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 2024-01-17.

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% 9% 4 7% 6% 4 5% 5% 6% 7% 34%	74481
	Night	16% 9% 4 7% 5% 3 5% 4 5% 7% 35%	75346
Transparency	Day	49% 7% 12% 20% 12% 1	74191
	Night	49% 6% 11% 20% 12% 1	75157
Seeing	Day	18% 11% 28% 23% 14% 6%	14310
	Night	19% 9% 29% 30% 12% 1	24462

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% 14% 5% 11% 7% 5% 5% 5% 5% 6% 20%	1517
	11th to 20th	Day	17% 16% 7% 10% 8% 3 5% 5% 5% 7% 17%	1488
	21st to end	Day	16% 18% 9% 9% 8% 3 4 4 5% 5% 17%	1751
February	1st to 10th	Day	16% 13% 6% 11% 9% 4 6% 6% 4 4 20%	1620
	11th to 20th	Day	15% 11% 5% 11% 9% 5% 6% 3 5% 6% 23%	1708
	21st to end	Day	17% 14% 5% 10% 6% 3 4 5% 4 6% 25%	1480
March	1st to 10th	Day	15% 11% 5% 8% 7% 4 5% 5% 6% 7% 28%	1836
	11th to 20th	Day	14% 12% 5% 7% 6% 3 4 3 5% 7% 34%	1980
	21st to end	Day	19% 11% 4 8% 6% 3 5% 5% 4 6% 28%	2178
April	1st to 10th	Day	16% 12% 5% 6% 5% 3 4 5% 5% 7% 31%	2098
	11th to 20th	Day	18% 9% 3 6% 6% 3 4 5% 5% 7% 34%	2271
	21st to end	Day	19% 7% 3 7% 6% 3 5% 3 5% 7% 34%	2340
May	1st to 10th	Day	18% 8% 3 6% 6% 4 5% 5% 5% 7% 32%	2340
	11th to 20th	Day	19% 9% 3 6% 6% 4 4 5% 5% 6% 35%	2301
	21st to end	Day	12% 5% 3 5% 5% 3 5% 5% 6% 9% 41%	2531
June	1st to 10th	Day	11% 5% 3 5% 5% 4 5% 6% 6% 9% 41%	2508
	11th to 20th	Day	11% 5% 2 5% 6% 4 5% 5% 7% 9% 40%	2520
	21st to end	Day	10% 6% 2 5% 6% 4 6% 6% 7% 9% 39%	2520
July	1st to 10th	Day	8% 5% 2 4 5% 4 5% 5% 6% 9% 47%	2520
	11th to 20th	Day	10% 5% 2 4 5% 3 5% 5% 7% 9% 44%	2520
	21st to end	Day	8% 5% 2 5% 6% 4 6% 6% 6% 8% 43%	2592
August	1st to 10th	Day	7% 5% 2 5% 6% 4 6% 6% 7% 9% 42%	2327
	11th to 20th	Day	10% 5% 3 6% 7% 5% 6% 6% 7% 8% 38%	2303
	21st to end	Day	11% 5% 2 5% 5% 4 6% 5% 6% 8% 43%	2538
September	1st to 10th	Day	12% 6% 2 5% 6% 4 6% 6% 7% 8% 39%	2039
	11th to 20th	Day	10% 4 2 5% 5% 3 4 6% 7% 9% 45%	1980
	21st to end	Day	15% 5% 2 6% 5% 4 5% 5% 6% 8% 40%	1980
October	1st to 10th	Day	13% 6% 2 6% 6% 3 5% 5% 5% 7% 41%	1938
	11th to 20th	Day	20% 7% 3 6% 6% 4 6% 4 5% 7% 32%	1800
	21st to end	Day	20% 10% 2 6% 6% 4 5% 5% 5% 6% 32%	1831
November	1st to 10th	Day	12% 9% 3 7% 5% 4 5% 5% 5% 7% 39%	1672
	11th to 20th	Day	15% 13% 4 7% 6% 4 5% 4 6% 6% 31%	1710
	21st to end	Day	19% 13% 4 7% 5% 4 4 5% 4 5% 30%	1653
December	1st to 10th	Day	15% 14% 7% 10% 8% 3 6% 5% 5% 6% 22%	1520
	11th to 20th	Day	16% 14% 5% 9% 6% 4 5% 5% 6% 7% 22%	1788
	21st to end	Day	27% 20% 8% 10% 7% 3 4 4 3 5% 10%	2783
January	1st to 10th	Night	17% 13% 6% 9% 7% 4 4 4 5% 6% 25%	2648
	11th to 20th	Night	18% 16% 6% 9% 7% 4 4 4 5% 5% 21%	2611
	21st to end	Night	13% 14% 8% 12% 7% 3 4 5% 5% 5% 23%	2602
February	1st to 10th	Night	17% 14% 5% 10% 8% 3 5% 4 4 5% 23%	2340
	11th to 20th	Night	15% 12% 6% 8% 8% 3 5% 4 5% 6% 28%	2138
