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 2024-12-24.

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% 34%	101195
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 36%	98926
Transparency	Day	47% 6% 12% 21% 13% 1	104809
	Night	47% 6% 11% 22% 13% 1	103452
Seeing	Day	17% 11% 28% 24% 14% 5%	15594
	Night	19% 9% 29% 29% 12% 1	25692

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	16% 18% 6% 11% 5% 5% 4 3 5% 5% 21%	2527
	11th to 20th	Day	17% 17% 6% 12% 7% 8% 7% 3 5% 7% 12%	2530
	21st to end	Day	28% 21% 7% 10% 6% 4 3 2 3 4 13%	3067
February	1st to 10th	Day	15% 15% 5% 12% 8% 5% 6% 5% 4 4 22%	1710
	11th to 20th	Day	14% 13% 5% 12% 7% 6% 4 3 5% 6% 25%	1805
	21st to end	Day	16% 16% 4 11% 5% 4 4 4 5% 6% 26%	1570
March	1st to 10th	Day	18% 17% 4 9% 6% 5% 3 3 4 6% 26%	3184
	11th to 20th	Day	13% 12% 4 8% 5% 5% 4 3 5% 7% 33%	3480
	21st to end	Day	19% 13% 4 9% 5% 4 5% 4 4 6% 27%	3815
April	1st to 10th	Day	16% 14% 3 7% 5% 5% 4 4 5% 7% 32%	2215
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 4 5% 7% 35%	2399
	21st to end	Day	19% 8% 3 8% 5% 4 4 3 4 7% 35%	2470
May	1st to 10th	Day	19% 8% 2 6% 5% 5% 5% 4 5% 7% 35%	4082
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	4081
	21st to end	Day	10% 6% 3 6% 5% 4 4 3 6% 8% 45%	4476
June	1st to 10th	Day	10% 6% 2 6% 5% 6% 5% 5% 6% 9% 41%	2648
	11th to 20th	Day	11% 6% 2 6% 6% 5% 5% 4 8% 8% 41%	2660
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 8% 9% 38%	2660
July	1st to 10th	Day	10% 6% 2 7% 5% 5% 6% 4 7% 9% 39%	4440
	11th to 20th	Day	7% 4 2 5% 4 4 5% 5% 7% 10% 46%	4440
	21st to end	Day	7% 5% 2 7% 5% 6% 6% 5% 7% 8% 42%	4539
August	1st to 10th	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 43%	4104
	11th to 20th	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 9% 39%	4044
	21st to end	Day	8% 5% 2 7% 5% 5% 7% 4 7% 9% 40%	4468
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 5% 6% 8% 41%	2152
	11th to 20th	Day	9% 4 2 5% 4 4 4 4 7% 9% 47%	2090
	21st to end	Day	15% 6% 3 7% 5% 5% 4 4 6% 8% 38%	2090
October	1st to 10th	Day	12% 6% 2 7% 5% 4 4 4 6% 7% 42%	2045
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 5% 7% 34%	1900
	21st to end	Day	19% 10% 2 7% 5% 5% 4 4 5% 6% 32%	1932
November	1st to 10th	Day	12% 9% 3 8% 5% 5% 4 4 5% 7% 39%	1762
	11th to 20th	Day	16% 14% 3 8% 5% 5% 5% 3 6% 6% 30%	1800
	21st to end	Day	18% 14% 4 8% 5% 5% 4 4 4 5% 28%	1740
December	1st to 10th	Day	14% 15% 6% 14% 6% 5% 5% 4 6% 5% 21%	1600
	11th to 20th	Day	16% 16% 5% 9% 6% 5% 4 4 6% 7% 21%	1863
	21st to end	Day	26% 21% 7% 11% 5% 4 4 3 3 5% 11%	2807
January	1st to 10th	Night	17% 16% 7% 10% 5% 4 4 3 5% 5% 24%	4428
	11th to 20th	Night	15% 18% 7% 14% 6% 5% 5% 4 6% 6% 15%	4440
	21st to end	Night	24% 17% 8% 11% 5% 4 3 3 4 4 18%	4588
February	1st to 10th	Night	16% 15% 5% 11% 7% 5% 4 4 5% 5% 24%	2470
