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 2024-11-25.

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	9% 9% 3 8% 6% 5% 5% 4 6% 7% 37%	105837
	Night	9% 11% 4 9% 6% 6% 5% 4 6% 7% 33%	102190
Transparency	Day	45% 9% 16% 19% 9% 2	105595
	Night	48% 10% 15% 18% 8% 1	102078
Seeing	Day	13% 13% 26% 27% 16% 4	15565
	Night	15% 9% 24% 32% 18% 3	23998

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% 20% 7% 13% 8% 8% 5% 5% 6% 5% 11%	2320
	11th to 20th	Day	10% 17% 4 9% 8% 6% 6% 6% 8% 7% 19%	2516
	21st to end	Day	8% 15% 5% 12% 9% 9% 8% 5% 7% 7% 14%	2794
February	1st to 10th	Day	10% 16% 4 14% 8% 6% 5% 5% 6% 4 22%	1531
	11th to 20th	Day	8% 13% 6% 12% 6% 6% 5% 5% 6% 7% 27%	1665
	21st to end	Day	8% 16% 4 11% 7% 7% 7% 5% 7% 8% 21%	1556
March	1st to 10th	Day	5% 12% 5% 12% 9% 7% 7% 6% 7% 8% 21%	3138
	11th to 20th	Day	5% 8% 3 7% 5% 5% 6% 5% 8% 8% 41%	3294
	21st to end	Day	8% 13% 4 10% 7% 7% 6% 5% 6% 7% 27%	4151
April	1st to 10th	Day	9% 15% 5% 8% 7% 7% 6% 5% 6% 8% 24%	2280
	11th to 20th	Day	9% 11% 4 10% 7% 6% 6% 4 7% 7% 30%	2291
	21st to end	Day	10% 11% 4 9% 6% 6% 6% 5% 6% 8% 28%	2596
May	1st to 10th	Day	8% 8% 3 8% 5% 5% 5% 5% 7% 8% 38%	4420
	11th to 20th	Day	9% 8% 3 8% 5% 6% 5% 5% 7% 10% 34%	4377
	21st to end	Day	14% 11% 4 11% 7% 6% 6% 5% 6% 7% 24%	4963
June	1st to 10th	Day	14% 9% 3 8% 7% 5% 5% 4 6% 7% 30%	2887
	11th to 20th	Day	16% 8% 4 8% 6% 6% 6% 5% 7% 7% 27%	3040
	21st to end	Day	10% 6% 3 7% 5% 5% 5% 4 7% 8% 41%	3040
July	1st to 10th	Day	5% 3 1 4 3 4 3 3 5% 6% 63%	4943
	11th to 20th	Day	3 2 1 3 3 3 3 3 4 6% 70%	4489
	21st to end	Day	4 3 1 4 3 3 4 4 5% 8% 62%	4790
August	1st to 10th	Day	2 2 1 3 3 3 4 4 7% 8% 62%	4401
	11th to 20th	Day	3 3 1 4 4 4 5% 5% 7% 8% 56%	4330
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 6% 8% 49%	4252
September	1st to 10th	Day	7% 4 1 6% 3 4 5% 3 6% 8% 52%	2280
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 4 6% 8% 44%	2249
	21st to end	Day	10% 9% 3 7% 5% 5% 6% 4 6% 8% 37%	2086
October	1st to 10th	Day	10% 8% 3 8% 5% 5% 4 3 6% 7% 39%	3180
	11th to 20th	Day	19% 9% 3 7% 4 5% 4 3 5% 6% 34%	3034
	21st to end	Day	11% 8% 4 9% 6% 5% 5% 4 6% 7% 35%	3101
November	1st to 10th	Day	15% 14% 4 11% 7% 6% 7% 5% 6% 7% 20%	1672
	11th to 20th	Day	10% 14% 4 10% 7% 7% 6% 5% 5% 7% 23%	1600
	21st to end	Day	11% 15% 5% 12% 8% 7% 7% 5% 7% 7% 16%	1373
December	1st to 10th	Day	9% 14% 5% 15% 11% 6% 6% 3 7% 6% 19%	1330
	11th to 20th	Day	10% 16% 5% 14% 9% 8% 6% 5% 8% 6% 12%	1682
	21st to end	Day	10% 15% 4 12% 9% 8% 7% 5% 7% 6% 16%	2186
January	1st to 10th	Night	13% 24% 7% 12% 7% 6% 5% 4 6% 5% 11%	4647
	11th to 20th	Night	10% 16% 5% 14% 8% 6% 7% 6% 4 6% 18%	4382
	21st to end	Night	10% 17% 6% 15% 9% 8% 6% 5% 5% 6% 13%	4862
February	1st to 10th	Night	13% 17% 5% 11% 7% 6% 5% 5% 6% 5% 20%	2646
	11th to 20th	Night	10% 16% 5% 10% 6% 5% 5% 4 5% 6% 29%	2402
