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 2025-04-11.

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% 7% 2 6% 4 4 4 4 5% 7% 41%	113276
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 43%	114976
Transparency	Day	41% 6% 11% 23% 17% 3	116851
	Night	42% 5% 11% 23% 17% 3	119565
Seeing	Day	20% 13% 31% 23% 11% 3	15479
	Night	20% 10% 31% 28% 9% 1	24755

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 4 3 4 4 3 5% 6% 50%	3597
	11th to 20th	Day	14% 9% 3 6% 4 3 5% 3 4 6% 42%	3570
	21st to end	Day	19% 9% 4 7% 4 4 4 2 4 4 40%	4282
February	1st to 10th	Day	18% 8% 2 6% 4 3 5% 3 5% 5% 40%	1818
	11th to 20th	Day	15% 7% 2 6% 4 4 4 3 4 6% 44%	1948
	21st to end	Day	20% 8% 2 5% 4 3 4 4 5% 6% 40%	1650
March	1st to 10th	Day	22% 7% 2 5% 3 3 3 4 4 5% 42%	4476
	11th to 20th	Day	12% 7% 2 7% 5% 4 4 4 5% 8% 42%	4745
	21st to end	Day	19% 6% 2 5% 4 4 3 3 5% 6% 42%	5942
April	1st to 10th	Day	20% 7% 3 6% 4 3 4 3 6% 7% 38%	2400
	11th to 20th	Day	17% 6% 2 6% 4 4 4 3 5% 7% 42%	2254
	21st to end	Day	19% 6% 2 4 4 5% 4 3 5% 8% 40%	2311
May	1st to 10th	Day	28% 9% 2 5% 3 3 3 3 5% 8% 30%	4106
	11th to 20th	Day	24% 8% 3 6% 5% 5% 4 3 5% 6% 31%	4356
	21st to end	Day	15% 5% 2 5% 4 3 4 4 5% 9% 44%	4837
June	1st to 10th	Day	16% 5% 2 5% 4 4 4 5% 7% 9% 37%	2660
	11th to 20th	Day	14% 7% 3 6% 5% 5% 5% 4 6% 8% 37%	2660
	21st to end	Day	12% 7% 2 6% 5% 4 6% 4 7% 9% 39%	2660
July	1st to 10th	Day	10% 5% 3 7% 6% 5% 6% 4 7% 10% 36%	4440
	11th to 20th	Day	12% 6% 2 6% 5% 5% 5% 5% 7% 10% 38%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 6% 5% 6% 8% 31%	4814
August	1st to 10th	Day	21% 6% 2 6% 5% 5% 5% 4 7% 8% 32%	4180
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 9% 41%	3730
	21st to end	Day	10% 5% 2 6% 4 5% 5% 3 6% 8% 47%	4180
September	1st to 10th	Day	13% 5% 2 6% 5% 5% 5% 4 5% 6% 44%	2280
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 5% 7% 49%	2192
	21st to end	Day	18% 6% 3 7% 5% 4 5% 3 5% 6% 40%	2090
October	1st to 10th	Day	21% 7% 2 5% 3 3 3 4 5% 7% 42%	1995
	11th to 20th	Day	15% 5% 2 6% 3 3 3 3 4 6% 49%	1900
	21st to end	Day	21% 5% 2 5% 4 3 3 3 4 5% 44%	1930
November	1st to 10th	Day	14% 6% 3 4 4 3 2 3 4 6% 52%	1761
	11th to 20th	Day	18% 5% 2 4 3 3 3 2 5% 6% 48%	1800
	21st to end	Day	16% 6% 2 5% 4 3 5% 3 5% 7% 44%	1726
December	1st to 10th	Day	16% 7% 2 6% 4 3 3 2 5% 6% 46%	2600
	11th to 20th	Day	24% 6% 2 4 2 2 2 3 4 6% 45%	2986
	21st to end	Day	19% 7% 2 7% 4 4 4 3 5% 6% 39%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 3 3 5% 6% 49%	6349
	11th to 20th	Night	14% 8% 2 6% 3 4 4 3 4 5% 47%	6315
	21st to end	Night	18% 9% 2 6% 3 4 4 3 5% 5% 40%	6656
February	1st to 10th	Night	17% 9% 3 5% 4 4 4 3 4 5% 42%	2580
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 6% 41%	2338
	21st to end	Night	19% 8% 2 6% 4 4 4 3 5% 5% 41%	1980
