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 2024-01-17.

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	11% 7% 3 6% 5% 3 4 4 5% 7% 44%	74646
	Night	11% 7% 3 5% 5% 2 4 4 5% 7% 48%	75183
Transparency	Day	39% 7% 14% 25% 14% 2	74369
	Night	35% 6% 13% 26% 17% 3	74980
Seeing	Day	15% 7% 29% 30% 15% 3	14634
	Night	14% 7% 32% 35% 11% 1	24654

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	11% 10% 4 7% 7% 3 5% 4 5% 8% 37%	1707
	11th to 20th	Day	10% 9% 5% 8% 5% 3 4 5% 5% 6% 39%	1674
	21st to end	Day	10% 11% 4 10% 7% 3 5% 5% 5% 7% 33%	1782
February	1st to 10th	Day	10% 11% 6% 8% 7% 4 5% 5% 5% 7% 33%	1620
	11th to 20th	Day	11% 9% 4 8% 7% 3 5% 4 6% 7% 38%	1596
	21st to end	Day	12% 7% 4 6% 6% 4 4 4 5% 8% 40%	1480
March	1st to 10th	Day	10% 10% 5% 6% 4 3 4 4 4 6% 43%	1957
	11th to 20th	Day	15% 9% 2 7% 5% 3 4 3 3 6% 41%	1980
	21st to end	Day	18% 13% 4 6% 5% 3 4 5% 5% 7% 30%	2178
April	1st to 10th	Day	16% 7% 3 6% 4 3 3 3 4 5% 44%	2038
	11th to 20th	Day	16% 7% 3 6% 5% 3 5% 4 5% 6% 39%	2225
	21st to end	Day	20% 7% 3 5% 5% 3 4 4 5% 6% 38%	2340
May	1st to 10th	Day	15% 7% 3 7% 6% 3 5% 4 5% 6% 38%	2340
	11th to 20th	Day	14% 7% 3 6% 6% 3 5% 4 4 9% 39%	2302
	21st to end	Day	15% 7% 3 7% 5% 4 5% 6% 6% 8% 34%	2607
June	1st to 10th	Day	7% 5% 2 5% 5% 3 4 5% 6% 10% 48%	2520
	11th to 20th	Day	8% 4 2 5% 5% 3 5% 5% 5% 9% 48%	2520
	21st to end	Day	8% 4 2 5% 5% 2 6% 6% 6% 8% 49%	2520
July	1st to 10th	Day	8% 4 2 3 3 3 4 4 7% 11% 51%	2377
	11th to 20th	Day	6% 4 2 4 4 2 5% 5% 5% 8% 54%	2340
	21st to end	Day	6% 3 2 4 5% 3 5% 5% 7% 11% 50%	2546
August	1st to 10th	Day	7% 4 2 6% 5% 4 6% 6% 7% 10% 43%	2328
	11th to 20th	Day	7% 4 2 4 5% 3 5% 5% 6% 9% 49%	2301
	21st to end	Day	8% 4 2 5% 4 3 4 5% 5% 9% 52%	2525
September	1st to 10th	Day	12% 5% 2 4 4 2 2 3 4 7% 54%	2100
	11th to 20th	Day	11% 5% 2 4 4 3 4 3 5% 7% 54%	1980
	21st to end	Day	11% 4 2 4 4 2 4 4 4 6% 55%	1980
October	1st to 10th	Day	15% 5% 2 4 3 3 3 4 4 7% 50%	1836
	11th to 20th	Day	12% 6% 2 4 4 2 3 3 5% 5% 56%	1800
	21st to end	Day	13% 8% 3 5% 5% 3 3 3 4 6% 47%	1980
November	1st to 10th	Day	9% 5% 3 4 4 1 4 4 4 6% 56%	1705
	11th to 20th	Day	10% 7% 3 6% 6% 2 5% 5% 4 6% 46%	1644
	21st to end	Day	13% 8% 4 4 3 3 4 4 4 8% 44%	1520
December	1st to 10th	Day	9% 7% 4 6% 6% 3 4 4 5% 7% 45%	1520
	11th to 20th	Day	9% 10% 3 8% 7% 3 5% 4 5% 8% 37%	1918
	21st to end	Day	16% 10% 5% 14% 8% 3 4 5% 4 5% 25%	2860
January	1st to 10th	Night	8% 7% 4 8% 7% 3 5% 5% 5% 7% 41%	2458
	11th to 20th	Night	9% 11% 6% 7% 5% 3 4 4 5% 7% 39%	2431
	21st to end	Night	12% 9% 4 7% 5% 2 4 4 4 7% 41%	2574
February	1st to 10th	Night	12% 9% 4 9% 6% 3 5% 4 4 7% 37%	2340
	11th to 20th	Night	10% 7% 4 7% 5% 3 5% 5% 6% 7% 42%	2264
	21st to end	Night	10% 7% 3 6% 5% 3 4 4 5% 6% 47%	1776
March	1st to 10th	Night	10% 7% 4 6% 3 2 4 4 5% 5% 49%	2002
