Brogdon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Brogdon Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	7% 5% 2 7% 6% 6% 6% 5% 7% 10% 39%	108959
	Night	8% 6% 2 7% 5% 5% 5% 4 7% 9% 42%	109633
Transparency	Day	38% 9% 15% 19% 14% 5%	112726
	Night	37% 8% 16% 20% 14% 5%	113993
Seeing	Day	8% 4 16% 25% 25% 22%	15924
	Night	9% 3 18% 34% 27% 9%	26066

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	7% 5% 2 6% 4 4 4 4 5% 8% 52%	4056
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 3 6% 7% 31%	4060
	21st to end	Day	14% 6% 2 5% 3 3 3 4 5% 8% 45%	4719
February	1st to 10th	Day	10% 6% 2 7% 5% 4 4 4 7% 8% 44%	2000
	11th to 20th	Day	6% 5% 2 4 4 4 5% 3 6% 8% 53%	1948
	21st to end	Day	8% 5% 2 5% 5% 4 5% 3 5% 7% 49%	1670
March	1st to 10th	Day	6% 6% 3 10% 6% 6% 7% 5% 6% 8% 37%	3435
	11th to 20th	Day	5% 4 2 5% 4 4 4 4 6% 11% 51%	3480
	21st to end	Day	9% 4 2 5% 4 3 4 4 5% 7% 52%	3815
April	1st to 10th	Day	5% 4 2 5% 4 6% 6% 5% 6% 10% 47%	2090
	11th to 20th	Day	7% 4 2 7% 6% 5% 5% 5% 7% 10% 42%	2074
	21st to end	Day	5% 4 1 5% 5% 5% 6% 5% 6% 9% 49%	2280
May	1st to 10th	Day	2 2 1 4 3 4 5% 4 7% 9% 58%	4005
	11th to 20th	Day	7% 5% 2 6% 5% 6% 5% 5% 8% 10% 39%	4081
	21st to end	Day	3 3 1 5% 5% 5% 5% 5% 7% 9% 51%	4476
June	1st to 10th	Day	12% 9% 3 10% 8% 6% 6% 6% 8% 9% 23%	2470
	11th to 20th	Day	7% 7% 3 10% 8% 8% 8% 6% 8% 11% 24%	2470
	21st to end	Day	8% 7% 3 11% 9% 8% 7% 6% 8% 9% 25%	2470
July	1st to 10th	Day	6% 5% 3 9% 8% 8% 8% 7% 9% 13% 22%	4120
	11th to 20th	Day	4 6% 3 10% 8% 8% 8% 7% 9% 13% 25%	4120
	21st to end	Day	5% 6% 3 10% 8% 9% 8% 7% 9% 11% 23%	4466
August	1st to 10th	Day	5% 7% 4 11% 7% 8% 8% 6% 9% 11% 25%	4104
	11th to 20th	Day	3 5% 2 9% 7% 8% 8% 6% 8% 13% 31%	3744
	21st to end	Day	7% 6% 3 9% 8% 8% 8% 6% 9% 13% 23%	3828
September	1st to 10th	Day	7% 7% 4 10% 9% 7% 8% 7% 7% 12% 22%	2090
	11th to 20th	Day	9% 7% 3 9% 8% 8% 7% 6% 8% 9% 25%	2090
	21st to end	Day	7% 6% 3 10% 8% 7% 7% 6% 7% 9% 29%	2090
October	1st to 10th	Day	10% 7% 3 8% 6% 6% 6% 5% 7% 8% 33%	2089
	11th to 20th	Day	6% 6% 2 6% 5% 6% 6% 5% 6% 8% 46%	1900
	21st to end	Day	6% 5% 2 6% 4 6% 5% 4 6% 8% 48%	2090
November	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 8% 48%	1957
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 7% 46%	2000
	21st to end	Day	8% 5% 2 6% 5% 6% 5% 4 6% 8% 45%	1903
December	1st to 10th	Day	4 3 2 4 4 3 4 4 5% 7% 59%	2930
	11th to 20th	Day	8% 7% 3 7% 4 4 5% 3 6% 9% 44%	3373
	21st to end	Day	9% 7% 3 9% 6% 5% 5% 5% 6% 8% 36%	4466
January	1st to 10th	Night	7% 6% 2 6% 4 4 4 3 5% 8% 50%	5889
	11th to 20th	Night	19% 11% 3 8% 5% 4 4 3 5% 7% 30%	5900
	21st to end	Night	15% 7% 2 6% 4 4 4 4 6% 8% 41%	6233
February	1st to 10th	Night	10% 7% 2 5% 4 4 4 4 6% 7% 47%	2400
	11th to 20th	Night	10% 6% 1 5% 4 3 4 3 6% 7% 51%	2338
	21st to end	Night	7% 6% 2 6% 4 4 4 4 5% 7% 52%	1784
