Plymouth Digital Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Plymouth Digital 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	14% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	130575
	Night	14% 10% 4 8% 5% 4 4 3 5% 6% 34%	131717
Transparency	Day	48% 6% 12% 20% 12% 1	134145
	Night	48% 6% 11% 21% 13% 1	136218
Seeing	Day	17% 12% 29% 23% 13% 5%	17328
	Night	19% 10% 29% 29% 12% 1	27507

By Month

Percentage Cloud Cover SE (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	12% 16% 6% 16% 6% 5% 4 3 5% 5% 20%	4691
	11th to 20th	Day	13% 15% 6% 18% 9% 9% 6% 3 4 5% 11%	4710
	21st to end	Day	18% 15% 5% 12% 9% 6% 6% 4 4 6% 15%	5697
February	1st to 10th	Day	14% 15% 5% 12% 8% 5% 6% 5% 4 4 22%	1890
	11th to 20th	Day	14% 13% 5% 12% 7% 6% 4 3 5% 6% 24%	2001
	21st to end	Day	15% 15% 4 11% 5% 4 4 4 5% 6% 26%	1730
March	1st to 10th	Day	16% 13% 4 10% 6% 5% 4 3 4 5% 30%	4674
	11th to 20th	Day	13% 11% 4 8% 5% 5% 4 3 6% 8% 35%	5090
	21st to end	Day	18% 14% 4 9% 5% 4 4 3 4 6% 27%	5586
April	1st to 10th	Day	16% 14% 3 7% 5% 4 4 3 5% 7% 31%	2449
	11th to 20th	Day	17% 9% 3 7% 5% 4 4 4 5% 7% 34%	2603
	21st to end	Day	18% 8% 3 8% 5% 4 4 3 5% 7% 35%	2600
May	1st to 10th	Day	20% 9% 3 7% 5% 5% 4 4 5% 6% 34%	5862
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 7% 8% 33%	5861
	21st to end	Day	14% 6% 2 6% 5% 4 4 3 6% 7% 41%	6433
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 9% 41%	2788
	11th to 20th	Day	11% 6% 2 7% 6% 5% 5% 4 8% 8% 39%	2800
	21st to end	Day	9% 6% 2 6% 5% 5% 6% 5% 8% 9% 39%	2800
July	1st to 10th	Day	9% 5% 2 7% 5% 5% 5% 4 7% 10% 40%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 10% 46%	6360
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	6525
August	1st to 10th	Day	11% 5% 2 5% 4 5% 5% 4 6% 8% 44%	4234
	11th to 20th	Day	9% 5% 2 7% 5% 5% 6% 5% 7% 9% 38%	4174
	21st to end	Day	8% 5% 2 7% 5% 5% 7% 4 7% 9% 41%	4609
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 4 6% 8% 42%	2265
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 7% 9% 48%	2200
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 4 6% 7% 38%	2200
October	1st to 10th	Day	12% 6% 2 7% 5% 4 4 4 6% 7% 44%	2152
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 4 6% 7% 34%	2000
	21st to end	Day	19% 10% 2 7% 5% 5% 4 4 5% 6% 32%	2033
November	1st to 10th	Day	12% 9% 3 8% 5% 5% 4 4 5% 6% 38%	1852
	11th to 20th	Day	16% 14% 3 8% 5% 5% 4 3 6% 5% 31%	1890
	21st to end	Day	18% 14% 4 9% 5% 6% 4 4 4 5% 28%	1827
December	1st to 10th	Day	13% 13% 6% 15% 6% 5% 4 4 6% 6% 21%	2680
	11th to 20th	Day	19% 19% 6% 9% 6% 5% 4 3 5% 6% 18%	2879
	21st to end	Day	26% 20% 7% 11% 5% 4 4 3 3 4 13%	4070
January	1st to 10th	Night	15% 15% 6% 13% 6% 4 4 3 4 5% 25%	8232
	11th to 20th	Night	9% 16% 7% 15% 8% 6% 6% 4 6% 6% 17%	8280
	21st to end	Night	15% 14% 7% 11% 6% 5% 5% 4 5% 5% 23%	8485
February	1st to 10th	Night	16% 14% 5% 11% 7% 5% 4 3 5% 5% 25%	2730
