Bobservatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bobservatory clear sky chart from 2005-11-06 to 2025-09-19.

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	16% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	126186
	Night	16% 9% 3 8% 5% 5% 5% 4 5% 7% 34%	122870
Transparency	Day	47% 7% 12% 20% 12% 2	129693
	Night	46% 7% 12% 20% 13% 2	127450
Seeing	Day	24% 19% 29% 18% 7% 2	15987
	Night	22% 18% 28% 24% 8% 1	24848

By Month

Percentage Cloud Cover NE (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% 10% 4 14% 8% 6% 6% 4 6% 5% 28%	3597
	11th to 20th	Day	12% 9% 5% 11% 5% 5% 5% 5% 6% 7% 31%	3570
	21st to end	Day	12% 12% 5% 8% 5% 5% 4 4 5% 5% 35%	3960
February	1st to 10th	Day	14% 11% 4 8% 6% 5% 6% 4 6% 6% 30%	1788
	11th to 20th	Day	10% 11% 4 10% 6% 6% 5% 3 6% 7% 32%	1948
	21st to end	Day	15% 13% 3 8% 5% 4 6% 4 5% 6% 31%	1650
March	1st to 10th	Day	18% 13% 4 10% 6% 5% 4 4 4 4 28%	4476
	11th to 20th	Day	15% 9% 4 8% 5% 5% 5% 4 6% 6% 33%	4695
	21st to end	Day	22% 10% 3 7% 4 4 5% 3 5% 7% 31%	5895
April	1st to 10th	Day	18% 11% 3 8% 5% 5% 4 3 4 6% 33%	2400
	11th to 20th	Day	17% 8% 2 7% 5% 5% 4 4 6% 7% 35%	2374
	21st to end	Day	21% 9% 3 8% 5% 5% 4 3 5% 6% 32%	2600
May	1st to 10th	Day	31% 10% 3 8% 5% 4 4 3 4 5% 21%	5886
	11th to 20th	Day	19% 10% 4 9% 5% 5% 5% 4 6% 7% 27%	6217
	21st to end	Day	18% 7% 2 7% 5% 5% 5% 4 7% 8% 31%	6949
June	1st to 10th	Day	18% 8% 3 8% 6% 5% 5% 4 6% 8% 30%	2800
	11th to 20th	Day	17% 6% 3 6% 6% 5% 4 4 6% 9% 35%	2800
	21st to end	Day	18% 8% 3 8% 6% 5% 5% 4 5% 7% 30%	2800
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 5% 7% 10% 33%	6360
	11th to 20th	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 10% 36%	6360
	21st to end	Day	15% 5% 2 6% 4 5% 5% 4 6% 8% 40%	6926
August	1st to 10th	Day	14% 6% 2 8% 6% 6% 6% 4 7% 9% 33%	4417
	11th to 20th	Day	14% 7% 3 8% 5% 5% 5% 4 7% 8% 33%	4086
	21st to end	Day	10% 4 2 6% 5% 5% 6% 5% 8% 9% 41%	4312
September	1st to 10th	Day	12% 5% 2 6% 4 5% 4 3 6% 8% 45%	2400
	11th to 20th	Day	14% 5% 2 5% 3 3 4 3 5% 6% 50%	2212
	21st to end	Day	15% 6% 2 6% 4 4 4 3 6% 8% 43%	2090
October	1st to 10th	Day	21% 9% 4 7% 4 3 4 3 5% 5% 36%	2090
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 4 5% 8% 39%	1815
	21st to end	Day	21% 11% 2 8% 4 4 5% 4 4 5% 32%	1881
November	1st to 10th	Day	12% 7% 3 7% 5% 4 4 4 5% 8% 42%	1761
	11th to 20th	Day	16% 7% 2 6% 5% 5% 4 3 5% 6% 39%	1800