	11th to 20th	Night	14% 9% 3 6% 3 2 3 4 4 6% 44%	1980
	21st to end	Night	18% 11% 4 7% 5% 2 4 5% 6% 7% 32%	2178
April	1st to 10th	Night	12% 8% 2 5% 5% 2 4 4 5% 6% 47%	1922
	11th to 20th	Night	12% 7% 3 6% 4 3 4 3 5% 7% 46%	1683
	21st to end	Night	17% 7% 3 5% 5% 2 4 3 3 8% 44%	1620
May	1st to 10th	Night	15% 6% 3 6% 6% 2 5% 5% 6% 7% 40%	1620
	11th to 20th	Night	14% 5% 2 5% 5% 3 3 5% 5% 7% 46%	1593
	21st to end	Night	14% 7% 3 5% 6% 3 5% 4 5% 9% 41%	1665
June	1st to 10th	Night	5% 3 1 5% 4 3 5% 4 5% 9% 55%	1440
	11th to 20th	Night	6% 4 2 3 4 2 5% 6% 8% 10% 51%	1440
	21st to end	Night	4 3 1 4 4 3 4 5% 6% 9% 56%	1440
July	1st to 10th	Night	6% 3 2 4 4 2 3 4 6% 9% 57%	1576
	11th to 20th	Night	3 2 1 3 3 2 3 4 7% 10% 62%	1620
	21st to end	Night	4 3 1 3 4 2 4 5% 6% 10% 59%	1772
August	1st to 10th	Night	5% 3 2 4 5% 3 5% 5% 6% 9% 53%	1611
	11th to 20th	Night	6% 4 1 4 4 2 4 4 5% 9% 58%	1600
	21st to end	Night	8% 3 2 3 3 2 4 4 4 7% 59%	1831
September	1st to 10th	Night	9% 4 2 3 4 2 4 5% 4 7% 57%	1857
	11th to 20th	Night	9% 3 1 3 3 3 3 4 6% 7% 58%	1980
	21st to end	Night	9% 4 2 4 3 2 3 3 5% 6% 60%	1980
October	1st to 10th	Night	13% 5% 2 4 4 2 3 5% 5% 6% 51%	2104
	11th to 20th	Night	14% 5% 1 3 4 2 3 3 4 6% 54%	2160
	21st to end	Night	13% 7% 2 4 3 3 4 4 4 5% 52%	2376
November	1st to 10th	Night	9% 4 2 3 4 2 4 3 5% 8% 57%	2369
	11th to 20th	Night	10% 6% 3 5% 5% 2 4 4 5% 6% 53%	2533
	21st to end	Night	12% 7% 3 5% 4 2 3 3 5% 6% 51%	2660
December	1st to 10th	Night	8% 7% 3 6% 6% 3 5% 4 6% 6% 45%	2660
	11th to 20th	Night	9% 9% 2 6% 5% 3 4 4 5% 6% 47%	3279
	21st to end	Night	18% 14% 6% 10% 5% 2 3 3 3 4 31%	4789

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	45% 6% 16% 21% 11% 1	1708
	11th to 20th	Day	44% 5% 16% 23% 11%	1674
	21st to end	Day	50% 6% 16% 19% 8% 1	1782
February	1st to 10th	Day	49% 6% 15% 18% 10% 1	1620
	11th to 20th	Day	45% 6% 11% 23% 12% 3	1605
	21st to end	Day	42% 6% 14% 22% 14% 2	1480
March	1st to 10th	Day	42% 6% 14% 24% 13% 2	1957
	11th to 20th	Day	45% 6% 11% 23% 13% 2	1980
	21st to end	Day	53% 8% 15% 16% 7% 1	2178
April	1st to 10th	Day	42% 6% 13% 25% 13% 1	2038
	11th to 20th	Day	44% 7% 12% 23% 12% 2	2225
	21st to end	Day	46% 7% 12% 23% 13% 1	2340
May	1st to 10th	Day	46% 8% 13% 19% 13% 1	2340
	11th to 20th	Day	42% 8% 15% 24% 12%	2306
	21st to end	Day	46% 9% 14% 19% 10% 1	2607
June	1st to 10th	Day	31% 8% 17% 27% 15% 2	2520
	11th to 20th	Day	33% 7% 14% 29% 16% 1	2520
	21st to end	Day	32% 9% 15% 27% 13% 4	2520
July	1st to 10th	Day	27% 7% 16% 32% 14% 3	2313
	11th to 20th	Day	27% 8% 14% 27% 19% 5%	2271
	21st to end	Day	26% 9% 18% 30% 16% 1	2405
August	1st to 10th	Day	32% 12% 19% 25% 11% 2	2314
	11th to 20th	Day	29% 9% 15% 30% 17% 1	2301
	21st to end	Day	30% 9% 11% 25% 20% 5%	2525
