Dartmoor Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dartmoor Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	16% 11% 4 9% 5% 5% 4 4 5% 7% 31%	126856
	Night	18% 11% 4 9% 5% 4 4 3 5% 6% 31%	124143
Transparency	Day	52% 5% 10% 21% 11% 1	130460
	Night	53% 5% 10% 20% 11% 2	128609
Seeing	Day	23% 14% 26% 20% 12% 5%	16143
	Night	23% 11% 25% 28% 12% 1	25863

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	18% 19% 7% 22% 5% 3 3 2 3 4 15%	4028
	11th to 20th	Day	14% 24% 10% 16% 9% 6% 3 1 2 2 14%	4060
	21st to end	Day	21% 21% 5% 11% 7% 4 4 2 4 4 18%	4466
February	1st to 10th	Day	21% 17% 5% 10% 7% 5% 4 3 3 4 21%	1800
	11th to 20th	Day	19% 17% 6% 12% 5% 4 3 3 4 6% 21%	1753
	21st to end	Day	21% 15% 4 12% 5% 3 3 4 5% 5% 24%	1655
March	1st to 10th	Day	17% 14% 5% 9% 4 4 4 3 4 5% 31%	4935
	11th to 20th	Day	13% 10% 4 8% 5% 5% 5% 4 5% 6% 36%	4980
	21st to end	Day	18% 10% 3 9% 6% 4 4 4 5% 6% 30%	5465
April	1st to 10th	Day	18% 13% 2 8% 5% 4 5% 4 5% 7% 30%	2319
	11th to 20th	Day	16% 12% 3 7% 5% 4 5% 3 5% 7% 34%	2379
	21st to end	Day	20% 10% 3 6% 4 5% 4 3 4 7% 33%	2600
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 6% 7% 27%	5862
	11th to 20th	Day	19% 8% 3 8% 5% 5% 4 4 6% 8% 30%	5861
	21st to end	Day	20% 7% 3 6% 5% 4 4 5% 7% 8% 32%	6839
June	1st to 10th	Day	14% 7% 3 7% 6% 5% 5% 5% 7% 8% 35%	2800
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 7% 9% 34%	2800
	21st to end	Day	13% 6% 3 6% 5% 5% 5% 4 5% 10% 37%	2800
July	1st to 10th	Day	11% 6% 2 7% 4 5% 5% 5% 6% 8% 41%	6190
	11th to 20th	Day	12% 7% 3 8% 5% 5% 5% 4 8% 11% 32%	5900
	21st to end	Day	12% 6% 3 8% 5% 5% 5% 4 6% 8% 38%	6424
August	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 5% 8% 9% 30%	4234
	11th to 20th	Day	12% 7% 3 7% 5% 5% 5% 5% 7% 8% 37%	4174
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 7% 9% 42%	4419
September	1st to 10th	Day	15% 6% 2 5% 4 4 4 3 5% 7% 45%	2400
	11th to 20th	Day	12% 6% 2 5% 4 3 5% 3 5% 7% 46%	2229
	21st to end	Day	21% 9% 3 6% 4 4 4 3 5% 6% 35%	2185
October	1st to 10th	Day	17% 6% 2 6% 4 4 3 4 6% 7% 41%	2000
	11th to 20th	Day	19% 7% 3 7% 5% 5% 5% 3 5% 6% 37%	2000
	21st to end	Day	22% 10% 3 7% 4 3 4 3 4 6% 35%	2200
November	1st to 10th	Day	12% 9% 4 7% 6% 3 4 3 5% 6% 42%	2001
	11th to 20th	Day	17% 14% 4 7% 5% 5% 4 4 4 6% 31%	1732
	21st to end	Day	20% 15% 6% 7% 5% 4 4 4 4 5% 26%	1680
December	1st to 10th	Day	18% 15% 7% 16% 6% 4 3 3 3 4 20%	2680
	11th to 20th	Day	24% 18% 6% 10% 4 3 3 2 3 5% 22%	3046
	21st to end	Day	22% 19% 5% 11% 5% 5% 4 3 5% 5% 16%	3960
January	1st to 10th	Night	16% 19% 7% 22% 5% 4 3 2 3 3 15%	5912
	11th to 20th	Night	15% 19% 9% 14% 9% 5% 4 2 3 4 15%	5900
	21st to end	Night	23% 16% 5% 12% 6% 4 4 2 3 3 21%	6490
February	1st to 10th	Night	21% 16% 5% 10% 5% 3 4 3 4 5% 24%	2600
	11th to 20th	Night	20% 15% 4 11% 6% 5% 4 2 4 5% 25%	2533
