Bell Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bell Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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% 8% 3 7% 5% 5% 5% 4 6% 7% 39%	125611
	Night	13% 8% 3 7% 4 4 4 3 5% 7% 41%	122535
Transparency	Day	41% 7% 11% 23% 16% 3	129277
	Night	40% 6% 10% 23% 18% 4	126940
Seeing	Day	16% 10% 23% 24% 18% 8%	16279
	Night	17% 9% 24% 30% 18% 2	25985

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% 10% 7% 12% 5% 5% 5% 3 5% 5% 29%	3617
	11th to 20th	Day	14% 9% 3 10% 5% 3 4 2 4 4 41%	3660
	21st to end	Day	19% 9% 5% 6% 3 3 4 3 4 6% 37%	4802
February	1st to 10th	Day	18% 11% 3 8% 4 4 3 3 5% 6% 34%	2000
	11th to 20th	Day	21% 10% 4 8% 5% 3 4 3 4 6% 34%	1948
	21st to end	Day	17% 11% 4 6% 4 3 3 3 5% 6% 38%	1650
March	1st to 10th	Day	17% 11% 4 8% 4 4 4 3 4 5% 36%	4480
	11th to 20th	Day	13% 8% 3 6% 4 3 3 3 5% 5% 47%	4784
	21st to end	Day	12% 7% 3 7% 5% 5% 4 4 5% 6% 43%	5755
April	1st to 10th	Day	14% 7% 2 6% 5% 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	13% 8% 3 7% 5% 4 3 4 5% 6% 42%	2374
	21st to end	Day	14% 7% 3 6% 5% 5% 4 4 5% 7% 39%	2472
May	1st to 10th	Day	17% 10% 3 9% 5% 5% 6% 5% 6% 8% 27%	5862
	11th to 20th	Day	15% 8% 3 9% 5% 5% 5% 4 6% 8% 32%	5861
	21st to end	Day	13% 7% 3 8% 5% 5% 5% 4 6% 8% 38%	6433
June	1st to 10th	Day	9% 5% 3 7% 5% 5% 5% 5% 6% 8% 41%	2667
	11th to 20th	Day	6% 5% 2 5% 4 5% 6% 6% 7% 10% 44%	2800
	21st to end	Day	5% 4 1 5% 5% 5% 6% 6% 8% 12% 42%	2800
July	1st to 10th	Day	7% 6% 2 8% 6% 6% 6% 5% 7% 10% 39%	6360
	11th to 20th	Day	6% 6% 3 7% 6% 6% 7% 5% 7% 12% 34%	6324
	21st to end	Day	11% 8% 3 9% 6% 6% 6% 5% 8% 10% 29%	6424
August	1st to 10th	Day	5% 4 1 4 3 4 5% 4 7% 10% 52%	4039
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 7% 10% 40%	3729
	21st to end	Day	5% 4 2 4 5% 5% 5% 4 7% 9% 52%	4180
September	1st to 10th	Day	12% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	2251
	11th to 20th	Day	12% 6% 2 6% 4 6% 4 3 5% 7% 44%	2090
	21st to end	Day	11% 6% 3 6% 4 3 4 3 5% 6% 49%	2090
October	1st to 10th	Day	9% 4 2 5% 4 3 3 3 4 6% 55%	2090
	11th to 20th	Day	12% 5% 1 6% 4 4 3 2 4 6% 53%	2036
	21st to end	Day	18% 6% 2 6% 3 3 3 3 4 5% 46%	1881
November	1st to 10th	Day	12% 5% 2 5% 3 3 3 3 4 6% 53%	1762
	11th to 20th	Day	14% 8% 3 5% 3 3 4 3 5% 6% 47%	1800
	21st to end	Day	16% 9% 3 6% 3 4 3 3 5% 4 44%	1800
December	1st to 10th	Day	16% 8% 2 7% 4 4 3 3 4 5% 44%	2930
	11th to 20th	Day	17% 12% 4 8% 5% 4 4 3 5% 6% 32%	3373
	21st to end	Day	18% 12% 5% 9% 5% 5% 5% 3 5% 6% 27%	4087
January	1st to 10th	Night	16% 12% 4 9% 5% 4 4 3 5% 6% 34%	6348
	11th to 20th	Night	18% 11% 3 8% 5% 5% 3 2 3 4 38%	6299
	21st to end	Night	19% 10% 4 6% 3 3 3 2 3 5% 40%	6150
February	1st to 10th	Night	21% 10% 4 9% 5% 4 4 3 4 5% 33%	2400
	11th to 20th	Night	21% 9% 3 9% 5% 4 4 2 4 5% 35%	2338