March	1st to 10th	Night	13% 9% 4 8% 5% 5% 4 4 5% 6% 36%	3418
	11th to 20th	Night	4 3 1 6% 3 4 4 5% 9% 12% 48%	3480
	21st to end	Night	8% 6% 2 6% 4 3 4 4 6% 7% 51%	3817
April	1st to 10th	Night	6% 4 2 4 3 3 4 3 7% 11% 52%	2090
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 7% 10% 49%	2054
	21st to end	Night	5% 4 1 5% 3 3 4 4 6% 8% 57%	1900
May	1st to 10th	Night	4 5% 2 6% 4 4 4 4 5% 8% 53%	2916
	11th to 20th	Night	7% 5% 2 5% 3 4 5% 4 7% 11% 47%	2812
	21st to end	Night	4 3 2 3 3 3 4 4 6% 9% 59%	3079
June	1st to 10th	Night	9% 6% 3 7% 6% 6% 6% 5% 7% 10% 35%	1710
	11th to 20th	Night	6% 8% 3 8% 6% 5% 6% 6% 8% 13% 31%	1710
	21st to end	Night	7% 6% 3 8% 6% 7% 6% 5% 9% 13% 29%	1710
July	1st to 10th	Night	7% 7% 3 11% 7% 8% 8% 5% 9% 10% 25%	2840
	11th to 20th	Night	4 6% 3 10% 7% 8% 8% 6% 9% 12% 26%	2840
	21st to end	Night	7% 10% 5% 12% 8% 7% 8% 6% 9% 9% 19%	3087
August	1st to 10th	Night	4 6% 4 9% 5% 6% 5% 6% 9% 12% 34%	2825
	11th to 20th	Night	3 5% 3 8% 6% 6% 7% 7% 10% 12% 35%	3029
	21st to end	Night	7% 5% 3 9% 7% 6% 7% 6% 10% 13% 28%	3828
September	1st to 10th	Night	7% 7% 3 8% 6% 7% 7% 7% 10% 12% 27%	2090
	11th to 20th	Night	11% 8% 3 11% 6% 6% 6% 4 9% 9% 27%	2090
	21st to end	Night	11% 7% 3 8% 6% 5% 5% 5% 7% 9% 33%	2090
October	1st to 10th	Night	10% 6% 3 9% 5% 4 5% 3 8% 8% 39%	2090
	11th to 20th	Night	6% 5% 2 6% 5% 5% 5% 4 6% 8% 50%	2280
	21st to end	Night	7% 4 2 7% 4 4 4 3 6% 8% 50%	2508
November	1st to 10th	Night	9% 4 2 5% 4 5% 4 4 5% 8% 50%	2341
	11th to 20th	Night	9% 6% 2 6% 4 5% 4 4 6% 8% 47%	2400
	21st to end	Night	8% 6% 2 7% 5% 4 4 4 6% 7% 47%	2493
December	1st to 10th	Night	4 2 1 4 3 3 3 4 5% 9% 60%	4250
	11th to 20th	Night	9% 8% 2 6% 3 4 4 3 5% 8% 48%	4822
	21st to end	Night	12% 8% 3 8% 5% 5% 5% 4 6% 7% 37%	6490

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	30% 4 7% 19% 28% 11%	4057
	11th to 20th	Day	52% 4 6% 17% 16% 6%	4060
	21st to end	Day	36% 4 6% 28% 20% 6%	4719
February	1st to 10th	Day	36% 5% 10% 19% 20% 11%	2000
	11th to 20th	Day	28% 5% 9% 21% 25% 11%	1958
	21st to end	Day	33% 4 10% 16% 27% 10%	1670
March	1st to 10th	Day	41% 10% 12% 17% 14% 6%	3435
	11th to 20th	Day	26% 6% 12% 30% 20% 6%	3480
	21st to end	Day	30% 5% 8% 30% 18% 9%	3815
April	1st to 10th	Day	30% 6% 15% 24% 18% 7%	2090
	11th to 20th	Day	35% 8% 12% 17% 20% 7%	2074
	21st to end	Day	29% 6% 10% 22% 25% 9%	2280
May	1st to 10th	Day	19% 7% 16% 33% 19% 6%	4042
	11th to 20th	Day	35% 10% 18% 23% 13% 2	4085
	21st to end	Day	26% 7% 13% 32% 19% 2	4489
June	1st to 10th	Day	54% 11% 16% 15% 5%	2470
	11th to 20th	Day	49% 12% 19% 17% 3	2470
	21st to end	Day	51% 12% 22% 12% 2 1	2470
July	1st to 10th	Day	47% 14% 29% 9% 1	4055
	11th to 20th	Day	44% 15% 32% 8%	4051
	21st to end	Day	47% 16% 26% 10% 1	4324
August	1st to 10th	Day	46% 14% 29% 10% 2	4091
