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 2024-09-29.

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% 6% 2 8% 6% 6% 6% 5% 7% 10% 37%	100476
	Night	8% 6% 3 7% 5% 5% 5% 4 7% 9% 41%	97702
Transparency	Day	39% 9% 16% 19% 13% 4	100288
	Night	37% 9% 17% 19% 13% 4	97561
Seeing	Day	8% 4 16% 24% 25% 22%	15432
	Night	9% 3 18% 34% 27% 9%	25489

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	9% 6% 3 8% 5% 5% 4 4 5% 8% 42%	2836
	11th to 20th	Day	19% 10% 3 7% 4 3 4 3 6% 7% 34%	2840
	21st to end	Day	11% 6% 3 7% 5% 4 4 4 5% 8% 44%	3293
February	1st to 10th	Day	10% 6% 2 6% 5% 4 4 4 7% 8% 44%	1900
	11th to 20th	Day	7% 5% 2 4 4 4 5% 3 6% 8% 54%	1848
	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% 28%	2069
October	1st to 10th	Day	9% 7% 3 8% 6% 6% 6% 4 7% 9% 35%	1980
	11th to 20th	Day	6% 6% 2 6% 5% 6% 6% 5% 6% 8% 45%	1800
	21st to end	Day	6% 5% 2 6% 4 6% 5% 4 6% 9% 47%	1980
November	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 4 7% 8% 49%	1857
	11th to 20th	Day	8% 5% 2 7% 6% 5% 5% 4 6% 7% 45%	1900
	21st to end	Day	8% 6% 2 7% 5% 6% 5% 4 7% 9% 43%	1808
December	1st to 10th	Day	7% 4 3 5% 5% 4 5% 5% 6% 8% 47%	1710
	11th to 20th	Day	10% 7% 2 7% 4 4 4 3 5% 8% 45%	2153
	21st to end	Day	11% 7% 3 10% 6% 5% 5% 5% 6% 7% 35%	3124
January	1st to 10th	Night	10% 8% 3 9% 5% 5% 5% 4 5% 8% 39%	4109
	11th to 20th	Night	18% 12% 3 8% 5% 3 4 3 5% 7% 32%	4120
	21st to end	Night	10% 7% 3 7% 4 4 5% 5% 7% 8% 39%	4359
February	1st to 10th	Night	10% 7% 2 5% 4 4 4 4 6% 7% 46%	2280
	11th to 20th	Night	9% 5% 1 5% 4 3 4 3 6% 8% 51%	2218
	21st to end	Night	7% 6% 2 6% 4 4 4 4 5% 7% 52%	1783
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	12% 8% 3 9% 6% 5% 5% 5% 7% 9% 33%	2079
October	1st to 10th	Night	8% 5% 2 8% 5% 4 5% 3 9% 8% 41%	1980
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 4 6% 8% 50%	2160
	21st to end	Night	7% 4 2 7% 4 4 5% 3 6% 8% 48%	2376
November	1st to 10th	Night	9% 4 2 5% 4 4 4 4 5% 8% 51%	2221
	11th to 20th	Night	9% 6% 2 6% 4 5% 4 4 6% 7% 47%	2280
	21st to end	Night	8% 7% 2 7% 5% 5% 5% 4 6% 8% 45%	2368
December	1st to 10th	Night	7% 4 2 6% 5% 4 4 5% 6% 8% 49%	2470
	11th to 20th	Night	11% 8% 3 5% 4 3 4 3 5% 8% 47%	3042
	21st to end	Night	13% 7% 3 8% 5% 5% 5% 4 6% 7% 37%	4532

