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Weather and climate

Be prepared!

Don’t knock the weather. If it didn’t change once in a while, nine out of ten people couldn’t start a conversation.
Phil Armstrong in 2 Promises

St Helena was first settled by the English, so of course we like to talk about the weather…

 

Clouds over the St Helena coastline

Below: Sun • Wind • Rain • Seasons • Expected weather • Current weather • In more technical terms… • Unexpected weather • Read More

All recent weather data recorded at, and kindly provided by, our Meteorological Station.

In excellence of climate, St Helena is perhaps without an equal; no heat of torrid zones, or cold blasts from frigid regions, approach its genial shores. There no thunder-storms terrify the timid, no cholera, no yellow-fever, no small-pox, scarlatina, or deadly lurking fever-germs pollute the air. Nor is its balmy atmosphere ever marred by scorching winds, hot vapours, typhoons, hurricanes, cyclones, or any other characteristic of tropical regions. Throughout the year bright sunshine, clear skies, gentle breezes and deep blue seas, all combine to make it one of the most charming spots that can be found.{a}

More recently, according to the Wikipedia The climate of Saint Helena island can be described as tropical, marine and mild. Although basically correct our weather is far more complex than that. Here are the main features:

Sun

Sun

St Helena is in the Southern Hemisphere, north of the Tropic of Capricorn, which means the midday sun is directly overhead during the summer (see the article ‘The shortest and longest days’ below). The temperature can reach 34°C in Jamestown on a sunny day, though you won’t swelter due to the wind. More normal temperatures are in the range 20-27°C. The hottest months are between January and March and the coolest are between June and September.

Tip: the tropical sun can be hot; sunscreen is recommended if going out for the day, especially on a boat{3}.

Wind

Wind

Wind direction {1}
Wind direction{1}

Being within the range of the South-easterly Trade Winds stops the weather from becoming uncomfortably hot and keeps the air fresh, even in the enclosed valleys such a Jamestown. This is one of the reasons{4} why our air-quality is unimaginably high throughout the island. Typical wind speeds are in the range 15-30Km/h. Wind speeds up to 80Km/h have been recorded but are very unusual. The lowest wind speed recorded in recent years was 17Km/h.

Rain

Rain

Surrounded by at least 1,900Km of ocean in any direction, you can expect a little rain. How much depends on the time of year, where you are and also, to a greater or lesser extent, on Sod’s Law{5}. That being said, the main wet periods are in March (the ‘Lent Rains’) and August (‘Scruffy August’{6}). Your location can be important - Jamestown receives less than 11cm of rainfall per year (only about 5% of the total across the rest of the island); the peaks can receive 102cm. And the weather is very variable; it can be raining in Longwood and sunny in Jamestown, and then an hour later the two can reverse. The good news is that a dull rainy morning can often lead onto a bright sunny afternoon.

The highest monthly rainfall in recent years was in February 2011: 120.8mm. August 2018 was the wettest August on record, with 106mm recorded at the Meteorological Station - that’s around 175% of the usual value; a truly ‘Scruffy August’. The record rainfall in a 24-hour period is 48.6mm, recorded on 26th April 2017, on which day the typical rainfall for a month fell in just 6 hours - when it does rain it can be heavy!.

Tip: bring a light waterproof, preferably one that packs down to fit in a pocket when not in use.

Sun, wind and rain

Seasons

Seasons

In effect St Helena doesn’t have any. It gets hotter in January-May and cooler in August-October but the difference is not great (see Expected weather (below)). There is no Autumn (‘Fall’) so trees shed their leaves when they feel like it, and no Spring so things grow at any time of the year. Christmas falls in the summer!

Tip: this means that there is no bad time of the year to come!

Sunrise and sunset

Sunrise

Sunrise

St Helena is located close to the equator so our day-length does not vary greatly around the year. In mid-winter sunrise is at around 06:50h and sunset at around 18:00h, a day-length of 11h10m. In mid-summer sunrise is at around 05:50h and sunset at around 18:50h, a day-length of 13h00m. The variation is therefore only around 15%.

If you like your information diagrammatically:

Weather chart

Expected weather

Here are the expected weather statistics for this and the next eleven months (based on recent history):

Figures, provided by the St Helena Meteorological Station, are monthly averages over the years 2001-2018, taken at Bottom Woods (North-west island temperatures - e.g. Jamestown or Half Tree Hollow - will be higher and rainfall lower). Figures in brackets [] are percentages of the annual total.

