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 2024-11-11.

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	17% 10% 4 8% 5% 4 4 4 5% 7% 32%	101182
	Night	18% 11% 4 8% 5% 4 4 3 5% 6% 32%	97912
Transparency	Day	50% 6% 11% 21% 11% 1	104830
	Night	51% 5% 10% 21% 11% 2	102404
Seeing	Day	23% 13% 26% 21% 13% 5%	14766
	Night	22% 11% 25% 28% 12% 1	24429

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	21% 20% 8% 11% 4 3 3 2 4 5% 19%	2808
	11th to 20th	Day	13% 23% 10% 15% 8% 4 3 2 3 3 17%	2840
	21st to end	Day	25% 21% 6% 13% 6% 4 3 2 3 4 14%	3124
February	1st to 10th	Day	21% 17% 5% 10% 7% 5% 3 3 4 4 21%	1710
	11th to 20th	Day	18% 17% 6% 12% 5% 4 3 3 5% 6% 22%	1663
	21st to end	Day	21% 15% 4 11% 5% 3 3 3 5% 5% 24%	1574
March	1st to 10th	Day	20% 12% 4 9% 4 4 4 3 5% 6% 29%	3435
	11th to 20th	Day	15% 11% 5% 9% 5% 5% 5% 3 4 5% 32%	3480
	21st to end	Day	19% 10% 3 9% 6% 4 4 4 5% 6% 31%	3815
April	1st to 10th	Day	17% 13% 2 8% 5% 4 5% 4 5% 7% 30%	2203
	11th to 20th	Day	16% 11% 3 7% 5% 4 5% 3 5% 7% 34%	2259
	21st to end	Day	21% 10% 3 6% 5% 5% 4 3 4 7% 32%	2470
May	1st to 10th	Day	24% 9% 3 7% 5% 4 4 4 5% 6% 28%	4082
	11th to 20th	Day	20% 8% 3 8% 5% 5% 4 4 6% 7% 31%	4081
	21st to end	Day	13% 6% 2 6% 5% 4 4 5% 7% 9% 37%	4756
June	1st to 10th	Day	14% 7% 3 7% 6% 5% 5% 5% 7% 8% 34%	2660
	11th to 20th	Day	13% 7% 3 7% 5% 5% 5% 4 7% 9% 35%	2660
	21st to end	Day	13% 6% 3 7% 6% 5% 5% 4 5% 10% 37%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 7% 8% 40%	4326
	11th to 20th	Day	12% 6% 3 6% 4 5% 5% 4 7% 10% 38%	4120
	21st to end	Day	13% 7% 3 8% 6% 5% 5% 5% 6% 8% 35%	4466
August	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 9% 30%	4104
	11th to 20th	Day	12% 7% 2 7% 5% 5% 6% 5% 7% 8% 37%	4044
	21st to end	Day	9% 4 2 6% 5% 5% 6% 5% 7% 9% 41%	4284
September	1st to 10th	Day	15% 6% 2 5% 4 5% 4 3 5% 7% 45%	2280
	11th to 20th	Day	12% 7% 2 5% 4 3 5% 4 6% 7% 46%	2118
	21st to end	Day	21% 9% 3 6% 4 4 4 3 5% 6% 36%	2076
October	1st to 10th	Day	18% 6% 2 6% 4 4 3 4 6% 7% 40%	1900
	11th to 20th	Day	20% 8% 3 7% 4 5% 5% 3 5% 5% 37%	1900
	21st to end	Day	23% 10% 3 7% 4 3 4 3 4 6% 35%	2090
November	1st to 10th	Day	12% 9% 4 7% 6% 3 3 2 5% 6% 42%	1904
	11th to 20th	Day	17% 14% 4 7% 5% 5% 4 4 4 6% 32%	1568
	21st to end	Day	20% 15% 6% 7% 5% 4 4 4 4 5% 27%	1520
December	1st to 10th	Day	22% 18% 6% 12% 6% 4 3 3 3 4 19%	1520
	11th to 20th	Day	21% 19% 6% 10% 4 3 3 3 4 6% 22%	1910
	21st to end	Day	24% 22% 6% 11% 5% 3 3 2 4 4 15%	2772
January	1st to 10th	Night	18% 22% 7% 12% 5% 4 4 3 4 4 19%	4132
	11th to 20th	Night	17% 21% 8% 12% 6% 5% 4 3 4 4 16%	4120
	21st to end	Night	28% 17% 6% 12% 5% 3 3 2 3 3 17%	4532
February	1st to 10th	Night	21% 16% 4 10% 5% 3 4 3 4 5% 25%	2470
	11th to 20th	Night	19% 15% 4 11% 6% 4 4 2 4 5% 25%	2403
	21st to end	Night	20% 15% 5% 11% 4 4 4 3 4 4 26%	1874