	21st to end	Night	19% 15% 5% 11% 4 4 4 3 4 4 26%	1969
March	1st to 10th	Night	22% 11% 4 8% 5% 3 3 2 4 5% 32%	4918
	11th to 20th	Night	14% 10% 3 7% 4 3 3 3 4 7% 42%	4980
	21st to end	Night	17% 10% 3 8% 5% 5% 4 3 5% 5% 34%	5467
April	1st to 10th	Night	19% 14% 3 7% 4 4 3 3 4 6% 34%	2080
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 6% 6% 38%	1975
	21st to end	Night	16% 9% 3 7% 4 4 3 3 6% 7% 37%	1810
May	1st to 10th	Night	26% 11% 3 7% 5% 4 4 3 5% 6% 24%	4033
	11th to 20th	Night	18% 9% 3 7% 5% 3 4 3 5% 8% 35%	4032
	21st to end	Night	21% 8% 2 7% 4 3 4 4 5% 8% 33%	4010
June	1st to 10th	Night	12% 5% 3 6% 5% 5% 4 4 6% 8% 43%	1600
	11th to 20th	Night	12% 7% 2 7% 5% 5% 5% 4 7% 8% 40%	1600
	21st to end	Night	11% 5% 2 5% 4 5% 5% 4 7% 10% 42%	1600
July	1st to 10th	Night	12% 7% 3 8% 4 4 4 3 5% 8% 40%	3735
	11th to 20th	Night	14% 6% 3 7% 5% 5% 6% 5% 6% 9% 34%	4057
	21st to end	Night	11% 7% 3 7% 5% 5% 5% 4 5% 9% 40%	4427
August	1st to 10th	Night	14% 8% 3 7% 5% 5% 6% 3 7% 8% 34%	2915
	11th to 20th	Night	14% 7% 2 7% 4 4 4 4 5% 7% 42%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 6% 5% 6% 9% 44%	3444
September	1st to 10th	Night	13% 5% 1 5% 4 4 4 4 5% 7% 48%	2000
	11th to 20th	Night	11% 5% 1 5% 3 4 4 3 5% 7% 52%	2168
	21st to end	Night	18% 8% 2 5% 5% 4 5% 3 5% 6% 39%	2215
October	1st to 10th	Night	15% 6% 2 6% 4 4 3 4 5% 6% 45%	2400
	11th to 20th	Night	17% 7% 2 7% 4 4 5% 4 5% 6% 39%	2400
	21st to end	Night	24% 7% 2 6% 4 3 3 2 4 6% 38%	2640
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 3 4 6% 45%	2514
	11th to 20th	Night	20% 13% 2 7% 4 4 5% 3 5% 5% 32%	2884
	21st to end	Night	21% 14% 4 7% 4 3 4 3 4 5% 29%	2940
December	1st to 10th	Night	20% 15% 11% 12% 4 4 3 3 5% 5% 19%	4720
	11th to 20th	Night	22% 17% 7% 9% 4 3 3 2 3 4 24%	5304
	21st to end	Night	22% 18% 5% 10% 5% 5% 4 3 4 5% 18%	6996

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	75% 4 9% 9% 4	4029
	11th to 20th	Day	79% 2 6% 8% 4 2	4060
	21st to end	Day	72% 3 7% 11% 6%	4466
February	1st to 10th	Day	67% 5% 8% 14% 6% 1	1800
	11th to 20th	Day	64% 5% 9% 15% 6%	1762
	21st to end	Day	62% 5% 10% 15% 7% 2	1655
March	1st to 10th	Day	55% 5% 9% 22% 8% 1	4935
	11th to 20th	Day	48% 5% 11% 25% 9% 2	4980
	21st to end	Day	54% 5% 7% 19% 13% 1	5465
April	1st to 10th	Day	54% 6% 10% 17% 11% 2	2319
	11th to 20th	Day	49% 5% 12% 17% 15% 2	2379
	21st to end	Day	50% 6% 11% 22% 11% 1	2600
May	1st to 10th	Day	55% 6% 9% 19% 8% 3	5897
	11th to 20th	Day	51% 7% 10% 23% 9% 1	5865
	21st to end	Day	48% 7% 13% 21% 10% 2	6854
June	1st to 10th	Day	44% 9% 13% 22% 12% 1	2800
	11th to 20th	Day	45% 8% 13% 21% 12% 1	2800
	21st to end	Day	42% 7% 13% 24% 13% 1	2800
July	1st to 10th	Day	41% 5% 10% 26% 16% 2	6123
	11th to 20th	Day	46% 4 14% 24% 11%	5831
	21st to end	Day	44% 4 10% 23% 19% 1	6240
August	1st to 10th	Day	49% 6% 14% 22% 9%	4221
