Bear Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bear Observatory clear sky chart from 2005-11-06 to 2025-03-12.

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	13% 8% 3 7% 4 4 4 4 5% 7% 40%	109004
	Night	14% 9% 3 7% 4 4 4 3 5% 6% 41%	110079
Transparency	Day	42% 5% 10% 24% 17% 2	112700
	Night	42% 4 9% 23% 18% 3	114522
Seeing	Day	17% 11% 24% 24% 17% 8%	15688
	Night	18% 8% 25% 29% 17% 2	25576

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% 12% 8% 11% 4 4 4 4 5% 5% 28%	3617
	11th to 20th	Day	14% 10% 4 13% 5% 4 4 3 3 4 36%	3681
	21st to end	Day	21% 9% 6% 6% 3 3 4 3 4 6% 35%	4673
February	1st to 10th	Day	18% 12% 4 7% 4 4 3 3 4 6% 34%	2000
	11th to 20th	Day	20% 9% 4 8% 5% 4 4 3 5% 6% 33%	1948
	21st to end	Day	18% 10% 4 7% 5% 4 4 4 5% 5% 34%	1650
March	1st to 10th	Day	23% 14% 3 6% 4 3 3 2 3 5% 32%	3220
	11th to 20th	Day	14% 7% 2 5% 4 3 3 3 4 7% 47%	3381
	21st to end	Day	12% 8% 3 7% 5% 5% 4 3 5% 6% 41%	3965
April	1st to 10th	Day	13% 7% 2 7% 5% 4 4 4 6% 7% 41%	2280
	11th to 20th	Day	14% 8% 3 7% 6% 3 4 4 4 5% 42%	2254
	21st to end	Day	16% 7% 3 6% 5% 5% 4 3 5% 8% 38%	2387
May	1st to 10th	Day	19% 8% 3 7% 4 5% 5% 4 6% 8% 31%	4082
	11th to 20th	Day	17% 8% 3 6% 4 4 4 4 5% 6% 38%	4081
	21st to end	Day	11% 6% 3 7% 5% 5% 5% 4 6% 9% 39%	4476
June	1st to 10th	Day	10% 5% 3 7% 5% 5% 5% 4 7% 7% 43%	2535
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 8% 10% 46%	2660
	21st to end	Day	7% 5% 2 5% 5% 5% 6% 5% 9% 10% 43%	2660
July	1st to 10th	Day	9% 5% 3 7% 6% 5% 5% 5% 7% 9% 39%	4440
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 4 7% 10% 44%	4413
	21st to end	Day	13% 12% 4 10% 6% 6% 5% 5% 7% 8% 25%	4466
August	1st to 10th	Day	5% 4 2 5% 4 4 4 4 7% 10% 51%	4086
	11th to 20th	Day	8% 5% 2 6% 4 5% 6% 5% 7% 10% 41%	3729
	21st to end	Day	5% 3 2 5% 4 3 5% 4 5% 9% 55%	4180
September	1st to 10th	Day	12% 6% 3 7% 4 5% 5% 4 5% 7% 42%	2234
	11th to 20th	Day	12% 6% 2 6% 5% 4 4 3 5% 8% 45%	2090
	21st to end	Day	14% 5% 2 6% 3 4 4 3 5% 7% 48%	2090
October	1st to 10th	Day	10% 5% 2 5% 3 3 3 2 4 7% 56%	2090
	11th to 20th	Day	13% 6% 2 4 4 4 4 3 4 6% 51%	2020
	21st to end	Day	18% 7% 2 6% 3 4 4 3 4 5% 44%	1881
November	1st to 10th	Day	12% 6% 2 5% 4 2 3 3 4 6% 54%	1762
	11th to 20th	Day	16% 8% 2 5% 4 3 3 3 5% 6% 45%	1800
	21st to end	Day	17% 9% 3 6% 4 3 4 3 4 6% 42%	1800
December	1st to 10th	Day	17% 9% 2 7% 4 4 3 2 4 5% 41%	2930
	11th to 20th	Day	18% 11% 5% 10% 5% 5% 4 3 4 5% 31%	3373
	21st to end	Day	20% 15% 4 8% 6% 5% 4 3 4 5% 25%	4070
January	1st to 10th	Night	16% 15% 5% 10% 5% 3 4 3 5% 6% 28%	6348
	11th to 20th	Night	17% 12% 5% 9% 5% 5% 4 2 3 4 33%	6292
	21st to end	Night	20% 11% 3 6% 4 3 3 2 3 5% 40%	6278
February	1st to 10th	Night	21% 11% 3 8% 4 4 4 3 5% 6% 30%	2400
	11th to 20th	Night	21% 9% 4 8% 5% 4 4 3 3 4 35%	2338
	21st to end	Night	21% 10% 3 8% 4 4 4 3 4 5% 33%	1980