	21st to end	Night	22% 9% 3 8% 4 3 3 4 4 6% 34%	1980
March	1st to 10th	Night	18% 11% 3 8% 4 3 4 3 4 6% 36%	5373
	11th to 20th	Night	11% 6% 2 5% 3 3 4 3 6% 7% 49%	5174
	21st to end	Night	13% 8% 3 8% 5% 4 5% 3 5% 7% 39%	5203
April	1st to 10th	Night	12% 6% 2 6% 3 4 5% 4 5% 8% 45%	2000
	11th to 20th	Night	15% 6% 2 6% 4 3 5% 3 5% 6% 45%	1975
	21st to end	Night	12% 8% 3 7% 4 4 4 4 6% 8% 40%	1925
May	1st to 10th	Night	15% 10% 3 9% 5% 4 5% 4 6% 8% 31%	4033
	11th to 20th	Night	13% 9% 3 9% 5% 5% 6% 4 7% 8% 31%	4032
	21st to end	Night	11% 7% 3 8% 5% 5% 5% 5% 6% 9% 37%	4421
June	1st to 10th	Night	10% 6% 2 6% 3 4 4 5% 6% 8% 47%	1728
	11th to 20th	Night	4 3 2 4 4 4 4 4 7% 10% 53%	1600
	21st to end	Night	3 3 1 4 5% 5% 6% 4 8% 12% 49%	1600
July	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 5% 7% 9% 45%	3618
	11th to 20th	Night	5% 5% 2 7% 5% 5% 5% 5% 8% 12% 43%	3634
	21st to end	Night	7% 7% 3 9% 6% 6% 7% 5% 7% 10% 32%	4427
August	1st to 10th	Night	4 3 1 5% 3 3 4 3 5% 8% 61%	2974
	11th to 20th	Night	4 3 2 5% 4 5% 6% 4 8% 11% 49%	3100
	21st to end	Night	4 3 1 4 3 3 4 3 6% 9% 60%	3476
September	1st to 10th	Night	10% 5% 1 5% 4 4 5% 4 4 6% 51%	1909
	11th to 20th	Night	10% 5% 2 7% 3 3 4 4 5% 8% 49%	2090
	21st to end	Night	11% 5% 2 5% 3 3 3 3 5% 6% 53%	2090
October	1st to 10th	Night	9% 4 1 3 2 3 3 2 4 6% 62%	2090
	11th to 20th	Night	14% 6% 2 5% 2 3 3 3 4 6% 52%	2142
	21st to end	Night	14% 7% 2 5% 4 3 3 3 5% 6% 48%	2717
November	1st to 10th	Night	12% 5% 2 4 4 3 4 3 5% 6% 52%	2536
	11th to 20th	Night	14% 8% 1 4 3 4 3 3 5% 7% 48%	2600
	21st to end	Night	16% 8% 3 6% 4 4 3 3 4 5% 46%	2600
December	1st to 10th	Night	17% 11% 3 7% 4 3 4 3 3 4 41%	4250
	11th to 20th	Night	19% 8% 4 7% 4 3 4 3 4 6% 37%	4822
	21st to end	Night	19% 12% 5% 10% 6% 5% 5% 3 5% 5% 26%	6881

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	56% 5% 10% 17% 10% 1	3618
	11th to 20th	Day	48% 3 5% 23% 19% 2	3660
	21st to end	Day	48% 4 7% 22% 16% 4	4802
February	1st to 10th	Day	52% 4 10% 21% 13% 2	2000
	11th to 20th	Day	53% 4 7% 20% 13% 3	1958
	21st to end	Day	47% 4 9% 19% 15% 5%	1650
March	1st to 10th	Day	51% 4 4 17% 19% 5%	4480
	11th to 20th	Day	40% 4 8% 25% 19% 5%	4784
	21st to end	Day	42% 5% 10% 28% 14% 1	5757
April	1st to 10th	Day	41% 6% 11% 22% 18% 2	2400
	11th to 20th	Day	42% 5% 9% 23% 19% 3	2374
	21st to end	Day	43% 6% 10% 25% 14% 2	2472
May	1st to 10th	Day	52% 9% 12% 17% 9%	5897
	11th to 20th	Day	48% 9% 12% 20% 10%	5865
	21st to end	Day	42% 7% 11% 21% 16% 3	6447
June	1st to 10th	Day	39% 7% 11% 23% 18% 2	2667
	11th to 20th	Day	31% 10% 16% 24% 18% 2	2800
	21st to end	Day	31% 10% 16% 23% 17% 4	2800
July	1st to 10th	Day	37% 9% 14% 23% 15% 1	6291
	11th to 20th	Day	38% 11% 17% 25% 7% 1	6249
	21st to end	Day	45% 10% 17% 21% 5% 1	6240
August	1st to 10th	Day	24% 8% 15% 33% 20%	4026
	11th to 20th	Day	36% 9% 12% 26% 17% 1	3765