	11th to 20th	Night	14% 13% 5% 10% 6% 4 5% 3 5% 6% 28%	2504
	21st to end	Night	18% 12% 5% 10% 4 4 4 3 5% 6% 29%	2076
March	1st to 10th	Night	15% 11% 3 8% 4 4 4 3 5% 6% 36%	5399
	11th to 20th	Night	10% 7% 3 6% 4 4 5% 4 6% 7% 42%	5090
	21st to end	Night	17% 9% 3 8% 4 5% 4 3 4 6% 36%	5588
April	1st to 10th	Night	16% 10% 3 7% 5% 4 4 3 6% 6% 35%	2189
	11th to 20th	Night	17% 9% 3 6% 4 4 4 5% 6% 6% 37%	1877
	21st to end	Night	17% 9% 2 6% 4 4 4 4 5% 5% 39%	1800
May	1st to 10th	Night	20% 10% 4 8% 4 4 4 3 4 7% 32%	4033
	11th to 20th	Night	19% 8% 2 6% 4 4 4 3 4 6% 39%	4032
	21st to end	Night	13% 6% 2 5% 4 3 4 3 6% 7% 46%	4421
June	1st to 10th	Night	12% 4 2 5% 4 3 5% 4 6% 8% 47%	1608
	11th to 20th	Night	11% 5% 2 7% 4 4 4 4 6% 10% 43%	1600
	21st to end	Night	9% 5% 3 5% 4 4 4 4 6% 9% 48%	1600
July	1st to 10th	Night	8% 4 2 5% 4 5% 5% 4 7% 10% 47%	3618
	11th to 20th	Night	7% 5% 2 5% 4 3 4 4 7% 9% 50%	3617
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 8% 49%	4328
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 9% 48%	2915
	11th to 20th	Night	8% 6% 2 5% 4 4 5% 3 6% 7% 49%	2875
	21st to end	Night	8% 5% 2 6% 5% 5% 4 3 6% 7% 49%	3285
September	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 6% 6% 50%	2115
	11th to 20th	Night	9% 5% 1 4 3 4 3 3 6% 7% 56%	2200
	21st to end	Night	14% 6% 3 6% 4 4 5% 4 4 6% 44%	2200
October	1st to 10th	Night	13% 6% 3 6% 4 3 3 4 4 6% 48%	2247
	11th to 20th	Night	18% 8% 3 7% 4 4 4 4 5% 5% 38%	2400
	21st to end	Night	20% 8% 3 6% 4 4 5% 3 5% 5% 36%	2805
November	1st to 10th	Night	13% 8% 3 6% 5% 4 4 4 5% 6% 43%	2666
	11th to 20th	Night	15% 12% 4 8% 4 5% 5% 4 6% 7% 31%	2730
	21st to end	Night	19% 14% 4 8% 4 4 4 3 5% 5% 28%	2787
December	1st to 10th	Night	12% 13% 5% 12% 6% 5% 5% 4 5% 7% 24%	4720
	11th to 20th	Night	16% 17% 7% 12% 6% 4 5% 3 4 5% 21%	5515
	21st to end	Night	27% 20% 6% 10% 4 4 4 3 4 4 15%	7150

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	64% 6% 11% 14% 5%	4692
	11th to 20th	Day	74% 4 8% 7% 6%	4710
	21st to end	Day	69% 5% 10% 10% 5%	5697
February	1st to 10th	Day	63% 7% 9% 14% 7%	1890
	11th to 20th	Day	61% 5% 9% 16% 7% 1	2011
	21st to end	Day	57% 6% 11% 16% 8% 2	1730
March	1st to 10th	Day	56% 5% 12% 17% 9% 2	4674
	11th to 20th	Day	47% 5% 15% 22% 9% 2	5090
	21st to end	Day	58% 5% 10% 15% 11% 1	5586
April	1st to 10th	Day	52% 6% 11% 18% 12%	2449
	11th to 20th	Day	49% 5% 11% 21% 12% 2	2603
	21st to end	Day	49% 6% 12% 19% 14% 1	2600
May	1st to 10th	Day	50% 6% 10% 19% 13% 2	5897
	11th to 20th	Day	46% 8% 12% 21% 12% 2	5865
	21st to end	Day	41% 6% 9% 22% 19% 4	6447
June	1st to 10th	Day	38% 7% 12% 26% 14% 3	2788
	11th to 20th	Day	39% 7% 13% 22% 17% 2	2800
	21st to end	Day	39% 8% 13% 21% 17% 2	2800
July	1st to 10th	Day	36% 7% 13% 26% 16% 2	6291
	11th to 20th	Day	31% 7% 14% 28% 18% 1	6285
	21st to end	Day	33% 7% 13% 23% 22% 1	6339