	11th to 20th	Day	43% 5% 11% 27% 13%	4213
	21st to end	Day	36% 6% 12% 29% 14% 2	4419
September	1st to 10th	Day	38% 6% 11% 22% 20% 3	2400
	11th to 20th	Day	36% 6% 12% 28% 16% 1	2229
	21st to end	Day	50% 4 11% 21% 14% 1	2185
October	1st to 10th	Day	35% 4 8% 32% 20% 1	3260
	11th to 20th	Day	46% 3 10% 23% 14% 5%	3240
	21st to end	Day	37% 3 11% 28% 19% 1	3538
November	1st to 10th	Day	44% 4 11% 21% 17% 3	2001
	11th to 20th	Day	54% 5% 8% 19% 11% 2	1732
	21st to end	Day	60% 5% 10% 18% 6% 1	1680
December	1st to 10th	Day	70% 3 12% 12% 3	2680
	11th to 20th	Day	67% 2 12% 14% 4	3042
	21st to end	Day	71% 4 9% 12% 4	3960
January	1st to 10th	Night	74% 3 7% 11% 4	5912
	11th to 20th	Night	73% 4 8% 10% 4 1	5900
	21st to end	Night	70% 3 7% 13% 6% 1	6490
February	1st to 10th	Night	63% 4 10% 18% 6%	2600
	11th to 20th	Night	62% 4 10% 16% 7%	2535
	21st to end	Night	62% 4 9% 16% 9% 1	1969
March	1st to 10th	Night	55% 3 8% 23% 9% 2	4918
	11th to 20th	Night	44% 4 12% 29% 9% 3	4980
	21st to end	Night	51% 4 10% 19% 15% 2	5467
April	1st to 10th	Night	53% 5% 10% 18% 13% 2	2080
	11th to 20th	Night	45% 6% 11% 19% 15% 3	1975
	21st to end	Night	46% 6% 10% 24% 11% 3	1810
May	1st to 10th	Night	60% 6% 9% 13% 9% 3	4060
	11th to 20th	Night	47% 7% 9% 25% 11% 1	4033
	21st to end	Night	48% 7% 12% 22% 8% 3	4017
June	1st to 10th	Night	38% 7% 12% 26% 15% 2	1600
	11th to 20th	Night	40% 7% 14% 25% 13% 1	1600
	21st to end	Night	37% 9% 13% 23% 16% 2	1600
July	1st to 10th	Night	43% 4 9% 23% 18% 3	3695
	11th to 20th	Night	47% 4 16% 21% 11% 1	4006
	21st to end	Night	42% 5% 10% 19% 22% 2	4297
August	1st to 10th	Night	47% 6% 14% 22% 10%	2904
	11th to 20th	Night	41% 6% 9% 27% 17% 1	2902
	21st to end	Night	37% 5% 13% 31% 12% 2	3444
September	1st to 10th	Night	34% 7% 10% 24% 21% 5%	2000
	11th to 20th	Night	30% 7% 11% 28% 23% 1	2168
	21st to end	Night	45% 5% 11% 21% 17% 1	2215
October	1st to 10th	Night	36% 4 7% 28% 23% 2	3920
	11th to 20th	Night	38% 4 9% 25% 16% 7%	3899
	21st to end	Night	37% 2 8% 28% 23% 2	4252
November	1st to 10th	Night	41% 4 10% 22% 18% 4	2514
	11th to 20th	Night	53% 5% 9% 19% 11% 3	2884
	21st to end	Night	57% 4 10% 21% 7% 1	2940
December	1st to 10th	Night	69% 3 10% 15% 3	4720
	11th to 20th	Night	66% 3 15% 14% 3	5307
	21st to end	Night	68% 5% 10% 14% 3	6996

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	32% 27% 24% 11% 4 1	293
	11th to 20th	Day	35% 26% 30% 5% 3	300
	21st to end	Day	31% 29% 27% 9% 3 1	330
February	1st to 10th	Day	33% 22% 35% 9% 1	300
	11th to 20th	Day	37% 29% 27% 6% 1	300
	21st to end	Day	34% 26% 30% 9%	299
March	1st to 10th	Day	28% 21% 31% 17% 4	500
	11th to 20th	Day	34% 21% 28% 12% 5% 1	500
	21st to end	Day	31% 18% 30% 16% 4 1	550
April	1st to 10th	Day	31% 18% 27% 16% 7% 1	500
	11th to 20th	Day	24% 22% 27% 15% 9% 4	492
	21st to end	Day	27% 16% 30% 17% 8% 2	500