March	1st to 10th	Night	22% 14% 3 6% 4 3 4 3 4 4 32%	3853
	11th to 20th	Night	13% 7% 2 6% 4 3 3 3 4 6% 49%	3606
	21st to end	Night	12% 8% 3 8% 5% 4 4 3 5% 6% 42%	3694
April	1st to 10th	Night	11% 6% 2 5% 5% 3 5% 4 6% 7% 45%	1900
	11th to 20th	Night	16% 7% 2 6% 4 3 4 3 5% 7% 43%	1875
	21st to end	Night	15% 9% 3 6% 5% 4 4 4 6% 7% 38%	1791
May	1st to 10th	Night	18% 10% 3 8% 5% 4 4 3 6% 7% 33%	2813
	11th to 20th	Night	15% 6% 2 6% 4 4 4 3 6% 7% 41%	2812
	21st to end	Night	7% 5% 2 7% 5% 5% 6% 4 7% 10% 42%	3079
June	1st to 10th	Night	9% 6% 1 5% 4 3 4 5% 6% 9% 47%	1640
	11th to 20th	Night	5% 4 2 4 4 4 4 4 7% 11% 52%	1520
	21st to end	Night	3 4 2 4 5% 4 5% 5% 8% 13% 48%	1520
July	1st to 10th	Night	8% 4 2 5% 4 5% 4 4 6% 10% 47%	2530
	11th to 20th	Night	6% 3 1 4 3 4 4 4 6% 11% 53%	2551
	21st to end	Night	9% 9% 3 11% 8% 7% 5% 5% 7% 7% 29%	3087
August	1st to 10th	Night	5% 4 2 5% 3 3 3 3 5% 8% 59%	2840
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 7% 10% 49%	3100
	21st to end	Night	4 3 1 3 3 3 3 3 5% 8% 63%	3476
September	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 5% 7% 50%	1925
	11th to 20th	Night	10% 5% 2 5% 4 4 3 3 5% 8% 50%	2090
	21st to end	Night	12% 4 2 5% 3 4 3 4 4 6% 52%	2090
October	1st to 10th	Night	9% 4 2 4 2 3 3 2 4 6% 61%	2090
	11th to 20th	Night	14% 5% 2 6% 3 3 3 3 4 5% 52%	2155
	21st to end	Night	16% 7% 2 5% 3 4 4 3 3 6% 47%	2717
November	1st to 10th	Night	13% 6% 2 5% 4 3 4 3 4 4 52%	2536
	11th to 20th	Night	14% 7% 2 5% 4 4 4 3 5% 7% 46%	2600
	21st to end	Night	17% 9% 3 5% 4 3 3 3 4 5% 42%	2600
December	1st to 10th	Night	17% 13% 3 7% 4 3 3 3 4 4 40%	4250
	11th to 20th	Night	22% 9% 4 7% 3 3 3 3 4 6% 36%	4822
	21st to end	Night	21% 16% 5% 10% 6% 5% 4 3 3 4 23%	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	58% 3 9% 20% 9% 1	3618
	11th to 20th	Day	54% 3 6% 20% 16% 2	3681
	21st to end	Day	52% 3 8% 20% 13% 4	4673
February	1st to 10th	Day	52% 4 9% 21% 13% 2	2000
	11th to 20th	Day	53% 4 8% 20% 12% 3	1958
	21st to end	Day	51% 4 9% 18% 14% 5%	1650
March	1st to 10th	Day	56% 3 5% 18% 13% 4	3220
	11th to 20th	Day	38% 4 10% 27% 18% 3	3381
	21st to end	Day	43% 5% 11% 25% 15% 1	3965
April	1st to 10th	Day	40% 6% 11% 23% 17% 2	2280
	11th to 20th	Day	44% 4 9% 21% 18% 3	2254
	21st to end	Day	44% 6% 10% 25% 12% 2	2387
May	1st to 10th	Day	49% 8% 13% 20% 10%	4120
	11th to 20th	Day	46% 7% 10% 23% 14%	4085
	21st to end	Day	41% 8% 12% 17% 20% 2	4489
June	1st to 10th	Day	38% 7% 11% 23% 19% 2	2535
	11th to 20th	Day	30% 9% 15% 25% 19% 2	2660
	21st to end	Day	33% 9% 17% 20% 18% 4	2660
July	1st to 10th	Day	41% 6% 14% 24% 13% 2	4371
	11th to 20th	Day	34% 6% 12% 35% 11% 2	4338
	21st to end	Day	56% 7% 13% 17% 7% 1	4324
August	1st to 10th	Day	27% 7% 14% 35% 17% 1	4073
	11th to 20th	Day	37% 5% 13% 27% 17% 1	3765
