19 December 2007

My latest ruminations have been about weather terminology. They were prompted during the last snow/sleet/freezing rain/rain/sleet/freezing rain/rain event by NOAA’s use of the terms sleet and freezing rain. I had a sense of what they were saying practically but wasn’t 100% sure what they were saying in a scientific sort of way. So I started digging out weather books at home and at the libraries and pouring over the glossaries to read all of the precipitation-related terms. I was amazed by the number of different types of fog there are. One of my favorite books was Weather: A Visual Guide. It is one of the newer popular books where every topic gets two facing pages that are half filled with pictures, a quarter filled with captions, and what isn’t taken up by headings is left for the real text. But it grouped types of precipitation the way I was starting to think of think of them as I copied more and more definitions out of the glossaries. The diagram below is adapted from a diagram in that book.

Basically it says that the more warm air the ice has to fall through after it leaves the cloud the wetter will be the precipitation when it reaches the ground.

For the definitions below I have used the NOAA National Weather Service’s online glossary because there are no copyright issues involved with quoting them and because they are the people I listen to for the weather so I should know what they mean by the terms. The one term that got a lot of play last week that they don’t define was SLUSH which I had to get from the Oxford dictionary. Some of the specific types of fog or other definitions not covered by NOAA may have come from:

• Everything Weather Book
• Firefly Guide to Weather
• Weather: A Visual Guide
• Wind and Weather

Liquid Precipitation

Drizzle

Precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter.

Rain

Precipitation that falls to earth in drops more than 0.5 mm in diameter.

Solid Precipitation

Blizzard

(abbrev. BLZD)- A blizzard means that the following conditions are expected to prevail for a period of 3 hours or longer:

• Sustained wind or frequent gusts to 35 miles an hour or greater; and
• Considerable falling and/or blowing snow (i.e., reducing visibility frequently to less than 1/4 mile)

Graupel

Same as snow pellets or small hail.

Hail

Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud.

Ice Pellets

(abbrev. IP) Same as Sleet; defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of 1/2" or more.

Snow

Precipitation in the form of ice crystals, mainly of intricately branched, hexagonal form and often agglomerated into snowflakes, formed directly from the freezing [deposition] of the water vapor in the air.

Blowing Snow

Blowing snow is wind-driven snow that reduces surface visibility. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds. Blowing snow is usually accompanied by drifting snow.

Corn Ice Snow

In hydrologic terms, rotten granular ice.

Drifting Snow

Drifting snow is an uneven distribution of snowfall/snow depth caused by strong surface winds. Drifting snow may occur during or after a snowfall. Drifting snow is usually associated with blowing snow.

Firn (Snow)

In hydrologic terms, old snow on top of glaciers, granular and compact and not yet converted into ice. It is a transitional stage between snow and ice. Also called Neve.

Heavy Snow

This generally means...

• snowfall accumulating to 4" or more in depth in 12 hours or less; or
• snowfall accumulating to 6" or more in depth in 24 hours or less

In forecasts, snowfall amounts are expressed as a range of values, e.g., "8 to 12 inches." However, in heavy snow situations where there is considerable uncertainty concerning the range of values, more appropriate phrases are used, such as "...up to 12 inches..." or alternatively "...8 inches or more...".

Powder Snow

Dry, loose, unconsolidated snow.

Lake Effect Snow

Snow showers that are created when cold, dry air passes over a large warmer lake, such as one of the Great Lakes, and picks up moisture and heat.

Snowflake

An agglomeration of snow crystals falling as a unit.

Snowflake Shape

Formation Temperature	Shape
14° – 21° F	Needle
	Column
10° - 14° F	Plate
10° - 3° F	Dendrite
3° - -8° F	Plate
	Column capped with Plates
<-8°F	Column

Snow Flurries

Snow flurries are an intermittent light snowfall of short duration (generally light snow showers) with no measurable accumulation (trace category).

Snow Grains

Precipitation consisting of white, opaque ice particles usually less than 1 mm in diameter.

Snow Pellets

Precipitation, usually of brief duration, consisting of crisp, white, opaque ice particles, round or conical in shape and about 2 to 5 mm in diameter. Same as graupel or small hail.

Snow Shower

A snow shower is a short duration of moderate snowfall. Some accumulation is possible.

Snow Squall

A snow squall is an intense, but limited duration, period of moderate to heavy snowfall, accompanied by strong, gusty surface winds and possibly lightning (generally moderate to heavy snow showers). Snow accumulation may be significant.

Snowflake

An agglomeration of snow crystals falling as a unit.

