Physci text 09

Weather, humidity, precipitation, and clouds

Weather information sites

A weekend of heavy rain, thunder, and lightning, always invites the inevitable questions of whether there is a storm present, approaching, or forming. Typhoons are the ancient fear here - the black typhoon circa 1905 that blotted out the sun in Kosrae for a week, and forced residents to seek shelter in mountain caves. There are many tools now available to the armchair weather observer. 

On the front line is the National Weather Service office on Guam. The front page includes weather warnings and alerts. Here on Pohnpei another US National Oceanic and Atmospheric Administration weather service office provides weather information in local languages. 

Once one understands some of the drivers of the local weather features including the InterTropical Convergence Zone and its migrations north and south with the sun, then sense can be made of satellite photos. Knowing to that weather tends to move from east to west along the equatorial tropics is also useful for those coming from temperate climates. 

Sites like Tropical Tidbits aggregate storm Invest and track information when storms are forming or have formed. 

The Joint Typhoon Warning Center is a leading indicator of what the NWS will announce - their track forecasts drive the public warnings and local statements that come from the NWS. JTWC data can give one a few hours of heads up on what the NWS is likely to announce.

A deeper dive into the meteorological data is available from the Tropical Cyclones page at the Cooperative Institute for Meteorological Satellite Studies of the Space Science and Engineering Center at the University of Wisconsin-Madison. Charts depicting lower level convergence, upper level divergence, vorticity, wind shear, and shear tendency provide details on meteorological conditions that influence storm formation.

Direct access to Himawari weather satellite bands is available on the Japanese Meteorological Satellite Center website.

Cyclocane provides "spaghetti" plots once a storm has formed, although these tend to be less useful than track products produced by the weather services.

Perhaps the most misunderstood tool is Windy which shows current weather and wind conditions, along with many other weather variables, and allows one to run one of the global models forward into the future. Windy is often misunderstood as being what will definitely happen, and there have been a number of rumors of impending storms that were based on these misunderstandings. Never rely on Windy for accurate weather information.

Relative humidity and precipitation

The residential class may start off with a demonstration of calculating the relative humidity, heat index, and risk level for activity using a wet and dry bulb thermometer. 

Relative humidity from dry bulb and wet bulb depression:

1. Look in the first column on the left. Find the row with the nearest dry bulb temperature to the measured dry bulb temperature.

2. Move to the right along the row until the wet bulb depression in the top row is matched.

3. Read the relative humidity provided by the table.

4. Look in the first row at the top. Find the column with the nearest dry bulb temperature to the measured dry bulb temperature.

6. Move down the column until the relative humidity nearest to the one you determined in the first table is found.

7. Read the effective temperature in °C (also called the Heat Index) provided by the table.

Heat index as an effective temperature in °C given a dry bulb temperature and Relative Humidity:

Precipitation

Warm air rises (convection) and cools. Cooler air holds less water than warmer air. The result is that water condenses out of the air as the air rises. This condensation is visible as clouds. Condensation requires dust, smoke, or other microscopic airborne particles for the water to condense upon. As a result of this condensation, precipitation can occur. Precipitation is produced by three different processes, although the first process is dominant in Micronesia.

    Collision-coalescence process occurs in warmer clouds, such as over tropical oceans. The tiny droplets of condensed water in the clouds collide and coalesce (stick together) forming larger drops of water. As the larger droplets fall they "sweep up" smaller droplets. This also occurs in colder clouds in the levels below where the ice crystals form. Cloud condensation nuclei are required. Collision-coalescence is the process that produces most of the daily rain in Micronesia.

    Bergeron process occurs in clouds that are cold enough to have supercooled water and ice crystals. Ice crystals form, and when they get large enough they fall to the surface. Freezing nuclei are required. The Bergeron process relies on supercooled liquid water interacting with ice nuclei to form larger particles. One by-product of this process are charge separations which lead to electrical discharges - lightning. The superheated air explodes outward generating a shock wave that dissipates as thunder. If there is lightning and thunder, then the Bergeron process is at work. In Micronesia, if there is no lightning and thunder, then the rain is being formed by the collision-coalescence process and not the Bergeron process.