	21st to end	Day	26% 5% 10% 39% 16% 3	4179
September	1st to 10th	Day	40% 7% 12% 19% 18% 4	2234
	11th to 20th	Day	37% 6% 9% 23% 22% 3	2090
	21st to end	Day	36% 5% 9% 26% 21% 4	2090
October	1st to 10th	Day	20% 3 5% 22% 46% 4	3480
	11th to 20th	Day	24% 3 10% 26% 32% 5%	3327
	21st to end	Day	32% 3 6% 29% 24% 8%	3081
November	1st to 10th	Day	33% 4 8% 28% 22% 5%	1762
	11th to 20th	Day	41% 3 8% 22% 21% 5%	1800
	21st to end	Day	45% 3 9% 23% 18% 2	1800
December	1st to 10th	Day	46% 2 7% 25% 18% 1	2930
	11th to 20th	Day	58% 2 8% 17% 13% 2	3370
	21st to end	Day	60% 4 11% 18% 7%	4070
January	1st to 10th	Night	57% 3 9% 21% 10% 1	6348
	11th to 20th	Night	57% 3 5% 18% 14% 3	6292
	21st to end	Night	49% 2 7% 19% 20% 2	6278
February	1st to 10th	Night	56% 5% 9% 18% 12% 1	2400
	11th to 20th	Night	53% 4 6% 20% 14% 2	2339
	21st to end	Night	54% 4 8% 17% 14% 3	1980
March	1st to 10th	Night	56% 4 6% 17% 14% 3	3853
	11th to 20th	Night	38% 4 10% 27% 14% 8%	3606
	21st to end	Night	43% 5% 11% 23% 17% 1	3694
April	1st to 10th	Night	36% 7% 11% 23% 20% 3	1900
	11th to 20th	Night	41% 5% 8% 17% 22% 6%	1875
	21st to end	Night	44% 7% 9% 24% 15% 2	1791
May	1st to 10th	Night	49% 8% 11% 22% 8% 2	2840
	11th to 20th	Night	41% 8% 12% 25% 13% 1	2813
	21st to end	Night	34% 10% 14% 22% 18% 2	3087
June	1st to 10th	Night	32% 7% 13% 26% 18% 3	1640
	11th to 20th	Night	26% 7% 15% 26% 24% 3	1520
	21st to end	Night	26% 10% 18% 22% 19% 6%	1520
July	1st to 10th	Night	32% 6% 14% 30% 16% 2	2490
	11th to 20th	Night	26% 6% 13% 37% 16% 3	2503
	21st to end	Night	53% 7% 12% 18% 9% 1	2988
August	1st to 10th	Night	24% 5% 12% 40% 18% 1	2829
	11th to 20th	Night	30% 6% 16% 29% 18% 1	3130
	21st to end	Night	20% 5% 11% 38% 23% 4	3476
September	1st to 10th	Night	34% 7% 9% 21% 24% 5%	1925
	11th to 20th	Night	33% 4 10% 25% 23% 4	2090
	21st to end	Night	32% 5% 8% 24% 25% 6%	2090
October	1st to 10th	Night	20% 3 5% 17% 49% 6%	3480
	11th to 20th	Night	23% 2 6% 24% 37% 7%	3587
	21st to end	Night	31% 2 7% 21% 31% 8%	4467
November	1st to 10th	Night	36% 4 7% 24% 24% 5%	2536
	11th to 20th	Night	39% 4 8% 21% 21% 7%	2600
	21st to end	Night	44% 4 8% 23% 18% 3	2600
December	1st to 10th	Night	50% 3 8% 21% 17% 1	4250
	11th to 20th	Night	51% 2 9% 20% 17% 2	4824
	21st to end	Night	65% 3 8% 16% 6% 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	28% 19% 30% 17% 5% 1	300
	11th to 20th	Day	28% 28% 27% 13% 4	300
	21st to end	Day	23% 22% 40% 12% 3	330
February	1st to 10th	Day	24% 21% 39% 12% 3	300
	11th to 20th	Day	32% 22% 34% 9% 3	300
	21st to end	Day	29% 21% 36% 12% 2	249
March	1st to 10th	Day	24% 23% 35% 14% 4 1	300
	11th to 20th	Day	25% 18% 36% 14% 7% 1	393
	21st to end	Day	24% 20% 33% 15% 6% 1	506
April	1st to 10th	Day	21% 18% 26% 25% 10% 1	460
	11th to 20th	Day	18% 19% 26% 22% 12% 3	452
	21st to end	Day	20% 16% 33% 21% 9% 3	460