	11th to 20th	Day	39% 14% 25% 16% 6%	3783
	21st to end	Day	47% 16% 25% 12% 1	3828
September	1st to 10th	Day	51% 16% 19% 12% 2	2090
	11th to 20th	Day	50% 13% 20% 13% 4	2090
	21st to end	Day	47% 12% 15% 15% 9% 2	2090
October	1st to 10th	Day	56% 11% 13% 9% 7% 4	3466
	11th to 20th	Day	27% 5% 9% 23% 31% 4	3140
	21st to end	Day	29% 5% 9% 22% 29% 7%	3428
November	1st to 10th	Day	31% 6% 11% 21% 20% 10%	1957
	11th to 20th	Day	36% 6% 9% 21% 20% 9%	2000
	21st to end	Day	35% 7% 8% 21% 20% 9%	1903
December	1st to 10th	Day	24% 4 15% 29% 19% 9%	2930
	11th to 20th	Day	36% 5% 12% 18% 20% 9%	3370
	21st to end	Day	43% 6% 12% 20% 15% 4	4466
January	1st to 10th	Night	32% 4 8% 20% 26% 11%	5889
	11th to 20th	Night	53% 5% 9% 17% 12% 4	5900
	21st to end	Night	39% 5% 9% 23% 16% 7%	6233
February	1st to 10th	Night	35% 6% 11% 18% 21% 9%	2400
	11th to 20th	Night	32% 5% 12% 20% 22% 9%	2339
	21st to end	Night	30% 5% 13% 19% 24% 8%	1784
March	1st to 10th	Night	45% 9% 11% 15% 15% 4	3418
	11th to 20th	Night	24% 8% 21% 28% 15% 5%	3480
	21st to end	Night	31% 5% 10% 25% 23% 6%	3817
April	1st to 10th	Night	25% 6% 19% 26% 17% 7%	2090
	11th to 20th	Night	28% 7% 15% 24% 21% 6%	2054
	21st to end	Night	24% 6% 14% 26% 23% 7%	1900
May	1st to 10th	Night	25% 11% 17% 27% 15% 5%	2945
	11th to 20th	Night	27% 11% 22% 28% 12% 1	2813
	21st to end	Night	21% 8% 17% 36% 18% 1	3087
June	1st to 10th	Night	41% 11% 23% 20% 5%	1710
	11th to 20th	Night	41% 12% 24% 19% 4	1710
	21st to end	Night	44% 14% 24% 13% 5%	1710
July	1st to 10th	Night	49% 14% 27% 9% 1	2795
	11th to 20th	Night	46% 19% 30% 5%	2786
	21st to end	Night	55% 15% 21% 8%	2988
August	1st to 10th	Night	38% 15% 33% 12% 2	2814
	11th to 20th	Night	36% 15% 28% 16% 5%	3056
	21st to end	Night	44% 16% 30% 10% 1	3828
September	1st to 10th	Night	44% 18% 24% 12% 2	2090
	11th to 20th	Night	49% 13% 20% 15% 2	2090
	21st to end	Night	44% 10% 18% 17% 9% 2	2090
October	1st to 10th	Night	57% 11% 10% 12% 7% 4	3480
	11th to 20th	Night	28% 6% 9% 29% 23% 5%	3779
	21st to end	Night	29% 5% 9% 23% 28% 6%	4120
November	1st to 10th	Night	30% 7% 13% 20% 21% 9%	2341
	11th to 20th	Night	34% 6% 10% 22% 19% 9%	2400
	21st to end	Night	35% 5% 9% 22% 21% 8%	2493
December	1st to 10th	Night	21% 5% 17% 31% 14% 12%	4250
	11th to 20th	Night	34% 5% 13% 22% 16% 10%	4824
	21st to end	Night	44% 7% 13% 19% 13% 5%	6490

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	10% 7% 39% 32% 12% 1	300
	11th to 20th	Day	8% 13% 33% 30% 13% 3	300
	21st to end	Day	15% 11% 40% 25% 8%	330
February	1st to 10th	Day	8% 10% 26% 32% 20% 4	300
	11th to 20th	Day	9% 7% 33% 31% 14% 7%	300
	21st to end	Day	7% 6% 28% 33% 22% 4	325
March	1st to 10th	Day	5% 5% 35% 35% 18% 1	460
	11th to 20th	Day	7% 6% 26% 32% 26% 3	460
	21st to end	Day	6% 6% 26% 34% 24% 4	506
April	1st to 10th	Day	6% 7% 30% 34% 18% 4	460
