Broadway Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Broadway Observatory clear sky chart from 2005-11-06 to 2026-08-15.

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% 6% 2 6% 4 4 4 3 5% 7% 40%	143856
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 42%	138015
Transparency	Day	41% 5% 11% 23% 17% 2	147385
	Night	42% 5% 10% 23% 17% 3	142575
Seeing	Day	20% 13% 31% 23% 11% 3	17274
	Night	20% 11% 31% 28% 9% 1	26343

By Month

Percentage Cloud Cover NE (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	13% 6% 2 5% 4 4 4 3 5% 6% 48%	4661
	11th to 20th	Day	16% 9% 2 6% 4 3 4 3 4 5% 45%	4650
	21st to end	Day	16% 7% 3 6% 4 4 4 3 5% 6% 42%	5541
February	1st to 10th	Day	17% 8% 2 6% 5% 3 5% 3 5% 5% 41%	1909
	11th to 20th	Day	16% 7% 2 6% 4 4 4 3 4 6% 43%	2048
	21st to end	Day	19% 8% 2 5% 4 3 4 4 5% 6% 40%	1730
March	1st to 10th	Day	22% 7% 2 5% 3 3 3 4 4 5% 42%	4576
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 4 5% 7% 42%	4850
	21st to end	Day	19% 6% 2 5% 4 4 4 3 5% 6% 42%	6073
April	1st to 10th	Day	20% 7% 3 5% 4 3 4 3 6% 6% 39%	2520
	11th to 20th	Day	17% 6% 2 6% 4 4 4 3 5% 7% 41%	2494
	21st to end	Day	19% 6% 2 5% 4 5% 4 3 6% 8% 40%	2555
May	1st to 10th	Day	26% 7% 2 5% 3 3 4 3 5% 7% 33%	7639
	11th to 20th	Day	20% 9% 2 6% 4 4 4 3 4 6% 39%	8138
	21st to end	Day	24% 6% 2 6% 4 4 5% 4 5% 7% 34%	9060
June	1st to 10th	Day	16% 5% 2 5% 4 4 4 5% 7% 9% 38%	2940
	11th to 20th	Day	15% 7% 3 5% 5% 5% 5% 4 6% 8% 38%	2940
	21st to end	Day	12% 7% 2 6% 5% 4 5% 4 7% 9% 39%	2940
July	1st to 10th	Day	14% 5% 2 6% 5% 5% 6% 4 6% 9% 37%	8280
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 5% 7% 11% 38%	8280
	21st to end	Day	14% 6% 2 6% 5% 4 5% 4 6% 8% 39%	8993
August	1st to 10th	Day	20% 5% 2 6% 5% 5% 5% 4 7% 8% 32%	4444
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 9% 41%	3898
	21st to end	Day	10% 5% 2 6% 4 5% 5% 3 6% 8% 47%	4312
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 7% 45%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 50%	2307
	21st to end	Day	18% 6% 2 7% 5% 4 5% 3 5% 6% 39%	2200
October	1st to 10th	Day	20% 6% 2 5% 3 2 3 4 5% 6% 43%	2100
	11th to 20th	Day	16% 5% 2 6% 3 3 4 3 4 7% 47%	2000
	21st to end	Day	20% 5% 2 5% 4 3 4 3 4 6% 45%	2031
November	1st to 10th	Day	14% 6% 2 4 4 3 2 3 4 6% 52%	1851
	11th to 20th	Day	18% 5% 2 4 3 3 4 2 5% 6% 48%	1890