	11th to 20th	Night	14% 13% 5% 10% 6% 4 5% 3 5% 6% 29%	2260
	21st to end	Night	19% 11% 5% 10% 5% 4 4 3 5% 6% 29%	1884
March	1st to 10th	Night	16% 12% 3 8% 5% 4 3 3 4 6% 35%	3669
	11th to 20th	Night	9% 7% 2 6% 4 5% 5% 5% 6% 8% 43%	3480
	21st to end	Night	17% 8% 3 8% 4 5% 4 3 4 6% 36%	3817
April	1st to 10th	Night	16% 10% 2 6% 5% 4 4 4 6% 6% 36%	1981
	11th to 20th	Night	18% 8% 2 5% 4 4 4 5% 5% 6% 37%	1729
	21st to end	Night	17% 9% 2 6% 4 4 4 4 5% 5% 38%	1710
May	1st to 10th	Night	17% 9% 3 7% 4 4 4 4 5% 8% 35%	2813
	11th to 20th	Night	16% 7% 2 7% 5% 4 4 3 5% 7% 41%	2812
	21st to end	Night	10% 6% 2 5% 4 3 4 3 6% 8% 49%	3079
June	1st to 10th	Night	12% 5% 2 5% 4 4 5% 4 6% 9% 47%	1528
	11th to 20th	Night	10% 5% 2 7% 4 5% 4 4 7% 9% 43%	1520
	21st to end	Night	9% 5% 3 5% 3 4 4 4 6% 9% 47%	1520
July	1st to 10th	Night	9% 3 2 5% 4 4 5% 4 7% 10% 48%	2530
	11th to 20th	Night	8% 4 2 4 3 3 3 4 5% 9% 55%	2530
	21st to end	Night	5% 4 2 6% 5% 5% 6% 4 7% 9% 48%	3014
August	1st to 10th	Night	11% 6% 2 5% 4 4 4 3 5% 9% 47%	2825
	11th to 20th	Night	8% 6% 2 6% 4 4 5% 3 6% 7% 49%	2785
	21st to end	Night	8% 5% 2 6% 5% 5% 4 4 6% 7% 48%	3184
September	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 6% 6% 50%	2009
	11th to 20th	Night	10% 5% 1 4 3 4 3 3 6% 7% 54%	2090
	21st to end	Night	14% 6% 2 6% 4 4 5% 3 4 6% 44%	2090
October	1st to 10th	Night	13% 6% 3 7% 4 3 3 4 4 6% 47%	2134
	11th to 20th	Night	18% 8% 3 8% 4 4 4 4 5% 5% 37%	2280
	21st to end	Night	21% 8% 3 6% 4 4 5% 3 5% 5% 35%	2665
November	1st to 10th	Night	13% 8% 3 6% 5% 4 4 4 5% 6% 44%	2536
	11th to 20th	Night	15% 12% 4 8% 5% 4 5% 4 6% 7% 31%	2600
	21st to end	Night	19% 13% 4 8% 4 4 4 3 5% 6% 28%	2654
December	1st to 10th	Night	17% 15% 6% 10% 6% 5% 5% 3 5% 7% 22%	2800
	11th to 20th	Night	15% 15% 5% 12% 5% 4 6% 3 5% 6% 24%	3539
	21st to end	Night	28% 21% 6% 10% 4 4 3 3 4 4 13%	4933

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	63% 7% 11% 13% 6%	2528
	11th to 20th	Day	71% 5% 11% 8% 4	2530
	21st to end	Day	76% 5% 9% 7% 3 1	3067
February	1st to 10th	Day	63% 7% 9% 14% 7%	1710
	11th to 20th	Day	60% 5% 9% 17% 8% 1	1815
	21st to end	Day	58% 6% 11% 16% 7% 2	1570
March	1st to 10th	Day	60% 5% 11% 17% 6% 1	3184
	11th to 20th	Day	50% 6% 13% 21% 8% 2	3480
	21st to end	Day	57% 6% 12% 14% 9% 2	3815
April	1st to 10th	Day	51% 6% 11% 18% 13%	2215
	11th to 20th	Day	48% 5% 11% 20% 13% 2	2399
	21st to end	Day	50% 6% 12% 18% 14% 1	2470
May	1st to 10th	Day	47% 7% 11% 23% 11% 2	4120
	11th to 20th	Day	45% 8% 13% 23% 9% 1	4085
	21st to end	Day	36% 6% 10% 20% 23% 4	4489
June	1st to 10th	Day	38% 7% 12% 26% 13% 3	2648
	11th to 20th	Day	39% 7% 13% 22% 17% 2	2660
	21st to end	Day	39% 8% 14% 21% 16% 2	2660
July	1st to 10th	Day	40% 7% 14% 20% 17% 2	4371
	11th to 20th	Day	29% 7% 13% 32% 17% 1	4365
	21st to end	Day	36% 8% 13% 18% 22% 2	4395
August	1st to 10th	Day	37% 7% 12% 27% 15% 1	4091