	11th to 20th	Day	10% 5% 24% 33% 21% 7%	452
	21st to end	Day	6% 4 18% 34% 27% 11%	460
May	1st to 10th	Day	5% 2 17% 28% 33% 15%	463
	11th to 20th	Day	9% 2 13% 28% 31% 18%	487
	21st to end	Day	7% 2 5% 21% 35% 31%	529
June	1st to 10th	Day	16% 3 7% 18% 28% 27%	490
	11th to 20th	Day	10% 3 6% 14% 30% 37%	490
	21st to end	Day	6% 2 2 12% 28% 50%	490
July	1st to 10th	Day	7% 2 2 7% 24% 58%	485
	11th to 20th	Day	5% 2 1 8% 27% 57%	484
	21st to end	Day	8% 1 2 13% 25% 50%	532
August	1st to 10th	Day	7% 1 5% 12% 25% 50%	486
	11th to 20th	Day	5% 1 3 9% 27% 56%	479
	21st to end	Day	9% 2 5% 19% 25% 41%	539
September	1st to 10th	Day	7% 3 5% 11% 28% 46%	490
	11th to 20th	Day	11% 4 3 16% 31% 34%	490
	21st to end	Day	9% 3 6% 18% 34% 31%	490
October	1st to 10th	Day	14% 3 12% 28% 33% 10%	490
	11th to 20th	Day	7% 3 18% 30% 31% 11%	490
	21st to end	Day	7% 3 16% 37% 29% 7%	539
November	1st to 10th	Day	9% 3 19% 31% 29% 8%	495
	11th to 20th	Day	12% 4 20% 36% 23% 5%	500
	21st to end	Day	9% 4 24% 35% 23% 5%	403
December	1st to 10th	Day	4 10% 28% 31% 20% 6%	300
	11th to 20th	Day	10% 9% 29% 32% 17% 3	300
	21st to end	Day	8% 6% 28% 36% 17% 4	330
January	1st to 10th	Night	10% 7% 31% 38% 14%	800
	11th to 20th	Night	13% 5% 26% 41% 13% 1	800
	21st to end	Night	15% 6% 34% 35% 11%	880
February	1st to 10th	Night	15% 6% 26% 39% 15%	800
	11th to 20th	Night	11% 7% 27% 41% 13% 1	783
	21st to end	Night	11% 3 32% 42% 12%	629
March	1st to 10th	Night	7% 5% 34% 43% 11% 1	760
	11th to 20th	Night	8% 5% 29% 41% 16%	760
	21st to end	Night	7% 4 29% 44% 15% 1	836
April	1st to 10th	Night	6% 4 29% 46% 16%	760
	11th to 20th	Night	6% 5% 28% 47% 14% 1	751
	21st to end	Night	6% 3 25% 44% 21%	760
May	1st to 10th	Night	5% 1 16% 51% 26% 2	636
	11th to 20th	Night	7% 2 13% 41% 32% 4	561
	21st to end	Night	6% 1 9% 35% 42% 6%	617
June	1st to 10th	Night	8% 1 7% 30% 44% 10%	570
	11th to 20th	Night	7% 4 4 20% 44% 22%	570
	21st to end	Night	8% 1 2 15% 46% 28%	570
July	1st to 10th	Night	9% 1 3 10% 46% 32%	555
	11th to 20th	Night	5% 2 1 9% 45% 39%	552
	21st to end	Night	6% 1 2 9% 45% 37%	624
August	1st to 10th	Night	5% 1 2 11% 49% 32%	567
	11th to 20th	Night	4 1 2 10% 44% 38%	593
	21st to end	Night	12% 2 5% 20% 38% 23%	836
September	1st to 10th	Night	10% 3 5% 19% 41% 23%	760
	11th to 20th	Night	12% 4 5% 24% 39% 16%	760
	21st to end	Night	15% 2 5% 28% 39% 12%	760
October	1st to 10th	Night	14% 2 12% 39% 29% 5%	760
	11th to 20th	Night	7% 3 15% 36% 36% 4	760
	21st to end	Night	10% 3 18% 39% 28% 3	836
November	1st to 10th	Night	10% 3 21% 40% 23% 3	780
	11th to 20th	Night	10% 4 20% 43% 22% 2	800
	21st to end	Night	11% 3 24% 44% 17% 1	800
December	1st to 10th	Night	7% 4 24% 42% 22% 1	800
	11th to 20th	Night	11% 5% 24% 37% 21% 1	800
	21st to end	Night	10% 5% 25% 42% 17% 1	880

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