March	1st to 10th	Night	21% 9% 2 5% 3 3 3 3 4 5% 43%	5364
	11th to 20th	Night	15% 9% 2 5% 4 3 4 3 4 5% 45%	5209
	21st to end	Night	20% 8% 2 6% 4 4 4 3 4 5% 40%	4976
April	1st to 10th	Night	19% 8% 2 7% 4 5% 4 3 6% 7% 36%	2000
	11th to 20th	Night	19% 7% 2 5% 4 3 4 4 5% 6% 40%	1882
	21st to end	Night	20% 6% 2 6% 3 3 4 3 4 6% 42%	1867
May	1st to 10th	Night	25% 10% 3 8% 3 4 3 3 5% 6% 30%	2834
	11th to 20th	Night	25% 9% 3 7% 4 4 3 3 3 6% 33%	2567
	21st to end	Night	17% 7% 2 5% 4 3 4 3 5% 9% 41%	2738
June	1st to 10th	Night	18% 6% 1 4 4 5% 6% 3 5% 8% 41%	1520
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 3 6% 9% 41%	1520
	21st to end	Night	11% 5% 2 7% 4 4 5% 3 7% 9% 42%	1520
July	1st to 10th	Night	10% 6% 2 8% 5% 6% 6% 4 7% 10% 37%	2520
	11th to 20th	Night	15% 5% 2 6% 4 5% 6% 4 8% 9% 36%	2520
	21st to end	Night	19% 8% 2 5% 4 4 5% 3 7% 7% 36%	2739
August	1st to 10th	Night	24% 6% 2 6% 4 4 5% 4 5% 7% 32%	2713
	11th to 20th	Night	14% 7% 2 7% 5% 4 4 4 7% 7% 39%	3097
	21st to end	Night	10% 5% 2 5% 3 4 4 4 5% 8% 51%	3476
September	1st to 10th	Night	15% 6% 2 6% 4 5% 4 4 4 7% 44%	1900
	11th to 20th	Night	14% 6% 1 5% 3 3 3 3 4 6% 52%	1987
	21st to end	Night	17% 7% 3 6% 4 4 3 3 4 6% 44%	2090
October	1st to 10th	Night	19% 7% 2 7% 4 4 3 2 4 7% 41%	2182
	11th to 20th	Night	15% 5% 2 5% 4 3 3 3 5% 6% 50%	2280
	21st to end	Night	21% 6% 3 5% 3 4 3 3 4 5% 44%	2666
November	1st to 10th	Night	13% 5% 2 5% 4 4 4 3 5% 6% 51%	2537
	11th to 20th	Night	19% 6% 2 4 3 2 3 3 4 5% 48%	2600
	21st to end	Night	16% 6% 2 5% 4 4 3 4 6% 7% 46%	2670
December	1st to 10th	Night	16% 7% 2 5% 3 3 3 3 4 5% 48%	4580
	11th to 20th	Night	26% 7% 2 4 3 3 3 2 3 4 43%	5208
	21st to end	Night	20% 9% 3 6% 4 4 4 3 4 5% 41%	6996

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	34% 2 8% 32% 21% 3	3598
	11th to 20th	Day	43% 3 9% 22% 17% 5%	3600
	21st to end	Day	49% 4 9% 24% 14% 1	4282
February	1st to 10th	Day	45% 5% 13% 23% 13% 2	1818
	11th to 20th	Day	43% 4 9% 25% 17% 3	1957
	21st to end	Day	45% 4 11% 22% 15% 3	1650
March	1st to 10th	Day	43% 4 11% 20% 14% 8%	4479
	11th to 20th	Day	38% 6% 13% 28% 12% 4	4745
	21st to end	Day	43% 5% 9% 13% 25% 5%	5942
April	1st to 10th	Day	45% 5% 10% 20% 16% 5%	2400
	11th to 20th	Day	42% 4 12% 22% 17% 3	2254
	21st to end	Day	43% 5% 10% 21% 17% 4	2311
May	1st to 10th	Day	53% 6% 13% 15% 13% 1	4120
	11th to 20th	Day	50% 8% 12% 19% 9% 2	4360
	21st to end	Day	35% 7% 12% 26% 19% 2	4837
June	1st to 10th	Day	40% 7% 13% 22% 16% 2	2660
	11th to 20th	Day	42% 8% 12% 25% 12% 2	2660
	21st to end	Day	38% 8% 15% 22% 16% 1	2660
July	1st to 10th	Day	41% 10% 12% 20% 16% 1	4370
	11th to 20th	Day	38% 8% 17% 23% 13% 1	4365
	21st to end	Day	47% 8% 12% 16% 15% 2	4661
August	1st to 10th	Day	47% 8% 15% 21% 9%	4166
	11th to 20th	Day	37% 7% 12% 30% 12% 1	3766
	21st to end	Day	33% 7% 10% 28% 20% 2	4180
