BX Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the BX Observatory clear sky chart from 2005-11-06 to 2026-01-17.

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	8% 9% 3 8% 5% 5% 5% 4 6% 8% 38%	131507
	Night	9% 11% 4 9% 6% 5% 5% 4 6% 7% 34%	128850
Transparency	Day	44% 8% 17% 21% 9% 2	131223
	Night	48% 9% 16% 18% 7% 1	128711
Seeing	Day	13% 13% 27% 28% 16% 4	16983
	Night	15% 9% 24% 32% 18% 2	25526

By Month

Percentage Cloud Cover NW (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	11% 19% 6% 12% 8% 7% 5% 5% 7% 6% 13%	3424
	11th to 20th	Day	7% 12% 3 7% 6% 5% 6% 6% 8% 9% 30%	3664
	21st to end	Day	8% 13% 4 11% 8% 8% 7% 4 6% 7% 25%	4004
February	1st to 10th	Day	10% 15% 4 14% 8% 6% 5% 5% 6% 5% 23%	1612
	11th to 20th	Day	8% 13% 6% 12% 6% 6% 5% 5% 6% 7% 27%	1755
	21st to end	Day	8% 17% 4 11% 7% 7% 7% 5% 7% 8% 21%	1636
March	1st to 10th	Day	5% 9% 4 10% 7% 6% 7% 6% 8% 9% 26%	4518
	11th to 20th	Day	4 11% 3 8% 6% 5% 6% 5% 8% 8% 36%	4726
	21st to end	Day	9% 14% 4 10% 6% 6% 6% 5% 7% 8% 26%	5942
April	1st to 10th	Day	9% 14% 5% 8% 7% 7% 6% 5% 6% 8% 25%	2400
	11th to 20th	Day	8% 11% 4 10% 7% 6% 6% 4 7% 7% 31%	2413
	21st to end	Day	10% 11% 4 9% 6% 6% 6% 5% 6% 8% 30%	2733
May	1st to 10th	Day	7% 8% 4 8% 5% 4 5% 4 6% 7% 43%	6320
	11th to 20th	Day	9% 10% 3 8% 5% 6% 6% 6% 8% 10% 28%	6277
	21st to end	Day	11% 9% 3 9% 6% 5% 5% 4 6% 8% 34%	7123
June	1st to 10th	Day	13% 9% 3 8% 6% 5% 5% 4 6% 7% 33%	3039
	11th to 20th	Day	15% 8% 4 8% 6% 6% 6% 5% 7% 7% 27%	3200
	21st to end	Day	9% 6% 3 7% 5% 5% 5% 4 7% 8% 41%	3200
July	1st to 10th	Day	5% 3 1 4 3 3 3 3 5% 7% 62%	7067
	11th to 20th	Day	4 2 1 3 3 3 3 3 4 7% 66%	6417
	21st to end	Day	4 3 1 3 3 3 4 4 6% 9% 59%	6874
August	1st to 10th	Day	2 2 1 3 3 4 4 4 7% 8% 61%	4541
	11th to 20th	Day	3 3 1 4 4 4 5% 4 7% 8% 56%	4470
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 6% 8% 49%	4386
September	1st to 10th	Day	7% 4 1 6% 3 4 4 3 6% 8% 53%	2400
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 4 6% 8% 45%	2367
	21st to end	Day	10% 8% 3 7% 5% 5% 6% 4 6% 8% 38%	2196
October	1st to 10th	Day	10% 8% 3 8% 5% 5% 5% 3 6% 7% 39%	3280
	11th to 20th	Day	19% 9% 3 8% 4 5% 4 3 5% 6% 34%	3130
	21st to end	Day	11% 9% 4 9% 6% 5% 5% 4 6% 7% 35%	3200
November	1st to 10th	Day	14% 13% 4 11% 6% 6% 7% 5% 6% 7% 21%	1757
	11th to 20th	Day	10% 14% 4 10% 7% 7% 7% 5% 6% 7% 22%	1680