	21st to end	Day	19% 10% 3 8% 4 4 4 4 5% 6% 33%	1725
December	1st to 10th	Day	13% 11% 4 11% 6% 7% 6% 3 6% 8% 26%	2600
	11th to 20th	Day	17% 13% 4 7% 4 4 3 3 5% 6% 33%	2986
	21st to end	Day	19% 12% 4 10% 4 4 4 3 4 5% 31%	3960
January	1st to 10th	Night	12% 7% 4 13% 9% 5% 5% 3 5% 6% 30%	6349
	11th to 20th	Night	10% 10% 3 11% 7% 6% 7% 5% 6% 7% 27%	6315
	21st to end	Night	13% 10% 4 8% 6% 5% 5% 4 6% 6% 32%	6996
February	1st to 10th	Night	15% 10% 4 7% 5% 5% 6% 5% 6% 7% 30%	2605
	11th to 20th	Night	11% 9% 4 8% 6% 6% 6% 5% 7% 8% 30%	2338
	21st to end	Night	13% 11% 3 8% 5% 5% 5% 4 6% 7% 34%	1980
March	1st to 10th	Night	20% 10% 3 7% 5% 5% 4 4 6% 6% 30%	5364
	11th to 20th	Night	14% 10% 4 8% 6% 5% 5% 4 5% 6% 33%	5263
	21st to end	Night	23% 11% 3 7% 4 4 4 3 6% 6% 31%	5022
April	1st to 10th	Night	19% 9% 2 7% 5% 5% 5% 3 5% 6% 33%	2000
	11th to 20th	Night	17% 11% 3 8% 4 4 3 3 5% 6% 36%	1975
	21st to end	Night	18% 9% 3 7% 5% 4 4 3 5% 7% 35%	1800
May	1st to 10th	Night	34% 10% 2 7% 4 4 4 3 4 5% 22%	4054
	11th to 20th	Night	16% 12% 4 9% 6% 5% 5% 4 5% 6% 29%	3718
	21st to end	Night	19% 8% 3 8% 6% 5% 5% 4 6% 7% 31%	3926
June	1st to 10th	Night	20% 6% 2 8% 5% 5% 5% 3 6% 7% 33%	1600
	11th to 20th	Night	18% 7% 3 6% 5% 4 6% 3 5% 6% 38%	1600
	21st to end	Night	18% 6% 2 7% 5% 5% 5% 5% 7% 7% 35%	1600
July	1st to 10th	Night	11% 6% 2 7% 4 4 5% 5% 7% 9% 40%	3600
	11th to 20th	Night	13% 7% 3 7% 5% 5% 5% 4 6% 7% 39%	3600
	21st to end	Night	12% 5% 2 5% 4 4 4 4 5% 7% 48%	3926
August	1st to 10th	Night	21% 7% 3 7% 5% 4 4 3 5% 7% 35%	2699
	11th to 20th	Night	14% 8% 4 7% 5% 4 4 3 5% 8% 38%	2957
	21st to end	Night	9% 4 1 5% 4 4 4 4 7% 10% 49%	3586
September	1st to 10th	Night	13% 5% 2 4 4 3 3 3 5% 6% 51%	2000
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 51%	2144
	21st to end	Night	15% 5% 2 5% 4 4 4 3 6% 7% 45%	2090
October	1st to 10th	Night	20% 7% 3 7% 4 5% 4 3 5% 5% 37%	2090
	11th to 20th	Night	15% 8% 2 8% 5% 5% 4 4 5% 6% 39%	2363
	21st to end	Night	22% 10% 2 6% 5% 4 4 3 5% 6% 32%	2717
November	1st to 10th	Night	14% 7% 3 7% 5% 4 4 3 5% 7% 41%	2537
	11th to 20th	Night	15% 8% 3 6% 5% 5% 4 4 5% 6% 39%	2600
	21st to end	Night	14% 10% 4 7% 4 5% 5% 4 5% 7% 34%	2672
December	1st to 10th	Night	14% 12% 5% 10% 6% 6% 5% 4 5% 6% 26%	4580
	11th to 20th	Night	21% 11% 3 7% 5% 4 4 4 5% 5% 32%	5208
	21st to end	Night	19% 12% 4 9% 4 4 4 4 5% 6% 29%	6996

