B&W Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the B&W Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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	11% 7% 3 7% 4 4 4 4 5% 7% 43%	104813
	Night	10% 7% 3 6% 4 3 4 3 5% 7% 47%	104334
Transparency	Day	37% 7% 14% 25% 16% 2	108490
	Night	35% 6% 13% 26% 18% 3	108784
Seeing	Day	15% 7% 30% 30% 15% 3	15991
	Night	14% 7% 32% 35% 11% 1	25958

By Month

Percentage Cloud Cover SE (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	10% 12% 5% 16% 8% 5% 4 3 5% 6% 26%	2901
	11th to 20th	Day	9% 12% 5% 9% 5% 5% 6% 4 6% 7% 32%	2850
	21st to end	Day	9% 14% 6% 14% 6% 6% 5% 4 4 5% 28%	3135
February	1st to 10th	Day	10% 12% 5% 9% 6% 6% 4 3 5% 7% 32%	1710
	11th to 20th	Day	10% 9% 3 9% 5% 4 4 3 6% 7% 40%	1687
	21st to end	Day	11% 8% 3 7% 5% 5% 4 3 5% 7% 42%	1570
March	1st to 10th	Day	13% 10% 3 6% 3 3 3 2 3 5% 48%	3402
	11th to 20th	Day	11% 7% 2 6% 4 4 4 3 4 6% 49%	3480
	21st to end	Day	21% 12% 4 8% 4 4 4 4 6% 7% 26%	3814
April	1st to 10th	Day	15% 8% 3 6% 4 4 3 3 5% 5% 44%	2152
	11th to 20th	Day	16% 8% 3 6% 4 4 4 3 5% 6% 40%	2351
	21st to end	Day	19% 7% 3 6% 4 4 4 3 6% 7% 38%	2470
May	1st to 10th	Day	15% 8% 3 8% 5% 5% 5% 4 5% 7% 36%	4082
	11th to 20th	Day	14% 8% 3 7% 4 3 4 3 4 7% 43%	4082
	21st to end	Day	13% 6% 2 6% 4 4 5% 5% 6% 8% 40%	4622
June	1st to 10th	Day	7% 5% 2 6% 4 5% 4 4 7% 10% 48%	2660
	11th to 20th	Day	8% 5% 2 6% 5% 3 5% 4 6% 9% 46%	2660
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 49%	2660
July	1st to 10th	Day	12% 6% 2 5% 3 4 4 4 7% 9% 43%	4185
	11th to 20th	Day	6% 4 2 5% 4 4 4 4 5% 8% 54%	4120
	21st to end	Day	4 3 2 4 3 4 4 4 7% 10% 54%	4465
August	1st to 10th	Day	6% 4 2 5% 4 5% 6% 5% 7% 10% 46%	4104
	11th to 20th	Day	12% 6% 2 7% 4 4 5% 4 6% 8% 42%	4041
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 6% 8% 49%	4441
September	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 4 7% 55%	2216
	11th to 20th	Day	11% 5% 2 4 4 3 3 3 5% 7% 54%	2090
	21st to end	Day	10% 4 2 5% 3 4 3 3 4 6% 56%	2090
October	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 6% 52%	1938
	11th to 20th	Day	11% 7% 1 4 3 3 2 2 5% 4 57%	1900
	21st to end	Day	13% 8% 3 5% 4 3 3 3 5% 6% 47%	2090
November	1st to 10th	Day	10% 6% 2 5% 4 3 4 3 5% 6% 53%	1796
	11th to 20th	Day	10% 8% 2 7% 5% 3 4 4 4 6% 47%	1730
	21st to end	Day	12% 9% 4 5% 4 3 4 3 5% 7% 44%	1600
December	1st to 10th	Day	6% 5% 2 6% 5% 5% 4 3 5% 6% 53%	2610
	11th to 20th	Day	9% 10% 3 9% 6% 4 4 3 5% 6% 42%	3008
	21st to end	Day	18% 10% 4 13% 6% 4 4 3 4 5% 29%	4101
January	1st to 10th	Night	8% 10% 5% 18% 9% 5% 5% 3 5% 5% 27%	4187
	11th to 20th	Night	8% 11% 4 7% 4 5% 8% 4 8% 6% 35%	4120
	21st to end	Night	12% 13% 6% 12% 4 4 4 3 5% 6% 31%	4532
February	1st to 10th	Night	11% 10% 4 9% 5% 4 4 3 4 7% 37%	2470
	11th to 20th	Night	9% 8% 4 7% 5% 4 4 4 6% 7% 43%	2394