	21st to end	Day	26% 8% 10% 34% 19% 2	4179
September	1st to 10th	Day	39% 9% 10% 19% 19% 4	2251
	11th to 20th	Day	38% 6% 10% 23% 21% 2	2090
	21st to end	Day	35% 6% 10% 23% 22% 4	2090
October	1st to 10th	Day	20% 4 6% 17% 47% 6%	3480
	11th to 20th	Day	24% 4 7% 27% 31% 8%	3355
	21st to end	Day	30% 3 6% 27% 27% 7%	3081
November	1st to 10th	Day	33% 4 8% 24% 26% 5%	1762
	11th to 20th	Day	38% 4 7% 23% 22% 6%	1800
	21st to end	Day	43% 4 8% 24% 19% 4	1800
December	1st to 10th	Day	42% 4 7% 26% 20% 1	2930
	11th to 20th	Day	52% 5% 8% 19% 13% 2	3370
	21st to end	Day	56% 6% 10% 20% 7%	4087
January	1st to 10th	Night	51% 5% 9% 22% 12% 1	6348
	11th to 20th	Night	52% 4 5% 19% 17% 3	6299
	21st to end	Night	48% 4 6% 20% 19% 4	6150
February	1st to 10th	Night	55% 4 9% 18% 13% 1	2400
	11th to 20th	Night	53% 4 6% 19% 14% 3	2339
	21st to end	Night	52% 5% 8% 16% 16% 3	1980
March	1st to 10th	Night	50% 4 7% 16% 18% 5%	5373
	11th to 20th	Night	33% 5% 9% 24% 19% 10%	5174
	21st to end	Night	44% 5% 9% 25% 14% 2	5203
April	1st to 10th	Night	36% 6% 11% 25% 19% 3	2000
	11th to 20th	Night	39% 5% 10% 19% 21% 6%	1975
	21st to end	Night	41% 6% 10% 24% 17% 2	1925
May	1st to 10th	Night	49% 8% 12% 21% 9% 1	4060
	11th to 20th	Night	48% 9% 13% 19% 11%	4033
	21st to end	Night	41% 9% 14% 22% 14% 1	4429
June	1st to 10th	Night	33% 7% 13% 26% 17% 4	1728
	11th to 20th	Night	25% 8% 14% 28% 23% 3	1600
	21st to end	Night	25% 10% 17% 27% 15% 6%	1600
July	1st to 10th	Night	31% 10% 14% 27% 17% 1	3580
	11th to 20th	Night	32% 10% 19% 28% 10% 2	3589
	21st to end	Night	42% 11% 17% 22% 7% 1	4297
August	1st to 10th	Night	21% 6% 12% 38% 21% 1	2963
	11th to 20th	Night	26% 10% 18% 29% 16% 1	3130
	21st to end	Night	20% 7% 12% 37% 23% 3	3476
September	1st to 10th	Night	32% 8% 10% 23% 22% 6%	1909
	11th to 20th	Night	33% 4 9% 26% 22% 5%	2090
	21st to end	Night	31% 5% 8% 24% 25% 7%	2090
October	1st to 10th	Night	17% 4 6% 17% 46% 10%	3480
	11th to 20th	Night	22% 3 5% 24% 36% 11%	3562
	21st to end	Night	27% 4 7% 23% 28% 11%	4467
November	1st to 10th	Night	34% 5% 7% 24% 25% 6%	2536
	11th to 20th	Night	35% 5% 7% 23% 22% 7%	2600
	21st to end	Night	42% 4 7% 25% 19% 4	2600
December	1st to 10th	Night	48% 3 8% 20% 20% 1	4250
	11th to 20th	Night	47% 4 9% 22% 16% 2	4824
	21st to end	Night	60% 6% 8% 18% 7% 1	6881

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	25% 20% 33% 16% 6% 1	300
	11th to 20th	Day	31% 22% 30% 12% 4	300
	21st to end	Day	21% 22% 41% 13% 3	330
February	1st to 10th	Day	27% 20% 38% 12% 3 1	300
	11th to 20th	Day	34% 22% 33% 9% 3	300
	21st to end	Day	30% 20% 34% 15% 1	249
March	1st to 10th	Day	28% 23% 30% 17% 2 1	300
	11th to 20th	Day	27% 19% 33% 15% 5% 1	408
	21st to end	Day	22% 17% 37% 15% 7% 1	550
April	1st to 10th	Day	21% 20% 26% 24% 8% 1	500
	11th to 20th	Day	19% 17% 28% 24% 10% 2	492