The data show a few interesting facts that do not accord with island folklore:

‘Sod’s Law’

It could be argued that, in some respects, our weather is controlled by ‘Sod’s Law’ - the rule that basically says that everything is out to get you. The primary holiday time for the entire northern hemisphere is August, which is one of our coldest and wettest months, and heavy seas are normal in January-February which is exactly when most Cruise Ships call and when The Governor’s Cup and other yachting rallies arrive. See also the Closing Humour image (below).

On the other hand, our winter may be colder than our summer but it's still often warmer here in August than it is in London, and we have a warm and mostly-sunny Christmas which makes St Helena an ideal place to escape the cold, dark northern hemisphere winter.

Some you win…{8}

For countryside this green you have to expect a little rain
For countryside this green you have to expect a little rain…

Current weather

There’s current time zone and weather information here or here and the site indexed below gives a general island view. You can also get information from BBC Weather.

Follow for a detailed forecast - new tab/window - links to WeatherUndergound.com/

Our Meteorological Station

Our Meteorological Station opened on 1st September 1976, replacing an older station at Hutts Gate. It is located in Bottom Woods, 15.93°S 5.66°W, 436m above sea level.

It takes daily ground weather readings, and releases a daily weather balloon to measure upper atmosphere conditions:

At approx. 11:15 daily, the Weather Station launches a weather balloon with a small lightweight device known as a Radiosonde. As the balloon/radiosonde ascends, measurements of temperature and relative humidity are made at 2-second intervals.

A GPS receiver in the device allows its location to be identified from which the pressure, wind speed and direction can be calculated. Data is transmitted back to a ground receiver for encoding and onward distribution to the UK MET Office.

Typically when launched, the balloon rises at 5.5 m/s to reach a height of about 25km. As pressure decreases with height, the balloon expands until it bursts at which point the radiosonde device returns to Earth.

Where measurements are required at very high levels in the atmosphere, larger balloons like the ones used on St. Helena are capable of reaching 35km.{b}

The Meteorological Station at Bottom Woods
The Meteorological Station at Bottom Woods

In more technical terms…

The following comes from www.io-warnemuende.de/en_hix-st-helena-island-climate-index.html:

The Benguela upwelling system in the sub-tropical southeast Atlantic is subject to dramatic inter-annual fluctuations sometimes termed ‘Benguela Niño’ events. The South Atlantic Anticyclone (SAA) is assumed to be the responsible climatic ‘activity centre’ for the south-east trade winds driving the upwelling processes along the Namibian and South-west African coasts.

Air temperature and humidity signals of this region, modulated by upwelling-controlled sea surface temperature (SST), are carried by the trades towards St Helena Island. The island’s 1893-1999 century-long monthly weather Records of temperature, pressure and rainfall have been assembled and homogenized. They exhibit trends for decreasing precipitation (10mm/100yr), increasing air temperature (0.9°C/100yr), and decreasing air pressure (0.6hPa/100yr).

Their first empirical orthogonal eigen function (EOF) covers 46% of the total variance; its associated temporal coefficient is proposed as a ‘St Helena Island Climate Index (HIX)’. Austral winter HIX has a 42% correlation with a remotely sensed SST-derived Benguela upwelling index, called IBU, for the time period 1982-1999.

Not yet identified Benguela Niños (1895, 1905, 1912, 1916, 1946) and years of strong Benguela upwelling (1911, 1922, 1967, 1976) are newly suggested by the HIX.{9}

Unexpected weather

As with weather elsewhere, conditions sometimes exceed normal expectations…

Below: Things we don’t get • Thunder & Lightning • ’Rollers’ • Aliens are coming! • It never rains… • Hypothermia. On St Helena?

Things we don’t get

Hurricanes, typhoons, tornados & dust storms

Global cyclone tracks
Global cyclone tracks

It is generally true that we don’t get any of these. Wind speeds up to 80Km/h have been recorded but are very unusual. See the diagram (right) regarding cyclones. However, listen to the report below to Radio St Helena in October 1993 by John Bailey (not a man given to fantasy)…

Whirlwind, John Bailey

Click here to hear this audio file

Click To listen

Frost, snow, hail & ice storms

G. C. Kitching claims the lowest temperature ever recorded on St Helena was 46°F in 1898; more recent records show a minimum of 18.6°C. G. C. Kitching also claims hail did once fall on St Helena, at Woodlands in a storm on 21st October 1897.