September	1st to 10th	Day	31% 6% 12% 28% 18% 5%	2100
	11th to 20th	Day	31% 6% 12% 32% 18% 1	1980
	21st to end	Day	29% 6% 10% 32% 23% 1	1980
October	1st to 10th	Day	34% 6% 13% 25% 21% 1	1836
	11th to 20th	Day	32% 4 10% 25% 24% 5%	1800
	21st to end	Day	38% 5% 12% 21% 19% 4	1980
November	1st to 10th	Day	29% 5% 14% 29% 20% 3	1705
	11th to 20th	Day	38% 5% 14% 24% 18% 1	1644
	21st to end	Day	40% 4 12% 28% 13% 1	1520
December	1st to 10th	Day	38% 7% 16% 26% 13% 1	1520
	11th to 20th	Day	44% 6% 16% 27% 6%	1915
	21st to end	Day	60% 7% 10% 17% 6%	2860
January	1st to 10th	Night	41% 6% 15% 22% 16% 1	2458
	11th to 20th	Night	43% 6% 15% 27% 9%	2431
	21st to end	Night	43% 6% 16% 24% 10% 2	2574
February	1st to 10th	Night	47% 5% 13% 19% 12% 3	2340
	11th to 20th	Night	39% 6% 16% 22% 15% 3	2265
	21st to end	Night	37% 6% 12% 24% 18% 4	1776
March	1st to 10th	Night	37% 6% 11% 28% 16% 3	2002
	11th to 20th	Night	41% 5% 10% 25% 17% 3	1980
	21st to end	Night	51% 7% 12% 20% 8% 2	2178
April	1st to 10th	Night	37% 6% 11% 25% 17% 3	1922
	11th to 20th	Night	38% 6% 12% 26% 16% 2	1683
	21st to end	Night	40% 6% 13% 22% 18% 1	1620
May	1st to 10th	Night	42% 7% 14% 22% 13% 2	1620
	11th to 20th	Night	39% 6% 14% 23% 17% 1	1594
	21st to end	Night	41% 8% 18% 18% 12% 3	1666
June	1st to 10th	Night	24% 10% 15% 33% 16% 3	1440
	11th to 20th	Night	24% 10% 14% 29% 19% 4	1440
	21st to end	Night	22% 8% 15% 30% 20% 3	1440
July	1st to 10th	Night	25% 6% 16% 30% 19% 4	1531
	11th to 20th	Night	18% 8% 18% 30% 21% 5%	1566
	21st to end	Night	18% 7% 17% 34% 20% 3	1673
August	1st to 10th	Night	27% 10% 17% 32% 14% 1	1601
	11th to 20th	Night	25% 5% 15% 33% 20% 3	1600
	21st to end	Night	24% 6% 14% 26% 21% 9%	1831
September	1st to 10th	Night	27% 6% 12% 27% 22% 7%	1857
	11th to 20th	Night	25% 6% 13% 34% 20% 3	1980
	21st to end	Night	25% 6% 9% 28% 28% 4	1980
October	1st to 10th	Night	32% 6% 12% 24% 23% 3	2104
	11th to 20th	Night	31% 4 9% 24% 26% 6%	2160
	21st to end	Night	35% 5% 12% 22% 24% 2	2376
November	1st to 10th	Night	26% 5% 15% 26% 24% 5%	2369
	11th to 20th	Night	32% 5% 13% 29% 18% 3	2533
	21st to end	Night	35% 4 12% 28% 19% 2	2660
December	1st to 10th	Night	37% 7% 13% 29% 13% 1	2660
	11th to 20th	Night	36% 5% 15% 34% 10%	3281
	21st to end	Night	57% 6% 10% 19% 7% 1	4789

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% 36% 22% 9% 2	270
	11th to 20th	Day	21% 12% 37% 21% 9% 1	258
	21st to end	Day	15% 11% 45% 23% 5%	264
February	1st to 10th	Day	17% 15% 45% 20% 3	240
	11th to 20th	Day	19% 15% 39% 21% 5% 1	240
	21st to end	Day	18% 16% 39% 23% 5%	198
March	1st to 10th	Day	18% 13% 44% 19% 6%	240
	11th to 20th	Day	31% 7% 34% 20% 7% 1	240
	21st to end	Day	34% 13% 32% 18% 2 1	264
April	1st to 10th	Day	22% 15% 39% 21% 3	298
	11th to 20th	Day	22% 12% 36% 21% 9%	503
