Fog that is formed when warm, moist air comes into contact with colder air: Advection fog
Fog caused by cold air flowing over a body of comparatively warm water, with vapor condensing in small convective columns near the water surface and giving the appearance of smoke or steam: “Sea smoke”
Notes
Participant leaders, team scores, and team MVP details are noted but not elaborated upon as per the main content focus here.