Old Broadwell Mill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Old Broadwell Mill Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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	13% 7% 3 6% 5% 4 5% 4 6% 8% 41%	120131
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	119021
Transparency	Day	40% 6% 11% 20% 19% 5%	123949
	Night	38% 5% 10% 20% 20% 6%	123341
Seeing	Day	16% 10% 22% 24% 19% 10%	15574
	Night	15% 8% 22% 30% 21% 4	25568

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% 11% 4 5% 2 3 3 2 4 4 45%	4056
	11th to 20th	Day	12% 7% 5% 6% 3 3 3 3 4 5% 51%	4060
	21st to end	Day	21% 10% 3 6% 4 3 3 2 4 6% 38%	4949
February	1st to 10th	Day	19% 9% 3 5% 5% 4 3 3 5% 5% 39%	2000
	11th to 20th	Day	22% 9% 2 6% 4 3 3 3 4 5% 39%	1948
	21st to end	Day	19% 8% 3 7% 5% 4 3 4 4 6% 38%	1650
March	1st to 10th	Day	20% 8% 3 7% 4 4 4 4 5% 6% 35%	4480
	11th to 20th	Day	12% 7% 2 5% 4 4 3 3 5% 7% 47%	4906
	21st to end	Day	16% 6% 3 6% 5% 3 4 4 5% 6% 42%	5505
April	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 5% 7% 45%	2400
	11th to 20th	Day	13% 6% 2 5% 4 4 3 4 5% 6% 47%	2374
	21st to end	Day	10% 5% 2 5% 5% 4 5% 4 6% 8% 47%	2443
May	1st to 10th	Day	14% 6% 2 7% 6% 5% 5% 5% 7% 8% 34%	5862
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 5% 7% 8% 31%	5861
	21st to end	Day	11% 6% 2 7% 4 5% 5% 4 6% 8% 41%	6433
June	1st to 10th	Day	11% 6% 2 8% 6% 6% 7% 5% 8% 10% 32%	2600
	11th to 20th	Day	5% 4 2 6% 5% 5% 6% 4 8% 11% 43%	2600
	21st to end	Day	8% 5% 2 6% 5% 6% 7% 6% 8% 11% 37%	2600
July	1st to 10th	Day	8% 6% 2 7% 6% 7% 8% 7% 10% 12% 28%	4250
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 8% 13% 36%	4250
	21st to end	Day	10% 9% 4 10% 7% 7% 7% 6% 8% 10% 20%	4466
August	1st to 10th	Day	4 2 1 5% 4 5% 6% 5% 8% 13% 48%	3882
	11th to 20th	Day	7% 4 2 5% 4 5% 5% 5% 7% 11% 45%	3729
	21st to end	Day	6% 4 2 5% 5% 5% 5% 5% 6% 8% 49%	4180
September	1st to 10th	Day	9% 5% 3 7% 6% 5% 5% 5% 6% 8% 40%	2180
	11th to 20th	Day	13% 6% 2 7% 4 5% 4 4 6% 6% 42%	2090
	21st to end	Day	12% 5% 2 6% 4 5% 3 3 5% 7% 48%	2090
October	1st to 10th	Day	11% 4 2 5% 3 4 4 3 5% 5% 54%	2090
	11th to 20th	Day	11% 5% 2 4 3 2 2 3 4 6% 57%	2085
	21st to end	Day	13% 5% 1 5% 3 3 4 3 4 5% 54%	1981
November	1st to 10th	Day	14% 6% 2 4 4 3 3 2 3 5% 54%	1762
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 6% 51%	1800
	21st to end	Day	14% 7% 2 6% 4 3 4 4 5% 7% 46%	1800
December	1st to 10th	Day	19% 7% 2 5% 3 2 3 3 4 5% 45%	2930
	11th to 20th	Day	15% 8% 4 8% 6% 5% 4 2 4 5% 41%	3373
	21st to end	Day	17% 9% 4 7% 4 4 4 2 4 6% 39%	4466
January	1st to 10th	Night	15% 10% 4 5% 3 3 3 3 4 5% 44%	5889
	11th to 20th	Night	16% 9% 3 6% 4 3 3 3 4 5% 45%	5900
	21st to end	Night	18% 11% 2 5% 3 3 3 3 4 4 43%	6002
February	1st to 10th	Night	17% 8% 2 6% 4 4 3 3 5% 6% 42%	2400
	11th to 20th	Night	22% 8% 2 6% 4 3 3 2 4 4 42%	2338