	21st to end	Night	20% 11% 5% 8% 5% 3 4 3 5% 6% 30%	1776
March	1st to 10th	Night	16% 9% 4 8% 6% 3 4 5% 5% 6% 35%	2124
	11th to 20th	Night	14% 9% 3 6% 5% 3 5% 4 6% 7% 38%	1980
	21st to end	Night	20% 8% 3 7% 5% 4 5% 4 4 6% 34%	2178
April	1st to 10th	Night	15% 8% 3 6% 6% 3 5% 4 6% 7% 37%	1877
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 6% 5% 6% 38%	1637
	21st to end	Night	18% 9% 2 5% 5% 2 4 5% 5% 6% 39%	1620
May	1st to 10th	Night	17% 7% 4 7% 6% 4 5% 5% 5% 7% 34%	1620
	11th to 20th	Night	19% 6% 2 5% 5% 3 4 4 5% 7% 40%	1592
	21st to end	Night	10% 5% 2 5% 5% 2 4 5% 7% 8% 46%	1744
June	1st to 10th	Night	11% 4 2 4 4 2 5% 5% 6% 9% 47%	1448
	11th to 20th	Night	10% 4 3 6% 5% 3 4 4 7% 9% 45%	1440
	21st to end	Night	9% 5% 2 4 4 2 4 5% 6% 9% 47%	1440
July	1st to 10th	Night	8% 3 2 4 4 3 6% 5% 5% 8% 53%	1440
	11th to 20th	Night	8% 4 2 4 4 2 4 5% 5% 9% 52%	1440
	21st to end	Night	5% 3 2 5% 4 3 5% 5% 6% 10% 52%	1724
August	1st to 10th	Night	7% 5% 2 3 5% 3 5% 5% 5% 10% 50%	1611
	11th to 20th	Night	8% 4 2 5% 5% 3 5% 5% 5% 8% 50%	1598
	21st to end	Night	9% 4 2 4 5% 3 4 4 7% 7% 52%	1814
September	1st to 10th	Night	11% 6% 3 5% 4 3 5% 4 6% 6% 48%	1903
	11th to 20th	Night	10% 4 2 4 3 3 3 4 6% 7% 53%	1980
	21st to end	Night	14% 5% 2 5% 4 3 5% 4 5% 6% 46%	1980
October	1st to 10th	Night	14% 6% 3 6% 4 2 4 5% 4 6% 45%	2021
	11th to 20th	Night	18% 8% 4 6% 6% 3 4 5% 5% 5% 36%	2160
	21st to end	Night	22% 8% 3 5% 5% 3 5% 4 5% 5% 34%	2524
November	1st to 10th	Night	13% 7% 3 5% 5% 3 5% 4 4 6% 44%	2406
	11th to 20th	Night	16% 10% 4 7% 6% 3 5% 5% 6% 7% 31%	2470
	21st to end	Night	20% 12% 5% 7% 4 3 4 4 5% 6% 30%	2521
December	1st to 10th	Night	18% 14% 7% 8% 7% 3 5% 4 5% 7% 22%	2660
	11th to 20th	Night	15% 14% 5% 11% 6% 3 6% 4 4 7% 25%	3395
	21st to end	Night	29% 20% 6% 9% 5% 3 4 3 4 4 12%	4884

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%	1518
	11th to 20th	Day	67% 6% 13% 9% 5%	1488
	21st to end	Day	69% 4 9% 12% 5% 1	1751
February	1st to 10th	Day	65% 6% 9% 13% 6%	1620
	11th to 20th	Day	62% 5% 9% 17% 6% 1	1718
	21st to end	Day	59% 6% 11% 15% 7% 2	1480
March	1st to 10th	Day	55% 6% 12% 18% 8% 1	1836
	11th to 20th	Day	51% 5% 11% 19% 12% 3	1980
	21st to end	Day	57% 5% 10% 20% 7% 1	2178
April	1st to 10th	Day	52% 6% 11% 18% 12%	2098
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2271
	21st to end	Day	50% 6% 12% 17% 14% 1	2340
May	1st to 10th	Day	51% 6% 12% 20% 10% 2	2340
	11th to 20th	Day	50% 6% 9% 19% 14% 2	2305
	21st to end	Day	40% 7% 14% 24% 14% 2	2531
June	1st to 10th	Day	38% 7% 12% 26% 14% 3	2508
	11th to 20th	Day	39% 7% 13% 22% 17% 2	2520
	21st to end	Day	39% 8% 14% 21% 16% 2	2520
July	1st to 10th	Day	33% 8% 11% 25% 19% 4	2451
	11th to 20th	Day	34% 8% 13% 24% 18% 2	2445
	21st to end	Day	36% 8% 12% 23% 19% 2	2448
August	1st to 10th	Day	35% 9% 13% 28% 13% 1	2314