March	1st to 10th	Night	23% 11% 4 7% 4 3 3 3 4 6% 33%	3418
	11th to 20th	Night	18% 14% 4 7% 4 3 3 2 4 6% 34%	3480
	21st to end	Night	18% 10% 3 7% 4 4 4 3 4 5% 38%	3817
April	1st to 10th	Night	18% 14% 3 7% 4 3 3 3 4 5% 35%	1976
	11th to 20th	Night	17% 8% 3 6% 4 4 4 4 6% 7% 38%	1875
	21st to end	Night	16% 9% 4 7% 4 4 3 3 5% 7% 37%	1720
May	1st to 10th	Night	27% 10% 4 7% 5% 4 4 3 5% 6% 26%	2813
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 3 4 6% 35%	2812
	21st to end	Night	12% 7% 2 7% 4 4 4 5% 6% 9% 38%	2794
June	1st to 10th	Night	12% 5% 3 6% 5% 5% 4 4 6% 8% 43%	1520
	11th to 20th	Night	11% 6% 2 7% 4 5% 5% 4 7% 9% 41%	1520
	21st to end	Night	12% 5% 2 5% 4 5% 5% 4 7% 11% 42%	1520
July	1st to 10th	Night	12% 7% 3 8% 4 4 5% 4 6% 9% 40%	2615
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 5% 7% 9% 40%	2840
	21st to end	Night	12% 8% 3 7% 5% 5% 5% 4 6% 8% 37%	3087
August	1st to 10th	Night	15% 8% 3 7% 5% 5% 6% 3 7% 9% 33%	2825
	11th to 20th	Night	14% 7% 2 7% 4 4 4 4 5% 7% 42%	2785
	21st to end	Night	9% 5% 2 6% 4 5% 6% 5% 6% 9% 44%	3338
September	1st to 10th	Night	13% 5% 1 5% 4 4 4 4 5% 7% 47%	1900
	11th to 20th	Night	11% 5% 1 5% 3 4 4 3 5% 7% 52%	2059
	21st to end	Night	18% 8% 2 5% 5% 4 5% 3 5% 6% 40%	2104
October	1st to 10th	Night	16% 7% 2 6% 4 4 3 4 5% 6% 44%	2280
	11th to 20th	Night	17% 7% 2 7% 4 4 5% 4 5% 6% 38%	2280
	21st to end	Night	25% 7% 2 6% 4 3 3 3 4 7% 37%	2508
November	1st to 10th	Night	13% 9% 3 6% 5% 4 4 3 3 6% 46%	2391
	11th to 20th	Night	19% 13% 2 7% 4 4 5% 3 5% 5% 33%	2615
	21st to end	Night	21% 14% 4 7% 4 3 4 3 5% 5% 31%	2660
December	1st to 10th	Night	23% 17% 6% 9% 5% 4 4 3 4 4 21%	2660
	11th to 20th	Night	21% 17% 6% 10% 5% 3 3 3 3 5% 24%	3285
	21st to end	Night	24% 19% 6% 10% 6% 5% 4 3 4 5% 15%	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	69% 5% 9% 13% 4	2809
	11th to 20th	Day	74% 3 7% 11% 4 1	2840
	21st to end	Day	76% 5% 7% 10% 2	3124
February	1st to 10th	Day	66% 5% 8% 14% 6% 1	1710
	11th to 20th	Day	63% 6% 10% 15% 7%	1672
	21st to end	Day	62% 5% 10% 15% 7% 2	1574
March	1st to 10th	Day	54% 7% 11% 21% 7%	3435
	11th to 20th	Day	52% 7% 12% 22% 7% 1	3480
	21st to end	Day	52% 7% 10% 20% 10% 1	3815
April	1st to 10th	Day	52% 6% 10% 18% 11% 2	2203
	11th to 20th	Day	49% 6% 12% 16% 15% 2	2259
	21st to end	Day	50% 6% 11% 21% 10% 2	2470
May	1st to 10th	Day	54% 8% 9% 20% 8% 1	4120
	11th to 20th	Day	50% 8% 11% 22% 8% 1	4085
	21st to end	Day	39% 10% 15% 22% 11% 2	4770
June	1st to 10th	Day	43% 9% 13% 22% 11% 1	2660
	11th to 20th	Day	43% 8% 13% 22% 12% 1	2660
	21st to end	Day	43% 7% 13% 25% 12% 1	2660
July	1st to 10th	Day	41% 6% 12% 23% 15% 3	4259
	11th to 20th	Day	40% 5% 12% 27% 15% 1	4051
	21st to end	Day	46% 5% 11% 21% 16% 1	4324
August	1st to 10th	Day	50% 6% 14% 21% 8%	4091
	11th to 20th	Day	43% 5% 11% 27% 13%	4083