May	1st to 10th	Day	19% 15% 29% 24% 8% 5%	463
	11th to 20th	Day	17% 8% 29% 26% 13% 7%	487
	21st to end	Day	12% 7% 21% 29% 20% 12%	529
June	1st to 10th	Day	11% 6% 23% 25% 24% 11%	555
	11th to 20th	Day	6% 4 19% 33% 27% 11%	680
	21st to end	Day	8% 4 22% 31% 23% 12%	680
July	1st to 10th	Day	9% 3 15% 28% 27% 18%	671
	11th to 20th	Day	9% 4 15% 29% 27% 16%	654
	21st to end	Day	12% 3 12% 31% 26% 17%	532
August	1st to 10th	Day	11% 1 14% 27% 30% 17%	486
	11th to 20th	Day	12% 5% 13% 33% 28% 9%	479
	21st to end	Day	11% 2 9% 24% 32% 22%	539
September	1st to 10th	Day	13% 3 14% 24% 32% 14%	490
	11th to 20th	Day	12% 2 13% 25% 33% 14%	490
	21st to end	Day	17% 7% 19% 24% 22% 11%	490
October	1st to 10th	Day	11% 4 19% 32% 26% 8%	490
	11th to 20th	Day	14% 13% 29% 24% 13% 7%	463
	21st to end	Day	22% 12% 30% 25% 9% 1	330
November	1st to 10th	Day	18% 9% 31% 28% 10% 3	300
	11th to 20th	Day	21% 15% 34% 23% 7%	300
	21st to end	Day	26% 13% 32% 21% 8% 1	300
December	1st to 10th	Day	23% 19% 31% 18% 8% 1	300
	11th to 20th	Day	23% 17% 31% 23% 6%	300
	21st to end	Day	29% 18% 32% 17% 4	330
January	1st to 10th	Night	24% 15% 31% 26% 4	1000
	11th to 20th	Night	28% 16% 31% 21% 4	971
	21st to end	Night	23% 13% 40% 23% 1	880
February	1st to 10th	Night	27% 12% 36% 22% 3	800
	11th to 20th	Night	26% 15% 37% 20% 2	783
	21st to end	Night	28% 13% 38% 19% 2	660
March	1st to 10th	Night	22% 15% 32% 26% 5%	800
	11th to 20th	Night	22% 13% 34% 24% 6% 1	767
	21st to end	Night	22% 11% 32% 30% 6%	764
April	1st to 10th	Night	14% 14% 33% 29% 9% 1	570
	11th to 20th	Night	21% 10% 28% 27% 12% 1	563
	21st to end	Night	19% 8% 31% 30% 10% 1	570
May	1st to 10th	Night	21% 6% 28% 31% 13% 2	570
	11th to 20th	Night	14% 4 21% 39% 19% 2	561
	21st to end	Night	6% 3 18% 38% 29% 6%	617
June	1st to 10th	Night	12% 1 18% 36% 30% 3	570
	11th to 20th	Night	6% 2 14% 41% 33% 4	570
	21st to end	Night	5% 2 16% 39% 33% 5%	570
July	1st to 10th	Night	11% 1 9% 29% 42% 8%	555
	11th to 20th	Night	9% 2 7% 33% 44% 6%	552
	21st to end	Night	10% 1 10% 37% 35% 7%	624
August	1st to 10th	Night	8% 2 7% 31% 45% 6%	567
	11th to 20th	Night	7% 1 11% 38% 38% 6%	564
	21st to end	Night	7% 1 7% 32% 45% 9%	627
September	1st to 10th	Night	12% 2 11% 33% 38% 5%	595
	11th to 20th	Night	12% 2 11% 32% 39% 5%	760
	21st to end	Night	14% 3 18% 31% 29% 4	760
October	1st to 10th	Night	11% 3 18% 33% 29% 5%	760
	11th to 20th	Night	17% 7% 23% 32% 17% 3	760
	21st to end	Night	20% 7% 31% 28% 14% 1	836
November	1st to 10th	Night	14% 9% 28% 32% 16% 1	780
	11th to 20th	Night	17% 11% 32% 29% 11%	800
	21st to end	Night	23% 13% 25% 28% 12%	800
December	1st to 10th	Night	27% 11% 29% 27% 5%	800
	11th to 20th	Night	26% 8% 34% 26% 6%	800
	21st to end	Night	29% 13% 29% 26% 3	1050

Page generated at 2025-03-09 07:08:45. Version ac07

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