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 2024-01-14.

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	10% 9% 4 7% 7% 4 6% 6% 6% 8% 34%	74791
	Night	10% 11% 4 8% 7% 4 6% 5% 6% 7% 31%	74926
Transparency	Day	48% 10% 15% 18% 8% 1	74483
	Night	50% 11% 14% 16% 7% 1	74752
Seeing	Day	13% 13% 26% 27% 16% 4	14351
	Night	15% 8% 23% 32% 19% 3	22976

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	16% 17% 6% 9% 8% 6% 6% 6% 7% 6% 13%	1391
	11th to 20th	Day	11% 13% 6% 8% 9% 5% 7% 7% 8% 7% 18%	1464
	21st to end	Day	8% 14% 5% 9% 10% 5% 9% 8% 7% 7% 18%	1584
February	1st to 10th	Day	11% 14% 5% 12% 10% 5% 6% 6% 6% 5% 23%	1451
	11th to 20th	Day	9% 12% 6% 11% 8% 5% 4 6% 6% 7% 25%	1575
	21st to end	Day	8% 14% 5% 9% 8% 5% 6% 7% 7% 8% 22%	1467
March	1st to 10th	Day	7% 11% 6% 9% 9% 5% 8% 7% 7% 8% 23%	1800
	11th to 20th	Day	9% 11% 5% 8% 8% 4 6% 6% 6% 8% 28%	1862
	21st to end	Day	9% 12% 5% 8% 7% 4 8% 7% 7% 8% 24%	2362
April	1st to 10th	Day	10% 14% 6% 7% 7% 6% 7% 6% 6% 8% 25%	2160
	11th to 20th	Day	10% 9% 5% 9% 8% 4 6% 6% 6% 7% 29%	2168
	21st to end	Day	10% 10% 5% 8% 7% 4 7% 7% 6% 8% 29%	2459
May	1st to 10th	Day	11% 9% 4 8% 6% 4 5% 6% 6% 8% 33%	2520
	11th to 20th	Day	9% 6% 3 6% 6% 4 6% 6% 6% 8% 38%	2477
	21st to end	Day	14% 8% 4 7% 7% 3 6% 6% 6% 7% 32%	2803
June	1st to 10th	Day	15% 8% 3 7% 8% 4 5% 6% 6% 7% 30%	2735
	11th to 20th	Day	16% 8% 4 7% 7% 4 7% 6% 6% 7% 26%	2880
	21st to end	Day	9% 5% 3 6% 6% 4 6% 6% 7% 8% 41%	2880
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 6% 7% 7% 49%	2819
	11th to 20th	Day	4 4 2 5% 5% 3 5% 5% 5% 9% 53%	2561
	21st to end	Day	3 2 1 3 3 2 3 4 5% 7% 65%	2741
August	1st to 10th	Day	3 4 2 4 4 3 5% 5% 6% 8% 57%	2507
	11th to 20th	Day	4 4 1 3 4 3 4 5% 5% 7% 58%	2475
	21st to end	Day	8% 4 2 4 6% 4 5% 6% 7% 9% 46%	2422
September	1st to 10th	Day	7% 5% 2 5% 4 3 5% 4 5% 8% 51%	2160
	11th to 20th	Day	9% 5% 2 6% 5% 4 6% 5% 6% 8% 43%	2131
	21st to end	Day	10% 8% 3 6% 6% 4 7% 5% 6% 8% 37%	1977
October	1st to 10th	Day	11% 8% 3 6% 6% 4 6% 4 7% 6% 37%	1800
	11th to 20th	Day	13% 9% 3 7% 6% 4 6% 6% 7% 7% 33%	1728
	21st to end	Day	16% 9% 4 8% 6% 3 5% 6% 6% 8% 30%	1782
November	1st to 10th	Day	15% 13% 5% 9% 8% 5% 6% 7% 6% 7% 21%	1587
	11th to 20th	Day	11% 11% 6% 9% 8% 5% 7% 6% 5% 7% 23%	1520
	21st to end	Day	11% 13% 7% 10% 9% 5% 8% 7% 6% 8% 16%	1345
December	1st to 10th	Day	9% 12% 6% 14% 11% 5% 6% 5% 7% 6% 19%	1330
	11th to 20th	Day	10% 15% 6% 12% 11% 5% 8% 6% 8% 6% 12%	1682
	21st to end	Day	11% 14% 5% 11% 11% 6% 7% 7% 7% 7% 16%	2186
January	1st to 10th	Night	15% 18% 6% 9% 8% 5% 6% 5% 7% 6% 13%	2783
	11th to 20th	Night	12% 16% 6% 13% 11% 6% 6% 7% 5% 6% 13%	2568
	21st to end	Night	9% 15% 6% 12% 10% 5% 7% 5% 5% 7% 18%	2772