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	39% 5% 7% 21% 20% 8%	2837
	11th to 20th	Day	49% 4 5% 17% 17% 7%	2840
	21st to end	Day	38% 5% 7% 19% 23% 8%	3293
February	1st to 10th	Day	36% 5% 10% 19% 20% 11%	1900
	11th to 20th	Day	28% 5% 9% 21% 26% 12%	1858
	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	48% 12% 14% 14% 9% 2	2069
October	1st to 10th	Day	43% 9% 12% 15% 12% 7%	1980
	11th to 20th	Day	35% 7% 10% 22% 21% 5%	1800
	21st to end	Day	33% 5% 12% 17% 22% 11%	1980
November	1st to 10th	Day	31% 6% 11% 20% 20% 11%	1857
	11th to 20th	Day	37% 6% 9% 21% 20% 8%	1900
	21st to end	Day	36% 8% 8% 21% 20% 7%	1808
December	1st to 10th	Day	33% 5% 14% 25% 18% 5%	1710
	11th to 20th	Day	37% 5% 11% 17% 18% 11%	2150
	21st to end	Day	47% 6% 11% 16% 15% 5%	3124
January	1st to 10th	Night	43% 6% 9% 18% 19% 6%	4109
	11th to 20th	Night	51% 4 7% 20% 13% 5%	4120
	21st to end	Night	40% 7% 11% 17% 17% 8%	4359
February	1st to 10th	Night	35% 6% 11% 18% 20% 10%	2280
	11th to 20th	Night	31% 5% 12% 20% 23% 10%	2219
	21st to end	Night	30% 5% 13% 19% 24% 8%	1783
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	45% 10% 19% 16% 9% 2	2079
October	1st to 10th	Night	37% 11% 14% 20% 12% 7%	1980
	11th to 20th	Night	31% 7% 11% 25% 21% 4	2160
	21st to end	Night	32% 5% 13% 20% 20% 10%	2376
November	1st to 10th	Night	30% 6% 13% 20% 21% 10%	2221
	11th to 20th	Night	35% 6% 10% 21% 19% 8%	2280
	21st to end	Night	36% 5% 9% 23% 20% 7%	2368
December	1st to 10th	Night	31% 6% 13% 28% 15% 6%	2470
	11th to 20th	Night	35% 5% 11% 20% 16% 13%	3044
	21st to end	Night	44% 7% 12% 17% 15% 5%	4532

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	11% 7% 37% 31% 13% 1	270
	11th to 20th	Day	8% 10% 33% 31% 15% 3	270
	21st to end	Day	15% 11% 41% 24% 8%	297
February	1st to 10th	Day	9% 11% 27% 33% 17% 3	270
	11th to 20th	Day	10% 6% 32% 30% 15% 7%	270
	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%	483
October	1st to 10th	Day	11% 2 12% 30% 33% 10%	450
	11th to 20th	Day	8% 3 16% 29% 32% 12%	450
	21st to end	Day	8% 3 15% 36% 31% 8%	495
November	1st to 10th	Day	10% 3 20% 29% 29% 9%	455
	11th to 20th	Day	12% 4 22% 36% 22% 5%	460
	21st to end	Day	10% 4 22% 37% 23% 4	368
December	1st to 10th	Day	5% 10% 26% 30% 22% 7%	270
	11th to 20th	Day	11% 9% 28% 31% 18% 3	270
	21st to end	Day	7% 6% 28% 36% 19% 4	297
January	1st to 10th	Night	10% 7% 29% 38% 15%	760
	11th to 20th	Night	12% 5% 25% 43% 14% 1	760
	21st to end	Night	14% 6% 34% 35% 11%	836
February	1st to 10th	Night	15% 6% 26% 40% 13%	760
	11th to 20th	Night	11% 6% 26% 42% 13% 1	743
	21st to end	Night	11% 4 32% 42% 12%	628
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 6% 28% 39% 11%	756
October	1st to 10th	Night	11% 2 12% 41% 29% 5%	720
	11th to 20th	Night	8% 3 14% 36% 37% 4	720
	21st to end	Night	10% 3 17% 37% 29% 3	792
November	1st to 10th	Night	10% 3 21% 40% 22% 4	740
	11th to 20th	Night	11% 4 20% 42% 22% 2	760
	21st to end	Night	12% 2 24% 45% 16% 1	760
December	1st to 10th	Night	8% 4 23% 42% 23% 1	760
	11th to 20th	Night	12% 6% 24% 36% 22% 1	760
	21st to end	Night	10% 5% 25% 41% 18% 1	836

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