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	53% 7% 10% 22% 9%	3598
	11th to 20th	Day	48% 8% 11% 19% 12% 3	3600
	21st to end	Day	48% 6% 8% 19% 15% 3	3960
February	1st to 10th	Day	51% 7% 13% 18% 9% 2	1788
	11th to 20th	Day	49% 7% 12% 19% 11% 2	1957
	21st to end	Day	51% 7% 11% 18% 13% 1	1650
March	1st to 10th	Day	57% 5% 8% 13% 10% 7%	4479
	11th to 20th	Day	47% 7% 16% 18% 9% 2	4695
	21st to end	Day	51% 6% 6% 15% 18% 4	5895
April	1st to 10th	Day	51% 6% 8% 19% 13% 3	2400
	11th to 20th	Day	46% 8% 10% 18% 16% 2	2374
	21st to end	Day	52% 6% 8% 17% 14% 3	2600
May	1st to 10th	Day	64% 6% 9% 14% 6% 1	5897
	11th to 20th	Day	54% 8% 13% 16% 8% 1	6221
	21st to end	Day	46% 9% 13% 22% 10% 1	6949
June	1st to 10th	Day	49% 8% 12% 19% 10% 1	2800
	11th to 20th	Day	44% 8% 13% 18% 15% 2	2800
	21st to end	Day	50% 9% 12% 17% 12% 1	2800
July	1st to 10th	Day	44% 7% 14% 21% 13%	6290
	11th to 20th	Day	39% 9% 15% 23% 14% 1	6285
	21st to end	Day	40% 7% 12% 27% 13%	6728
August	1st to 10th	Day	45% 8% 13% 22% 11%	4403
	11th to 20th	Day	45% 8% 16% 24% 7%	4125
	21st to end	Day	34% 8% 14% 28% 14% 1	4312
September	1st to 10th	Day	36% 7% 13% 27% 16% 1	2400
	11th to 20th	Day	35% 6% 9% 28% 21% 2	2212
	21st to end	Day	39% 6% 13% 25% 15% 2	2090
October	1st to 10th	Day	45% 6% 9% 22% 17% 1	3480
	11th to 20th	Day	40% 5% 13% 24% 15% 3	2992
	21st to end	Day	39% 4 8% 31% 14% 3	3081
November	1st to 10th	Day	40% 5% 15% 23% 15% 1	1761
	11th to 20th	Day	42% 7% 13% 18% 18% 2	1800
	21st to end	Day	49% 7% 16% 19% 8% 1	1725
December	1st to 10th	Day	51% 7% 16% 21% 5% 1	2600
	11th to 20th	Day	49% 6% 10% 18% 16% 1	2986
	21st to end	Day	55% 5% 7% 16% 13% 5%	3960
January	1st to 10th	Night	51% 7% 11% 20% 10% 1	6349
	11th to 20th	Night	50% 10% 13% 16% 10% 2	6360
	21st to end	Night	48% 7% 10% 16% 15% 4	6996
February	1st to 10th	Night	48% 9% 12% 19% 11% 1	2605
	11th to 20th	Night	46% 9% 13% 19% 12% 2	2340
	21st to end	Night	46% 7% 13% 21% 9% 3	1980
March	1st to 10th	Night	50% 6% 10% 15% 15% 4	5374
	11th to 20th	Night	49% 7% 16% 16% 10% 2	5264
	21st to end	Night	51% 6% 6% 14% 20% 3	5022
April	1st to 10th	Night	50% 7% 10% 18% 14% 2	2000
	11th to 20th	Night	48% 5% 11% 20% 15% 1	1975
	21st to end	Night	47% 6% 12% 17% 16% 3	1800
May	1st to 10th	Night	62% 8% 10% 12% 7% 1	4060
	11th to 20th	Night	53% 9% 12% 17% 8% 1	3719
	21st to end	Night	50% 8% 14% 17% 10% 1	3927
June	1st to 10th	Night	48% 8% 12% 20% 12%	1600
	11th to 20th	Night	44% 9% 15% 19% 12% 2	1600
	21st to end	Night	45% 10% 12% 22% 11% 1	1600
July	1st to 10th	Night	38% 7% 17% 26% 11%	3560
	11th to 20th	Night	41% 9% 14% 22% 14%	3552
	21st to end	Night	34% 6% 13% 33% 12% 1	3809
August	1st to 10th	Night	49% 7% 13% 20% 10%	2689
	11th to 20th	Night	44% 7% 17% 21% 11% 1	2984
	21st to end	Night	28% 7% 18% 32% 15% 1	3586
September	1st to 10th	Night	33% 6% 14% 28% 18% 1	2000
	11th to 20th	Night	34% 6% 11% 28% 21% 1	2144
	21st to end	Night	37% 6% 12% 28% 15% 2	2090
October	1st to 10th	Night	43% 6% 15% 23% 11% 1	3480
	11th to 20th	Night	36% 6% 12% 24% 19% 3	3925
	21st to end	Night	42% 5% 10% 30% 12% 1	4467
November	1st to 10th	Night	42% 6% 13% 23% 15% 2	2537
	11th to 20th	Night	42% 6% 13% 21% 16% 2	2600
	21st to end	Night	44% 9% 16% 21% 9% 1	2672
December	1st to 10th	Night	52% 8% 13% 22% 5%	4580
	11th to 20th	Night	51% 6% 12% 13% 17% 1	5208
	21st to end	Night	53% 6% 10% 16% 11% 4	6996