	21st to end	Day	17% 6% 2 5% 4 4 5% 3 5% 6% 44%	1812
December	1st to 10th	Day	16% 7% 2 6% 4 3 3 2 5% 6% 45%	2680
	11th to 20th	Day	23% 6% 2 4 3 2 2 3 4 7% 45%	3066
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 6% 38%	4048
January	1st to 10th	Night	13% 8% 3 6% 4 4 3 3 5% 6% 46%	8234
	11th to 20th	Night	16% 9% 2 6% 3 4 3 2 4 5% 45%	8235
	21st to end	Night	16% 9% 2 5% 3 4 4 3 5% 5% 44%	8622
February	1st to 10th	Night	17% 9% 3 5% 4 4 4 3 4 5% 42%	2709
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	2458
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 5% 41%	2076
March	1st to 10th	Night	21% 9% 2 5% 3 3 3 3 4 5% 43%	5484
	11th to 20th	Night	15% 9% 2 5% 4 3 4 3 4 6% 45%	5324
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 5% 40%	5087
April	1st to 10th	Night	20% 8% 2 6% 4 4 3 3 6% 6% 37%	2100
	11th to 20th	Night	19% 7% 2 6% 4 4 4 4 5% 7% 39%	2075
	21st to end	Night	19% 6% 2 6% 3 3 4 3 4 6% 43%	2063
May	1st to 10th	Night	25% 8% 3 7% 4 4 4 3 5% 6% 31%	5259
	11th to 20th	Night	20% 9% 3 6% 4 4 4 3 4 7% 37%	4765
	21st to end	Night	25% 8% 3 6% 4 4 4 3 4 7% 33%	5114
June	1st to 10th	Night	17% 6% 1 4 4 5% 5% 3 5% 7% 42%	1680
	11th to 20th	Night	14% 6% 2 6% 5% 5% 5% 3 7% 8% 39%	1680
	21st to end	Night	11% 5% 2 7% 4 4 5% 3 7% 9% 42%	1680
July	1st to 10th	Night	14% 7% 2 8% 5% 5% 5% 4 6% 10% 37%	4680
	11th to 20th	Night	14% 7% 2 7% 4 5% 6% 4 7% 9% 36%	4680
	21st to end	Night	15% 6% 2 5% 4 4 4 3 6% 8% 42%	5084
August	1st to 10th	Night	23% 6% 2 6% 4 4 5% 4 5% 7% 33%	2887
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 4 7% 7% 39%	3238
	21st to end	Night	10% 5% 2 5% 3 4 4 4 5% 8% 52%	3586
September	1st to 10th	Night	15% 6% 2 6% 4 5% 4 4 4 6% 44%	2000
	11th to 20th	Night	13% 6% 1 5% 3 3 4 3 4 6% 52%	2092
	21st to end	Night	17% 7% 3 6% 4 4 3 3 4 7% 43%	2200
October	1st to 10th	Night	18% 7% 2 7% 4 4 3 2 4 7% 42%	2297
	11th to 20th	Night	16% 5% 2 5% 4 3 3 3 5% 6% 50%	2400
	21st to end	Night	20% 6% 3 5% 3 3 3 3 4 5% 44%	2807
November	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 51%	2667
	11th to 20th	Night	19% 6% 2 4 3 2 3 3 4 5% 48%	2730
	21st to end	Night	15% 6% 2 5% 4 4 3 4 5% 7% 46%	2804
December	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 5% 48%	4720
	11th to 20th	Night	26% 7% 2 4 3 3 3 2 3 4 43%	5348
	21st to end	Night	20% 9% 2 6% 4 4 4 3 4 5% 40%	7150

