The Sun has reached its highest point in the sky, marking the beginning of astronomical summer and the longest day of the year. Solstice means “standing still” – a day when the path of the Sun across the sky does not appear to be moving up or down.
From now until 21 December, the Sun’s path will appear to move closer to the horizon and days will become shorter. This will happen in small amounts (a few minutes) especially at first, but the nights are already drawing in.
The atmosphere and oceans take time to warm up, so this is not usually the warmest part of the year, which is still to come.
We are coming into the season of “white nights” around the summer solstice, when there is no true darkness because the sun is close below the horizon still illuminating the atmosphere even though the sun itself is not visible. In effect, the sun has started rising as soon as it has finished setting. Of course, if you were to travel further north, this phenomenon is clearer and more dramatic until, at the arctic circle, there are times when the sun does not set at all. We are at 51 degrees North here and the Arctic Circle is roughly 66.5 degrees North so we are about 930 miles (1,722km) short of the midnight sun, which is not that far compared with the size of the Earth.
There are degrees of darkness. We tend to call darkness that is not complete “twilight”. Civil twilight is when the sun is 6 degrees (or a little more than the width of a hand held at arms length) below the horizon. The horizon is clearly visible as are objects (such as mountains) even though, on a very clear night, the brightest stars are becoming apparent. Nautical Twilight is when the sun is 12 degrees below the horizon. The horizon is no longer clear although some objects may be discerned. Astronomical twilight is when the sun is more than 18 degrees below the horizon – complete darkness (apart from light pollution, of course)
Around the summer solstice here, we have a few weeks where there is no astronomical darkness. This is well shown on the “skylight clock” on the weather station web site.
Yellow is daylight with the sun above the horizon, the light blue is civil twilight, (sunset until the sun reaches 6 degrees below the horizon or from that point until sunrise), the mid blue is nautical twilight (sun reaching 12 degrees below the horizon) and the dark blue is astronomical twilight (the sun reaches 18 degrees below the horizon). Black is darkness. The red hand indicates what time it is relative to this. At the time of writing, there is a sliver of black left on the clock, but this will soon disappear until the evenings begin (slowly) to draw out in July.
All this affects nature. Animals (and small children) go to sleep later and get up earlier. Most British animals are crepuscular: being most active through twilight and in the morning and evening. As people tend to stick to the clock, we miss their most active times through this period of the year. perhaps we should get up and go to bed with the sun, but have a sleep in the day at this time of year!
Early this morning (22 Feb 2015) my camera recorded two bright meteors. These are not connected, but it after weeks of cloudy skies and very little activity when it was clear (this is the quietest time of the year for meteors) it is nice to have captured two impressive meteors.
The first was at 00:28 (UTC). This is probably a February Eta Draconid meteor (see here for details of this recently discovered meteor shower) and had a magnitude of -3.4 making it about the brightest thing in the sky. The path appears to be curved because of the wide angle lens in the camera. In fact, the meteor passed almost overhead here – its path started above the North Downs and ended over the South Coast. It was still probably about 35km high when it burnt out.
The second came in at 03:39 and was a sporadic – not associated with any known meteor stream. It had a magnitude of -2.9:
Both meteors show a small explosion during their path.
Songs allow birds to mark out a territory, which protects the resources they need to breed. The song is a challenge to other birds of the same species, but also prevents conflict because birds who do not wish to challenge for the territory can avoid it. Peak activity for birdsong in our climate is crepuscular – in the time around and after dawn and again in the evening. The dawn (and dusk) chorus, where birds of many species sing, is one of the glories of nature and one of the most powerful signs of spring.
It is rather difficult to pinpoint when birdsong begins again. Robins, for example, sing a little all through winter. It is a sound of hope in the darkness and also reflects that Robins maintain a territory through the winter. Some species will begin to sing in mild weather, but stop again if the weather worsens.
This year, the first day I heard a substantial amount of bird song was on 27 January. I noted that not only did I hear a Robin, but that there was a great tit (whose song is very monotonous) and several blackbirds singing in the dusk. In 2014, I experienced something similar on 8th January – weeks earlier. It was much milder in 2014, of course.
Last week, here and at College on the South Downs, there were a couple of days with lots of birdsong, which I am hearing much better now I have hearing aids to help with high frequencies. There were lots of thrushes singing at College – quite wonderful.
The weather station began to operate in August so I do not have data on a full year yet, but I thought it might be useful to mention a few figures.
The highest temperature was 27.2C at 15:00 on 18 September and the coldest was -5.1C at 07:33 on 31 December. 18 September was the only day when the temperature was above 27C (or 80F). The highest minimum was 15.9C on 6 August ( an uncomfortably warm night) There were 13 air frosts (2 in November and 11 in December)
The strongest gust of wind was 47kts (54mph or 87kph) at 03:41 on 12 December. December was the windiest month too.
It rained on 82 days (out of 153 recorded days). Six of these days had more than 20mm rain and 49 more than 2mm (which is what most people would notice as “raining”). November was the wettest month with 156mm, but October was close behind with 147mm. The wettest day was 23 November with 29.6mm.