	21st to end	Night	9% 7% 3 7% 3 4 4 3 5% 5% 49%	1884
March	1st to 10th	Night	11% 6% 3 5% 4 3 3 3 5% 5% 52%	3453
	11th to 20th	Night	11% 6% 2 5% 2 2 3 3 4 5% 56%	3480
	21st to end	Night	19% 9% 3 9% 4 4 4 4 6% 7% 32%	3817
April	1st to 10th	Night	12% 8% 2 5% 4 3 3 3 5% 7% 46%	2028
	11th to 20th	Night	11% 7% 2 6% 3 4 4 3 5% 7% 46%	1777
	21st to end	Night	17% 7% 2 6% 4 4 4 2 4 7% 45%	1710
May	1st to 10th	Night	19% 6% 2 6% 4 4 3 3 5% 6% 44%	2823
	11th to 20th	Night	13% 5% 2 5% 4 3 3 3 4 8% 50%	2823
	21st to end	Night	16% 7% 2 5% 4 4 4 3 5% 6% 44%	2941
June	1st to 10th	Night	5% 4 1 5% 3 4 5% 3 6% 9% 55%	1520
	11th to 20th	Night	6% 5% 1 3 4 3 4 5% 8% 10% 50%	1520
	21st to end	Night	4 3 1 4 4 4 4 5% 6% 9% 56%	1520
July	1st to 10th	Night	7% 4 2 6% 4 3 4 4 6% 8% 52%	2778
	11th to 20th	Night	5% 2 1 3 2 2 3 3 6% 8% 64%	2850
	21st to end	Night	3 2 1 4 3 3 4 4 7% 10% 59%	3098
August	1st to 10th	Night	4 3 1 4 4 3 4 3 6% 10% 59%	2836
	11th to 20th	Night	11% 5% 1 5% 3 3 4 3 5% 8% 51%	2798
	21st to end	Night	7% 4 2 4 3 3 4 3 6% 8% 57%	3223
September	1st to 10th	Night	9% 4 2 4 3 2 3 4 4 6% 59%	1961
	11th to 20th	Night	9% 3 1 3 2 4 3 3 6% 7% 58%	2090
	21st to end	Night	9% 5% 2 4 2 4 3 3 5% 5% 59%	2090
October	1st to 10th	Night	12% 5% 2 4 3 2 3 4 5% 6% 53%	2221
	11th to 20th	Night	13% 5% 1 4 4 3 3 2 4 6% 56%	2280
	21st to end	Night	13% 7% 2 4 3 4 4 3 4 5% 52%	2508
November	1st to 10th	Night	10% 4 2 4 4 3 4 3 5% 7% 55%	2497
	11th to 20th	Night	9% 6% 2 5% 4 3 3 3 5% 6% 53%	2667
	21st to end	Night	12% 8% 2 5% 4 3 3 3 5% 5% 51%	2800
December	1st to 10th	Night	5% 6% 2 6% 4 4 4 4 6% 7% 53%	4580
	11th to 20th	Night	10% 8% 3 7% 4 4 4 4 6% 7% 44%	5199
	21st to end	Night	17% 13% 5% 10% 4 3 3 3 4 5% 33%	6859

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	55% 8% 14% 15% 7%	2902
	11th to 20th	Day	49% 5% 16% 21% 8%	2850
	21st to end	Day	54% 8% 15% 15% 7%	3135
February	1st to 10th	Day	49% 7% 15% 18% 10% 1	1710
	11th to 20th	Day	43% 6% 12% 24% 13% 3	1696
	21st to end	Day	40% 6% 13% 23% 15% 3	1570
March	1st to 10th	Day	38% 5% 10% 26% 18% 3	3402
	11th to 20th	Day	35% 6% 12% 28% 17% 2	3480
	21st to end	Day	52% 12% 19% 12% 4 1	3814
April	1st to 10th	Day	41% 6% 12% 26% 13% 2	2152
	11th to 20th	Day	43% 7% 13% 22% 13% 2	2351
	21st to end	Day	45% 8% 12% 23% 13% 1	2470
May	1st to 10th	Day	42% 10% 14% 20% 12% 2	4120
	11th to 20th	Day	40% 8% 11% 23% 17% 1	4086
	21st to end	Day	37% 10% 14% 23% 14% 1	4635
June	1st to 10th	Day	30% 8% 18% 28% 15% 2	2660
	11th to 20th	Day	33% 7% 14% 29% 16% 1	2660
	21st to end	Day	31% 9% 15% 28% 13% 4	2660
July	1st to 10th	Day	37% 6% 15% 28% 13% 2	4121
	11th to 20th	Day	28% 6% 13% 32% 19% 3	4051
	21st to end	Day	22% 8% 18% 30% 20% 2	4325
August	1st to 10th	Day	30% 8% 15% 28% 18% 1	4090
	11th to 20th	Day	38% 7% 15% 24% 16% 1	4080
