Gower Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gower Observatory clear sky chart from 2005-11-06 to 2026-04-17.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 32%	131441
	Night	20% 11% 3 7% 5% 4 4 3 5% 6% 33%	130872
Transparency	Day	50% 7% 12% 20% 10% 1	134954
	Night	51% 6% 12% 19% 10% 1	135446
Seeing	Day	27% 18% 30% 17% 6% 1	18361
	Night	26% 16% 31% 22% 5%	26262

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 20% 6% 12% 4 3 4 3 3 5% 27%	4093
	11th to 20th	Day	14% 16% 6% 14% 6% 4 3 2 3 4 26%	4406
	21st to end	Day	9% 10% 3 8% 5% 4 4 4 6% 7% 38%	5679
February	1st to 10th	Day	15% 13% 4 7% 6% 4 5% 3 5% 6% 30%	1890
	11th to 20th	Day	16% 13% 3 9% 5% 5% 4 3 5% 7% 31%	1843
	21st to end	Day	11% 13% 4 7% 5% 5% 4 4 5% 5% 37%	1640
March	1st to 10th	Day	23% 13% 4 12% 4 3 3 2 4 4 26%	4893
	11th to 20th	Day	20% 13% 3 8% 5% 4 4 4 5% 6% 27%	5090
	21st to end	Day	27% 13% 4 8% 4 3 4 2 5% 6% 24%	5587
April	1st to 10th	Day	23% 15% 3 7% 5% 4 4 3 4 5% 28%	2453
	11th to 20th	Day	20% 11% 3 7% 4 5% 4 3 5% 7% 32%	2650
	21st to end	Day	25% 10% 3 7% 4 3 3 3 5% 6% 32%	2600
May	1st to 10th	Day	27% 14% 3 7% 4 4 4 3 4 6% 24%	5886
	11th to 20th	Day	25% 10% 3 7% 4 4 4 3 4 6% 31%	6313
	21st to end	Day	22% 9% 3 7% 4 4 5% 4 6% 7% 29%	7077
June	1st to 10th	Day	26% 8% 3 7% 5% 5% 5% 3 5% 6% 27%	3000
	11th to 20th	Day	21% 7% 3 7% 4 4 5% 4 6% 7% 33%	3000
	21st to end	Day	28% 8% 3 6% 4 4 4 3 5% 6% 30%	3000
July	1st to 10th	Day	17% 7% 3 7% 5% 6% 5% 5% 7% 9% 29%	6820
	11th to 20th	Day	17% 5% 2 6% 4 4 5% 4 6% 8% 40%	6820
	21st to end	Day	14% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	6660
August	1st to 10th	Day	21% 8% 2 6% 5% 4 5% 4 6% 7% 33%	4235
	11th to 20th	Day	15% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	4175
	21st to end	Day	15% 4 2 5% 4 4 4 4 6% 7% 45%	4652
September	1st to 10th	Day	13% 5% 2 5% 4 4 4 4 5% 8% 45%	2400
	11th to 20th	Day	17% 6% 2 5% 4 3 4 3 4 6% 46%	2210
	21st to end	Day	20% 5% 2 5% 4 4 4 3 4 6% 44%	2200
October	1st to 10th	Day	17% 6% 3 7% 4 4 4 3 5% 6% 41%	2009
	11th to 20th	Day	20% 8% 3 6% 4 4 4 3 4 8% 36%	2000
	21st to end	Day	22% 10% 3 7% 5% 4 4 3 4 6% 32%	2103
November	1st to 10th	Day	22% 9% 3 7% 5% 4 4 3 4 5% 34%	1732
	11th to 20th	Day	21% 10% 3 8% 5% 4 3 3 5% 5% 34%	1680
	21st to end	Day	21% 13% 3 7% 5% 4 3 4 5% 4 30%	1680
December	1st to 10th	Day	19% 12% 4 9% 6% 3 3 3 4 5% 32%	2690
	11th to 20th	Day	21% 13% 4 9% 5% 3 3 2 3 4 34%	2722
	21st to end	Day	18% 14% 3 8% 4 3 4 3 5% 4 33%	3553
January	1st to 10th	Night	16% 17% 4 8% 4 4 4 3 5% 6% 30%	8831
	11th to 20th	Night	20% 14% 5% 11% 6% 4 4 3 5% 5% 24%	8476
	21st to end	Night	8% 13% 4 7% 6% 5% 4 4 5% 6% 38%	8486
February	1st to 10th	Night	15% 13% 4 9% 5% 4 4 3 6% 6% 32%	2730
	11th to 20th	Night	16% 13% 4 10% 6% 5% 4 3 4 4 30%	2663