	21st to end	Day	37% 6% 13% 28% 14% 2	4284
September	1st to 10th	Day	39% 6% 12% 21% 19% 3	2280
	11th to 20th	Day	37% 6% 12% 28% 16% 1	2118
	21st to end	Day	49% 4 11% 22% 14% 1	2076
October	1st to 10th	Day	36% 4 9% 32% 19% 1	3160
	11th to 20th	Day	46% 3 10% 23% 13% 5%	3140
	21st to end	Day	37% 3 11% 29% 19% 1	3428
November	1st to 10th	Day	43% 4 11% 21% 17% 3	1904
	11th to 20th	Day	53% 6% 9% 19% 12% 2	1568
	21st to end	Day	58% 6% 11% 18% 6% 1	1520
December	1st to 10th	Day	70% 4 9% 12% 4	1520
	11th to 20th	Day	65% 4 12% 16% 4	1906
	21st to end	Day	72% 5% 10% 10% 3	2772
January	1st to 10th	Night	68% 5% 9% 13% 4 1	4132
	11th to 20th	Night	71% 5% 11% 10% 3	4120
	21st to end	Night	74% 4 6% 13% 3	4532
February	1st to 10th	Night	62% 4 10% 17% 7%	2470
	11th to 20th	Night	62% 4 10% 16% 7%	2405
	21st to end	Night	62% 5% 9% 15% 9% 1	1874
March	1st to 10th	Night	53% 5% 10% 25% 8%	3418
	11th to 20th	Night	51% 6% 11% 25% 6% 1	3480
	21st to end	Night	46% 6% 10% 23% 13% 2	3817
April	1st to 10th	Night	52% 5% 10% 18% 13% 2	1976
	11th to 20th	Night	44% 7% 11% 19% 15% 4	1875
	21st to end	Night	46% 6% 10% 24% 11% 3	1720
May	1st to 10th	Night	57% 7% 8% 16% 10% 2	2840
	11th to 20th	Night	48% 8% 11% 23% 9% 2	2813
	21st to end	Night	38% 10% 15% 22% 11% 4	2801
June	1st to 10th	Night	38% 8% 12% 26% 15% 2	1520
	11th to 20th	Night	38% 8% 14% 26% 13% 1	1520
	21st to end	Night	36% 9% 14% 24% 16% 2	1520
July	1st to 10th	Night	42% 5% 10% 23% 17% 4	2573
	11th to 20th	Night	40% 6% 14% 25% 15% 1	2786
	21st to end	Night	44% 6% 10% 16% 20% 3	2988
August	1st to 10th	Night	48% 7% 14% 22% 10%	2814
	11th to 20th	Night	42% 6% 9% 27% 17% 1	2812
	21st to end	Night	37% 5% 13% 31% 12% 2	3338
September	1st to 10th	Night	35% 7% 10% 23% 20% 5%	1900
	11th to 20th	Night	30% 7% 12% 27% 23% 1	2059
	21st to end	Night	43% 5% 11% 22% 17% 1	2104
October	1st to 10th	Night	37% 4 7% 28% 22% 2	3800
	11th to 20th	Night	38% 4 9% 25% 16% 7%	3779
	21st to end	Night	37% 2 8% 28% 23% 2	4120
November	1st to 10th	Night	41% 5% 10% 22% 18% 4	2391
	11th to 20th	Night	52% 5% 9% 19% 12% 2	2615
	21st to end	Night	55% 5% 10% 22% 7% 1	2660
December	1st to 10th	Night	66% 5% 11% 13% 5%	2660
	11th to 20th	Night	64% 4 11% 17% 4	3288
	21st to end	Night	70% 7% 10% 11% 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	30% 26% 26% 11% 5% 2	263
	11th to 20th	Day	35% 24% 31% 6% 3	270
	21st to end	Day	31% 28% 28% 9% 3 1	297
February	1st to 10th	Day	33% 19% 36% 10% 1	270
	11th to 20th	Day	35% 29% 28% 7% 1	270
	21st to end	Day	35% 26% 29% 10%	272
March	1st to 10th	Day	27% 20% 32% 17% 4	460
	11th to 20th	Day	34% 20% 28% 12% 5% 1	460
	21st to end	Day	30% 18% 29% 17% 5% 1	506
April	1st to 10th	Day	29% 19% 27% 16% 8% 1	460
	11th to 20th	Day	23% 21% 28% 15% 9% 4	452
	21st to end	Day	27% 16% 30% 17% 8% 2	460