	21st to end	Day	32% 8% 15% 25% 16% 5%	4440
September	1st to 10th	Day	30% 6% 12% 29% 19% 5%	2216
	11th to 20th	Day	30% 6% 12% 32% 19% 1	2090
	21st to end	Day	29% 5% 9% 31% 24% 1	2090
October	1st to 10th	Day	20% 3 7% 19% 47% 3	3224
	11th to 20th	Day	20% 3 12% 31% 27% 7%	3140
	21st to end	Day	34% 4 9% 28% 22% 3	3428
November	1st to 10th	Day	31% 5% 14% 28% 19% 3	1796
	11th to 20th	Day	38% 5% 13% 24% 19% 1	1730
	21st to end	Day	40% 4 13% 28% 13% 1	1600
December	1st to 10th	Day	32% 5% 17% 29% 16% 1	2610
	11th to 20th	Day	43% 5% 17% 25% 10%	3005
	21st to end	Day	57% 5% 12% 19% 6% 1	4101
January	1st to 10th	Night	54% 8% 12% 15% 10%	4187
	11th to 20th	Night	44% 7% 15% 23% 10% 1	4120
	21st to end	Night	50% 7% 17% 17% 8% 1	4532
February	1st to 10th	Night	46% 6% 14% 19% 12% 3	2470
	11th to 20th	Night	39% 6% 15% 22% 16% 2	2395
	21st to end	Night	36% 5% 11% 24% 20% 4	1884
March	1st to 10th	Night	32% 6% 9% 27% 22% 3	3453
	11th to 20th	Night	30% 5% 9% 30% 20% 5%	3480
	21st to end	Night	48% 10% 21% 16% 5% 1	3817
April	1st to 10th	Night	36% 6% 12% 26% 17% 3	2028
	11th to 20th	Night	37% 6% 12% 25% 17% 3	1777
	21st to end	Night	40% 6% 12% 23% 17% 1	1710
May	1st to 10th	Night	40% 6% 12% 22% 17% 2	2850
	11th to 20th	Night	34% 7% 12% 25% 21% 2	2824
	21st to end	Night	39% 8% 13% 19% 19% 3	2949
June	1st to 10th	Night	25% 9% 14% 32% 16% 4	1520
	11th to 20th	Night	24% 10% 14% 29% 19% 4	1520
	21st to end	Night	23% 8% 15% 30% 21% 3	1520
July	1st to 10th	Night	30% 6% 14% 30% 17% 3	2733
	11th to 20th	Night	19% 6% 15% 35% 22% 3	2796
	21st to end	Night	19% 8% 17% 32% 23% 2	2999
August	1st to 10th	Night	21% 6% 15% 35% 20% 2	2826
	11th to 20th	Night	33% 5% 14% 28% 19% 2	2825
	21st to end	Night	26% 5% 16% 27% 18% 8%	3223
September	1st to 10th	Night	26% 6% 11% 28% 23% 7%	1961
	11th to 20th	Night	25% 6% 13% 33% 20% 3	2090
	21st to end	Night	26% 6% 8% 27% 28% 4	2090
October	1st to 10th	Night	20% 4 8% 17% 48% 4	3702
	11th to 20th	Night	21% 3 7% 29% 30% 9%	3779
	21st to end	Night	34% 3 9% 29% 23% 1	4120
November	1st to 10th	Night	27% 5% 15% 25% 23% 4	2497
	11th to 20th	Night	32% 4 13% 29% 19% 3	2667
	21st to end	Night	35% 4 12% 28% 18% 2	2800
December	1st to 10th	Night	29% 5% 14% 35% 16% 1	4580
	11th to 20th	Night	38% 5% 16% 28% 13%	5201
	21st to end	Night	54% 5% 11% 22% 7% 1	6859

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	21% 10% 37% 22% 9% 2	288
	11th to 20th	Day	21% 12% 35% 22% 9% 1	270
	21st to end	Day	15% 10% 43% 22% 8% 2	297
February	1st to 10th	Day	17% 14% 44% 20% 4	270
	11th to 20th	Day	17% 14% 39% 23% 6% 1	270
	21st to end	Day	16% 16% 40% 22% 6%	225
March	1st to 10th	Day	19% 12% 42% 21% 6%	270
	11th to 20th	Day	29% 8% 35% 20% 7% 1	270
	21st to end	Day	33% 12% 34% 18% 2 1	297
April	1st to 10th	Day	23% 15% 39% 20% 3	332
	11th to 20th	Day	21% 13% 36% 21% 8%	549