	11th to 20th	Day	41% 9% 12% 23% 14% 2	2303
	21st to end	Day	37% 8% 11% 25% 17% 2	2538
September	1st to 10th	Day	41% 8% 16% 19% 14% 2	2039
	11th to 20th	Day	34% 7% 12% 33% 14% 1	1980
	21st to end	Day	41% 6% 13% 26% 14%	1980
October	1st to 10th	Day	41% 7% 11% 26% 13% 1	1938
	11th to 20th	Day	51% 6% 13% 19% 11% 1	1800
	21st to end	Day	53% 6% 12% 18% 10% 1	1831
November	1st to 10th	Day	45% 7% 11% 23% 14% 1	1672
	11th to 20th	Day	53% 5% 12% 19% 8% 2	1710
	21st to end	Day	57% 5% 12% 19% 7%	1653
December	1st to 10th	Day	63% 6% 12% 14% 4	1520
	11th to 20th	Day	61% 6% 13% 16% 5%	1784
	21st to end	Day	78% 4 8% 6% 3	2783
January	1st to 10th	Night	60% 5% 10% 18% 6% 1	2648
	11th to 20th	Night	65% 5% 12% 11% 6%	2611
	21st to end	Night	63% 5% 11% 14% 6% 1	2602
February	1st to 10th	Night	63% 6% 11% 13% 6% 1	2340
	11th to 20th	Night	58% 5% 10% 17% 9% 1	2139
	21st to end	Night	56% 4 12% 17% 9% 2	1776
March	1st to 10th	Night	50% 7% 11% 22% 10% 1	2124
	11th to 20th	Night	45% 7% 11% 20% 15% 2	1980
	21st to end	Night	52% 5% 9% 22% 11% 2	2178
April	1st to 10th	Night	46% 6% 11% 21% 14% 2	1877
	11th to 20th	Night	44% 7% 10% 18% 19% 2	1637
	21st to end	Night	46% 6% 10% 19% 16% 3	1620
May	1st to 10th	Night	50% 6% 11% 19% 12% 2	1620
	11th to 20th	Night	44% 6% 9% 20% 18% 3	1593
	21st to end	Night	35% 7% 15% 25% 15% 3	1745
June	1st to 10th	Night	35% 5% 12% 28% 18% 3	1448
	11th to 20th	Night	36% 8% 12% 25% 17% 2	1440
	21st to end	Night	32% 7% 13% 27% 20% 1	1440
July	1st to 10th	Night	30% 7% 10% 29% 20% 4	1400
	11th to 20th	Night	29% 6% 12% 30% 21% 2	1392
	21st to end	Night	26% 8% 16% 24% 20% 4	1628
August	1st to 10th	Night	30% 7% 14% 31% 17% 2	1600
	11th to 20th	Night	31% 8% 13% 30% 14% 3	1598
	21st to end	Night	31% 7% 13% 27% 20% 2	1814
September	1st to 10th	Night	34% 8% 12% 27% 17% 2	1903
	11th to 20th	Night	31% 6% 9% 32% 21% 1	1980
	21st to end	Night	39% 6% 11% 28% 15% 1	1980
October	1st to 10th	Night	40% 5% 10% 25% 19% 1	2021
	11th to 20th	Night	49% 6% 11% 20% 14% 1	2160
	21st to end	Night	51% 6% 10% 23% 9% 1	2524
November	1st to 10th	Night	40% 6% 11% 26% 16% 1	2406
	11th to 20th	Night	51% 6% 13% 21% 9% 1	2470
	21st to end	Night	56% 6% 12% 19% 7%	2521
December	1st to 10th	Night	63% 6% 12% 14% 6%	2660
	11th to 20th	Night	60% 6% 13% 15% 7% 1	3398
	21st to end	Night	76% 5% 10% 7% 2	4884

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	22% 15% 43% 17% 2 1	258
	21st to end	Day	23% 17% 36% 18% 5% 1	264
February	1st to 10th	Day	27% 20% 38% 14% 1	240
	11th to 20th	Day	25% 21% 39% 13% 1	240
	21st to end	Day	27% 27% 35% 11% 1	198
March	1st to 10th	Day	25% 20% 33% 17% 4	276
	11th to 20th	Day	22% 12% 32% 25% 7% 1	420
	21st to end	Day	25% 17% 30% 22% 6% 1	462
April	1st to 10th	Day	20% 15% 34% 21% 9% 1	420
	11th to 20th	Day	23% 17% 31% 15% 8% 5%	412