	21st to end	Night	14% 14% 4 8% 4 5% 4 3 4 5% 35%	2165
March	1st to 10th	Night	26% 14% 4 9% 5% 5% 3 2 3 3 26%	5167
	11th to 20th	Night	22% 13% 4 8% 4 4 4 4 5% 6% 25%	5089
	21st to end	Night	31% 12% 3 7% 4 3 3 3 4 4 25%	5587
April	1st to 10th	Night	25% 13% 3 8% 4 4 4 4 4 4 28%	2163
	11th to 20th	Night	23% 10% 3 8% 4 4 4 3 4 5% 32%	1833
	21st to end	Night	24% 10% 3 6% 5% 5% 4 2 5% 6% 30%	1800
May	1st to 10th	Night	29% 12% 2 6% 4 4 4 3 5% 6% 26%	4054
	11th to 20th	Night	25% 8% 2 5% 4 4 4 3 4 6% 33%	3601
	21st to end	Night	18% 10% 3 7% 4 4 4 4 5% 6% 35%	3861
June	1st to 10th	Night	26% 9% 2 5% 5% 4 4 4 5% 7% 29%	1400
	11th to 20th	Night	21% 8% 2 6% 4 4 5% 4 5% 5% 35%	1400
	21st to end	Night	27% 8% 3 6% 5% 3 4 4 5% 5% 31%	1400
July	1st to 10th	Night	18% 7% 2 7% 5% 4 6% 5% 6% 8% 30%	3160
	11th to 20th	Night	17% 6% 2 7% 4 5% 5% 4 6% 8% 35%	3160
	21st to end	Night	16% 6% 2 6% 5% 4 5% 4 6% 8% 37%	4149
August	1st to 10th	Night	19% 8% 3 8% 5% 4 5% 4 6% 7% 30%	2915
	11th to 20th	Night	12% 8% 3 8% 5% 4 5% 4 7% 8% 37%	2872
	21st to end	Night	15% 4 2 5% 4 4 4 2 5% 7% 47%	3246
September	1st to 10th	Night	17% 6% 2 6% 3 4 4 4 5% 7% 44%	2000
	11th to 20th	Night	16% 6% 3 5% 3 3 3 3 4 6% 48%	2185
	21st to end	Night	20% 6% 2 5% 3 4 3 3 5% 6% 44%	2200
October	1st to 10th	Night	20% 7% 3 6% 3 3 3 3 5% 5% 42%	2390
	11th to 20th	Night	18% 8% 3 7% 4 4 4 3 5% 7% 38%	2400
	21st to end	Night	24% 8% 2 7% 5% 4 4 3 4 5% 34%	2735
November	1st to 10th	Night	23% 9% 2 6% 5% 3 4 3 4 5% 35%	2785
	11th to 20th	Night	22% 10% 3 7% 4 4 4 3 5% 6% 32%	2940
	21st to end	Night	23% 11% 4 7% 5% 4 4 2 4 5% 30%	2940
December	1st to 10th	Night	17% 11% 4 9% 5% 4 4 3 4 6% 34%	4720
	11th to 20th	Night	22% 12% 3 7% 4 4 3 2 3 4 36%	5702
	21st to end	Night	20% 14% 4 9% 4 3 3 3 5% 5% 29%	7667

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	60% 5% 11% 14% 9%	4094
	11th to 20th	Day	62% 5% 11% 13% 8% 1	4435
	21st to end	Day	42% 6% 10% 22% 17% 2	5679
February	1st to 10th	Day	53% 6% 11% 18% 10% 2	1890
	11th to 20th	Day	53% 6% 12% 19% 10% 1	1851
	21st to end	Day	49% 5% 11% 21% 11% 2	1640
March	1st to 10th	Day	62% 4 9% 13% 10% 2	4897
	11th to 20th	Day	56% 8% 14% 14% 6% 2	5090
	21st to end	Day	61% 4 6% 18% 9% 2	5587
April	1st to 10th	Day	59% 5% 8% 14% 11% 2	2453
	11th to 20th	Day	52% 5% 11% 19% 11% 1	2650
	21st to end	Day	54% 5% 10% 16% 12% 4	2600
May	1st to 10th	Day	61% 6% 8% 16% 9%	5897
	11th to 20th	Day	53% 7% 13% 20% 6% 2	6317
	21st to end	Day	50% 9% 11% 17% 11%	7077
June	1st to 10th	Day	56% 7% 11% 17% 9% 1	3000
	11th to 20th	Day	49% 8% 12% 19% 12% 1	3000
	21st to end	Day	55% 8% 11% 16% 9% 1	3000
July	1st to 10th	Day	48% 8% 17% 19% 8%	6745
	11th to 20th	Day	41% 8% 16% 26% 9% 1	6739
	21st to end	Day	42% 8% 14% 28% 8%	6470
August	1st to 10th	Day	49% 8% 13% 23% 7%	4222
	11th to 20th	Day	43% 10% 15% 25% 7%	4214
