3B Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the 3B Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	125510
	Night	13% 6% 2 6% 4 4 4 3 5% 7% 44%	123883
Transparency	Day	40% 4 12% 21% 18% 5%	129113
	Night	40% 4 10% 20% 20% 6%	128350
Seeing	Day	16% 8% 23% 26% 18% 9%	16582
	Night	16% 7% 24% 33% 18% 2	26937

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	15% 5% 2 5% 3 3 2 2 3 3 56%	4056
	11th to 20th	Day	13% 6% 2 5% 2 3 3 2 3 4 58%	4060
	21st to end	Day	13% 8% 2 6% 4 4 4 3 4 5% 47%	4466
February	1st to 10th	Day	15% 6% 3 6% 5% 4 4 3 5% 8% 44%	1800
	11th to 20th	Day	22% 6% 2 5% 3 3 3 3 3 5% 46%	1797
	21st to end	Day	17% 8% 3 6% 4 4 3 4 4 5% 43%	1770
March	1st to 10th	Day	20% 8% 2 5% 4 3 4 3 5% 6% 40%	4935
	11th to 20th	Day	10% 5% 2 6% 4 4 3 3 4 6% 53%	4980
	21st to end	Day	14% 5% 2 5% 4 4 5% 3 5% 5% 48%	5465
April	1st to 10th	Day	11% 4 1 5% 4 3 3 3 6% 7% 53%	2345
	11th to 20th	Day	11% 5% 2 5% 4 3 4 3 5% 8% 50%	2374
	21st to end	Day	13% 5% 2 6% 4 3 4 3 6% 7% 46%	2400
May	1st to 10th	Day	9% 6% 3 8% 5% 5% 6% 5% 8% 10% 36%	5712
	11th to 20th	Day	18% 7% 3 8% 5% 5% 5% 5% 7% 9% 29%	5861
	21st to end	Day	12% 4 2 6% 4 5% 5% 4 7% 9% 42%	6433
June	1st to 10th	Day	9% 6% 3 6% 5% 4 6% 4 8% 9% 39%	2600
	11th to 20th	Day	9% 4 2 6% 4 5% 6% 5% 7% 10% 42%	2600
	21st to end	Day	8% 4 2 6% 5% 6% 6% 6% 8% 10% 39%	2600
July	1st to 10th	Day	8% 6% 2 9% 5% 6% 7% 6% 9% 11% 31%	5900
	11th to 20th	Day	8% 6% 3 9% 7% 7% 7% 6% 9% 12% 26%	5900
	21st to end	Day	8% 7% 2 8% 6% 6% 6% 6% 9% 11% 31%	6424
August	1st to 10th	Day	20% 10% 4 8% 5% 5% 6% 5% 7% 9% 21%	4234
	11th to 20th	Day	9% 5% 3 9% 5% 6% 6% 5% 8% 9% 33%	3987
	21st to end	Day	6% 4 2 6% 4 5% 5% 4 7% 10% 47%	4312
September	1st to 10th	Day	8% 5% 2 5% 4 5% 5% 5% 7% 8% 46%	2400
	11th to 20th	Day	15% 5% 2 6% 5% 4 5% 3 6% 8% 41%	2305
	21st to end	Day	19% 8% 2 7% 4 4 4 3 5% 7% 37%	2135
October	1st to 10th	Day	14% 6% 2 6% 3 3 4 3 4 6% 49%	1930
	11th to 20th	Day	11% 5% 1 5% 3 2 3 3 4 5% 57%	1900
	21st to end	Day	12% 4 2 5% 4 3 4 2 4 5% 57%	2090
November	1st to 10th	Day	14% 6% 2 4 3 3 3 3 4 6% 53%	1957
	11th to 20th	Day	14% 7% 2 4 3 3 3 2 5% 6% 51%	2000
	21st to end	Day	13% 5% 2 6% 3 3 3 3 5% 6% 52%	1657
December	1st to 10th	Day	12% 6% 2 6% 4 3 4 3 4 5% 52%	2600
	11th to 20th	Day	16% 6% 2 6% 4 4 4 3 5% 5% 46%	3059
	21st to end	Day	16% 6% 2 6% 4 4 3 3 5% 6% 45%	4466
January	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 5% 55%	5889
	11th to 20th	Night	14% 6% 2 5% 3 3 2 2 4 4 54%	5900
	21st to end	Night	14% 7% 2 5% 3 4 3 2 5% 6% 47%	6490
February	1st to 10th	Night	18% 6% 2 5% 4 3 4 4 5% 5% 44%	2600
	11th to 20th	Night	20% 7% 3 6% 4 3 4 3 4 6% 42%	2488
	21st to end	Night	15% 6% 2 4 4 4 3 4 5% 6% 48%	1855
March	1st to 10th	Night	18% 9% 3 7% 4 4 3 2 4 4 42%	4918
	11th to 20th	Night	12% 6% 2 4 3 3 4 3 4 6% 53%	4980