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	29% 27% 28% 12% 4	200
	11th to 20th	Day	30% 25% 33% 11% 1 1	200
	21st to end	Day	25% 33% 27% 11% 3	220
February	1st to 10th	Day	33% 32% 25% 9% 1	323
	11th to 20th	Day	32% 29% 26% 11% 1	496
	21st to end	Day	28% 30% 28% 13% 2	414
March	1st to 10th	Day	32% 22% 32% 12% 2	500
	11th to 20th	Day	31% 28% 25% 12% 4 1	500
	21st to end	Day	33% 22% 28% 10% 5% 2	550
April	1st to 10th	Day	38% 19% 26% 11% 5% 1	500
	11th to 20th	Day	32% 24% 26% 12% 5% 1	492
	21st to end	Day	28% 25% 28% 12% 5% 2	500
May	1st to 10th	Day	31% 23% 27% 14% 4 1	503
	11th to 20th	Day	20% 21% 36% 16% 5% 2	527
	21st to end	Day	21% 16% 32% 23% 8% 1	573
June	1st to 10th	Day	20% 16% 37% 16% 9% 1	530
	11th to 20th	Day	19% 18% 32% 19% 9% 2	530
	21st to end	Day	23% 16% 34% 18% 8% 2	530
July	1st to 10th	Day	17% 13% 32% 24% 12% 2	525
	11th to 20th	Day	15% 9% 34% 31% 9% 2	524
	21st to end	Day	13% 12% 38% 28% 7% 2	576
August	1st to 10th	Day	12% 11% 31% 28% 15% 3	526
	11th to 20th	Day	15% 11% 31% 24% 13% 6%	519
	21st to end	Day	12% 14% 31% 26% 13% 4	583
September	1st to 10th	Day	15% 12% 29% 23% 15% 5%	530
	11th to 20th	Day	16% 14% 26% 19% 16% 9%	522
	21st to end	Day	18% 16% 28% 21% 12% 5%	490
October	1st to 10th	Day	26% 15% 25% 18% 12% 3	490
	11th to 20th	Day	22% 20% 28% 21% 7% 2	445
	21st to end	Day	32% 18% 26% 18% 6% 1	429
November	1st to 10th	Day	24% 24% 22% 20% 10% 1	395
	11th to 20th	Day	30% 23% 25% 17% 5% 1	400
	21st to end	Day	36% 20% 25% 14% 4 1	325
December	1st to 10th	Day	27% 29% 26% 16% 3	200
	11th to 20th	Day	35% 27% 27% 11% 2	200
	21st to end	Day	33% 22% 28% 12% 4	220
January	1st to 10th	Night	23% 26% 31% 19% 2	800
	11th to 20th	Night	25% 24% 30% 18% 3 1	800
	21st to end	Night	20% 25% 33% 19% 3	880
February	1st to 10th	Night	26% 23% 33% 17% 2	800
	11th to 20th	Night	24% 26% 34% 15% 2	783
	21st to end	Night	26% 22% 31% 19% 2	660
March	1st to 10th	Night	27% 21% 31% 19% 2	800
	11th to 20th	Night	25% 22% 28% 21% 4	800
	21st to end	Night	28% 20% 27% 19% 5%	880
April	1st to 10th	Night	28% 20% 28% 20% 3	800
	11th to 20th	Night	24% 25% 27% 19% 6%	791
	21st to end	Night	25% 19% 31% 21% 4	600
May	1st to 10th	Night	28% 15% 31% 22% 5%	600
	11th to 20th	Night	19% 17% 31% 28% 5%	459
	21st to end	Night	19% 8% 29% 33% 10%	433
June	1st to 10th	Night	22% 10% 27% 31% 10% 1	400
	11th to 20th	Night	19% 11% 29% 31% 10%	400
	21st to end	Night	19% 9% 26% 39% 8%	400
July	1st to 10th	Night	15% 6% 28% 38% 12% 1	390
	11th to 20th	Night	13% 7% 23% 39% 16% 2	388
	21st to end	Night	13% 5% 27% 39% 14% 1	438
August	1st to 10th	Night	15% 5% 21% 42% 16% 1	459
	11th to 20th	Night	12% 8% 29% 32% 16% 3	638
	21st to end	Night	15% 8% 24% 32% 19% 2	880
September	1st to 10th	Night	15% 10% 28% 29% 18% 2	800
	11th to 20th	Night	14% 10% 24% 28% 20% 4	793
	21st to end	Night	17% 10% 27% 28% 16% 2	760
October	1st to 10th	Night	24% 14% 20% 27% 13% 1	760
	11th to 20th	Night	19% 18% 27% 24% 11% 1	760
	21st to end	Night	27% 19% 26% 21% 6%	836
November	1st to 10th	Night	19% 21% 27% 25% 8% 1	780
	11th to 20th	Night	22% 23% 28% 19% 7% 1	800
	21st to end	Night	24% 22% 29% 19% 5% 1	800
December	1st to 10th	Night	23% 24% 33% 15% 4	800
	11th to 20th	Night	28% 23% 29% 17% 3	800
	21st to end	Night	28% 24% 28% 18% 2	880

Page generated at 2025-09-16 01:49:13. Version ac07

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