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	37% 2 10% 30% 19% 2	4662
	11th to 20th	Day	42% 3 9% 22% 18% 6%	4680
	21st to end	Day	44% 3 11% 25% 16% 1	5541
February	1st to 10th	Day	44% 5% 13% 24% 13% 2	1909
	11th to 20th	Day	43% 4 9% 25% 17% 2	2057
	21st to end	Day	45% 4 11% 22% 15% 3	1730
March	1st to 10th	Day	43% 4 11% 20% 14% 8%	4579
	11th to 20th	Day	38% 6% 13% 28% 12% 3	4850
	21st to end	Day	43% 5% 9% 13% 24% 5%	6073
April	1st to 10th	Day	45% 5% 10% 20% 16% 4	2520
	11th to 20th	Day	41% 4 13% 21% 18% 3	2494
	21st to end	Day	42% 5% 10% 22% 17% 4	2555
May	1st to 10th	Day	50% 6% 12% 17% 14% 1	7651
	11th to 20th	Day	45% 6% 10% 23% 15% 1	8142
	21st to end	Day	48% 5% 11% 21% 14% 1	9061
June	1st to 10th	Day	40% 7% 12% 22% 17% 2	2940
	11th to 20th	Day	42% 7% 12% 25% 13% 2	2940
	21st to end	Day	39% 8% 15% 21% 16% 1	2940
July	1st to 10th	Day	43% 9% 12% 20% 16% 1	8210
	11th to 20th	Day	38% 7% 16% 26% 13% 1	8205
	21st to end	Day	40% 6% 10% 22% 20% 1	8795
August	1st to 10th	Day	46% 7% 15% 21% 9% 1	4430
	11th to 20th	Day	37% 7% 12% 30% 12% 1	3934
	21st to end	Day	33% 7% 10% 28% 21% 2	4312
September	1st to 10th	Day	37% 7% 11% 23% 19% 3	2400
	11th to 20th	Day	32% 6% 11% 24% 24% 2	2307
	21st to end	Day	45% 6% 9% 18% 19% 3	2200
October	1st to 10th	Day	36% 5% 10% 20% 24% 6%	3425
	11th to 20th	Day	27% 4 10% 23% 31% 5%	3240
	21st to end	Day	28% 4 13% 26% 25% 3	3256
November	1st to 10th	Day	35% 4 12% 23% 23% 3	1851
	11th to 20th	Day	38% 4 9% 24% 22% 4	1890
	21st to end	Day	40% 4 14% 28% 11% 2	1812
December	1st to 10th	Day	39% 4 15% 28% 12% 1	2680
	11th to 20th	Day	42% 3 13% 22% 17% 2	3066
	21st to end	Day	46% 5% 7% 23% 15% 3	4048
January	1st to 10th	Night	40% 3 11% 24% 19% 3	8234
	11th to 20th	Night	43% 3 11% 21% 17% 6%	8280
	21st to end	Night	43% 3 8% 21% 20% 4	8622
February	1st to 10th	Night	44% 5% 11% 25% 13% 2	2709
	11th to 20th	Night	46% 4 8% 20% 17% 5%	2460
	21st to end	Night	44% 4 8% 24% 16% 4	2076
March	1st to 10th	Night	44% 4 9% 21% 13% 9%	5494
	11th to 20th	Night	40% 6% 13% 24% 12% 4	5325
	21st to end	Night	47% 5% 6% 14% 23% 5%	5087
April	1st to 10th	Night	47% 4 9% 19% 16% 4	2100
	11th to 20th	Night	44% 6% 9% 19% 20% 2	2075
	21st to end	Night	42% 5% 9% 20% 19% 4	2063
May	1st to 10th	Night	53% 5% 11% 17% 12% 1	5265
	11th to 20th	Night	48% 6% 12% 19% 13% 2	4766
	21st to end	Night	53% 4 10% 21% 10% 2	5115
June	1st to 10th	Night	40% 7% 9% 22% 21% 2	1680
	11th to 20th	Night	41% 7% 12% 27% 12% 1	1680
	21st to end	Night	37% 8% 14% 24% 15% 2	1680
July	1st to 10th	Night	44% 8% 12% 21% 14% 2	4640
	11th to 20th	Night	43% 7% 12% 24% 12% 1	4632
	21st to end	Night	39% 6% 14% 22% 18% 1	4967
August	1st to 10th	Night	50% 8% 14% 17% 10% 1	2877
	11th to 20th	Night	42% 7% 12% 28% 10% 1	3268
	21st to end	Night	31% 6% 11% 31% 19% 1	3586
September	1st to 10th	Night	41% 7% 8% 24% 19% 1	2000
	11th to 20th	Night	33% 5% 10% 23% 25% 3	2092
	21st to end	Night	42% 5% 8% 22% 21% 2	2200
October	1st to 10th	Night	39% 5% 10% 23% 20% 3	3752
	11th to 20th	Night	25% 4 10% 27% 30% 4	3899
	21st to end	Night	30% 4 12% 29% 23% 2	4532
November	1st to 10th	Night	35% 4 11% 24% 22% 4	2667
	11th to 20th	Night	39% 4 7% 24% 21% 5%	2730
	21st to end	Night	38% 5% 11% 28% 16% 2	2804
December	1st to 10th	Night	39% 4 11% 30% 13% 2	4720
	11th to 20th	Night	46% 3 9% 22% 16% 3	5348
	21st to end	Night	46% 5% 9% 20% 17% 4	7150