	21st to end	Night	13% 5% 2 5% 4 4 4 3 4 6% 50%	5467
April	1st to 10th	Night	14% 6% 2 4 3 3 3 3 5% 8% 48%	2054
	11th to 20th	Night	12% 5% 2 5% 4 4 3 3 5% 8% 49%	1975
	21st to end	Night	11% 4 1 5% 4 5% 5% 5% 6% 7% 47%	2000
May	1st to 10th	Night	11% 8% 3 8% 5% 5% 5% 4 6% 9% 37%	4224
	11th to 20th	Night	18% 8% 3 8% 5% 5% 4 3 5% 8% 34%	4032
	21st to end	Night	15% 5% 2 6% 4 4 4 3 5% 8% 43%	4421
June	1st to 10th	Night	11% 5% 2 5% 5% 5% 6% 5% 7% 9% 39%	1800
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 4 8% 9% 39%	1800
	21st to end	Night	6% 7% 2 6% 5% 5% 6% 6% 8% 11% 38%	1800
July	1st to 10th	Night	10% 7% 3 7% 5% 5% 6% 5% 9% 11% 30%	4058
	11th to 20th	Night	12% 8% 3 9% 6% 6% 6% 5% 9% 12% 24%	4057
	21st to end	Night	12% 8% 3 8% 5% 5% 6% 5% 8% 10% 30%	4427
August	1st to 10th	Night	22% 8% 3 9% 5% 5% 5% 5% 6% 8% 23%	2915
	11th to 20th	Night	7% 7% 3 8% 6% 6% 6% 5% 8% 10% 33%	3028
	21st to end	Night	6% 4 1 6% 4 5% 5% 4 8% 11% 46%	3586
September	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 9% 45%	2000
	11th to 20th	Night	13% 6% 3 7% 5% 4 5% 4 4 8% 42%	2090
	21st to end	Night	19% 6% 3 8% 4 5% 4 3 5% 6% 37%	2265
October	1st to 10th	Night	14% 6% 3 6% 3 4 3 3 5% 6% 50%	2317
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 4 6% 54%	2280
	21st to end	Night	12% 4 2 4 3 3 2 3 4 6% 57%	2508
November	1st to 10th	Night	13% 6% 2 4 3 3 3 3 4 5% 53%	2341
	11th to 20th	Night	14% 5% 2 5% 4 3 4 3 5% 6% 50%	2400
	21st to end	Night	13% 4 2 4 4 3 3 3 5% 6% 52%	2735
December	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 4 4 53%	4580
	11th to 20th	Night	16% 8% 3 8% 4 4 4 3 5% 5% 39%	5113
	21st to end	Night	14% 6% 2 5% 4 4 3 3 5% 6% 48%	6490

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	36% 1 3 20% 32% 8%	4057
	11th to 20th	Day	33% 2 3 16% 37% 9%	4060
	21st to end	Day	42% 3 4 21% 21% 9%	4466
February	1st to 10th	Day	41% 2 10% 21% 20% 5%	1800
	11th to 20th	Day	43% 3 6% 18% 23% 7%	1806
	21st to end	Day	44% 3 8% 17% 23% 6%	1770
March	1st to 10th	Day	47% 2 5% 17% 18% 10%	4935
	11th to 20th	Day	33% 2 7% 17% 25% 15%	4980
	21st to end	Day	40% 2 9% 23% 20% 7%	5465
April	1st to 10th	Day	30% 4 9% 25% 24% 8%	2345
	11th to 20th	Day	34% 3 10% 25% 21% 7%	2374
	21st to end	Day	38% 4 12% 24% 18% 4	2400
May	1st to 10th	Day	43% 3 19% 23% 11% 1	5745
	11th to 20th	Day	52% 4 12% 21% 10% 1	5865
	21st to end	Day	38% 4 13% 24% 19% 3	6447
June	1st to 10th	Day	40% 5% 15% 23% 16% 1	2600
	11th to 20th	Day	35% 7% 16% 25% 15% 2	2600
	21st to end	Day	36% 8% 16% 23% 15% 2	2600
July	1st to 10th	Day	44% 6% 19% 20% 9% 2	5835
	11th to 20th	Day	48% 8% 28% 12% 3 1	5831
	21st to end	Day	43% 7% 19% 21% 10%	6240
August	1st to 10th	Day	58% 5% 18% 14% 5%	4221
	11th to 20th	Day	44% 6% 17% 24% 9% 1	4026
	21st to end	Day	33% 4 14% 32% 16% 2	4312