February	1st to 10th	Night	12% 16% 6% 10% 9% 5% 6% 6% 6% 6% 20%	2506
	11th to 20th	Night	10% 15% 6% 9% 7% 4 5% 5% 4 7% 28%	2272
	21st to end	Night	10% 15% 5% 10% 7% 4 6% 5% 5% 7% 27%	1788
March	1st to 10th	Night	9% 12% 5% 8% 8% 4 6% 5% 6% 8% 28%	2160
	11th to 20th	Night	9% 10% 5% 8% 7% 4 6% 5% 6% 7% 32%	2094
	21st to end	Night	8% 11% 5% 8% 7% 5% 7% 5% 6% 8% 31%	1994
April	1st to 10th	Night	8% 9% 4 6% 7% 5% 6% 5% 7% 9% 35%	1800
	11th to 20th	Night	10% 9% 4 8% 8% 6% 7% 6% 6% 6% 30%	1741
	21st to end	Night	10% 9% 4 7% 7% 4 5% 6% 5% 7% 36%	1499
May	1st to 10th	Night	8% 10% 3 9% 8% 5% 6% 5% 5% 6% 36%	1440
	11th to 20th	Night	7% 6% 3 6% 6% 4 6% 5% 8% 10% 39%	1416
	21st to end	Night	13% 8% 3 7% 4 3 5% 6% 5% 8% 38%	1467
June	1st to 10th	Night	12% 9% 4 5% 6% 3 6% 6% 6% 8% 35%	1221
	11th to 20th	Night	15% 6% 3 7% 6% 5% 5% 5% 8% 8% 32%	1080
	21st to end	Night	6% 6% 2 6% 5% 4 5% 8% 6% 9% 43%	1080
July	1st to 10th	Night	6% 5% 1 4 5% 3 6% 5% 6% 9% 50%	1138
	11th to 20th	Night	4 3 2 4 4 3 4 5% 6% 8% 58%	1398
	21st to end	Night	4 2 1 2 3 2 3 4 5% 7% 67%	1576
August	1st to 10th	Night	4 2 1 4 3 3 4 4 6% 8% 61%	1432
	11th to 20th	Night	4 2 1 2 4 2 3 4 4 8% 65%	1424
	21st to end	Night	5% 3 2 5% 4 3 4 6% 6% 8% 53%	1930
September	1st to 10th	Night	6% 4 2 5% 5% 3 5% 4 6% 8% 53%	1800
	11th to 20th	Night	8% 5% 3 4 5% 3 4 5% 6% 8% 50%	1828
	21st to end	Night	11% 6% 3 5% 5% 3 5% 4 5% 8% 44%	1980
October	1st to 10th	Night	9% 7% 3 6% 5% 4 5% 5% 6% 7% 42%	2160
	11th to 20th	Night	13% 8% 2 7% 7% 4 5% 5% 5% 7% 37%	2232
	21st to end	Night	14% 9% 3 8% 7% 3 6% 4 6% 7% 34%	2574
November	1st to 10th	Night	17% 14% 5% 7% 7% 4 6% 5% 6% 6% 23%	2490
	11th to 20th	Night	14% 13% 5% 8% 9% 5% 6% 5% 5% 6% 23%	2660
	21st to end	Night	13% 14% 7% 9% 8% 4 6% 6% 7% 6% 19%	2816
December	1st to 10th	Night	11% 13% 6% 11% 8% 4 6% 6% 6% 6% 21%	2850
	11th to 20th	Night	12% 18% 7% 14% 10% 6% 7% 6% 5% 5% 11%	3510
	21st to end	Night	10% 15% 6% 12% 10% 5% 8% 7% 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	69% 14% 11% 4 2	1391
	11th to 20th	Day	61% 14% 15% 9% 1 1	1464
	21st to end	Day	60% 15% 16% 8% 1 1	1584
February	1st to 10th	Day	62% 11% 12% 9% 4 1	1451
	11th to 20th	Day	58% 12% 14% 12% 3	1584
	21st to end	Day	59% 13% 13% 12% 3	1467
March	1st to 10th	Day	59% 12% 13% 12% 4 1	1800
	11th to 20th	Day	55% 10% 15% 16% 5%	1862
	21st to end	Day	58% 10% 17% 11% 4	2362
April	1st to 10th	Day	58% 12% 14% 14% 3	2160
	11th to 20th	Day	54% 9% 14% 15% 6% 1	2168
	21st to end	Day	54% 9% 14% 17% 6%	2459
May	1st to 10th	Day	48% 9% 17% 17% 8% 1	2520
	11th to 20th	Day	44% 10% 16% 21% 8%	2480
	21st to end	Day	51% 8% 17% 19% 5% 1	2804
June	1st to 10th	Day	53% 9% 12% 15% 8% 2	2735
	11th to 20th	Day	57% 9% 11% 16% 6% 1	2880