Yellow Snow

Snow given a golden or yellow appearance by the presence in it of pine, cypress pollen, or anthropogenic material or animal-produced material.

Freezing Precipitation

Freezing Drizzle

A drizzle that falls as a liquid but freezes into glaze or rime upon contact with the cold ground or surface structures.

Freezing Rain

Rain that falls as a liquid but freezes into glaze upon contact with the ground.

Freezing Spray

An accumulation of freezing water droplets on a vessel caused by some appropriate combination of cold water, wind, cold air temperature, and vessel movement.

Glaze

Ice formed by freezing precipitation covering the ground or exposed objects.

Hail

Showery precipitation in the form of irregular pellets or balls of ice more than 5 mm in diameter, falling from a cumulonimbus cloud.

Ice Crystals

A barely visible crystalline form of ice that has the shape of needles, columns or plates. Ice crystals are so small that they seem to be suspended in air. Ice crystals occur at very low temperatures in a stable atmosphere.

Ice Storm

An ice storm is used to describe occasions when damaging accumulations of ice are expected during freezing rain situations. Significant accumulations of ice pull down trees and utility lines resulting in loss of power and communication. These accumulations of ice make walking and driving extremely dangerous. Significant ice accumulations are usually accumulations of 1/4" or greater.

Rime Ice

An opaque coating of tiny, white, granular ice particles caused by the rapid freezing of supercooled water droplets on impact with an object. See also clear ice.

Sleet

(PL) - Sleet is defined as pellets of ice composed of frozen or mostly frozen raindrops or refrozen partially melted snowflakes. These pellets of ice usually bounce after hitting the ground or other hard surfaces. Heavy sleet is a relatively rare event defined as an accumulation of ice pellets covering the ground to a depth of 1/2" or more.

Slush

Partially melted snow or ice; watery mud.

Supercooled Liquid Water

In the atmosphere, liquid water can survive at temperatures colder than 0 degrees Celsius; many vigorous storms contain large amounts of supercooled liquid water at cold temperatures. Important in the formation of graupel and hail.

Other Airborne Moisture

Dew

Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

Fog

(abbrev. F) Fog is water droplets suspended in the air at the Earth's surface. Fog is often hazardous when the visibility is reduced to 1/4 mile or less.

Advection Fog

A fog that forms when warm air flows over a cold surface and cools from below until saturation is reached.

Arctic Sea Smoke

A stream of fog that occurs over the ocean in high-latitude regions.

Evaporation-mixing Fog

Fog that forms when the evaporation of water raises the dew point of the adjacent air.

Freezing Fog

A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0° Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Ground Fog

(abbrev. GF) Fog produced over the land by the cooling of the lower atmosphere as it comes in contact with the ground. Also known as radiation fog, and in parts of California as tule fog.

Hill Fog

Fog caused by stratiform cloud intersecting hille.

Ice Fog

Same as Freezing Fog; a suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0° Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Precipitation Fog

Fog produced by precipitation falling through a layer of cold air.

Radiation Fog

A fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.

Rain Induced Fog

When warm rain falls through cooler air, water evaporates from the warm rain. It subsequently condenses in the cool air forming fog. Such fog can be quite dense. It generally will persist as long as the rain continues. Since temperature rises little during the day, there is little diurnal variation in rain induced fog. Improvement in visibility cannot be expected until the rain stops or moves out of the affected area.

Sea Fog

Common advection fog caused by transport of moist air over a cold body of water.

Sea Smoke

Visible, steamlike evaporation from a warm body of water into cold air, also called steam fog.

Shallow Fog

Fog in which the visibility at 6 feet above ground level is 5/8ths statute mile or more and the apparent visibility in the fog layer is less than 5/8ths statute mile.

Steam Fog

Fog formed when water vapor is added to air which is much colder than the source of the vapor. It may be formed when very cold air drifts across relatively warm water. At temperatures below about-20°F, ice particles or droxtals may be formed in the air producing a type of ice fog known as frost smoke.

Tule Fog

Radiation fog in the Central Valley of California. It forms during night and morning hours in late fall and winter months following the first significant rainfall. A leading cause of weather related casualties in California.

Upslope Fog

A fog that forms when moist, stable air is carried up a mountain slope.

Frost

(Abbrev. FRST) - Frost describes the formation of thin ice crystals on the ground or other surfaces in the form of scales, needles, feathers, or fans. Frost develops under conditions similar to dew, except the temperatures of the Earth's surface and earthbound objects falls below 32°F. As with the term "freeze," this condition is primarily significant during the growing season. If a frost period is sufficiently severe to end the growing season or delay its beginning, it is commonly referred to as a "killing frost." Because frost is primarily an event that occurs as the result of radiational cooling, it frequently occurs with a thermometer level temperature in the mid-30s.