	21st to end	Day	10% 16% 5% 12% 8% 7% 7% 5% 6% 7% 16%	1485
December	1st to 10th	Day	10% 11% 4 11% 8% 6% 5% 4 8% 7% 27%	2360
	11th to 20th	Day	10% 17% 5% 13% 8% 7% 6% 4 7% 7% 14%	2712
	21st to end	Day	9% 16% 4 11% 8% 7% 7% 5% 7% 7% 17%	3199
January	1st to 10th	Night	13% 22% 6% 12% 7% 6% 5% 4 6% 5% 13%	6777
	11th to 20th	Night	7% 12% 4 11% 7% 5% 6% 6% 5% 7% 29%	6366
	21st to end	Night	9% 13% 5% 12% 8% 7% 5% 4 5% 6% 27%	6952
February	1st to 10th	Night	12% 17% 5% 11% 8% 6% 5% 4 6% 5% 20%	2785
	11th to 20th	Night	10% 16% 5% 10% 5% 5% 4 4 5% 6% 30%	2532
	21st to end	Night	10% 17% 5% 11% 6% 5% 6% 4 5% 7% 26%	1993
March	1st to 10th	Night	6% 10% 4 9% 6% 6% 6% 5% 7% 10% 32%	5376
	11th to 20th	Night	5% 11% 4 9% 6% 5% 5% 4 7% 8% 37%	5270
	21st to end	Night	7% 12% 4 10% 6% 6% 7% 5% 7% 8% 27%	5033
April	1st to 10th	Night	8% 10% 3 8% 7% 6% 6% 4 7% 9% 33%	2000
	11th to 20th	Night	9% 10% 4 9% 7% 7% 6% 5% 6% 5% 31%	1936
	21st to end	Night	9% 9% 3 8% 7% 5% 5% 4 5% 7% 37%	1665
May	1st to 10th	Night	5% 9% 3 9% 6% 6% 6% 4 5% 7% 41%	3640
	11th to 20th	Night	8% 8% 4 9% 5% 6% 5% 5% 8% 9% 34%	3616
	21st to end	Night	8% 7% 3 7% 5% 5% 6% 4 6% 8% 41%	3747
June	1st to 10th	Night	11% 10% 3 6% 5% 5% 5% 4 6% 8% 36%	1356
	11th to 20th	Night	14% 7% 2 9% 5% 5% 5% 5% 7% 8% 33%	1200
	21st to end	Night	6% 7% 2 6% 4 6% 6% 6% 6% 9% 42%	1200
July	1st to 10th	Night	6% 3 1 4 4 4 5% 4 6% 7% 56%	2876
	11th to 20th	Night	3 3 1 4 3 4 3 3 5% 6% 65%	3542
	21st to end	Night	5% 4 1 4 3 4 4 3 6% 7% 61%	3969
August	1st to 10th	Night	3 1 1 4 2 3 4 5% 8% 11% 59%	2609
	11th to 20th	Night	3 3 2 6% 4 4 5% 4 6% 10% 54%	2577
	21st to end	Night	8% 5% 2 5% 3 4 4 4 6% 7% 51%	3528
September	1st to 10th	Night	6% 4 1 5% 4 4 4 3 6% 8% 55%	2000
	11th to 20th	Night	8% 5% 2 5% 4 3 4 4 6% 7% 51%	2032
	21st to end	Night	10% 6% 3 7% 5% 4 5% 3 5% 7% 45%	2200
October	1st to 10th	Night	8% 7% 3 8% 5% 5% 5% 4 6% 6% 43%	3920
	11th to 20th	Night	19% 7% 2 6% 4 4 3 3 5% 7% 40%	4024
	21st to end	Night	12% 10% 4 9% 6% 4 4 3 5% 6% 37%	4600
November	1st to 10th	Night	16% 15% 4 8% 6% 6% 5% 4 6% 6% 23%	2760
	11th to 20th	Night	13% 14% 4 10% 8% 7% 6% 4 5% 6% 22%	2940
	21st to end	Night	12% 17% 5% 11% 7% 6% 6% 5% 7% 6% 19%	3113
December	1st to 10th	Night	10% 11% 4 9% 6% 5% 5% 4 6% 7% 34%	5040
	11th to 20th	Night	14% 19% 6% 14% 8% 6% 6% 4 5% 5% 15%	5700
	21st to end	Night	11% 16% 5% 13% 7% 7% 7% 5% 5% 6% 18%	7976