Heatwaves

G. C. Kitching claims an absolute maximum of 87°F was recorded at Woodlands in 1893; more recent records show a maximum of 27.5°C.

Droughts

We did have a significant water shortage in 2013 - the first for more than a decade - and another in the summer of 2016/17, but in neither case were drinking water supplies in danger so you couldn’t call either a drought by any means.

Monsoons

The highest monthly rainfall in recent years was in February 2011: 120.8mm.

We rarely get thunderstorms; when we do they cause great excitement!

Snow in Blue Hill? No, just clever photo editing!
Snow in Blue Hill? No, just clever photo editing!

Thunder & Lightning

Thunderstorms are sometimes seen out at sea, but until recently the last recorded thunderstorm over the island was on 9th June 1981. No damage was reported.

Amusingly, on 25th October 2016 the Government of St Helena issued a warning for consumers to reduce water consumption, saying there was no significant rainfall forecast over the coming weeks. This was followed 5 days later by a full-blown thunderstorm over the island, in the evening of Sunday 30th October. It was first noticed around dusk and continued well into the early hours of Monday morning, with bright intra-cloud lightning and loud thunder. A few Saints were terrified, never previously having experienced a storm such as this. No damage was reported. After the storm passed away from the island lightning was seen discharging into the sea.

Previous big storms include that of 21st October 1897, which lasted two days and produced hail stones a ½inch in diameter. Others recorded were on 22nd November 1874, 2nd October 1888, 22nd September 1890, 29th September 1891, 16th September 1894, 18th October 1905, 19th November 1914 and 4th November 1945 (there are no earlier records; either they didn’t occur or The East India Company didn’t consider thunderstorms worthy of note). It can be seen that they mostly occur in our Spring. The cluster in the late 1800s is interesting but probably not significant.

There are some reports of thunderstorms sometime in 1979, and another on 20th November 1985, but these are not in the Records.

The images below are from the storm on 30th October 2016:

Storm, 30th October 2016
{c}

Storm, 30th October 2016
{c}

Storm, 30th October 2016
{c}

Storm, 30th October 2016
{d}

Storm, 30th October 2016
{e}

 

’Rollers’

Storms out at sea sometimes result in an abnormally high swell in James Bay, and in recorded history a few times the waves have been powerful enough to cause damage. But nothing in recorded history ever matched the Rollers of February 1846. Read more on our Rollers page.

The aliens are coming!

The unusual cloud formation seen below was observed over St Helena on 6th January 2016. As far as we can tell it didn’t precede an alien invasion{10}. It appears to be of type altocumulus lenticularis.

Auto-Cumulus cloud seen over St Helena, 6th January 2016
Auto-Cumulus cloud seen over St Helena, 6th January 2016

It never rains…

An exceptional flood in Jamestown; usually the driest part of the island.
An exceptional flood in Jamestown; usually the driest part of the island.

Hypothermia. On St Helena?

Napoleon’s retreat from Moscow {2}
Napoleon’s retreat from Moscow{2}{f}

We understand that the lowest temperature ever officially recorded on St Helena was 8°C, and yet there is a story of somebody suffering from Hypothermia. It goes like this…

An ‘elderly’ gentleman (thought to have been in his 70s) was visiting the island. Despite his age he was fit and strong and an avid walker. One day he decided to attempt The Barn. All went well but when he was on the top low cloud descended without warning (as it often does), reducing visibility to near-zero. Aware that The Barn is basically an uneven plateau surrounded by steep cliffs, and being unsure of the path and his orientation, he decided to camp-down and wait for the visibility to improve; which it did not, and darkness fell. Missed from his accommodation a search party was sent out the following morning. Fortunately he had told others his route, so he was quickly located atop the barn, but after a night in the cold with no warm clothing he was found to be suffering from mild Hypothermia. He was taken down to the Hospital where he quickly made a full recovery. None the worst for his adventure, our unknown walker has the distinction of being St Helena’s only recorded case of Hypothermia. If you can help us verify this story please contact us

Read More

Below: Article: Four seasons in one day - Winter in St Helena and El Niño • Article: Stunning Cloud Swirls Spotted by Satellite • Shortest and longest days