	21st to end	Night	18% 8% 3 6% 4 4 4 3 4 7% 39%	1980
March	1st to 10th	Night	21% 11% 3 6% 5% 4 4 4 4 6% 33%	5373
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 6% 7% 50%	5053
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 4 5% 44%	5453
April	1st to 10th	Night	12% 4 2 4 4 5% 4 4 6% 8% 47%	2000
	11th to 20th	Night	14% 5% 2 4 3 3 3 3 6% 6% 51%	1975
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 6% 52%	1955
May	1st to 10th	Night	12% 7% 3 8% 5% 5% 5% 4 6% 9% 36%	4033
	11th to 20th	Night	16% 9% 3 8% 6% 5% 5% 4 6% 8% 31%	4032
	21st to end	Night	10% 6% 2 5% 4 5% 5% 4 7% 10% 41%	4421
June	1st to 10th	Night	5% 5% 2 7% 5% 6% 6% 6% 9% 12% 38%	1800
	11th to 20th	Night	4 4 2 3 5% 4 5% 6% 7% 12% 49%	1800
	21st to end	Night	5% 6% 1 6% 4 4 5% 5% 7% 13% 45%	1800
July	1st to 10th	Night	7% 5% 2 7% 6% 6% 6% 6% 9% 13% 33%	2930
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 5% 10% 13% 39%	2930
	21st to end	Night	8% 9% 4 11% 7% 7% 7% 6% 8% 10% 24%	3087
August	1st to 10th	Night	4 3 1 5% 3 4 4 4 7% 12% 53%	3042
	11th to 20th	Night	7% 5% 2 6% 4 3 5% 4 5% 9% 51%	3100
	21st to end	Night	7% 4 2 4 3 4 4 3 5% 8% 55%	3476
September	1st to 10th	Night	7% 4 3 6% 5% 5% 6% 5% 7% 8% 44%	1978
	11th to 20th	Night	11% 5% 3 7% 4 4 4 4 5% 8% 46%	2090
	21st to end	Night	8% 4 2 5% 3 4 3 4 6% 8% 52%	2090
October	1st to 10th	Night	9% 4 1 4 3 3 3 2 4 6% 61%	2090
	11th to 20th	Night	11% 5% 2 4 3 4 3 3 4 6% 55%	2094
	21st to end	Night	13% 5% 1 5% 4 3 3 3 3 4 55%	2612
November	1st to 10th	Night	11% 5% 1 4 4 3 3 3 5% 6% 55%	2536
	11th to 20th	Night	12% 7% 2 4 3 3 3 2 4 6% 54%	2600
	21st to end	Night	13% 7% 2 5% 4 3 4 3 4 6% 49%	2600
December	1st to 10th	Night	18% 7% 2 5% 3 3 3 3 4 5% 47%	4250
	11th to 20th	Night	17% 9% 3 6% 4 3 4 3 4 5% 45%	4822
	21st to end	Night	16% 9% 3 7% 4 4 4 3 5% 6% 38%	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	44% 3 5% 23% 22% 3	4057
	11th to 20th	Day	37% 3 6% 23% 28% 3	4060
	21st to end	Day	50% 2 7% 15% 19% 7%	4949
February	1st to 10th	Day	48% 3 8% 18% 19% 4	2000
	11th to 20th	Day	49% 3 5% 18% 18% 6%	1958
	21st to end	Day	48% 5% 6% 13% 22% 7%	1650
March	1st to 10th	Day	51% 4 8% 12% 18% 7%	4480
	11th to 20th	Day	37% 3 7% 15% 23% 14%	4906
	21st to end	Day	43% 4 9% 20% 20% 4	5507
April	1st to 10th	Day	38% 6% 10% 21% 18% 7%	2400
	11th to 20th	Day	37% 5% 7% 18% 25% 7%	2374
	21st to end	Day	34% 6% 10% 24% 22% 3	2443
May	1st to 10th	Day	44% 8% 14% 21% 12% 2	5897
	11th to 20th	Day	48% 8% 15% 20% 8% 1	5865
	21st to end	Day	40% 8% 10% 24% 17% 1	6447
June	1st to 10th	Day	43% 11% 14% 18% 13% 1	2600
	11th to 20th	Day	32% 9% 20% 22% 16% 1	2600
	21st to end	Day	36% 11% 17% 22% 11% 2	2600
July	1st to 10th	Day	42% 13% 21% 17% 6% 1	4185
	11th to 20th	Day	36% 12% 23% 22% 6% 1	4181
	21st to end	Day	56% 13% 14% 13% 5%	4324
August	1st to 10th	Day	26% 8% 14% 36% 15%	3869