	21st to end	Day	23% 14% 32% 21% 9% 2	600
May	1st to 10th	Day	21% 9% 33% 25% 10% 1	603
	11th to 20th	Day	20% 6% 33% 26% 14% 2	625
	21st to end	Day	20% 5% 28% 29% 16% 2	679
June	1st to 10th	Day	7% 5% 22% 34% 26% 6%	630
	11th to 20th	Day	8% 5% 28% 37% 17% 4	630
	21st to end	Day	10% 5% 32% 35% 15% 3	630
July	1st to 10th	Day	11% 1 13% 34% 31% 10%	621
	11th to 20th	Day	8% 1 22% 38% 23% 8%	618
	21st to end	Day	7% 2 16% 42% 28% 4	684
August	1st to 10th	Day	8% 1 17% 43% 26% 5%	626
	11th to 20th	Day	9% 2 16% 40% 29% 4	615
	21st to end	Day	10% 2 20% 37% 25% 7%	644
September	1st to 10th	Day	13% 3 23% 36% 20% 5%	390
	11th to 20th	Day	8% 2 30% 33% 20% 7%	270
	21st to end	Day	16% 6% 33% 29% 15% 2	270
October	1st to 10th	Day	22% 5% 27% 26% 13% 6%	270
	11th to 20th	Day	14% 15% 42% 20% 8% 1	270
	21st to end	Day	19% 14% 36% 19% 10% 2	297
November	1st to 10th	Day	16% 9% 35% 30% 8% 3	270
	11th to 20th	Day	16% 14% 42% 21% 8%	270
	21st to end	Day	22% 10% 33% 25% 8% 1	270
December	1st to 10th	Day	9% 12% 40% 31% 6% 2	270
	11th to 20th	Day	15% 12% 33% 29% 10% 1	270
	21st to end	Day	21% 13% 41% 21% 3	297
January	1st to 10th	Night	12% 11% 35% 34% 8% 1	950
	11th to 20th	Night	15% 12% 35% 30% 8%	935
	21st to end	Night	17% 8% 37% 31% 6%	990
February	1st to 10th	Night	16% 10% 39% 30% 5%	900
	11th to 20th	Night	13% 10% 42% 30% 5%	870
	21st to end	Night	15% 9% 44% 26% 6% 1	592
March	1st to 10th	Night	14% 12% 38% 28% 7% 1	562
	11th to 20th	Night	20% 8% 32% 31% 8%	540
	21st to end	Night	24% 9% 36% 25% 6%	594
April	1st to 10th	Night	16% 14% 40% 24% 5%	540
	11th to 20th	Night	15% 12% 39% 29% 5% 1	533
	21st to end	Night	21% 8% 34% 29% 7% 1	540
May	1st to 10th	Night	18% 9% 30% 36% 7%	540
	11th to 20th	Night	17% 9% 29% 35% 10% 1	531
	21st to end	Night	18% 5% 28% 33% 16% 1	584
June	1st to 10th	Night	8% 2 24% 49% 17%	540
	11th to 20th	Night	9% 2 30% 44% 15% 1	540
	21st to end	Night	5% 3 30% 46% 15% 1	540
July	1st to 10th	Night	8% 16% 42% 32% 2	525
	11th to 20th	Night	4 1 19% 49% 26% 1	522
	21st to end	Night	5% 1 15% 54% 27%	591
August	1st to 10th	Night	6% 1 17% 49% 26% 2	537
	11th to 20th	Night	7% 2 16% 49% 26%	534
	21st to end	Night	11% 1 18% 41% 25% 4	594
September	1st to 10th	Night	10% 3 22% 40% 23% 2	540
	11th to 20th	Night	10% 2 29% 38% 20% 1	540
	21st to end	Night	12% 3 32% 36% 15% 1	540
October	1st to 10th	Night	15% 6% 32% 33% 12% 3	664
	11th to 20th	Night	15% 9% 35% 33% 8%	720
	21st to end	Night	19% 9% 31% 33% 9%	792
November	1st to 10th	Night	10% 10% 32% 36% 11% 1	889
	11th to 20th	Night	14% 11% 38% 31% 6%	950
	21st to end	Night	16% 9% 34% 31% 9%	950
December	1st to 10th	Night	13% 10% 38% 32% 7%	950
	11th to 20th	Night	17% 8% 34% 34% 6% 1	950
	21st to end	Night	21% 11% 32% 31% 4	1045

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