	11th to 20th	Day	38% 9% 11% 29% 12% 1	4083
	21st to end	Day	37% 7% 14% 28% 13% 1	4467
September	1st to 10th	Day	39% 8% 15% 21% 15% 2	2152
	11th to 20th	Day	32% 7% 11% 32% 16% 1	2090
	21st to end	Day	43% 6% 12% 24% 14%	2090
October	1st to 10th	Day	31% 5% 11% 31% 21% 1	3396
	11th to 20th	Day	39% 5% 12% 25% 19%	3140
	21st to end	Day	44% 5% 13% 23% 13% 1	3156
November	1st to 10th	Day	45% 6% 11% 23% 14% 1	1762
	11th to 20th	Day	54% 6% 12% 19% 8% 1	1800
	21st to end	Day	58% 5% 11% 18% 7%	1740
December	1st to 10th	Day	64% 6% 12% 14% 4	1600
	11th to 20th	Day	61% 6% 13% 16% 4	1859
	21st to end	Day	78% 4 8% 6% 3	2807
January	1st to 10th	Night	61% 6% 11% 16% 5% 1	4428
	11th to 20th	Night	68% 6% 11% 10% 5%	4440
	21st to end	Night	71% 5% 9% 11% 3	4588
February	1st to 10th	Night	62% 6% 11% 14% 7% 1	2470
	11th to 20th	Night	57% 5% 10% 19% 9% 1	2261
	21st to end	Night	56% 4 12% 18% 8% 2	1884
March	1st to 10th	Night	50% 5% 13% 24% 8%	3669
	11th to 20th	Night	37% 7% 15% 21% 20% 1	3480
	21st to end	Night	48% 5% 12% 23% 11% 1	3817
April	1st to 10th	Night	47% 6% 11% 21% 14% 2	1981
	11th to 20th	Night	44% 7% 10% 18% 18% 2	1729
	21st to end	Night	46% 6% 10% 19% 16% 3	1710
May	1st to 10th	Night	47% 6% 10% 21% 14% 2	2840
	11th to 20th	Night	42% 7% 10% 26% 12% 2	2813
	21st to end	Night	33% 7% 12% 22% 21% 5%	3087
June	1st to 10th	Night	34% 5% 12% 28% 18% 3	1528
	11th to 20th	Night	36% 8% 12% 24% 17% 2	1520
	21st to end	Night	32% 7% 14% 26% 20% 1	1520
July	1st to 10th	Night	31% 6% 12% 31% 14% 6%	2490
	11th to 20th	Night	27% 6% 14% 31% 21% 1	2482
	21st to end	Night	30% 9% 18% 19% 22% 3	2918
August	1st to 10th	Night	35% 6% 12% 28% 17% 1	2814
	11th to 20th	Night	32% 7% 11% 36% 12% 2	2812
	21st to end	Night	34% 7% 12% 31% 15% 1	3184
September	1st to 10th	Night	33% 7% 12% 27% 18% 2	2009
	11th to 20th	Night	29% 6% 9% 32% 22% 2	2090
	21st to end	Night	41% 6% 11% 28% 14% 1	2090
October	1st to 10th	Night	27% 4 9% 33% 28% 1	3563
	11th to 20th	Night	41% 4 11% 24% 19%	3779
	21st to end	Night	39% 5% 10% 32% 14% 1	4391
November	1st to 10th	Night	41% 6% 11% 26% 16% 1	2536
	11th to 20th	Night	51% 6% 13% 20% 9% 1	2600
	21st to end	Night	57% 6% 12% 18% 7%	2654
December	1st to 10th	Night	62% 6% 12% 14% 6%	2800
	11th to 20th	Night	60% 6% 12% 15% 6% 1	3542
	21st to end	Night	75% 5% 10% 7% 3	4933

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	24% 15% 37% 17% 6% 1	270
	11th to 20th	Day	23% 16% 42% 16% 2 1	270
	21st to end	Day	27% 16% 33% 19% 4 1	297
February	1st to 10th	Day	25% 19% 37% 15% 2 1	270
	11th to 20th	Day	24% 20% 38% 16% 2	270
	21st to end	Day	25% 26% 36% 12% 1	225
March	1st to 10th	Day	24% 21% 33% 17% 4	308
	11th to 20th	Day	23% 13% 32% 25% 7% 1	460
	21st to end	Day	24% 17% 32% 21% 5% 1	506
April	1st to 10th	Day	20% 16% 35% 20% 8% 1	460
	11th to 20th	Day	23% 19% 32% 15% 7% 4	452