    Orographic precipitation is precipitation from clouds formed as a result of air being forced up a slope. Orographic precipitation is rain being generated by the terrain, typically by air (wind) being forced up a mountain. The air cools, water condenses, and then collision-coalescence produces rain. The precipitation is referred to as "orographic precipitation." This is the generator of the rain on the windward side of high islands such as Oahu and Hawaii. Thus Waimanalo and Hilo are rainy, Waianae and Kona coasts (leeward) are dry.

What am I?

I bring fresh showers for the thirsting flowers,
From the seas and the streams;
I bear light shade for the leaves when laid
In their noonday dreams.
From my wings are shaken the dews that waken
The sweet buds every one,
When rocked to rest on their mother's breast,
As she dances about the sun.
I wield the flail of the lashing hail,
And whiten the green plains under,
And then again I dissolve it in rain,
And laugh as I pass in thunder.

I sift the snow on the mountains below,
And their great pines groan aghast;
And all the night 'tis my pillow white,
While I sleep in the arms of the blast.
Sublime on the towers of my skyey bowers,
Lightning, my pilot, sits;
In a cavern under is fettered the thunder,
It struggles and howls at fits;
Over earth and ocean, with gentle motion,
This pilot is guiding me,
Lured by the love of the genii that move
In the depths of the purple sea;
Over the rills, and the crags, and the hills,
Over the lakes and the plains,
Wherever he dream, under mountain or stream,
The Spirit he loves remains;
And I all the while bask in Heaven's blue smile,
Whilst he is dissolving in rains.

The sanguine Sunrise, with his meteor eyes,
And his burning plumes outspread,
Leaps on the back of my sailing rack,
When the morning star shines dead;
As on the jag of a mountain crag,
Which an earthquake rocks and swings,
An eagle alit one moment may sit
In the light of its golden wings.
And when Sunset may breathe,
from the lit sea beneath,
Its ardors of rest and of love,
And the crimson pall of eve may fall
From the depth of Heaven above,
With wings folded I rest, on mine aery nest,
As still as a brooding dove.

That orbed maiden with white fire laden,
Whom mortals call the Moon,
Glides glimmering o'er my fleece-like floor,
By the midnight breezes strewn;
And wherever the beat of her unseen feet,
Which only the angels hear,
May have broken the woof of my tent's thin roof,
The stars peep behind her and peer;
And I laugh to see them whirl and flee,
Like a swarm of golden bees,
When I widen the rent in my wind-built tent,
Till the calm rivers, lakes, and seas,
Like strips of the sky fallen through me on high,
Are each paved with the moon and these.

I bind the Sun's throne with a burning zone,
And the Moon's with a girdle of pearl;
The volcanoes are dim, and the stars reel and swim
When the whirlwinds my banner unfurl.
From cape to cape, with a bridge-like shape,
Over a torrent sea,
Sunbeam-proof, I hang like a roof,--
The mountains its columns be.
The triumphal arch through which I march
With hurricane, fire, and snow,
When the Powers of the air are chained to my chair,
Is the million-colored bow;
The sphere-fire above its soft colors wove,
While the moist Earth was laughing below.

I am the daughter of Earth and Water,
And the nursling of the Sky;
I pass through the pores of the ocean and shores;
I change, but I cannot die.
For after the rain when with never a stain
The pavilion of Heaven is bare,
And the winds and sunbeams with their convex gleams
Build up the blue dome of air,
I silently laugh at my own cenotaph,
And out of the caverns of rain,
Like a child from the womb,
like a ghost from the tomb,
I arise and unbuild it again.
- Percy Bysshe Shelley (1792-1822)

Clouds in Shakespeare

Hamlet: Do you see yonder cloud that's almost in shape of a camel?

Polonius: By the mass, and 'tis like a camel, indeed.