May	1st to 10th	Day	28% 15% 29% 16% 10% 3	503
	11th to 20th	Day	23% 15% 26% 21% 10% 5%	527
	21st to end	Day	18% 10% 25% 23% 16% 7%	573
June	1st to 10th	Day	14% 11% 30% 27% 16% 3	530
	11th to 20th	Day	17% 9% 27% 25% 18% 3	530
	21st to end	Day	15% 8% 27% 27% 18% 4	530
July	1st to 10th	Day	15% 6% 16% 30% 21% 13%	525
	11th to 20th	Day	14% 6% 23% 29% 19% 9%	524
	21st to end	Day	16% 4 23% 30% 20% 8%	576
August	1st to 10th	Day	13% 6% 21% 28% 23% 9%	526
	11th to 20th	Day	16% 5% 17% 27% 23% 12%	519
	21st to end	Day	13% 5% 21% 26% 22% 14%	583
September	1st to 10th	Day	17% 7% 19% 27% 20% 11%	530
	11th to 20th	Day	13% 3 15% 29% 27% 12%	530
	21st to end	Day	27% 9% 23% 21% 15% 5%	530
October	1st to 10th	Day	18% 9% 24% 22% 21% 6%	530
	11th to 20th	Day	22% 13% 30% 23% 10% 2	530
	21st to end	Day	27% 17% 29% 18% 9% 1	583
November	1st to 10th	Day	22% 14% 31% 22% 9% 3	479
	11th to 20th	Day	34% 17% 30% 14% 5% 1	247
	21st to end	Day	33% 19% 34% 11% 3	220
December	1st to 10th	Day	30% 29% 27% 10% 3 1	220
	11th to 20th	Day	35% 22% 29% 11% 2	214
	21st to end	Day	35% 22% 25% 13% 5%	220
January	1st to 10th	Night	29% 21% 29% 17% 3	800
	11th to 20th	Night	33% 16% 31% 17% 3	800
	21st to end	Night	30% 18% 29% 20% 2	880
February	1st to 10th	Night	30% 16% 27% 24% 3	800
	11th to 20th	Night	31% 18% 33% 17% 1	783
	21st to end	Night	28% 16% 35% 19% 2	660
March	1st to 10th	Night	25% 16% 31% 23% 4	800
	11th to 20th	Night	27% 16% 28% 22% 6% 1	800
	21st to end	Night	24% 16% 32% 23% 5% 1	880
April	1st to 10th	Night	27% 16% 30% 22% 6%	800
	11th to 20th	Night	23% 12% 31% 25% 10% 1	791
	21st to end	Night	19% 10% 33% 31% 7%	610
May	1st to 10th	Night	23% 9% 22% 34% 11% 1	600
	11th to 20th	Night	22% 9% 25% 32% 12%	591
	21st to end	Night	13% 3 24% 33% 24% 2	472
June	1st to 10th	Night	12% 5% 22% 41% 19% 1	400
	11th to 20th	Night	13% 4 23% 42% 18% 1	400
	21st to end	Night	12% 3 27% 37% 21% 2	400
July	1st to 10th	Night	9% 2 17% 44% 24% 4	447
	11th to 20th	Night	12% 2 17% 35% 30% 4	582
	21st to end	Night	12% 3 14% 40% 29% 2	657
August	1st to 10th	Night	11% 3 14% 41% 29% 2	597
	11th to 20th	Night	12% 4 12% 39% 29% 4	594
	21st to end	Night	13% 3 15% 35% 29% 5%	790
September	1st to 10th	Night	14% 4 16% 36% 26% 4	800
	11th to 20th	Night	13% 3 13% 38% 28% 5%	800
	21st to end	Night	22% 5% 22% 28% 21% 3	800
October	1st to 10th	Night	17% 6% 21% 31% 22% 3	800
	11th to 20th	Night	22% 7% 29% 31% 11% 1	800
	21st to end	Night	27% 11% 23% 31% 7% 1	880
November	1st to 10th	Night	18% 10% 27% 32% 11% 1	820
	11th to 20th	Night	27% 14% 25% 25% 7% 1	840
	21st to end	Night	30% 13% 28% 23% 6%	840
December	1st to 10th	Night	33% 15% 28% 19% 5%	840
	11th to 20th	Night	29% 18% 29% 21% 3	829
	21st to end	Night	28% 17% 30% 23% 2	880

Page generated at 2025-12-15 01:13:54. Version ac07

Copyright © 2025 Attilla Danko.