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% 27% 30% 15% 5%	220
	11th to 20th	Day	25% 26% 36% 10% 1	220
	21st to end	Day	19% 22% 40% 12% 6% 1	319
February	1st to 10th	Day	26% 18% 39% 15% 2	349
	11th to 20th	Day	26% 19% 36% 17% 2	536
	21st to end	Day	26% 16% 37% 17% 4	446
March	1st to 10th	Day	24% 20% 31% 21% 4	540
	11th to 20th	Day	20% 23% 35% 16% 5% 1	540
	21st to end	Day	27% 20% 33% 13% 6% 1	594
April	1st to 10th	Day	28% 20% 32% 13% 6% 1	540
	11th to 20th	Day	23% 18% 34% 17% 8% 2	532
	21st to end	Day	26% 15% 34% 19% 5% 2	540
May	1st to 10th	Day	29% 17% 29% 16% 6% 2	543
	11th to 20th	Day	20% 12% 35% 23% 9% 1	567
	21st to end	Day	22% 7% 30% 24% 14% 3	617
June	1st to 10th	Day	17% 11% 28% 28% 14% 3	570
	11th to 20th	Day	17% 12% 31% 28% 10% 3	570
	21st to end	Day	14% 9% 30% 28% 17% 3	570
July	1st to 10th	Day	13% 4 23% 35% 19% 6%	565
	11th to 20th	Day	11% 3 27% 31% 23% 5%	564
	21st to end	Day	13% 7% 26% 31% 18% 4	620
August	1st to 10th	Day	13% 5% 22% 34% 18% 7%	566
	11th to 20th	Day	14% 5% 21% 35% 18% 6%	535
	21st to end	Day	12% 5% 29% 31% 17% 7%	583
September	1st to 10th	Day	17% 7% 27% 25% 17% 6%	530
	11th to 20th	Day	14% 5% 27% 29% 16% 9%	530
	21st to end	Day	24% 8% 26% 26% 10% 5%	530
October	1st to 10th	Day	24% 7% 27% 21% 15% 7%	530
	11th to 20th	Day	19% 15% 31% 23% 10% 2	530
	21st to end	Day	22% 14% 30% 22% 9% 2	489
November	1st to 10th	Day	14% 14% 35% 23% 11% 2	425
	11th to 20th	Day	21% 18% 33% 23% 5%	430
	21st to end	Day	20% 16% 37% 21% 5% 1	352
December	1st to 10th	Day	20% 26% 32% 16% 5% 1	220
	11th to 20th	Day	29% 21% 35% 11% 4	220
	21st to end	Day	26% 18% 34% 12% 9%	242
January	1st to 10th	Night	19% 16% 41% 20% 4	840
	11th to 20th	Night	26% 16% 32% 22% 3	840
	21st to end	Night	20% 16% 40% 22% 2	924
February	1st to 10th	Night	21% 13% 37% 26% 3	840
	11th to 20th	Night	23% 17% 37% 21% 1	823
	21st to end	Night	23% 13% 37% 23% 3	692
March	1st to 10th	Night	22% 14% 37% 22% 5%	840
	11th to 20th	Night	22% 16% 36% 23% 4 1	840
	21st to end	Night	23% 13% 38% 22% 4	924
April	1st to 10th	Night	22% 16% 35% 22% 5%	840
	11th to 20th	Night	22% 12% 35% 25% 6%	831
	21st to end	Night	22% 12% 32% 28% 6% 1	803
May	1st to 10th	Night	23% 11% 35% 23% 7% 1	630
	11th to 20th	Night	22% 9% 33% 29% 7%	464
	21st to end	Night	22% 5% 21% 39% 13% 1	455
June	1st to 10th	Night	18% 6% 25% 36% 14%	420
	11th to 20th	Night	16% 5% 24% 40% 15%	420
	21st to end	Night	12% 3 22% 48% 14% 1	420
July	1st to 10th	Night	12% 4 22% 42% 20% 1	410
	11th to 20th	Night	14% 2 19% 42% 22% 1	408
	21st to end	Night	14% 3 20% 43% 18% 1	460
August	1st to 10th	Night	15% 2 18% 43% 20% 2	548
	11th to 20th	Night	15% 3 18% 38% 24% 2	807
	21st to end	Night	15% 3 20% 37% 23% 2	880
September	1st to 10th	Night	17% 4 23% 35% 19% 2	800
	11th to 20th	Night	17% 3 23% 33% 20% 4	800
	21st to end	Night	21% 4 25% 33% 15% 2	800
October	1st to 10th	Night	22% 7% 27% 25% 17% 3	800
	11th to 20th	Night	18% 11% 29% 30% 10% 1	800
	21st to end	Night	24% 11% 33% 24% 8%	880
November	1st to 10th	Night	15% 12% 34% 27% 11% 1	820
	11th to 20th	Night	22% 15% 32% 24% 7%	840
	21st to end	Night	19% 14% 34% 26% 6% 1	840
December	1st to 10th	Night	24% 14% 34% 21% 5% 1	840
	11th to 20th	Night	24% 15% 34% 23% 4	840
	21st to end	Night	24% 14% 35% 25% 2	924

Page generated at 2026-08-12 03:37:16. Version ac07

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