September	1st to 10th	Day	35% 6% 15% 20% 19% 4	2400
	11th to 20th	Day	42% 5% 9% 20% 21% 4	2305
	21st to end	Day	47% 4 9% 21% 15% 4	2135
October	1st to 10th	Day	35% 3 8% 21% 24% 7%	3190
	11th to 20th	Day	21% 2 5% 20% 27% 25%	3140
	21st to end	Day	25% 2 5% 25% 25% 17%	3428
November	1st to 10th	Day	35% 2 7% 22% 27% 6%	1957
	11th to 20th	Day	35% 2 8% 21% 27% 7%	2000
	21st to end	Day	35% 2 7% 24% 25% 7%	1657
December	1st to 10th	Day	35% 3 8% 21% 25% 8%	2600
	11th to 20th	Day	42% 2 10% 19% 21% 5%	3055
	21st to end	Day	41% 2 10% 25% 20% 1	4466
January	1st to 10th	Night	33% 2 4 17% 34% 10%	5889
	11th to 20th	Night	36% 2 5% 16% 34% 8%	5900
	21st to end	Night	40% 2 5% 18% 28% 6%	6490
February	1st to 10th	Night	42% 2 8% 22% 21% 5%	2600
	11th to 20th	Night	46% 2 8% 16% 21% 7%	2490
	21st to end	Night	38% 3 7% 20% 23% 9%	1855
March	1st to 10th	Night	48% 3 5% 13% 22% 10%	4918
	11th to 20th	Night	34% 3 7% 14% 25% 18%	4980
	21st to end	Night	37% 2 8% 24% 23% 6%	5467
April	1st to 10th	Night	36% 4 9% 18% 24% 9%	2054
	11th to 20th	Night	35% 3 9% 24% 21% 7%	1975
	21st to end	Night	36% 4 11% 27% 18% 4	2000
May	1st to 10th	Night	45% 4 17% 19% 13% 2	4254
	11th to 20th	Night	51% 4 12% 20% 13% 1	4033
	21st to end	Night	39% 4 13% 24% 17% 2	4429
June	1st to 10th	Night	39% 7% 16% 21% 16% 1	1800
	11th to 20th	Night	39% 7% 16% 21% 15% 2	1800
	21st to end	Night	36% 10% 17% 21% 13% 3	1800
July	1st to 10th	Night	44% 8% 22% 17% 8% 1	4015
	11th to 20th	Night	51% 11% 23% 11% 4 1	4006
	21st to end	Night	47% 6% 17% 20% 9%	4297
August	1st to 10th	Night	58% 5% 17% 16% 4 1	2904
	11th to 20th	Night	44% 6% 20% 23% 7% 1	3055
	21st to end	Night	32% 5% 13% 30% 18% 2	3586
September	1st to 10th	Night	36% 7% 12% 23% 16% 5%	2000
	11th to 20th	Night	41% 5% 11% 19% 20% 3	2090
	21st to end	Night	49% 5% 11% 17% 15% 4	2265
October	1st to 10th	Night	38% 3 7% 21% 25% 6%	3837
	11th to 20th	Night	25% 2 5% 17% 28% 23%	3779
	21st to end	Night	22% 2 7% 29% 23% 17%	4120
November	1st to 10th	Night	35% 3 7% 20% 28% 8%	2341
	11th to 20th	Night	37% 3 7% 16% 26% 11%	2400
	21st to end	Night	34% 3 7% 23% 26% 8%	2735
December	1st to 10th	Night	36% 2 5% 19% 28% 9%	4580
	11th to 20th	Night	48% 2 9% 17% 17% 8%	5116
	21st to end	Night	38% 2 10% 26% 21% 2	6490

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	22% 21% 35% 17% 5%	300
	11th to 20th	Day	21% 24% 33% 16% 6%	300
	21st to end	Day	18% 19% 40% 18% 4	330
February	1st to 10th	Day	19% 16% 39% 21% 4 1	300
	11th to 20th	Day	35% 14% 34% 15% 2	344
	21st to end	Day	23% 17% 32% 21% 6%	414
March	1st to 10th	Day	23% 14% 31% 25% 7%	500
	11th to 20th	Day	20% 15% 37% 21% 6% 1	500
	21st to end	Day	19% 14% 35% 23% 6% 2	550
April	1st to 10th	Day	18% 16% 33% 22% 10% 2	500
	11th to 20th	Day	14% 14% 28% 25% 15% 3	492
	21st to end	Day	15% 9% 27% 27% 19% 2	500