	21st to end	Day	40% 10% 16% 20% 12% 2	2880
July	1st to 10th	Day	33% 9% 13% 23% 17% 4	2743
	11th to 20th	Day	29% 7% 15% 28% 17% 4	2483
	21st to end	Day	18% 6% 15% 29% 28% 5%	2589
August	1st to 10th	Day	24% 9% 16% 27% 20% 4	2490
	11th to 20th	Day	25% 7% 17% 30% 18% 3	2475
	21st to end	Day	34% 9% 17% 28% 11% 1	2422
September	1st to 10th	Day	31% 8% 15% 31% 12% 2	2160
	11th to 20th	Day	36% 10% 15% 24% 13% 2	2131
	21st to end	Day	43% 10% 20% 18% 7% 1	1977
October	1st to 10th	Day	45% 9% 19% 22% 5%	1800
	11th to 20th	Day	48% 12% 18% 19% 3	1728
	21st to end	Day	53% 10% 18% 15% 4	1782
November	1st to 10th	Day	63% 12% 12% 11% 3	1587
	11th to 20th	Day	60% 14% 14% 8% 3 1	1520
	21st to end	Day	63% 13% 15% 7% 1	1345
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	68% 10% 11% 8% 3	2783
	11th to 20th	Night	66% 12% 14% 6% 1	2568
	21st to end	Night	62% 12% 14% 9% 3 1	2772
February	1st to 10th	Night	62% 11% 12% 11% 4 1	2506
	11th to 20th	Night	57% 8% 16% 15% 4	2274
	21st to end	Night	58% 9% 14% 12% 7%	1788
March	1st to 10th	Night	55% 11% 14% 12% 7% 1	2160
	11th to 20th	Night	51% 11% 15% 17% 6% 1	2095
	21st to end	Night	52% 9% 16% 18% 5%	1994
April	1st to 10th	Night	45% 15% 18% 17% 5%	1800
	11th to 20th	Night	53% 10% 14% 17% 7% 1	1741
	21st to end	Night	47% 10% 12% 22% 9%	1499
May	1st to 10th	Night	48% 10% 14% 17% 10% 1	1440
	11th to 20th	Night	38% 10% 17% 26% 9%	1416
	21st to end	Night	44% 8% 15% 23% 9%	1467
June	1st to 10th	Night	47% 9% 14% 18% 9% 4	1221
	11th to 20th	Night	47% 11% 13% 20% 9%	1080
	21st to end	Night	35% 14% 16% 21% 11% 2	1080
July	1st to 10th	Night	30% 8% 13% 25% 17% 6%	1105
	11th to 20th	Night	24% 8% 16% 29% 19% 4	1353
	21st to end	Night	15% 6% 12% 30% 30% 6%	1488
August	1st to 10th	Night	21% 7% 14% 27% 26% 5%	1424
	11th to 20th	Night	19% 7% 15% 32% 23% 4	1424
	21st to end	Night	27% 8% 16% 31% 16% 2	1930
September	1st to 10th	Night	30% 9% 15% 27% 16% 4	1800
	11th to 20th	Night	33% 9% 14% 23% 18% 3	1828
	21st to end	Night	40% 8% 17% 23% 10% 2	1980
October	1st to 10th	Night	41% 8% 17% 25% 8% 1	2160
	11th to 20th	Night	47% 10% 15% 21% 7%	2232
	21st to end	Night	51% 9% 17% 19% 4	2574
November	1st to 10th	Night	59% 12% 11% 13% 4	2490
	11th to 20th	Night	60% 13% 12% 10% 4 1	2660
	21st to end	Night	61% 13% 14% 9% 3	2816
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% 9% 29% 23% 19% 4	166
	21st to end	Day	18% 7% 30% 30% 13% 3	176
February	1st to 10th	Day	17% 21% 36% 20% 5% 1	171
	11th to 20th	Day	14% 19% 31% 28% 7% 1	335
	21st to end	Day	11% 15% 30% 30% 12% 2	341
March	1st to 10th	Day	14% 23% 29% 22% 10% 2	420
	11th to 20th	Day	14% 20% 25% 24% 13% 3	420
	21st to end	Day	18% 21% 26% 18% 15% 2	462
April	1st to 10th	Day	20% 21% 27% 20% 11% 1	420