	21st to end	Day	23% 14% 32% 21% 9% 2	650
May	1st to 10th	Day	21% 10% 33% 26% 9% 1	653
	11th to 20th	Day	20% 6% 32% 27% 13% 2	675
	21st to end	Day	19% 6% 29% 29% 15% 2	734
June	1st to 10th	Day	7% 5% 22% 35% 25% 5%	680
	11th to 20th	Day	9% 5% 30% 36% 17% 4	680
	21st to end	Day	9% 5% 34% 35% 14% 3	680
July	1st to 10th	Day	12% 1 14% 34% 30% 9%	671
	11th to 20th	Day	8% 1 22% 39% 23% 8%	668
	21st to end	Day	7% 2 17% 41% 28% 5%	739
August	1st to 10th	Day	8% 1 18% 43% 26% 4	676
	11th to 20th	Day	10% 2 16% 41% 27% 4	665
	21st to end	Day	10% 1 21% 37% 24% 6%	696
September	1st to 10th	Day	12% 2 23% 38% 19% 5%	426
	11th to 20th	Day	8% 2 30% 33% 20% 6%	300
	21st to end	Day	15% 7% 31% 29% 15% 3	300
October	1st to 10th	Day	20% 5% 26% 29% 14% 6%	300
	11th to 20th	Day	12% 14% 42% 21% 9% 1	300
	21st to end	Day	18% 14% 38% 20% 9% 2	330
November	1st to 10th	Day	19% 9% 34% 28% 8% 2	300
	11th to 20th	Day	17% 14% 39% 22% 7%	300
	21st to end	Day	21% 12% 34% 23% 8% 1	300
December	1st to 10th	Day	8% 12% 41% 31% 6% 2	300
	11th to 20th	Day	15% 14% 34% 28% 9% 1	300
	21st to end	Day	21% 12% 39% 22% 4 1	330
January	1st to 10th	Night	13% 11% 35% 34% 8% 1	981
	11th to 20th	Night	15% 12% 35% 30% 8%	950
	21st to end	Night	17% 7% 36% 32% 6%	1045
February	1st to 10th	Night	16% 10% 39% 31% 5%	950
	11th to 20th	Night	12% 10% 41% 31% 5%	920
	21st to end	Night	14% 9% 45% 26% 5%	628
March	1st to 10th	Night	14% 11% 39% 29% 7% 1	593
	11th to 20th	Night	20% 8% 34% 31% 8%	570
	21st to end	Night	24% 9% 37% 24% 6%	627
April	1st to 10th	Night	17% 14% 40% 24% 5%	570
	11th to 20th	Night	15% 13% 38% 29% 4 1	563
	21st to end	Night	21% 8% 35% 29% 7% 1	570
May	1st to 10th	Night	19% 9% 30% 35% 7%	570
	11th to 20th	Night	16% 8% 29% 37% 10% 1	561
	21st to end	Night	18% 5% 29% 33% 15%	617
June	1st to 10th	Night	8% 2 24% 49% 16%	570
	11th to 20th	Night	9% 2 30% 44% 14% 1	570
	21st to end	Night	6% 3 31% 46% 14% 1	570
July	1st to 10th	Night	9% 16% 42% 31% 2	555
	11th to 20th	Night	4 1 18% 50% 26% 1	552
	21st to end	Night	4 15% 53% 27% 1	624
August	1st to 10th	Night	5% 1 17% 49% 25% 2	567
	11th to 20th	Night	7% 2 16% 49% 25%	564
	21st to end	Night	11% 1 19% 41% 25% 4	627
September	1st to 10th	Night	9% 3 22% 40% 24% 2	570
	11th to 20th	Night	10% 2 30% 39% 19% 1	570
	21st to end	Night	13% 3 31% 36% 16% 1	570
October	1st to 10th	Night	14% 6% 31% 34% 12% 2	701
	11th to 20th	Night	14% 9% 35% 34% 8%	760
	21st to end	Night	18% 9% 31% 32% 9%	836
November	1st to 10th	Night	12% 10% 31% 37% 10% 1	937
	11th to 20th	Night	14% 11% 38% 31% 6%	1000
	21st to end	Night	16% 10% 34% 31% 9%	1000
December	1st to 10th	Night	13% 10% 38% 33% 7%	1000
	11th to 20th	Night	17% 9% 34% 34% 6% 1	1000
	21st to end	Night	21% 11% 32% 32% 5%	1100

Page generated at 2025-01-04 02:23:57. Version ac07

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