May	1st to 10th	Day	29% 14% 27% 16% 10% 3	463
	11th to 20th	Day	22% 14% 27% 22% 10% 5%	487
	21st to end	Day	15% 9% 26% 25% 18% 8%	529
June	1st to 10th	Day	14% 11% 28% 26% 17% 4	490
	11th to 20th	Day	16% 9% 28% 25% 19% 3	490
	21st to end	Day	16% 8% 28% 27% 17% 3	490
July	1st to 10th	Day	14% 6% 16% 30% 20% 13%	485
	11th to 20th	Day	14% 6% 23% 30% 19% 9%	484
	21st to end	Day	16% 4 23% 30% 19% 8%	532
August	1st to 10th	Day	14% 6% 21% 27% 23% 9%	486
	11th to 20th	Day	16% 6% 18% 27% 22% 12%	479
	21st to end	Day	13% 5% 21% 26% 22% 14%	539
September	1st to 10th	Day	18% 6% 20% 27% 20% 10%	490
	11th to 20th	Day	14% 3 16% 29% 26% 12%	490
	21st to end	Day	27% 10% 23% 20% 15% 6%	490
October	1st to 10th	Day	19% 9% 25% 22% 20% 6%	490
	11th to 20th	Day	22% 14% 28% 23% 10% 2	490
	21st to end	Day	27% 16% 29% 19% 9% 1	539
November	1st to 10th	Day	21% 12% 31% 22% 10% 3	442
	11th to 20th	Day	30% 20% 28% 15% 6% 1	203
	21st to end	Day	29% 19% 37% 11% 3 1	180
December	1st to 10th	Day	29% 26% 29% 11% 3 1	180
	11th to 20th	Day	34% 22% 30% 12% 2	180
	21st to end	Day	37% 22% 25% 13% 4 1	198
January	1st to 10th	Night	29% 21% 29% 18% 4	760
	11th to 20th	Night	34% 15% 31% 17% 3	760
	21st to end	Night	30% 18% 29% 21% 2	836
February	1st to 10th	Night	30% 16% 27% 24% 3	760
	11th to 20th	Night	31% 18% 34% 17% 1	743
	21st to end	Night	29% 16% 34% 19% 2	628
March	1st to 10th	Night	25% 16% 31% 24% 4	760
	11th to 20th	Night	28% 16% 28% 21% 6% 1	760
	21st to end	Night	24% 15% 31% 24% 5% 1	836
April	1st to 10th	Night	25% 16% 30% 22% 6%	760
	11th to 20th	Night	22% 11% 32% 24% 10% 1	751
	21st to end	Night	19% 10% 32% 32% 7%	580
May	1st to 10th	Night	22% 10% 22% 35% 11% 1	570
	11th to 20th	Night	22% 9% 25% 33% 12%	561
	21st to end	Night	11% 3 23% 35% 25% 2	448
June	1st to 10th	Night	11% 5% 22% 42% 19% 1	380
	11th to 20th	Night	13% 4 23% 41% 19% 1	380
	21st to end	Night	12% 3 27% 38% 20% 1	380
July	1st to 10th	Night	9% 3 17% 43% 23% 4	424
	11th to 20th	Night	11% 3 18% 36% 28% 4	552
	21st to end	Night	13% 3 15% 40% 28% 2	624
August	1st to 10th	Night	12% 3 13% 41% 29% 2	567
	11th to 20th	Night	12% 4 12% 39% 29% 4	564
	21st to end	Night	13% 3 15% 35% 30% 5%	750
September	1st to 10th	Night	14% 4 15% 36% 27% 4	760
	11th to 20th	Night	13% 3 13% 39% 28% 4	760
	21st to end	Night	22% 5% 22% 28% 20% 3	760
October	1st to 10th	Night	17% 7% 22% 31% 21% 2	760
	11th to 20th	Night	22% 7% 28% 30% 11% 1	760
	21st to end	Night	28% 11% 22% 30% 8% 1	836
November	1st to 10th	Night	18% 11% 27% 32% 12% 1	780
	11th to 20th	Night	26% 15% 26% 25% 8% 1	763
	21st to end	Night	29% 13% 28% 24% 6%	760
December	1st to 10th	Night	35% 14% 26% 19% 5%	760
	11th to 20th	Night	28% 19% 28% 22% 4	760
	21st to end	Night	28% 17% 30% 22% 2	836

Page generated at 2024-11-08 06:44:48. Version ac07

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