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	67% 15% 11% 6% 1	3424
	11th to 20th	Day	45% 10% 27% 13% 3 2	3688
	21st to end	Day	58% 11% 17% 11% 3	4004
February	1st to 10th	Day	62% 11% 12% 9% 5% 1	1612
	11th to 20th	Day	56% 12% 14% 13% 5% 1	1764
	21st to end	Day	60% 12% 14% 12% 3	1636
March	1st to 10th	Day	53% 8% 15% 17% 6%	4518
	11th to 20th	Day	45% 8% 17% 26% 5%	4726
	21st to end	Day	57% 8% 14% 15% 6%	5942
April	1st to 10th	Day	56% 11% 15% 15% 3	2400
	11th to 20th	Day	52% 9% 15% 16% 8% 1	2413
	21st to end	Day	52% 9% 15% 17% 7%	2733
May	1st to 10th	Day	39% 8% 11% 26% 15% 1	6320
	11th to 20th	Day	48% 10% 18% 19% 5%	6280
	21st to end	Day	47% 8% 19% 22% 4	7124
June	1st to 10th	Day	50% 9% 12% 17% 10% 2	3039
	11th to 20th	Day	56% 8% 11% 18% 6% 1	3200
	21st to end	Day	41% 9% 16% 20% 12% 2	3200
July	1st to 10th	Day	23% 4 13% 26% 24% 10%	6991
	11th to 20th	Day	22% 3 17% 27% 22% 9%	6339
	21st to end	Day	23% 3 21% 31% 16% 5%	6681
August	1st to 10th	Day	20% 5% 13% 37% 21% 3	4524
	11th to 20th	Day	27% 4 27% 30% 10% 2	4512
	21st to end	Day	33% 5% 23% 29% 9% 1	4385
September	1st to 10th	Day	30% 8% 16% 32% 12% 3	2400
	11th to 20th	Day	35% 9% 16% 26% 13% 2	2367
	21st to end	Day	43% 9% 22% 18% 7% 1	2196
October	1st to 10th	Day	45% 7% 23% 21% 4	3280
	11th to 20th	Day	51% 7% 14% 25% 3	3130
	21st to end	Day	49% 7% 24% 17% 3	3200
November	1st to 10th	Day	63% 11% 13% 11% 3	1757
	11th to 20th	Day	62% 13% 14% 8% 3 1	1680
	21st to end	Day	64% 13% 14% 7% 1	1485
December	1st to 10th	Day	55% 10% 24% 11%	2360
	11th to 20th	Day	67% 12% 13% 7% 1	2714
	21st to end	Day	60% 10% 17% 11% 1	3199
January	1st to 10th	Night	70% 11% 12% 5% 2	6777
	11th to 20th	Night	51% 8% 24% 12% 5%	6404
	21st to end	Night	57% 11% 17% 14% 2	6952
February	1st to 10th	Night	63% 10% 11% 10% 4 1	2785
	11th to 20th	Night	55% 8% 16% 15% 6% 1	2534
	21st to end	Night	58% 9% 14% 12% 6%	1993
March	1st to 10th	Night	46% 8% 17% 22% 6%	5376
	11th to 20th	Night	44% 8% 18% 21% 8%	5271
	21st to end	Night	54% 10% 13% 17% 6%	5033
April	1st to 10th	Night	45% 15% 18% 17% 5%	2000
	11th to 20th	Night	51% 9% 14% 18% 7% 1	1936
	21st to end	Night	46% 10% 12% 23% 9%	1665
May	1st to 10th	Night	41% 7% 10% 23% 18% 1	3637
	11th to 20th	Night	42% 10% 18% 24% 5%	3616
	21st to end	Night	40% 10% 17% 27% 6% 1	3747
June	1st to 10th	Night	45% 9% 13% 19% 11% 3	1356
	11th to 20th	Night	47% 10% 12% 20% 10%	1200
	21st to end	Night	36% 13% 17% 22% 10% 2	1200
July	1st to 10th	Night	28% 4 13% 22% 20% 12%	2843
	11th to 20th	Night	22% 4 18% 24% 24% 9%	3497
	21st to end	Night	24% 3 15% 35% 16% 7%	3857
August	1st to 10th	Night	20% 5% 11% 36% 24% 3	2601
	11th to 20th	Night	29% 4 22% 28% 14% 2	2601
	21st to end	Night	33% 5% 23% 26% 13% 1	3528
September	1st to 10th	Night	28% 8% 15% 28% 18% 3	2000
	11th to 20th	Night	33% 9% 14% 25% 17% 3	2032
	21st to end	Night	40% 7% 17% 24% 10% 1	2200
October	1st to 10th	Night	42% 6% 21% 23% 8%	3920
	11th to 20th	Night	45% 6% 17% 25% 7%	4024
	21st to end	Night	49% 6% 19% 23% 3	4600
November	1st to 10th	Night	59% 12% 12% 13% 4	2760
	11th to 20th	Night	61% 12% 12% 11% 4	2940
	21st to end	Night	62% 12% 15% 8% 2	3113
December	1st to 10th	Night	49% 9% 27% 14% 2	5040
	11th to 20th	Night	69% 10% 13% 7% 2	5697
	21st to end	Night	62% 9% 15% 11% 2	7976