	11th to 20th	Day	19% 20% 25% 19% 12% 4	412
	21st to end	Day	13% 20% 31% 21% 10% 5%	420
May	1st to 10th	Day	11% 18% 31% 27% 11% 3	423
	11th to 20th	Day	13% 12% 24% 28% 20% 5%	448
	21st to end	Day	16% 11% 21% 28% 19% 5%	565
June	1st to 10th	Day	18% 12% 26% 29% 14% 2	665
	11th to 20th	Day	16% 13% 28% 28% 13% 2	810
	21st to end	Day	10% 11% 25% 29% 21% 4	810
July	1st to 10th	Day	9% 9% 21% 35% 22% 3	738
	11th to 20th	Day	5% 11% 27% 32% 21% 4	482
	21st to end	Day	5% 7% 24% 35% 24% 7%	487
August	1st to 10th	Day	4 8% 21% 35% 23% 9%	447
	11th to 20th	Day	6% 7% 25% 34% 20% 8%	437
	21st to end	Day	10% 9% 25% 29% 19% 7%	495
September	1st to 10th	Day	10% 12% 24% 32% 17% 4	450
	11th to 20th	Day	11% 12% 25% 28% 20% 4	450
	21st to end	Day	18% 13% 26% 26% 14% 3	450
October	1st to 10th	Day	16% 12% 25% 25% 18% 5%	450
	11th to 20th	Day	16% 16% 26% 23% 16% 3	378
	21st to end	Day	23% 11% 28% 25% 12% 2	297
November	1st to 10th	Day	19% 12% 29% 25% 12% 2	228
	11th to 20th	Day	16% 17% 28% 22% 13% 4	180
	21st to end	Day	16% 12% 29% 23% 14% 6%	180
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	18% 9% 26% 30% 15% 1	734
	21st to end	Night	16% 8% 27% 30% 16% 3	792
February	1st to 10th	Night	20% 11% 25% 30% 12% 2	720
	11th to 20th	Night	17% 10% 25% 35% 13% 1	704
	21st to end	Night	15% 8% 25% 30% 18% 4	592
March	1st to 10th	Night	15% 12% 25% 29% 17% 2	720
	11th to 20th	Night	14% 12% 27% 32% 15% 1	720
	21st to end	Night	13% 10% 26% 30% 17% 3	792
April	1st to 10th	Night	12% 11% 24% 31% 20% 2	720
	11th to 20th	Night	15% 10% 26% 31% 18% 1	673
	21st to end	Night	13% 9% 29% 32% 15% 2	419
May	1st to 10th	Night	13% 8% 19% 37% 21% 1	360
	11th to 20th	Night	12% 4 19% 38% 24% 2	354
	21st to end	Night	15% 6% 18% 31% 26% 4	389
June	1st to 10th	Night	16% 5% 28% 33% 16% 1	360
	11th to 20th	Night	15% 7% 25% 32% 19% 3	360
	21st to end	Night	8% 6% 20% 39% 24% 3	360
July	1st to 10th	Night	9% 5% 15% 35% 34% 2	350
	11th to 20th	Night	5% 7% 20% 34% 33% 2	348
	21st to end	Night	6% 1 14% 40% 36% 4	394
August	1st to 10th	Night	5% 1 13% 35% 37% 8%	358
	11th to 20th	Night	4 2 12% 40% 35% 6%	356
	21st to end	Night	6% 4 17% 37% 32% 3	742
September	1st to 10th	Night	7% 5% 18% 37% 27% 5%	720
	11th to 20th	Night	11% 5% 16% 28% 33% 7%	720
	21st to end	Night	15% 8% 18% 35% 21% 4	720
October	1st to 10th	Night	13% 5% 18% 36% 24% 3	720
	11th to 20th	Night	17% 9% 19% 30% 22% 3	720
	21st to end	Night	18% 9% 26% 31% 13% 3	792
November	1st to 10th	Night	23% 10% 21% 35% 9% 1	740
	11th to 20th	Night	20% 14% 26% 25% 13% 2	760
	21st to end	Night	18% 11% 25% 30% 13% 3	916
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 2024-01-11 01:22:37. Version ac07

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