	21st to end	Day	20% 13% 33% 20% 11% 3	500
May	1st to 10th	Day	20% 16% 28% 23% 10% 4	503
	11th to 20th	Day	16% 9% 25% 28% 13% 9%	527
	21st to end	Day	12% 6% 23% 29% 17% 13%	573
June	1st to 10th	Day	11% 6% 21% 26% 23% 13%	597
	11th to 20th	Day	7% 5% 17% 31% 27% 13%	730
	21st to end	Day	6% 3 18% 32% 26% 14%	730
July	1st to 10th	Day	11% 3 13% 26% 28% 19%	721
	11th to 20th	Day	7% 5% 14% 26% 31% 17%	708
	21st to end	Day	10% 3 12% 28% 27% 19%	576
August	1st to 10th	Day	12% 3 10% 28% 30% 17%	502
	11th to 20th	Day	11% 4 13% 31% 30% 10%	479
	21st to end	Day	10% 2 7% 25% 32% 24%	539
September	1st to 10th	Day	13% 2 12% 28% 32% 14%	490
	11th to 20th	Day	13% 3 11% 26% 33% 15%	490
	21st to end	Day	14% 8% 19% 24% 25% 10%	490
October	1st to 10th	Day	10% 3 19% 31% 29% 9%	490
	11th to 20th	Day	15% 10% 31% 25% 13% 5%	445
	21st to end	Day	24% 14% 26% 26% 9% 1	330
November	1st to 10th	Day	17% 11% 31% 25% 13% 2	300
	11th to 20th	Day	18% 20% 31% 23% 7%	300
	21st to end	Day	23% 15% 30% 19% 12% 1	300
December	1st to 10th	Day	23% 19% 32% 18% 7% 1	300
	11th to 20th	Day	25% 16% 30% 23% 6%	300
	21st to end	Day	28% 17% 34% 17% 4	330
January	1st to 10th	Night	22% 17% 32% 25% 5%	1000
	11th to 20th	Night	27% 17% 30% 22% 4	978
	21st to end	Night	24% 14% 39% 22% 2	880
February	1st to 10th	Night	28% 14% 33% 21% 4	800
	11th to 20th	Night	28% 17% 33% 20% 2	783
	21st to end	Night	26% 14% 37% 20% 2	660
March	1st to 10th	Night	24% 13% 30% 27% 6%	800
	11th to 20th	Night	21% 15% 33% 25% 6% 1	800
	21st to end	Night	20% 14% 30% 28% 7%	771
April	1st to 10th	Night	16% 14% 32% 30% 9%	600
	11th to 20th	Night	18% 10% 31% 29% 11% 1	593
	21st to end	Night	16% 9% 34% 31% 10% 1	600
May	1st to 10th	Night	16% 7% 30% 30% 15% 3	600
	11th to 20th	Night	13% 7% 22% 36% 18% 4	591
	21st to end	Night	6% 2 20% 38% 28% 5%	650
June	1st to 10th	Night	12% 3 16% 37% 28% 4	600
	11th to 20th	Night	6% 2 10% 42% 38% 3	600
	21st to end	Night	4 2 13% 40% 35% 7%	600
July	1st to 10th	Night	8% 2 8% 30% 42% 9%	585
	11th to 20th	Night	8% 3 9% 28% 46% 6%	582
	21st to end	Night	9% 3 10% 33% 39% 6%	657
August	1st to 10th	Night	8% 2 5% 35% 43% 7%	579
	11th to 20th	Night	7% 1 8% 43% 37% 4	564
	21st to end	Night	7% 1 5% 30% 49% 7%	627
September	1st to 10th	Night	11% 1 10% 31% 41% 4	579
	11th to 20th	Night	10% 3 9% 32% 39% 6%	760
	21st to end	Night	13% 4 16% 31% 31% 5%	760
October	1st to 10th	Night	10% 4 16% 36% 29% 4	760
	11th to 20th	Night	17% 7% 24% 32% 18% 2	760
	21st to end	Night	16% 8% 31% 29% 15% 1	836
November	1st to 10th	Night	14% 8% 26% 34% 16% 2	780
	11th to 20th	Night	19% 10% 31% 29% 11%	800
	21st to end	Night	20% 12% 27% 30% 11% 1	800
December	1st to 10th	Night	26% 10% 30% 28% 6%	800
	11th to 20th	Night	24% 11% 32% 27% 7% 1	800
	21st to end	Night	27% 13% 31% 25% 4	1050

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