	21st to end	Day	26% 12% 31% 18% 10% 3	420
May	1st to 10th	Day	24% 13% 28% 21% 10% 4	423
	11th to 20th	Day	21% 11% 25% 23% 13% 6%	447
	21st to end	Day	14% 8% 21% 28% 21% 9%	485
June	1st to 10th	Day	13% 6% 29% 32% 16% 4	618
	11th to 20th	Day	13% 7% 29% 30% 16% 4	630
	21st to end	Day	11% 8% 30% 28% 16% 6%	630
July	1st to 10th	Day	9% 4 21% 29% 27% 10%	621
	11th to 20th	Day	11% 7% 24% 31% 19% 8%	618
	21st to end	Day	9% 3 25% 28% 24% 11%	533
August	1st to 10th	Day	8% 2 22% 26% 27% 15%	446
	11th to 20th	Day	14% 3 19% 29% 22% 13%	439
	21st to end	Day	11% 4 19% 28% 26% 11%	495
September	1st to 10th	Day	12% 5% 22% 26% 22% 12%	450
	11th to 20th	Day	10% 4 17% 26% 29% 14%	450
	21st to end	Day	15% 11% 23% 22% 21% 9%	450
October	1st to 10th	Day	17% 9% 25% 23% 17% 8%	450
	11th to 20th	Day	20% 16% 29% 26% 8% 1	450
	21st to end	Day	29% 14% 34% 18% 5% 1	346
November	1st to 10th	Day	13% 20% 33% 23% 9% 2	270
	11th to 20th	Day	20% 21% 31% 21% 6% 1	270
	21st to end	Day	27% 15% 40% 14% 5%	243
December	1st to 10th	Day	21% 19% 38% 16% 7% 1	180
	11th to 20th	Day	26% 14% 44% 12% 3 1	189
	21st to end	Day	29% 19% 34% 16% 2	297
January	1st to 10th	Night	23% 14% 35% 21% 6% 1	950
	11th to 20th	Night	27% 13% 34% 21% 5% 1	933
	21st to end	Night	19% 13% 34% 28% 6%	820
February	1st to 10th	Night	26% 11% 37% 23% 3	720
	11th to 20th	Night	24% 13% 33% 26% 3	703
	21st to end	Night	26% 13% 38% 21% 2	592
March	1st to 10th	Night	21% 11% 33% 28% 6%	720
	11th to 20th	Night	18% 16% 35% 25% 5% 1	720
	21st to end	Night	23% 10% 30% 28% 8% 1	792
April	1st to 10th	Night	18% 14% 34% 29% 4 1	617
	11th to 20th	Night	21% 10% 34% 26% 8% 1	533
	21st to end	Night	23% 7% 31% 31% 9%	540
May	1st to 10th	Night	19% 9% 30% 31% 10% 1	540
	11th to 20th	Night	22% 4 27% 33% 13% 1	531
	21st to end	Night	13% 2 23% 37% 22% 2	584
June	1st to 10th	Night	13% 2 23% 47% 14% 1	540
	11th to 20th	Night	12% 4 22% 44% 18%	540
	21st to end	Night	10% 4 21% 44% 20% 1	540
July	1st to 10th	Night	8% 3 16% 40% 29% 4	525
	11th to 20th	Night	9% 3 18% 40% 26% 4	522
	21st to end	Night	5% 2 18% 48% 25% 2	591
August	1st to 10th	Night	9% 2 16% 42% 28% 3	537
	11th to 20th	Night	10% 2 17% 37% 31% 3	534
	21st to end	Night	9% 2 16% 38% 32% 3	594
September	1st to 10th	Night	13% 4 18% 33% 27% 5%	643
	11th to 20th	Night	11% 4 18% 33% 30% 4	720
	21st to end	Night	15% 6% 21% 31% 23% 4	720
October	1st to 10th	Night	16% 6% 25% 32% 18% 3	720
	11th to 20th	Night	22% 11% 31% 26% 8% 2	720
	21st to end	Night	25% 11% 32% 26% 5%	792
November	1st to 10th	Night	17% 11% 31% 27% 12% 1	740
	11th to 20th	Night	23% 14% 34% 22% 7% 1	760
	21st to end	Night	26% 12% 33% 22% 6%	760
December	1st to 10th	Night	25% 14% 34% 21% 5%	760
	11th to 20th	Night	24% 13% 37% 22% 4	864
	21st to end	Night	32% 12% 33% 21% 2	1045

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