My meteor video system captured sprites early on the morning of 8 November 2014.
Sprites are one form of transient luminous event or upper atmospheric lightning. In the upper atmosphere, very high above active thunderstorms, there are various forms of electrical discharge (sprites, elves, blue jets and even gnomes and pixies!) that are poorly understood and which were not even recognised until the last 20 years or so. In the last few years, it has been realised that video systems designed to capture meteors may also capture these fleeting phenomena which seem to occur at similar heights to meteors. Early this morning, my meteor camera caught two (and possibly three) sprites associated with thunderstorms that were over Northern France and the English Channel (120 – 150km away from here) at the time. I am thrilled to have video of one of the atmosphere’s most elusive sights.
At 03:20 the system captured a “columnar sprite” or possibly a “blue jet” (Clearly visible in the lower centre of the image captured from the video). As I look at the video in detail (frame by frame) I can see a very faint general flash along the lower right of the screen one frame (1/25 second) before the sprite appears. I think this was the very distant lightning reflected in clouds that were just below the horizon here and possibly from some clouds that are just visible in the image, between here and the storm. (The video camera is extremely sensitive, so this seems possible. Lightning is easily bright enough to be seen from 150km away at night if you are high enough. I was once thrilled to watch from the window of an airliner at night as lightning coursed backwards and forwards through a line of towering African thunderstorms. The captain reassured us that we were over 100miles away from them). The sprite lasts three video frames or 3/25th second, with only one frame being bright as shown in the image. A blue jet is emitted from the top of the thunderstorm cloud, so this is more likely to be a column sprite – an emission high in the upper atmosphere (perhaps up to 100km) above the storm.
The video shows a very much fainter sprite happening about 1/5 second before this one, to its left and lower down.
At 03:37 (see main image) the system captured a classic “carrot” sprite – very clear and bright. Again, it is possible to see a brightening along the horizon, which I assume to be from lightning, about 1/25 second before the sprite appears. This sprite appears on four video frames (4/25 second in total). The central column appears first then the “carrot”. Like lightning, sprites are a very short lived phenomenon.
Such sensitive video systems are very noisy – the backgrounds look like snowstorms, and trying to convert the video to web format makes it even worse. I will try to process a viewable video and share it. On my computer, it is possible to work through frame by frame to see the details. The sprites are fairly bright. If you had been looking in exactly the right place (and had you eyes open and focused at the split second the sprites occurred) you could have seen them with the naked eye.
I have reported the sprites to researchers at Bath University, as well as to the UK meteor network.
Today is the windiest it has been since I set up the weather station at the beginning of August. Top gust so far is 33kts ( 38mph or 61kph a near gale) and the average wind has been at Beaufort force 4 (moderate breeze) for quite a lot of the day. Nothing extreme here, but the liveliest yet. The weather station is in the garden – fairly sheltered – so official wind speeds are likely to be higher. This is simply a typical autumn storm as a low pressure area sits on top of us. As well as the wind, this gives the unstable and showery, even thundery, conditions.
I made a video from one of the captures by my automatic meteor camera. This is basically the raw video from the camera, with a little noise reduction and colorising the monochrome video to a blue hue. This is real time video (not slowed or sped up). I shared it because I thought it was a particularly pretty meteor.
It was a brightly moonlit night and, as you can see, partly cloudy. The bright star in the lower centre of the image is Aldebaran – the brightest in the constellation of Taurus, the bull.
The meteor analyses as part of the lambda cygnid meteor shower. This is a minor shower, possibly associated with debris from a minor comet – 2005CA. Details of this meteor (and all others observed) are in the ukmon archive.
Today’s weather came from the passage of a cold front: where cold air pushing in, lifts the warmer, moister air in front of it, to create rain. Although the front was not particularly remarkable, it fitted the textbook very well so is worth reporting.
Firstly, the analysis chart shows the situation. Over Europe there is still the high pressure which has dominated our weather for some time, bringing light winds from the south and south-east. This air has been in place for a while, so was warm and humid. The air held quite a lot of water moisture, but there was no rain as the high pressure meant that air was descending and there was no mechanism to lift the air to create clouds and rain.
Behind the cold front, a very different air mass is pushing in from the atlantic. The air is much colder and so holds much less moisture. The front is where the colder air is pushing at the warmer air, lifting it from the surface and so making rain.
That’s the “synoptic view” (which means “seeing it all”). What did we see here in Horley, on the ground? My weather station saw a sudden drop in temperature, a sudden drop in dew point (which reflects how much moisture is in the air) and a change in wind direction. There was also a sudden increase in wind speed at the front itself.
There was a drop in temperature from about 14C to about 10C over less than an hour. The dew point dropped from about 15C to about 10C in the same time. A clear change in air mass.
The wind speed peaked and the wind direction changed at the same time. This was when the bulk of the rain (3mm of the total of just over 4mm today) fell.
After such a long period of dry and warm weather, it is tempting to see this clear event as the arrival of autumn. We shall see.