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	17% 13% 25% 25% 12% 7%	220
	11th to 20th	Day	14% 7% 32% 26% 18% 3	212
	21st to end	Day	16% 8% 29% 31% 14% 3	220
February	1st to 10th	Day	15% 19% 35% 22% 8% 2	212
	11th to 20th	Day	12% 17% 32% 27% 9% 2	395
	21st to end	Day	14% 17% 28% 28% 11% 2	408
March	1st to 10th	Day	13% 21% 28% 25% 10% 2	500
	11th to 20th	Day	13% 19% 26% 25% 13% 3	500
	21st to end	Day	17% 20% 27% 18% 16% 2	550
April	1st to 10th	Day	19% 21% 27% 21% 10% 1	500
	11th to 20th	Day	18% 20% 25% 21% 11% 4	492
	21st to end	Day	13% 19% 29% 24% 10% 5%	500
May	1st to 10th	Day	10% 17% 33% 25% 11% 3	503
	11th to 20th	Day	13% 12% 24% 28% 19% 5%	528
	21st to end	Day	14% 12% 23% 28% 17% 5%	663
June	1st to 10th	Day	16% 12% 28% 29% 13% 1	769
	11th to 20th	Day	15% 13% 28% 28% 14% 2	930
	21st to end	Day	11% 11% 25% 30% 20% 4	930
July	1st to 10th	Day	9% 9% 22% 35% 22% 3	850
	11th to 20th	Day	5% 10% 26% 33% 22% 4	566
	21st to end	Day	5% 7% 25% 33% 24% 7%	575
August	1st to 10th	Day	4 7% 21% 35% 23% 9%	527
	11th to 20th	Day	7% 7% 25% 33% 21% 8%	517
	21st to end	Day	9% 9% 23% 31% 21% 7%	583
September	1st to 10th	Day	9% 11% 22% 31% 22% 6%	530
	11th to 20th	Day	10% 11% 24% 30% 21% 4	530
	21st to end	Day	18% 14% 28% 25% 13% 3	530
October	1st to 10th	Day	15% 12% 26% 27% 16% 4	530
	11th to 20th	Day	17% 14% 28% 23% 14% 3	450
	21st to end	Day	21% 11% 32% 24% 10% 1	363
November	1st to 10th	Day	19% 14% 31% 24% 10% 1	278
	11th to 20th	Day	15% 20% 28% 22% 12% 3	220
	21st to end	Day	16% 13% 29% 24% 14% 5%	220
December	1st to 10th	Day	17% 10% 20% 26% 21% 5%	220
	11th to 20th	Day	20% 11% 30% 23% 11% 4	220
	21st to end	Day	16% 8% 27% 35% 10% 5%	242
January	1st to 10th	Night	22% 12% 27% 27% 11% 2	931
	11th to 20th	Night	17% 9% 29% 31% 14% 1	824
	21st to end	Night	15% 9% 28% 30% 15% 3	880
February	1st to 10th	Night	20% 11% 25% 31% 11% 2	800
	11th to 20th	Night	16% 9% 27% 34% 13% 1	784
	21st to end	Night	16% 10% 25% 28% 18% 3	660
March	1st to 10th	Night	15% 12% 25% 30% 17% 2	800
	11th to 20th	Night	13% 11% 28% 32% 15% 1	800
	21st to end	Night	13% 10% 27% 32% 16% 3	880
April	1st to 10th	Night	12% 12% 26% 30% 18% 2	800
	11th to 20th	Night	15% 10% 28% 30% 16% 1	748
	21st to end	Night	12% 11% 28% 32% 15% 2	465
May	1st to 10th	Night	13% 10% 20% 36% 20% 1	400
	11th to 20th	Night	12% 5% 20% 39% 22% 2	394
	21st to end	Night	14% 7% 20% 30% 25% 3	433
June	1st to 10th	Night	15% 6% 29% 35% 15% 1	400
	11th to 20th	Night	14% 7% 26% 32% 19% 2	400
	21st to end	Night	8% 6% 22% 39% 22% 3	400
July	1st to 10th	Night	8% 6% 15% 37% 31% 2	390
	11th to 20th	Night	4 6% 22% 35% 31% 2	388
	21st to end	Night	6% 2 16% 41% 33% 3	438
August	1st to 10th	Night	5% 2 14% 36% 35% 8%	398
	11th to 20th	Night	5% 2 13% 40% 34% 6%	396
	21st to end	Night	6% 4 18% 39% 30% 2	826
September	1st to 10th	Night	7% 5% 17% 40% 27% 5%	800
	11th to 20th	Night	11% 5% 17% 30% 31% 7%	800
	21st to end	Night	15% 8% 22% 33% 19% 4	800
October	1st to 10th	Night	12% 5% 22% 36% 22% 3	800
	11th to 20th	Night	17% 9% 22% 29% 20% 2	800
	21st to end	Night	18% 10% 27% 30% 13% 3	880
November	1st to 10th	Night	22% 11% 24% 33% 8% 1	820
	11th to 20th	Night	20% 14% 26% 26% 12% 2	840
	21st to end	Night	17% 11% 27% 30% 12% 3	1013
December	1st to 10th	Night	14% 10% 29% 31% 13% 2	1050
	11th to 20th	Night	19% 11% 29% 30% 11% 1	1050
	21st to end	Night	20% 10% 29% 32% 8% 1	1238

Page generated at 2026-01-13 22:14:27. Version ac07

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