Article: Four seasons in one day - Winter in St Helena and El Niño

From Notes from a (very) small island 29th August 2015{11}{12}

Wind-pruning example
Wind-pruning example

I have blogged about the weather on Saint Helena before. But being British it’s obviously a pretty much inexhaustible subject and always a conversation saver. The musically educated amongst you will also recognise yet another song title here and Crowded House themselves seemed to like singing about the weather, particularly using it has metaphors for feelings in their writing. But for this blog ‘four seasons in one day’ isn’t a metaphor it’s the daily reality of winter in St Helena. My last blog on the weather was about the micro-climates on the island, where you can be in a cloud forest in area but within a mile be in bright sunshine and not be able to see a cloud in the sky. At least though to experience this you had to travel somewhere, now it’s got to winter you can just choose a spot, stay there and let all the different weather patterns come to you.

This is my first full winter here, last year I returned to the UK for the whole of August which is what would be described as mid-winter here. In fact, it’s not described as mid-winter at all, it’s called ‘Scruffy August’ in a lovely turn of phrase that is meant to describe the higher winds, more cloud and greater levels of rain that are meant to hit the island during this month. In reality that has hardly been the case. This morning, at the end of winter the sun is shining, there isn’t a cloud in the sky and it’s about 24°C. Winter here has basically been like a British summer, it’s never got lower than 16°C in Jamestown, I’ve even be sunburned a couple of times. ‘Scruffy August’ itself really hasn’t materialised at all, in fact August has been pretty much like August at home, the odd scorcher, the odd cloudy day and the odd spot of rain.

The bad weather seemed to come a bit earlier. June and July were a bit cooler, a bit greyer than they usually would be and it was these days that would often have the ‘four seasons in one day’, apart from the snow, I don’t think it has ever snowed here{13}. Generally the days followed a similar patter, wake up to a grey and rainy morning, which has been blown out to sea by mid morning leaving a lovely warm afternoon before turning windy, grey and wet again in the early evening. In all honesty, I can cope with that, get the rain out of the way during the night leaving lovely sunny days for us to enjoy.

One of the causes of this slightly different weather pattern of an earlier winter but with a much nicer August could be that this is an El Niño year. El Niño impacts on the climate because of changes to the world’s ocean flows, so being a small island in the middle of thousands of Km² of open ocean then it would be bound to have an impact here. The recent weather patterns certainly seem to have not matched past experience, although this is purely anecdotal. Perhaps more interestingly the Humpback Whales started appearing in the island’s waters much earlier than usual. Prime whale spotting time is generally from about now until November, but they’ve been here for about the past 3 months. Of course, whales are in the waters all the time, but not in sufficient numbers to guarantee spotting them and not usually breaching to make the spotting even easier. But I have been seeing them from the beach, from my walk up Jacob’s Ladder and even from my office window. It’s one of the real treats of living on an oceanic island and something I can’t imagine that I will ever tire of.

Article: Stunning Cloud Swirls Spotted by Satellite

Three wonderful images of St Helena taken from space. Explanations below.

Image 1
Image 1{g}

Image 2
Image 2{g}

Image 3
Image 3{g}

Image 4
Image 4{g}

Image 5
Image 5{g}

 

Image 1

From Our Amazing Planet (livescience.com) 27th November 2012{11}

Alone in the South Atlantic Ocean sits the small volcanic island of Saint Helena. The towering peak of the island disrupts clouds as they pass overhead, creating swirling patterns called von Karman vortices that can be seen by satellites overhead.

The swirling clouds, moving to the northwest over Saint Helena, were snapped by NASA’s Terra satellite on Nov. 15, 2012, according to NASA’s Earth Observatory.

Von Karman vortices are created when a mass of fluid, such as water or air, encounters an obstacle, and creates swirls going in alternating directions. These so-called ‘Von Karman streets’ can be seen in satellite photographs of clouds around the world.

Saint Helena is dominated by Mount Actaeon, which reaches up to 818 metres.

Image 2

Cmdr Chris Hadfield, one of the astronauts on the International Space Station, snapped this shot of St Helena and posted it on Twitter. Given the importance to St Helena of Fishing we think maybe this image should become part of the national flag.

Image 3

An even more dramatic example of von Karman vortices, taken by NASA’s Earth Observatory, 10th November 2015.