September	1st to 10th	Day	38% 7% 11% 23% 19% 3	2280
	11th to 20th	Day	32% 6% 11% 24% 24% 2	2192
	21st to end	Day	44% 6% 9% 19% 19% 3	2090
October	1st to 10th	Day	37% 5% 10% 19% 23% 6%	3320
	11th to 20th	Day	26% 4 10% 23% 32% 5%	3140
	21st to end	Day	28% 4 14% 26% 25% 4	3155
November	1st to 10th	Day	35% 4 11% 23% 24% 3	1761
	11th to 20th	Day	38% 4 9% 24% 22% 4	1800
	21st to end	Day	39% 4 14% 29% 11% 2	1726
December	1st to 10th	Day	39% 4 15% 28% 12% 1	2600
	11th to 20th	Day	42% 3 13% 22% 17% 2	2986
	21st to end	Day	46% 5% 8% 23% 15% 3	3960
January	1st to 10th	Night	37% 3 8% 26% 23% 4	6349
	11th to 20th	Night	40% 3 13% 22% 16% 5%	6360
	21st to end	Night	46% 3 9% 21% 17% 4	6656
February	1st to 10th	Night	45% 5% 12% 24% 12% 2	2580
	11th to 20th	Night	45% 5% 8% 20% 18% 5%	2340
	21st to end	Night	44% 4 8% 24% 17% 4	1980
March	1st to 10th	Night	44% 4 9% 21% 13% 9%	5374
	11th to 20th	Night	40% 6% 13% 24% 12% 4	5210
	21st to end	Night	47% 5% 6% 14% 23% 5%	4976
April	1st to 10th	Night	47% 5% 9% 20% 16% 4	2000
	11th to 20th	Night	45% 6% 9% 19% 21% 2	1882
	21st to end	Night	42% 6% 9% 21% 19% 4	1867
May	1st to 10th	Night	55% 6% 10% 17% 10% 1	2840
	11th to 20th	Night	53% 7% 14% 14% 10% 2	2568
	21st to end	Night	41% 6% 13% 23% 14% 3	2739
June	1st to 10th	Night	41% 7% 9% 21% 20% 2	1520
	11th to 20th	Night	38% 7% 13% 29% 12% 1	1520
	21st to end	Night	36% 8% 14% 25% 16% 1	1520
July	1st to 10th	Night	43% 9% 14% 18% 14% 2	2480
	11th to 20th	Night	42% 9% 13% 24% 11% 1	2472
	21st to end	Night	45% 6% 14% 15% 17% 2	2651
August	1st to 10th	Night	51% 8% 15% 16% 10% 1	2703
	11th to 20th	Night	42% 7% 12% 28% 11% 1	3127
	21st to end	Night	31% 6% 11% 31% 19% 1	3476
September	1st to 10th	Night	40% 7% 9% 23% 19% 1	1900
	11th to 20th	Night	33% 5% 11% 24% 24% 3	1987
	21st to end	Night	42% 6% 8% 21% 21% 2	2090
October	1st to 10th	Night	39% 5% 11% 23% 20% 2	3637
	11th to 20th	Night	24% 4 10% 27% 31% 4	3779
	21st to end	Night	30% 4 12% 29% 22% 2	4391
November	1st to 10th	Night	35% 4 11% 24% 22% 4	2537
	11th to 20th	Night	40% 4 7% 24% 21% 5%	2600
	21st to end	Night	38% 5% 12% 28% 15% 2	2670
December	1st to 10th	Night	39% 4 11% 30% 13% 2	4580
	11th to 20th	Night	46% 3 9% 22% 16% 3	5208
	21st to end	Night	46% 5% 9% 20% 17% 4	6996

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	24% 28% 29% 15% 5%	200
	11th to 20th	Day	26% 24% 38% 11% 2 1	200
	21st to end	Day	21% 20% 40% 12% 6% 1	290
February	1st to 10th	Day	27% 19% 39% 13% 2	318
	11th to 20th	Day	26% 20% 37% 16% 2	496
	21st to end	Day	26% 17% 37% 16% 4	414
March	1st to 10th	Day	24% 21% 31% 20% 4	500
	11th to 20th	Day	20% 22% 36% 16% 5% 1	500
	21st to end	Day	28% 19% 32% 14% 6% 1	550
April	1st to 10th	Day	28% 20% 33% 12% 6% 1	500
	11th to 20th	Day	24% 17% 33% 16% 8% 2	452
	21st to end	Day	27% 15% 34% 17% 5% 2	460