May	1st to 10th	Day	13% 12% 32% 24% 14% 6%	503
	11th to 20th	Day	17% 6% 20% 29% 20% 8%	527
	21st to end	Day	20% 5% 15% 27% 20% 13%	573
June	1st to 10th	Day	11% 4 18% 32% 24% 11%	530
	11th to 20th	Day	10% 3 17% 30% 23% 16%	530
	21st to end	Day	8% 3 14% 29% 32% 14%	530
July	1st to 10th	Day	10% 2 7% 26% 33% 20%	525
	11th to 20th	Day	7% 2 7% 25% 35% 23%	524
	21st to end	Day	9% 2 9% 29% 33% 18%	576
August	1st to 10th	Day	16% 4 13% 24% 29% 14%	526
	11th to 20th	Day	12% 3 8% 25% 32% 19%	519
	21st to end	Day	11% 3 9% 29% 28% 21%	583
September	1st to 10th	Day	12% 2 16% 26% 24% 20%	530
	11th to 20th	Day	17% 2 13% 27% 27% 14%	530
	21st to end	Day	24% 5% 20% 26% 18% 7%	530
October	1st to 10th	Day	16% 5% 20% 24% 25% 10%	502
	11th to 20th	Day	12% 9% 22% 32% 19% 7%	490
	21st to end	Day	12% 8% 22% 31% 21% 6%	539
November	1st to 10th	Day	18% 6% 26% 30% 16% 4	495
	11th to 20th	Day	17% 9% 32% 28% 12% 2	500
	21st to end	Day	17% 10% 29% 27% 15% 3	245
December	1st to 10th	Day	19% 13% 37% 25% 6% 2	200
	11th to 20th	Day	21% 7% 35% 23% 12% 1	215
	21st to end	Day	23% 12% 34% 20% 10% 2	330
January	1st to 10th	Night	21% 12% 35% 26% 5% 1	800
	11th to 20th	Night	19% 11% 33% 30% 7%	800
	21st to end	Night	20% 9% 38% 29% 5%	880
February	1st to 10th	Night	21% 8% 40% 28% 4	800
	11th to 20th	Night	25% 11% 40% 21% 2	783
	21st to end	Night	19% 13% 36% 29% 4	660
March	1st to 10th	Night	16% 14% 32% 33% 4	800
	11th to 20th	Night	20% 10% 37% 26% 6% 1	800
	21st to end	Night	17% 10% 37% 30% 5%	880
April	1st to 10th	Night	19% 13% 34% 29% 7%	800
	11th to 20th	Night	15% 10% 32% 31% 11% 1	791
	21st to end	Night	13% 5% 32% 39% 10%	800
May	1st to 10th	Night	10% 9% 31% 37% 12%	690
	11th to 20th	Night	18% 4 20% 41% 15% 1	591
	21st to end	Night	13% 3 15% 38% 29% 2	650
June	1st to 10th	Night	13% 2 14% 44% 26% 2	600
	11th to 20th	Night	13% 3 14% 36% 30% 4	600
	21st to end	Night	8% 4 11% 40% 33% 4	600
July	1st to 10th	Night	10% 2 8% 37% 39% 4	585
	11th to 20th	Night	8% 3 6% 37% 38% 8%	582
	21st to end	Night	9% 3 10% 35% 39% 4	657
August	1st to 10th	Night	13% 2 8% 40% 34% 4	597
	11th to 20th	Night	12% 2 9% 34% 38% 5%	682
	21st to end	Night	12% 3 9% 33% 37% 7%	880
September	1st to 10th	Night	14% 2 9% 34% 37% 6%	800
	11th to 20th	Night	15% 3 11% 34% 34% 3	800
	21st to end	Night	23% 4 15% 32% 25% 2	800
October	1st to 10th	Night	17% 3 16% 32% 27% 4	773
	11th to 20th	Night	15% 6% 22% 34% 21% 2	760
	21st to end	Night	13% 5% 26% 32% 21% 2	836
November	1st to 10th	Night	18% 4 24% 35% 16% 3	780
	11th to 20th	Night	17% 7% 27% 35% 14% 1	800
	21st to end	Night	15% 7% 29% 33% 15% 1	800
December	1st to 10th	Night	22% 9% 31% 33% 6%	800
	11th to 20th	Night	19% 7% 30% 36% 9%	800
	21st to end	Night	19% 9% 29% 33% 9%	880

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