	11th to 20th	Day	31% 9% 18% 26% 14% 2	3765
	21st to end	Day	31% 9% 15% 17% 25% 3	4179
September	1st to 10th	Day	39% 10% 14% 16% 15% 6%	2180
	11th to 20th	Day	41% 8% 11% 19% 18% 3	2090
	21st to end	Day	36% 5% 12% 20% 21% 6%	2090
October	1st to 10th	Day	25% 4 7% 10% 36% 18%	3480
	11th to 20th	Day	20% 3 4 16% 29% 28%	3441
	21st to end	Day	23% 3 7% 21% 35% 10%	3244
November	1st to 10th	Day	35% 3 8% 16% 28% 10%	1762
	11th to 20th	Day	34% 4 7% 18% 27% 10%	1800
	21st to end	Day	38% 5% 7% 18% 26% 6%	1800
December	1st to 10th	Day	42% 4 5% 18% 27% 5%	2930
	11th to 20th	Day	48% 3 8% 19% 17% 5%	3370
	21st to end	Day	48% 3 7% 24% 16% 2	4466
January	1st to 10th	Night	44% 2 5% 23% 22% 3	5889
	11th to 20th	Night	43% 3 5% 20% 23% 6%	5900
	21st to end	Night	45% 2 5% 17% 26% 5%	6002
February	1st to 10th	Night	44% 4 7% 19% 20% 7%	2400
	11th to 20th	Night	48% 4 5% 17% 19% 8%	2339
	21st to end	Night	44% 5% 7% 14% 19% 10%	1980
March	1st to 10th	Night	53% 3 7% 13% 18% 5%	5373
	11th to 20th	Night	33% 4 9% 16% 23% 16%	5053
	21st to end	Night	43% 3 6% 22% 20% 7%	5453
April	1st to 10th	Night	34% 6% 11% 21% 20% 8%	2000
	11th to 20th	Night	34% 4 7% 20% 23% 11%	1975
	21st to end	Night	33% 5% 11% 25% 20% 5%	1955
May	1st to 10th	Night	42% 9% 14% 19% 14% 2	4060
	11th to 20th	Night	50% 7% 13% 20% 8% 1	4033
	21st to end	Night	36% 9% 13% 25% 16% 1	4429
June	1st to 10th	Night	34% 12% 16% 21% 16% 2	1800
	11th to 20th	Night	24% 12% 22% 23% 17% 2	1800
	21st to end	Night	29% 10% 18% 22% 16% 4	1800
July	1st to 10th	Night	38% 13% 22% 18% 7% 1	2885
	11th to 20th	Night	32% 13% 24% 23% 6% 2	2876
	21st to end	Night	53% 13% 14% 14% 6% 1	2988
August	1st to 10th	Night	23% 8% 16% 39% 13% 1	3031
	11th to 20th	Night	29% 7% 17% 29% 15% 2	3130
	21st to end	Night	27% 7% 14% 19% 27% 5%	3476
September	1st to 10th	Night	33% 10% 17% 18% 16% 6%	1978
	11th to 20th	Night	36% 9% 10% 21% 21% 3	2090
	21st to end	Night	30% 6% 13% 21% 23% 7%	2090
October	1st to 10th	Night	23% 3 7% 16% 33% 19%	3480
	11th to 20th	Night	19% 2 4 16% 30% 28%	3477
	21st to end	Night	22% 2 5% 24% 33% 15%	4299
November	1st to 10th	Night	30% 4 10% 19% 24% 13%	2536
	11th to 20th	Night	32% 3 7% 19% 28% 11%	2600
	21st to end	Night	37% 4 7% 20% 24% 8%	2600
December	1st to 10th	Night	41% 4 5% 21% 27% 4	4250
	11th to 20th	Night	43% 3 9% 16% 24% 4	4824
	21st to end	Night	46% 3 8% 24% 17% 2	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	23% 25% 37% 13% 2	300
	11th to 20th	Day	25% 22% 32% 16% 4 1	300
	21st to end	Day	24% 25% 35% 12% 3 1	330
February	1st to 10th	Day	25% 20% 36% 15% 5%	300
	11th to 20th	Day	36% 22% 28% 12% 3	300
	21st to end	Day	26% 25% 36% 12% 1	249
March	1st to 10th	Day	25% 25% 31% 14% 4 1	300
	11th to 20th	Day	23% 19% 33% 17% 7% 1	465
	21st to end	Day	22% 15% 35% 21% 7%	550
April	1st to 10th	Day	21% 15% 32% 19% 11% 2	500
	11th to 20th	Day	18% 13% 31% 26% 11% 1	492