Hoar Frost

A deposit of interlocking crystals formed by direct sublimation on objects, usually those of small diameter freely exposed to the air, such as tree branches, plants, wires, poles, etc. The deposition of hoar frost is similar to the process by which dew is formed, except that the temperature of the frosted object must be below freezing. It forms when air with a dew point below freezing is brought to saturation by cooling.

Haze

(abbrev. HZ)- An aggregation in the atmosphere of very fine, widely dispersed, solid or liquid particles, or both, giving the air an opalescent appearance that subdues colors.

Layered Haze

Haze produced when air pollution from multiple line, area or point sources is transported long distances to form distinguishable layers of discoloration in a stable atmosphere.

Regional Haze

Haze that is mixed uniformly between the surface and the top of a convective boundary layer.

Virga

Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground. In certain cases, shafts of virga may precede a microburst.

Precipitation-Related Terms

Aerosols

Minute particles suspended in a gas.

Bergemon-Findeisen Process

Process in clouds whereby snowflakes grow at the expense of liquid water droplets where both are present.

Cloud Droplets

Very small liquid droplets that compose a cloud. Typically 0.001 in. (0.02 mm).

Condensation

In general, the physical process by which a vapor becomes a liquid or solid; the opposite of evaporation, although on the molecular scale, both processes are always occurring.

Condensation Nuclei

Found in every cloud, droplet or ice crystal; critical microscopic particle on which condensation occurs.

Dew Point

(Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).

Freeze

A freeze is when the surface air temperature is expected to be 32°F or below over a widespread area for a climatologically significant period of time. Use of the term is usually restricted to advective situations or to occasions when wind or other conditions prevent frost. "Killing" may be used during the growing season when the temperature is expected to be low enough for a sufficient duration to kill all but the hardiest herbaceous crops.

Freezing Nuclei

Small particles in the air that serve as cores for the formation of ice.

Humidity

Generally, a measure of the water vapor content of the air. Popularly, it is used synonymously with relative humidity.

Relative Humidity

A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present. See dew point.

Specific Humidity

n a system of moist air, the ratio of the mass of water vapor to the total mass of the system.

Hygroscopic Nuclei

Small particles of matter on which water begins condensing even before the air is saturated.

Melting

The process by which a solid transforms to a liquid.

Mist

A visible aggregate of minute water particles suspended in the atmosphere that reduces visibility to less than 7 statute miles, but greater than or equal to 5/8 statute miles. It does not reduce visibility as much as fog and is often confused with drizzle.

Shower

A descriptor, SH, used to qualify precipitation characterized by the suddenness with which they start and stop, by the rapid changes of intensity, and usually by rapid changes in the appearance of the sky.

Smog

Originally smog meant a mixture of smoke and fog. Now, it means air that has restricted visibility due to pollution or pollution formed in the presence of sunlight--photochemical smog.

Sublimation

The transition of a substance from the solid phase directly to the vapor phase, or vice versa, without passing through an intermediate liquid phase. Thus an ice crystal or icicle sublimes under low relative humidity at temperatures below 0°C. The process is analogous to evaporation of a liquid.

Water Vapor

Water in gaseous form.

Severe Weather Warnings

Advisory

(Abbrev. ADVY)- Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property.

Watch

A watch is used when the risk of a hazardous weather or hydrologic event has increased significantly, but its occurrence, location, and/or timing is still uncertain. It is intended to provide enough lead time so that those who need to set their plans in motion can do so.

Warning

A warning is issued when a hazardous weather or hydrologic event is occurring, is imminent, or has a very high probability of occurring. A warning is used for conditions posing a threat to life or property.

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It appears that you can only back up your most recent 1000 posts with blogger. At least I haven't found a way to get to any older than that with the backup procedure they recommend. Since I have over 800 posts in this blog I thought the end of the year would be a good time to create Volume 2 of Astronomical Observations. Not that I back anything up, but just in case. So I created a new blog called starsights2.blogspot.com. Then I went in to start to customize it so that it looks just like this blog. I copied my template from this blog, which is working fine and can be saved any time I modify it, and pasted it into the new blog and Blogger tells me it can't be saved. Apparently something in XML hasn't been closed. And they won't show me what it is. I am not sure I know enough XML to sort it out.

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It is not looking good for the McGraw-Hill Sun Triangle alignment Friday or Saturday or for the full Moon on Sunday. Clouds are supposed to be prevelant through at least Sunday when we are due for rain with the temperature peaking at 50°F.