	21st to end	Day	25% 13% 31% 18% 10% 2	460
May	1st to 10th	Day	23% 13% 29% 21% 10% 4	463
	11th to 20th	Day	20% 11% 25% 24% 14% 7%	487
	21st to end	Day	13% 8% 22% 28% 21% 9%	529
June	1st to 10th	Day	13% 8% 28% 32% 16% 4	668
	11th to 20th	Day	13% 8% 30% 30% 16% 4	680
	21st to end	Day	12% 9% 31% 27% 16% 6%	680
July	1st to 10th	Day	10% 4 23% 28% 26% 9%	671
	11th to 20th	Day	10% 7% 24% 31% 19% 9%	668
	21st to end	Day	9% 3 24% 29% 24% 11%	579
August	1st to 10th	Day	9% 3 22% 27% 26% 14%	486
	11th to 20th	Day	13% 4 19% 30% 21% 12%	479
	21st to end	Day	11% 4 19% 28% 26% 12%	539
September	1st to 10th	Day	11% 5% 23% 26% 22% 12%	490
	11th to 20th	Day	9% 4 16% 26% 30% 15%	490
	21st to end	Day	17% 10% 23% 22% 19% 8%	490
October	1st to 10th	Day	16% 10% 26% 24% 17% 8%	490
	11th to 20th	Day	19% 16% 29% 26% 10% 1	490
	21st to end	Day	28% 14% 33% 19% 6% 1	381
November	1st to 10th	Day	13% 21% 33% 23% 8% 2	300
	11th to 20th	Day	22% 20% 32% 20% 5% 1	300
	21st to end	Day	26% 16% 40% 13% 5%	270
December	1st to 10th	Day	19% 20% 38% 17% 7% 1	200
	11th to 20th	Day	28% 15% 40% 13% 3 1	210
	21st to end	Day	28% 18% 35% 17% 2	306
January	1st to 10th	Night	23% 14% 35% 21% 6% 1	950
	11th to 20th	Night	26% 13% 34% 21% 5% 1	950
	21st to end	Night	20% 13% 34% 27% 5%	866
February	1st to 10th	Night	25% 12% 36% 24% 3	760
	11th to 20th	Night	23% 14% 33% 27% 3	743
	21st to end	Night	25% 14% 37% 21% 2	628
March	1st to 10th	Night	22% 11% 33% 28% 6%	760
	11th to 20th	Night	18% 17% 36% 24% 5% 1	760
	21st to end	Night	23% 10% 31% 28% 8%	836
April	1st to 10th	Night	19% 14% 35% 28% 4 1	651
	11th to 20th	Night	21% 11% 35% 25% 7% 1	563
	21st to end	Night	22% 8% 31% 31% 8%	570
May	1st to 10th	Night	19% 9% 30% 31% 10% 1	570
	11th to 20th	Night	22% 4 27% 33% 13% 1	561
	21st to end	Night	13% 3 24% 37% 21% 2	617
June	1st to 10th	Night	13% 2 24% 45% 14% 1	570
	11th to 20th	Night	12% 4 22% 44% 18%	570
	21st to end	Night	10% 4 21% 43% 20% 1	570
July	1st to 10th	Night	8% 3 17% 41% 27% 4	555
	11th to 20th	Night	9% 3 20% 39% 26% 4	552
	21st to end	Night	5% 2 18% 48% 25% 2	624
August	1st to 10th	Night	8% 3 17% 41% 28% 2	567
	11th to 20th	Night	10% 2 18% 37% 31% 3	564
	21st to end	Night	9% 2 16% 38% 31% 3	627
September	1st to 10th	Night	12% 5% 19% 33% 27% 5%	679
	11th to 20th	Night	10% 4 17% 33% 32% 4	760
	21st to end	Night	16% 6% 21% 31% 22% 4	760
October	1st to 10th	Night	15% 6% 26% 32% 18% 3	760
	11th to 20th	Night	21% 11% 31% 26% 9% 2	760
	21st to end	Night	24% 11% 32% 27% 6%	836
November	1st to 10th	Night	17% 11% 31% 27% 12% 1	780
	11th to 20th	Night	22% 14% 34% 22% 7% 1	800
	21st to end	Night	26% 13% 33% 22% 6%	800
December	1st to 10th	Night	24% 15% 34% 21% 5%	800
	11th to 20th	Night	25% 13% 37% 21% 4	910
	21st to end	Night	32% 12% 33% 21% 3	1063

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