	21st to end	Night	9% 17% 5% 11% 6% 5% 6% 4 5% 7% 26%	1897
March	1st to 10th	Night	7% 11% 5% 10% 7% 6% 6% 5% 7% 9% 28%	3736
	11th to 20th	Night	6% 9% 3 7% 5% 4 4 3 6% 7% 45%	3682
	21st to end	Night	7% 13% 4 10% 6% 6% 6% 4 6% 7% 31%	3502
April	1st to 10th	Night	8% 10% 3 7% 6% 6% 5% 4 8% 9% 33%	1900
	11th to 20th	Night	9% 11% 4 9% 7% 7% 6% 5% 6% 5% 30%	1838
	21st to end	Night	10% 9% 4 8% 7% 5% 5% 4 5% 7% 35%	1582
May	1st to 10th	Night	6% 10% 3 10% 7% 6% 6% 4 5% 7% 36%	2540
	11th to 20th	Night	9% 7% 3 7% 5% 6% 5% 5% 8% 10% 37%	2516
	21st to end	Night	10% 9% 3 9% 5% 6% 6% 5% 7% 7% 32%	2607
June	1st to 10th	Night	11% 10% 3 6% 5% 5% 6% 4 6% 8% 35%	1288
	11th to 20th	Night	14% 7% 2 9% 5% 6% 5% 5% 7% 9% 32%	1140
	21st to end	Night	5% 7% 2 6% 4 6% 6% 7% 6% 9% 41%	1140
July	1st to 10th	Night	4 3 1 4 4 4 5% 4 6% 7% 57%	2014
	11th to 20th	Night	2 2 1 3 3 3 2 3 5% 5% 71%	2470
	21st to end	Night	4 3 1 3 2 3 3 3 5% 7% 64%	2762
August	1st to 10th	Night	2 1 1 3 2 3 4 5% 8% 10% 60%	2529
	11th to 20th	Night	3 2 2 6% 4 4 5% 4 6% 10% 54%	2497
	21st to end	Night	8% 5% 2 6% 3 4 4 4 6% 8% 50%	3420
September	1st to 10th	Night	6% 4 2 5% 4 4 5% 3 6% 8% 54%	1900
	11th to 20th	Night	8% 6% 3 5% 4 4 4 4 7% 7% 49%	1930
	21st to end	Night	10% 6% 3 7% 5% 4 5% 3 5% 8% 45%	2090
October	1st to 10th	Night	8% 8% 3 8% 5% 5% 5% 4 6% 6% 42%	3800
	11th to 20th	Night	20% 7% 2 6% 4 4 4 3 5% 7% 39%	3900
	21st to end	Night	12% 10% 4 9% 6% 4 4 3 5% 6% 37%	4457
November	1st to 10th	Night	16% 15% 4 8% 6% 6% 5% 4 6% 6% 23%	2625
	11th to 20th	Night	13% 14% 4 10% 8% 7% 6% 4 5% 6% 23%	2800
	21st to end	Night	12% 17% 5% 11% 7% 6% 6% 5% 7% 6% 19%	2882
December	1st to 10th	Night	11% 15% 5% 12% 7% 6% 6% 4 6% 6% 21%	2850
	11th to 20th	Night	11% 19% 6% 15% 9% 7% 7% 5% 4 5% 11%	3510
	21st to end	Night	10% 16% 5% 14% 8% 8% 8% 5% 5% 6% 15%	5447

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	66% 19% 10% 3 1	2320
	11th to 20th	Day	58% 12% 17% 9% 2 2	2540
	21st to end	Day	64% 16% 14% 5% 1	2794
February	1st to 10th	Day	63% 11% 12% 9% 4 1	1531
	11th to 20th	Day	57% 12% 14% 12% 5% 1	1674
	21st to end	Day	60% 13% 13% 12% 3	1556
March	1st to 10th	Day	57% 11% 13% 15% 4	3138
	11th to 20th	Day	39% 10% 20% 27% 4	3294
	21st to end	Day	56% 10% 15% 13% 6%	4151
April	1st to 10th	Day	57% 12% 14% 14% 3	2280
	11th to 20th	Day	53% 9% 14% 16% 7% 1	2291
	21st to end	Day	54% 9% 14% 16% 7%	2596
May	1st to 10th	Day	41% 10% 15% 22% 11% 1	4420
	11th to 20th	Day	40% 14% 17% 22% 7%	4380
	21st to end	Day	56% 11% 16% 14% 3	4964
June	1st to 10th	Day	52% 10% 12% 15% 8% 2	2887
	11th to 20th	Day	57% 9% 11% 16% 6% 1	3040
	21st to end	Day	40% 10% 16% 20% 12% 2	3040
July	1st to 10th	Day	23% 5% 11% 22% 26% 13%	4867
	11th to 20th	Day	18% 4 9% 26% 30% 14%	4411
	21st to end	Day	21% 4 15% 30% 22% 7%	4639
August	1st to 10th	Day	19% 5% 13% 38% 21% 3	4384
	11th to 20th	Day	27% 4 27% 29% 10% 2	4372