August	1st to 10th	Day	36% 7% 13% 27% 16% 1	4221
	11th to 20th	Day	38% 9% 11% 30% 11% 1	4213
	21st to end	Day	37% 7% 14% 28% 13% 1	4608
September	1st to 10th	Day	38% 8% 15% 23% 15% 2	2265
	11th to 20th	Day	31% 6% 12% 33% 16% 1	2200
	21st to end	Day	43% 6% 12% 24% 14%	2200
October	1st to 10th	Day	30% 5% 11% 31% 21% 1	3503
	11th to 20th	Day	39% 5% 12% 26% 19%	3240
	21st to end	Day	44% 5% 13% 23% 14% 1	3257
November	1st to 10th	Day	45% 6% 11% 23% 14% 1	1852
	11th to 20th	Day	54% 5% 12% 19% 8% 2	1890
	21st to end	Day	58% 5% 11% 19% 6%	1827
December	1st to 10th	Day	61% 7% 15% 12% 5%	2680
	11th to 20th	Day	67% 5% 10% 14% 4	2875
	21st to end	Day	76% 4 7% 7% 5%	4070
January	1st to 10th	Night	61% 5% 10% 16% 7% 1	8233
	11th to 20th	Night	65% 5% 11% 11% 7%	8280
	21st to end	Night	62% 5% 10% 15% 7%	8485
February	1st to 10th	Night	60% 6% 10% 16% 7% 1	2730
	11th to 20th	Night	57% 5% 10% 19% 9% 1	2505
	21st to end	Night	55% 4 13% 18% 8% 1	2076
March	1st to 10th	Night	48% 5% 14% 22% 11% 1	5399
	11th to 20th	Night	38% 6% 14% 26% 16% 1	5090
	21st to end	Night	48% 5% 11% 20% 14% 1	5588
April	1st to 10th	Night	48% 6% 10% 20% 13% 2	2189
	11th to 20th	Night	45% 7% 11% 18% 18% 1	1877
	21st to end	Night	45% 6% 10% 19% 17% 3	1800
May	1st to 10th	Night	52% 5% 10% 16% 13% 4	4060
	11th to 20th	Night	46% 6% 10% 25% 11% 3	4033
	21st to end	Night	36% 6% 12% 24% 18% 4	4429
June	1st to 10th	Night	34% 5% 11% 27% 20% 3	1608
	11th to 20th	Night	37% 8% 12% 25% 17% 2	1600
	21st to end	Night	31% 7% 14% 26% 20% 1	1600
July	1st to 10th	Night	31% 7% 13% 30% 15% 4	3580
	11th to 20th	Night	29% 7% 14% 29% 19% 1	3572
	21st to end	Night	32% 7% 14% 22% 24% 2	4201
August	1st to 10th	Night	34% 6% 13% 28% 18% 1	2904
	11th to 20th	Night	32% 7% 11% 36% 12% 2	2902
	21st to end	Night	33% 7% 12% 32% 15% 1	3285
September	1st to 10th	Night	32% 7% 12% 29% 18% 2	2115
	11th to 20th	Night	28% 6% 10% 33% 22% 2	2200
	21st to end	Night	41% 6% 11% 27% 15% 1	2200
October	1st to 10th	Night	26% 4 9% 33% 28% 1	3676
	11th to 20th	Night	41% 4 11% 24% 19%	3899
	21st to end	Night	38% 5% 10% 32% 15% 1	4531
November	1st to 10th	Night	41% 6% 11% 26% 16% 1	2666
	11th to 20th	Night	50% 6% 13% 21% 9% 1	2730
	21st to end	Night	57% 5% 12% 18% 7%	2787
December	1st to 10th	Night	58% 6% 16% 15% 4	4720
	11th to 20th	Night	65% 5% 11% 13% 5% 1	5518
	21st to end	Night	74% 5% 9% 7% 6%	7150

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	23% 15% 40% 16% 5% 1	330
	11th to 20th	Day	22% 19% 42% 14% 2 1	330
	21st to end	Day	26% 17% 34% 18% 4 1	363
February	1st to 10th	Day	24% 19% 38% 15% 3 1	330
	11th to 20th	Day	24% 21% 37% 15% 2	330
	21st to end	Day	25% 25% 37% 12% 1	273
March	1st to 10th	Day	26% 20% 35% 16% 3	372
	11th to 20th	Day	25% 14% 31% 23% 6% 1	540
	21st to end	Day	23% 18% 34% 20% 5% 1	594
April	1st to 10th	Day	22% 18% 34% 19% 7% 1	540
	11th to 20th	Day	22% 20% 32% 15% 7% 4	516