May	1st to 10th	Day	30% 18% 27% 17% 6% 2	463
	11th to 20th	Day	22% 12% 33% 23% 9% 1	487
	21st to end	Day	19% 6% 29% 26% 16% 3	529
June	1st to 10th	Day	17% 10% 28% 28% 13% 3	490
	11th to 20th	Day	16% 11% 31% 29% 10% 3	490
	21st to end	Day	14% 9% 31% 27% 16% 3	490
July	1st to 10th	Day	12% 5% 24% 34% 19% 6%	485
	11th to 20th	Day	11% 3 26% 32% 23% 5%	484
	21st to end	Day	14% 7% 27% 30% 17% 4	532
August	1st to 10th	Day	14% 5% 22% 35% 17% 7%	486
	11th to 20th	Day	14% 6% 20% 34% 20% 5%	479
	21st to end	Day	12% 6% 27% 31% 17% 7%	539
September	1st to 10th	Day	17% 7% 27% 25% 17% 7%	490
	11th to 20th	Day	14% 5% 28% 28% 15% 10%	490
	21st to end	Day	25% 9% 28% 24% 10% 5%	490
October	1st to 10th	Day	25% 7% 27% 20% 15% 5%	490
	11th to 20th	Day	17% 14% 31% 24% 11% 2	490
	21st to end	Day	23% 13% 30% 22% 10% 2	454
November	1st to 10th	Day	15% 13% 35% 24% 12% 2	395
	11th to 20th	Day	22% 17% 33% 23% 5% 1	400
	21st to end	Day	19% 16% 36% 22% 6% 1	326
December	1st to 10th	Day	21% 25% 32% 17% 5% 1	200
	11th to 20th	Day	31% 21% 35% 10% 5%	200
	21st to end	Day	25% 17% 35% 13% 10%	220
January	1st to 10th	Night	19% 17% 41% 20% 4	800
	11th to 20th	Night	25% 15% 33% 23% 3	800
	21st to end	Night	20% 15% 40% 23% 2	880
February	1st to 10th	Night	22% 13% 38% 25% 3	800
	11th to 20th	Night	23% 18% 38% 20% 1	783
	21st to end	Night	23% 13% 37% 23% 3	660
March	1st to 10th	Night	22% 15% 37% 21% 5%	800
	11th to 20th	Night	22% 15% 36% 22% 4 1	800
	21st to end	Night	23% 13% 37% 22% 4	880
April	1st to 10th	Night	22% 16% 35% 22% 4	800
	11th to 20th	Night	22% 12% 34% 26% 6%	754
	21st to end	Night	22% 12% 31% 27% 6% 1	727
May	1st to 10th	Night	23% 12% 34% 23% 8% 1	570
	11th to 20th	Night	23% 9% 33% 28% 7%	420
	21st to end	Night	20% 4 21% 40% 14% 1	411
June	1st to 10th	Night	19% 6% 25% 36% 14%	380
	11th to 20th	Night	14% 4 23% 42% 16% 1	380
	21st to end	Night	11% 3 23% 49% 13% 1	380
July	1st to 10th	Night	12% 4 22% 43% 19% 1	370
	11th to 20th	Night	14% 2 18% 42% 23% 1	368
	21st to end	Night	15% 3 20% 44% 17% 1	416
August	1st to 10th	Night	14% 2 19% 43% 20% 2	494
	11th to 20th	Night	15% 3 19% 37% 24% 2	750
	21st to end	Night	15% 3 19% 37% 24% 2	836
September	1st to 10th	Night	17% 4 23% 35% 19% 2	760
	11th to 20th	Night	17% 3 24% 31% 20% 4	760
	21st to end	Night	21% 4 26% 33% 15% 2	760
October	1st to 10th	Night	23% 8% 27% 24% 16% 3	760
	11th to 20th	Night	17% 10% 30% 31% 10% 1	760
	21st to end	Night	24% 10% 33% 24% 8%	836
November	1st to 10th	Night	14% 11% 34% 27% 12% 1	780
	11th to 20th	Night	23% 13% 32% 25% 7%	800
	21st to end	Night	20% 13% 34% 26% 6% 1	800
December	1st to 10th	Night	25% 13% 34% 22% 5% 1	800
	11th to 20th	Night	24% 14% 34% 24% 4	800
	21st to end	Night	24% 13% 35% 25% 3	880

Page generated at 2025-04-08 09:17:30. Version ac07

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