Image 4

Taken by Oleg Artemyev from the International Space Station, 14th August 2018.

Image 5

Taken from the International Space Station, 8th October 2018.

The shortest and longest days

The Wikipedia inclusion on our On This Day page on 21st June 2018 prompted the following email:

Here’s a nerdy subject!

Your page ‘On this day’ for June 21st makes an embedded reference to the Solstice page of Wikipedia. One would hope that that was definitive enough, but it is not (an issue I have been grappling with for a long time). The problem is, the authors are well in the Northern Hemisphere (between the Tropic of Cancer and the Arctic Circle) and what they say is all well and good for such locations. For analogous locations in the Southern Hemisphere, the terms ‘Summer’ and ‘Winter’ need only be interchanged. But we are located within the tropics and things are different there (here).

It is all to do with solar system mechanics. The basic model (which underpins the general preconception as per Wiki) is that the sun’s highest point goes up and down in the sky during the seasons. All else being ignored, if you were to plot the sun’s highest point along a time axis covering a year, it should resemble a sine wave, and the approximations are good enough for most applications. The closer you are to a tropic, the higher the sun’s highest point (altitude) will reach (the amplitude of the sine wave). The Summer Solstice is then defined as the day on which the sun reaches its highest point.

On a tropic, the highest point is the zenith (that is pretty much what defines the tropic). What happens between the tropics though? For such locations as ours, the sun’s wandering up the sky actually overshoots - during the year, the highest reach of the sun gets higher and higher in the north and then… actually moves over to the south of us! Briefly. Then it returns to the north and declines again into winter.

Again, all else being ignored, we should have two longest days and two shortest days (the two spanning the relevant solstices by a few days in each case). But this is not quite what happens. And the reason that it does not happen like this is the following:

What this means is that the graph of the sun’s highest points does a strange wobble around the solstices and the length of the day graph is somewhat unexpected. For 2018, on St.Helena, the Shortest day (between sunrise and sunset) occurs on 13th and 14th June, then again on 17th - 24th June, and then again on 26th - 28th June. The Longest days are similarly spread out (17th, 19th, 21st, 23rd and 25th December).

Weird eh?

Have a look at www.gaisma.com/en/location/half-tree-hollow.html. The graph of sunrise and sunset times is apparently bizarre - but it explains why the day length over the year (the distance between the two wavy lines) is not what you’d simplistically expect.

Editor’s Note: Thanks, Stuart, for that fascinating and comprehensive explanation!

Laugh at funny Weather and climate humour - LOL

Credits:
{a} St Helena: A Physical, Historical and Topographical Description of the Island, including the Geology, Fauna, Flora and Meteorology, by John Melliss, published in 1875, 1875{11}{b} South Atlantic Media Services Ltd (SAMS){11}{c} Bruce Peters{d} Simon Ashley Yon{e} Dion Yon{f} Adolph Northen, 1828-1876{g} Earth Observatory, taken from the ISS

Footnotes:
{1} Showing the wind direction on each day of a year.{2} Many of his troops died of Hypothermia.{3} It’s not always realised but when you are on the water you get a double-dose of the sun - once from above and again reflected off the surface of the sea.{4} Also there is no heavy industry on the island.{5} Which means if you organise a major outdoor event, even in the middle of the dry season, you should anticipate rain.{6} So named because in the olden days there was no point in wearing your finest for Church on Sunday because you would get all muddy walking there, so people habitually ‘dressed down’ at this time of year.{7} The data show that March has the most rainfall, and also the most sun-hours. We cannot explain this apparent contradiction. If you can help, please contact us.{8} Incidentally, our Editor firmly believes that Sod’s Law is the unifying theory of everything that scientists have been looking for. Why do planets not obey the same physical laws as sub-atomic particles? To make physics difficult. Sod’s Law. Is Schrödinger’s cat alive or dead? If you want it to be dead, it’s alive, and vice-verse. Sod’s Law. Why is there not enough matter in the universe? Sod’s Law. randomthoughtssthljt.blogspot.com/2019/01/the-unifying-theory-of-everything.html.{9} So now you know. If you can translate this into layperson’s terms, please contact us!{10} Though if the aliens have secretly taken over people’s bodies in preparation for an invasion, nobody has yet noticed.{11} Reproduced for educational non-commercial use only; all copyrights are acknowledged.{12} See more blogs.{13} Correct - it hasn’t.

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