	21st to end	Day	25% 12% 31% 18% 11% 2	500
May	1st to 10th	Day	23% 14% 29% 21% 10% 4	503
	11th to 20th	Day	20% 11% 25% 24% 14% 6%	527
	21st to end	Day	14% 10% 23% 27% 19% 8%	573
June	1st to 10th	Day	13% 8% 29% 31% 15% 4	718
	11th to 20th	Day	13% 8% 30% 30% 15% 3	730
	21st to end	Day	11% 9% 30% 29% 16% 5%	730
July	1st to 10th	Day	10% 5% 23% 28% 25% 9%	721
	11th to 20th	Day	10% 7% 25% 31% 19% 8%	718
	21st to end	Day	9% 3 24% 29% 24% 11%	625
August	1st to 10th	Day	8% 3 21% 28% 26% 14%	526
	11th to 20th	Day	13% 4 19% 31% 22% 12%	519
	21st to end	Day	10% 4 19% 29% 26% 12%	583
September	1st to 10th	Day	11% 6% 24% 25% 21% 13%	530
	11th to 20th	Day	8% 4 15% 25% 31% 16%	530
	21st to end	Day	16% 10% 22% 23% 20% 9%	530
October	1st to 10th	Day	15% 9% 25% 24% 18% 8%	530
	11th to 20th	Day	18% 14% 30% 27% 9% 1	530
	21st to end	Day	26% 16% 33% 19% 6%	416
November	1st to 10th	Day	16% 20% 32% 23% 7% 2	330
	11th to 20th	Day	22% 21% 31% 19% 5% 1	330
	21st to end	Day	27% 16% 39% 14% 5%	297
December	1st to 10th	Day	19% 20% 37% 17% 7%	220
	11th to 20th	Day	29% 16% 39% 13% 3 1	231
	21st to end	Day	29% 20% 33% 15% 2	363
January	1st to 10th	Night	23% 14% 36% 22% 5% 1	1050
	11th to 20th	Night	25% 15% 34% 21% 4 1	1050
	21st to end	Night	20% 15% 34% 26% 5%	956
February	1st to 10th	Night	24% 12% 36% 24% 3	840
	11th to 20th	Night	23% 15% 33% 26% 3	823
	21st to end	Night	24% 14% 39% 20% 2	692
March	1st to 10th	Night	22% 12% 33% 27% 6%	840
	11th to 20th	Night	19% 18% 35% 23% 5% 1	840
	21st to end	Night	23% 12% 31% 27% 7%	924
April	1st to 10th	Night	21% 14% 35% 26% 3 1	719
	11th to 20th	Night	21% 11% 35% 25% 7% 1	611
	21st to end	Night	22% 9% 31% 31% 8%	600
May	1st to 10th	Night	19% 10% 30% 31% 9% 1	600
	11th to 20th	Night	22% 5% 27% 32% 13% 1	591
	21st to end	Night	13% 3 24% 38% 20% 2	650
June	1st to 10th	Night	13% 3 25% 44% 14% 1	600
	11th to 20th	Night	12% 4 22% 44% 17%	600
	21st to end	Night	10% 4 21% 43% 21% 1	600
July	1st to 10th	Night	8% 3 17% 41% 27% 4	585
	11th to 20th	Night	9% 3 20% 38% 26% 4	582
	21st to end	Night	6% 2 17% 48% 25% 2	657
August	1st to 10th	Night	8% 3 16% 41% 29% 2	597
	11th to 20th	Night	9% 2 18% 37% 31% 3	594
	21st to end	Night	9% 2 16% 38% 32% 3	660
September	1st to 10th	Night	12% 5% 19% 33% 27% 5%	715
	11th to 20th	Night	10% 4 17% 33% 33% 5%	800
	21st to end	Night	16% 6% 21% 32% 22% 4	800
October	1st to 10th	Night	14% 6% 25% 33% 18% 3	800
	11th to 20th	Night	21% 11% 31% 27% 9% 2	800
	21st to end	Night	24% 11% 33% 28% 5%	880
November	1st to 10th	Night	17% 11% 32% 27% 11% 1	820
	11th to 20th	Night	22% 14% 34% 22% 7% 1	840
	21st to end	Night	27% 13% 34% 21% 6%	840
December	1st to 10th	Night	24% 15% 34% 22% 5%	840
	11th to 20th	Night	24% 14% 38% 21% 3	956
	21st to end	Night	32% 12% 33% 20% 2	1155

Page generated at 2026-04-14 02:44:30. Version ac07

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