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Forecast Terms ..........                      back to today's forecast
 

 

Understanding the weather forecast  

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since 11 January 2006
. . . can be a challenge especially if you are not familiar with the terms or measurement units  used. The summary below has been set up to  help you understand what the terms and the numbers often used in the weather forecast mean.
 
Troughs, Ridges, and Convergence Zones
A trough is an elongated area of relatively low atmospheric pressure. 
Low atmospheric pressure around the Samoa area normally brings about cloudy skies and occasional rain and at times brief, but often at times, gusty winds. On a weather map, an area of low atmospheric pressure (called a 'Low') is represented by the letter 'L'. During the wet season in Samoa (November - April) Lows are more common in and around the Samoa area.

A ridge (of high pressure) is an elongated area of relatively high atmospheric pressure, almost always associated with wind flow. 
High atmospheric pressure around the Samoa area is normally associated with clear skies, persistent gentle breeze to strong winds, and generally fine weather. On a weather map, an area of high atmospheric pressure (called a 'High') is represented by the letter 'H'. During the dry season in Samoa (May - October), Highs are more common in and around the Samoa. 

The convergence zone mentioned often in the weather forecast refers to the South Pacific Convergence Zone (SPCZ), a band of wind convergence, cloudiness and precipitation extending from the west Pacific near Papua New Guinea, south-eastwards towards French Polynesia. This convergence zone occurs where the southeast trade winds from transitory Highs to the south-west Pacific (southern area generally between Australia to east of New Zealand) meet with the semi-permanent easterly flow from the south-east Pacific High (generally south of French Polynesia).
Wind speed and direction
Wind is the element that is most influenced by surrounding hills, valleys, and buildings. It flows over the ground like water down a stream ducking and diving over hills. Eddies form in the shelter of the ranges , while fast running sections flow through narrow gorges and around the end of hilly chains

A northerly wind means the wind blows from the north, while a southeast wind comes from somewhere between south and east. The term variable refers to winds coming from no particular direction - it does not refer to the wind speed.

Wind speed guide for use on land
Explanatory titles Impact of this wind on land Kilometers 
per hour

Miles per hour
Not significant
Calm Calm; smoke rises vertically less than 1 less than 1
Light  air Direction of wind indicated by smoke drift, but not by wind vanes 1-5 1 - 4
Slight breeze Wind felt on face; leaves rustle; ordinary vane moved by wind 6-11 5 - 7
Gentle breeze Leaves and small twigs in constant motion; wind extends light flag 12-19 8 - 11
Moderate breeze Raises dust and loose paper; small branches moved 20-28 12 - 18
The Shakers
Fresh Breeze Small trees in leaf begin to sway; wavelets form on inland waters 29-38 19 - 24
Strong Breeze large branches in motion; whistling heard in telegraph wires; umbrellas used with difficulty 39-49 25 - 31
Near gale Whole trees in motion; inconvenience felt when walking against wind 50-61 32 - 38
The Breakers
Gale Breaks twigs off trees; generally impedes progress 62-74 39 - 46
Strong gale Slight structural damage (roofing iron raised slightly) 75-88 47 - 54
Storm Trees are uprooted; considerable structural damage occurs 89-102 55 - 63
Damaging Winds
Violent storm Very rarely experienced; accompanied by widespread damage 103-117

64 - 72
Hurricane

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 above 118  above 73
Weather words and phrases
Rain, Showers and Drizzle
To most people when it's wet it's wet, and it doesn't make much difference whether you call rain or showers. Forecasters use these terms to add extra information about the character of the precipitation.

Rain comes with generally cloudy skies, while there are bright periods between individual showers. 
Drizzle  consists of very small raindrops that appear to float on the wind.

Term

Comment
Dry a period without precipitation. There may or may not be  cloud, dew or mist
Fine Dry weather during which conditions are mostly sunny or bright. Even if there are scattered clouds or a thin veil of high clouds over the sky, shadows are cast by the sun most of the time
Dull Gloomy overcast day
Wet A period during which rain or showers occur most of the time
Showery A period where there are showers and temporary clearances
Stormy A period which rain and high winds predominate
Settled A period of several days during which the weather is either: dry, fine: or dry apart from a few patches of rain. If it does rain, the rain either falls as morning drizzle or as afternoon showers near the high country. It is unlikely to be windy
Unsettled a period of several days during which precipitation is likely to occur. Although there may be breaks in the precipitation, these breaks will seldom last more than a day. Strong winds may occur sometimes.
 

Some Useful Unit Conversions
Multiply the line unit by the indicated number to get the column unit; for example on the first line, 12 inches gives 1 ft in the second column and on the last line (time) 24 hours gives 1 day in column 4. Only conversions to higher values are shown.

Calculator? One is available here

LENGTH

in ft mi nm mm m km olat
in 1 12 6.336x104 7.30x104 * 39.37 393.70 4.38x106
ft * 1 5280 6080 * 3.28 3281 364320
mi * * 1 1.15 * * * 69
nm * * * 1 * * * 60
mm 25.4 304.8 1.61x106 1.85x106 1 103 106 1.11137x108
m * * 1609 1853 * 1 103 1.11137x105
km * * 1.61 1.853 * * 1 111.137

TIME

sec min hour day
sec 1 60 3.6x103 8.64x103
min * 1 60 1.44x103
hour * * 1 24

SPEED

km h-1 mph kt ms-1
km h-1 1 1.61 1.85 3.6
mph * 1 1.15 2.24
kt * * 1 1.95

CONVERSION oC to oF

F = 9/5C + 32
C = 5/9 (F - 32)