	21st to end	Day	36% 8% 15% 26% 15%	4652
September	1st to 10th	Day	36% 9% 16% 26% 13%	2400
	11th to 20th	Day	39% 5% 13% 26% 15% 2	2210
	21st to end	Day	41% 7% 10% 26% 15% 1	2200
October	1st to 10th	Day	45% 7% 13% 25% 10%	3269
	11th to 20th	Day	45% 6% 11% 31% 7%	3240
	21st to end	Day	41% 6% 16% 21% 16%	3379
November	1st to 10th	Day	52% 7% 10% 21% 10% 1	1732
	11th to 20th	Day	52% 6% 13% 16% 12% 1	1680
	21st to end	Day	57% 6% 15% 16% 6%	1680
December	1st to 10th	Day	55% 5% 16% 19% 5%	2690
	11th to 20th	Day	57% 4 8% 19% 9% 3	2722
	21st to end	Day	53% 5% 14% 16% 10% 2	3553
January	1st to 10th	Night	55% 5% 12% 21% 7% 1	8831
	11th to 20th	Night	60% 7% 10% 16% 6%	8522
	21st to end	Night	44% 6% 11% 22% 16% 2	8486
February	1st to 10th	Night	53% 5% 11% 17% 12% 2	2730
	11th to 20th	Night	55% 6% 9% 17% 11% 1	2666
	21st to end	Night	51% 5% 10% 21% 12% 2	2165
March	1st to 10th	Night	63% 5% 6% 11% 11% 3	5176
	11th to 20th	Night	56% 9% 14% 12% 8% 1	5090
	21st to end	Night	62% 4 6% 15% 11% 2	5587
April	1st to 10th	Night	59% 6% 7% 15% 11% 4	2163
	11th to 20th	Night	52% 6% 10% 19% 12% 1	1833
	21st to end	Night	54% 6% 11% 14% 13% 2	1800
May	1st to 10th	Night	58% 6% 12% 14% 9% 1	4060
	11th to 20th	Night	51% 8% 11% 21% 7% 2	3602
	21st to end	Night	47% 10% 18% 18% 8% 1	3862
June	1st to 10th	Night	53% 8% 11% 18% 9% 1	1400
	11th to 20th	Night	49% 7% 12% 22% 8% 2	1400
	21st to end	Night	55% 7% 12% 16% 9% 1	1400
July	1st to 10th	Night	49% 8% 14% 19% 9%	3125
	11th to 20th	Night	46% 7% 16% 24% 7%	3118
	21st to end	Night	43% 9% 15% 23% 10%	4025
August	1st to 10th	Night	52% 8% 13% 20% 7%	2903
	11th to 20th	Night	41% 10% 18% 23% 7%	2899
	21st to end	Night	35% 7% 18% 24% 13% 3	3245
September	1st to 10th	Night	40% 7% 12% 28% 13% 1	2000
	11th to 20th	Night	37% 6% 14% 28% 14% 1	2185
	21st to end	Night	41% 7% 9% 27% 14% 1	2200
October	1st to 10th	Night	47% 5% 12% 24% 11%	3910
	11th to 20th	Night	40% 6% 12% 29% 12%	3899
	21st to end	Night	44% 5% 15% 22% 13%	4409
November	1st to 10th	Night	51% 6% 13% 20% 9% 1	2785
	11th to 20th	Night	54% 6% 13% 18% 9% 1	2940
	21st to end	Night	56% 7% 13% 15% 9%	2940
December	1st to 10th	Night	51% 5% 13% 23% 8%	4720
	11th to 20th	Night	53% 4 10% 19% 12% 1	5703
	21st to end	Night	57% 5% 9% 18% 10% 1	7667

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	31% 20% 35% 13% 1	220
	11th to 20th	Day	37% 21% 28% 8% 6%	220
	21st to end	Day	20% 19% 37% 19% 4	436
February	1st to 10th	Day	27% 20% 31% 15% 6% 1	430
	11th to 20th	Day	29% 25% 28% 13% 3 1	426
	21st to end	Day	27% 24% 32% 13% 5%	399
March	1st to 10th	Day	31% 27% 27% 12% 3 1	540
	11th to 20th	Day	31% 23% 29% 13% 3 1	540
	21st to end	Day	35% 23% 26% 12% 3 1	594
April	1st to 10th	Day	40% 24% 24% 9% 3	540
	11th to 20th	Day	34% 25% 24% 13% 4 1	516
	21st to end	Day	35% 22% 25% 12% 5% 1	500