	21st to end	Day	14% 10% 31% 30% 13% 2	500
May	1st to 10th	Day	15% 10% 28% 24% 17% 5%	503
	11th to 20th	Day	12% 6% 23% 31% 18% 9%	527
	21st to end	Day	10% 6% 21% 30% 19% 13%	573
June	1st to 10th	Day	13% 5% 15% 31% 23% 12%	530
	11th to 20th	Day	7% 3 15% 26% 27% 21%	530
	21st to end	Day	11% 4 12% 23% 29% 21%	530
July	1st to 10th	Day	9% 2 8% 25% 30% 25%	525
	11th to 20th	Day	6% 1 6% 22% 34% 31%	524
	21st to end	Day	8% 3 10% 23% 33% 24%	532
August	1st to 10th	Day	8% 2 11% 21% 34% 24%	486
	11th to 20th	Day	8% 4 10% 27% 33% 18%	479
	21st to end	Day	15% 2 9% 21% 34% 19%	539
September	1st to 10th	Day	8% 3 10% 30% 35% 14%	490
	11th to 20th	Day	14% 4 13% 24% 32% 13%	490
	21st to end	Day	13% 3 17% 26% 30% 10%	490
October	1st to 10th	Day	10% 4 22% 33% 20% 10%	490
	11th to 20th	Day	11% 8% 23% 31% 23% 3	490
	21st to end	Day	16% 10% 20% 33% 19% 2	430
November	1st to 10th	Day	20% 9% 27% 30% 10% 4	300
	11th to 20th	Day	16% 14% 38% 24% 8% 1	300
	21st to end	Day	14% 19% 34% 22% 9% 1	300
December	1st to 10th	Day	27% 19% 32% 18% 4 1	300
	11th to 20th	Day	22% 17% 34% 19% 6% 1	300
	21st to end	Day	27% 18% 31% 19% 5% 1	330
January	1st to 10th	Night	22% 17% 32% 26% 4	800
	11th to 20th	Night	23% 16% 30% 24% 7%	800
	21st to end	Night	22% 15% 34% 25% 3	880
February	1st to 10th	Night	21% 12% 30% 32% 5%	800
	11th to 20th	Night	26% 17% 31% 24% 3	783
	21st to end	Night	22% 16% 34% 25% 2	660
March	1st to 10th	Night	21% 14% 31% 29% 5%	800
	11th to 20th	Night	19% 12% 35% 25% 9% 1	800
	21st to end	Night	19% 13% 30% 28% 9%	878
April	1st to 10th	Night	15% 11% 34% 28% 12% 1	600
	11th to 20th	Night	16% 11% 31% 30% 12% 1	593
	21st to end	Night	12% 7% 24% 40% 17% 1	600
May	1st to 10th	Night	11% 5% 26% 33% 22% 3	600
	11th to 20th	Night	12% 5% 19% 34% 26% 5%	591
	21st to end	Night	10% 2 11% 38% 33% 6%	650
June	1st to 10th	Night	7% 2 12% 37% 37% 5%	600
	11th to 20th	Night	4 1 11% 31% 39% 14%	600
	21st to end	Night	7% 1 9% 28% 45% 10%	600
July	1st to 10th	Night	8% 2 6% 23% 46% 15%	585
	11th to 20th	Night	5% 4 22% 51% 17%	582
	21st to end	Night	7% 1 7% 26% 45% 14%	624
August	1st to 10th	Night	6% 2 6% 27% 45% 14%	567
	11th to 20th	Night	10% 2 7% 28% 44% 9%	564
	21st to end	Night	14% 1 7% 26% 40% 11%	627
September	1st to 10th	Night	9% 2 8% 29% 41% 11%	648
	11th to 20th	Night	13% 3 12% 28% 38% 5%	760
	21st to end	Night	10% 2 15% 34% 36% 3	760
October	1st to 10th	Night	10% 4 20% 35% 26% 5%	760
	11th to 20th	Night	12% 7% 24% 34% 21% 2	760
	21st to end	Night	15% 8% 20% 37% 18% 2	836
November	1st to 10th	Night	13% 7% 21% 37% 21% 2	780
	11th to 20th	Night	17% 9% 28% 33% 13% 1	800
	21st to end	Night	17% 11% 26% 34% 11% 1	800
December	1st to 10th	Night	22% 13% 29% 29% 8% 1	800
	11th to 20th	Night	25% 10% 28% 29% 8% 1	800
	21st to end	Night	24% 14% 27% 28% 7%	880

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