	21st to end	Day	34% 6% 23% 29% 8% 1	4251
September	1st to 10th	Day	30% 8% 16% 31% 12% 3	2280
	11th to 20th	Day	36% 9% 16% 25% 13% 2	2249
	21st to end	Day	43% 10% 21% 17% 7% 1	2086
October	1st to 10th	Day	45% 7% 22% 22% 4	3180
	11th to 20th	Day	51% 7% 14% 26% 2	3034
	21st to end	Day	49% 7% 24% 17% 3	3101
November	1st to 10th	Day	63% 11% 12% 11% 2	1672
	11th to 20th	Day	61% 14% 13% 8% 3 1	1600
	21st to end	Day	64% 13% 15% 7% 1	1373
December	1st to 10th	Day	62% 14% 10% 12% 1	1330
	11th to 20th	Day	64% 18% 10% 6% 2	1684
	21st to end	Day	60% 13% 14% 11% 1	2186
January	1st to 10th	Night	69% 14% 10% 5% 2	4647
	11th to 20th	Night	61% 10% 16% 8% 3 1	4420
	21st to end	Night	67% 15% 11% 5% 2	4862
February	1st to 10th	Night	63% 11% 12% 10% 4 1	2646
	11th to 20th	Night	55% 8% 15% 15% 6% 1	2404
	21st to end	Night	59% 9% 14% 12% 6%	1897
March	1st to 10th	Night	48% 12% 14% 18% 7%	3736
	11th to 20th	Night	36% 11% 22% 25% 6%	3683
	21st to end	Night	50% 11% 14% 18% 8%	3502
April	1st to 10th	Night	45% 15% 18% 16% 5%	1900
	11th to 20th	Night	52% 9% 14% 17% 7% 1	1838
	21st to end	Night	47% 10% 12% 21% 9%	1582
May	1st to 10th	Night	44% 10% 10% 21% 14%	2540
	11th to 20th	Night	34% 14% 17% 26% 7%	2516
	21st to end	Night	46% 14% 14% 20% 5%	2607
June	1st to 10th	Night	46% 9% 13% 18% 10% 3	1288
	11th to 20th	Night	47% 11% 12% 20% 10%	1140
	21st to end	Night	36% 14% 16% 21% 11% 2	1140
July	1st to 10th	Night	27% 6% 10% 21% 22% 15%	1981
	11th to 20th	Night	17% 5% 9% 23% 33% 13%	2425
	21st to end	Night	21% 4 11% 33% 21% 10%	2674
August	1st to 10th	Night	19% 5% 11% 37% 25% 3	2521
	11th to 20th	Night	28% 4 23% 29% 14% 2	2521
	21st to end	Night	33% 5% 24% 26% 11% 1	3420
September	1st to 10th	Night	29% 8% 14% 28% 17% 4	1900
	11th to 20th	Night	33% 9% 14% 24% 17% 3	1930
	21st to end	Night	40% 7% 17% 24% 10% 2	2090
October	1st to 10th	Night	43% 6% 21% 22% 8%	3800
	11th to 20th	Night	46% 6% 17% 25% 6%	3900
	21st to end	Night	49% 6% 19% 23% 3	4457
November	1st to 10th	Night	59% 12% 12% 13% 4	2625
	11th to 20th	Night	60% 12% 12% 11% 4 1	2800
	21st to end	Night	62% 13% 14% 9% 3	2882
December	1st to 10th	Night	60% 11% 15% 12% 3	2850
	11th to 20th	Night	68% 15% 9% 6% 1	3507
	21st to end	Night	62% 13% 15% 9% 1	5447

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	16% 13% 26% 24% 13% 8%	180
	11th to 20th	Day	16% 8% 31% 23% 18% 4	180
	21st to end	Day	17% 8% 29% 30% 14% 3	198
February	1st to 10th	Day	16% 21% 35% 20% 6% 1	191
	11th to 20th	Day	13% 18% 31% 27% 9% 2	365
	21st to end	Day	13% 16% 28% 30% 11% 2	376
March	1st to 10th	Day	13% 22% 28% 24% 10% 2	460
	11th to 20th	Day	13% 19% 25% 26% 14% 3	460
	21st to end	Day	17% 20% 26% 18% 16% 2	506
April	1st to 10th	Day	20% 22% 27% 20% 10% 1	460
	11th to 20th	Day	18% 20% 25% 21% 12% 4	452
	21st to end	Day	13% 20% 30% 22% 10% 5%	460
May	1st to 10th	Day	10% 17% 32% 25% 12% 3	463