May	1st to 10th	Day	34% 21% 27% 13% 5% 1	503
	11th to 20th	Day	30% 15% 29% 19% 5% 1	719
	21st to end	Day	29% 18% 31% 18% 4 1	790
June	1st to 10th	Day	31% 17% 28% 18% 5% 1	730
	11th to 20th	Day	28% 16% 29% 20% 6% 1	730
	21st to end	Day	30% 14% 30% 21% 6%	730
July	1st to 10th	Day	23% 13% 35% 21% 8%	720
	11th to 20th	Day	19% 10% 36% 25% 9% 1	718
	21st to end	Day	19% 11% 37% 23% 9% 1	638
August	1st to 10th	Day	17% 12% 37% 24% 9% 2	526
	11th to 20th	Day	20% 11% 30% 26% 11% 2	519
	21st to end	Day	19% 13% 30% 25% 10% 3	583
September	1st to 10th	Day	13% 17% 34% 26% 8% 2	530
	11th to 20th	Day	24% 11% 31% 21% 11% 3	530
	21st to end	Day	21% 17% 33% 17% 9% 3	530
October	1st to 10th	Day	21% 15% 26% 23% 13% 2	424
	11th to 20th	Day	25% 19% 27% 18% 9% 2	420
	21st to end	Day	27% 21% 32% 14% 5% 1	462
November	1st to 10th	Day	34% 17% 32% 13% 4 1	425
	11th to 20th	Day	34% 19% 26% 15% 5% 1	430
	21st to end	Day	32% 22% 29% 13% 3	430
December	1st to 10th	Day	33% 17% 30% 15% 5%	430
	11th to 20th	Day	37% 27% 26% 9% 1	241
	21st to end	Day	31% 24% 31% 12% 3	242
January	1st to 10th	Night	28% 20% 31% 18% 3	950
	11th to 20th	Night	28% 18% 28% 20% 4 1	884
	21st to end	Night	20% 20% 34% 21% 5%	924
February	1st to 10th	Night	25% 19% 31% 20% 4	840
	11th to 20th	Night	24% 24% 32% 17% 4	823
	21st to end	Night	23% 19% 32% 23% 4	692
March	1st to 10th	Night	29% 18% 33% 18% 2	840
	11th to 20th	Night	26% 20% 30% 19% 5%	840
	21st to end	Night	27% 18% 34% 17% 4	924
April	1st to 10th	Night	32% 19% 29% 17% 2	712
	11th to 20th	Night	31% 17% 32% 17% 4	613
	21st to end	Night	30% 18% 30% 18% 5%	600
May	1st to 10th	Night	32% 15% 29% 21% 4	600
	11th to 20th	Night	29% 14% 33% 21% 3	592
	21st to end	Night	20% 17% 41% 18% 3	614
June	1st to 10th	Night	26% 11% 34% 27% 3	400
	11th to 20th	Night	23% 13% 32% 27% 5%	400
	21st to end	Night	30% 7% 30% 29% 4 1	400
July	1st to 10th	Night	19% 7% 41% 28% 5%	390
	11th to 20th	Night	19% 6% 33% 33% 9%	388
	21st to end	Night	20% 9% 33% 31% 6% 1	593
August	1st to 10th	Night	17% 6% 34% 33% 8% 1	598
	11th to 20th	Night	17% 8% 30% 35% 9% 1	592
	21st to end	Night	15% 10% 29% 32% 12% 1	660
September	1st to 10th	Night	20% 9% 38% 25% 7%	600
	11th to 20th	Night	20% 11% 25% 31% 12% 1	785
	21st to end	Night	22% 13% 30% 26% 9%	800
October	1st to 10th	Night	24% 11% 29% 27% 8% 1	800
	11th to 20th	Night	22% 14% 33% 22% 9% 1	800
	21st to end	Night	29% 19% 28% 19% 4	880
November	1st to 10th	Night	30% 17% 27% 21% 5%	883
	11th to 20th	Night	30% 18% 29% 17% 5% 1	950
	21st to end	Night	30% 19% 29% 18% 4	950
December	1st to 10th	Night	32% 15% 30% 19% 4	950
	11th to 20th	Night	32% 22% 28% 16% 2	950
	21st to end	Night	31% 18% 28% 20% 3 1	1045

Page generated at 2026-04-14 03:44:29. Version ac07

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