Hamlet: Methinks it is like a weasel.

Polonius: It is backed like a weasel.

Hamlet: Or like a whale?

Polonius: Very like a whale.

- Hamlet, Act iii Scene 2, William Shakespeare, 1602

Luke Howard and the scientific classification of clouds

Prior to 1800 clouds were seen as ephemeral, ever changing, impermanent, and thus difficult to categorize or name. In 1802 an Englishman named Luke Howard developed a systematic way to name clouds. Impressed by the Latin naming system designed by Carl von Linné for classifying plants and animals, Howard devised a Latin-based language system for classifying and naming clouds. Howard's system made sense to scientists. With some modifications, this is the system the world uses to this day. The descriptions are Howard's original descriptions.

• Cumulus: Latin for "heap." Convex or conical heaps, increasing upward from a horizontal base.

• Stratus: Latin for "layer." Widely extended horizontal sheets.

Cirrus: Latin for "curl." Flexuous fibres extensible by increase in any or all directions' - could also be described as "wispy."

• Nimbus: Latin for "rain." Systems of clouds from which rain falls (usually appear dark grey).

• Howard extended his system with descriptions for unusual types of clouds.

• Altus (alto) meaning 'high'

• Capillatus meaning 'having hair'

• Castellanus meaning 'castle-like battlements'

• Congestus meaning 'piled up'

• Floccus meaning 'tufted'

• Fractus meaning 'broken, irregular or ragged'

• Humilis meaning 'flattened'

• Lenticularis meaning 'lens or almond shaped'

• Mamma (mammatus) meaning 'udder or protuberance'

• Pileus meaning 'cap or hood'

One modification has been to add a prefix to the middle and high level clouds. Thus we now speak of altocumulus, altostratus, and cirrostratus clouds. Alto- refers to middle level clouds, cirro- to the highest level clouds. Low clouds get no prefix. Some clouds have bottoms at the low level and tops at the high level. These are vertically developed clouds and usually produce precipitation. These clouds are usually cumulonimbus clouds, a term that combines two of Howard's cloud terms. You and I think of the largest cumulonimbus as clouds that produce heavy rain, wind gusts, and occasional lightning and thunder.

• Low clouds: below 2000 meters (under 10,000 feet)

• Middle (alto) clouds: 2000 to 6000 meters (10,000 to 25,000 feet)

• High (cirro) clouds: 6000 meters and up (25,000 feet and up, not often higher than 35,000 feet)

Here on Pohnpei the top of our ridges are 700 to 800 meters high (2000 to 2500 feet). The bottoms of cumulus clouds are often also around this high. Cloud bottoms can be lower, and when the cloud reaches the ground the term used is "fog."

A second modification is that Howard's extended cloud names are often now used to modify one of the four basic cloud types. Cumulus humilis and cumulus congestus are examples of this naming system.

Howard is perhaps best known for his many cloud drawings that categorized the cloud types he had named. His skill as scientist was as a careful observer and recorder of physical nature.

Clouds, weather, sky phenomenon albums

Clouds and weather album

Cumulonimbus album

Altocumulus album

Weather words

Among all animals, only humans appear to attempt to name objects in their surroundings. Western science has spent a great deal of effort on naming and classifying objects and phenomenon. Western science is not the only system that has produced naming and classification systems.

All cultures have weather words. Words such as sky, cloud, fog, rain, wind, thunder, lightning, morning dew, storm, typhoon, and rainbow. Think about your own language. Is there only one word for each or many words? In English rain words include drizzle, mist, downpour, precipitation, shower, and deluge. Lightning is described sometimes by modifier words such as bolt, spear, and flash. Thunder includes variations such as "rolling thunder." Is there more than one kind of cloud? Do the languages of Micronesia include such distinctions?

The following are some of the weather words in some parts of Micronesia listed from east to west.

Fohru is more accurately a waterspout or tornado. The loan word typhoon from Asian languages is also used for typhoons. 

Typhoons of Kosrae and Pohnpei