	11th to 20th	Day	12% 12% 24% 28% 19% 5%	488
	21st to end	Day	15% 12% 22% 28% 18% 5%	614
June	1st to 10th	Day	17% 12% 27% 29% 13% 2	717
	11th to 20th	Day	15% 13% 28% 27% 14% 2	870
	21st to end	Day	11% 11% 25% 29% 20% 4	870
July	1st to 10th	Day	9% 9% 21% 36% 23% 3	794
	11th to 20th	Day	5% 10% 25% 33% 23% 4	524
	21st to end	Day	5% 7% 24% 35% 23% 6%	531
August	1st to 10th	Day	4 8% 21% 35% 23% 9%	487
	11th to 20th	Day	6% 7% 24% 33% 21% 9%	477
	21st to end	Day	10% 9% 25% 30% 19% 7%	539
September	1st to 10th	Day	9% 11% 23% 32% 19% 5%	490
	11th to 20th	Day	11% 11% 26% 28% 21% 3	490
	21st to end	Day	18% 14% 27% 25% 13% 3	490
October	1st to 10th	Day	16% 12% 26% 25% 17% 5%	490
	11th to 20th	Day	18% 15% 26% 23% 15% 3	414
	21st to end	Day	21% 11% 30% 25% 11% 2	330
November	1st to 10th	Day	19% 13% 30% 25% 11% 2	253
	11th to 20th	Day	16% 20% 28% 21% 12% 4	200
	21st to end	Day	15% 12% 30% 23% 14% 6%	188
December	1st to 10th	Day	17% 9% 20% 24% 23% 7%	180
	11th to 20th	Day	17% 9% 32% 23% 13% 5%	180
	21st to end	Day	17% 7% 26% 35% 10% 5%	198
January	1st to 10th	Night	23% 12% 26% 26% 11% 2	839
	11th to 20th	Night	17% 9% 26% 31% 15% 1	760
	21st to end	Night	15% 9% 27% 30% 16% 3	836
February	1st to 10th	Night	21% 11% 25% 30% 12% 2	760
	11th to 20th	Night	16% 9% 25% 35% 13% 1	744
	21st to end	Night	16% 9% 24% 29% 19% 4	628
March	1st to 10th	Night	15% 12% 25% 29% 18% 2	760
	11th to 20th	Night	13% 11% 26% 33% 15% 1	760
	21st to end	Night	13% 9% 27% 31% 17% 3	836
April	1st to 10th	Night	12% 12% 25% 30% 19% 2	760
	11th to 20th	Night	15% 10% 27% 31% 17% 1	710
	21st to end	Night	13% 11% 28% 31% 15% 2	442
May	1st to 10th	Night	13% 9% 19% 36% 21% 1	380
	11th to 20th	Night	12% 5% 20% 38% 23% 2	374
	21st to end	Night	15% 7% 18% 30% 26% 4	411
June	1st to 10th	Night	16% 5% 28% 34% 16% 1	380
	11th to 20th	Night	15% 7% 25% 32% 19% 2	380
	21st to end	Night	8% 6% 22% 38% 23% 3	380
July	1st to 10th	Night	8% 5% 16% 36% 33% 2	370
	11th to 20th	Night	5% 6% 20% 35% 32% 2	368
	21st to end	Night	6% 1 14% 41% 34% 3	416
August	1st to 10th	Night	5% 1 14% 36% 35% 8%	378
	11th to 20th	Night	4 2 13% 40% 35% 6%	376
	21st to end	Night	7% 4 19% 37% 31% 3	784
September	1st to 10th	Night	7% 5% 18% 39% 26% 5%	760
	11th to 20th	Night	11% 5% 16% 28% 33% 7%	760
	21st to end	Night	15% 8% 19% 34% 19% 4	760
October	1st to 10th	Night	13% 6% 20% 35% 23% 3	760
	11th to 20th	Night	18% 9% 20% 30% 21% 2	760
	21st to end	Night	18% 9% 26% 30% 13% 3	836
November	1st to 10th	Night	23% 11% 23% 34% 9% 1	780
	11th to 20th	Night	20% 13% 26% 26% 13% 2	800
	21st to end	Night	17% 11% 26% 30% 13% 3	938
December	1st to 10th	Night	14% 9% 28% 32% 14% 2	950
	11th to 20th	Night	18% 10% 29% 32% 11% 1	950
	21st to end	Night	21% 9% 28% 32% 9% 1	1112

Page generated at 2024-11-22 05:02:27. Version ac07

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