Bellevue Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bellevue Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	16% 9% 3 8% 5% 5% 5% 4 5% 7% 32%	132006
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 30%	130817
Transparency	Day	34% 37% 8% 14% 7% 1	135451
	Night	37% 36% 7% 12% 7% 1	135459
Seeing	Day	20% 11% 28% 24% 12% 4	16424
	Night	22% 9% 29% 29% 10% 1	25506

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	15% 17% 6% 13% 6% 5% 4 3 4 6% 22%	4661
	11th to 20th	Day	12% 15% 6% 13% 6% 6% 5% 4 5% 5% 24%	4650
	21st to end	Day	10% 13% 5% 11% 9% 7% 7% 4 5% 5% 24%	5124
February	1st to 10th	Day	13% 15% 4 10% 7% 4 5% 4 4 6% 27%	1859
	11th to 20th	Day	13% 12% 4 11% 7% 5% 5% 5% 6% 6% 27%	1943
	21st to end	Day	17% 16% 4 9% 6% 5% 4 4 4 5% 27%	1730
March	1st to 10th	Day	19% 12% 5% 10% 6% 5% 5% 3 4 5% 26%	4576
	11th to 20th	Day	10% 13% 4 10% 6% 5% 5% 4 6% 6% 31%	4826
	21st to end	Day	18% 9% 4 8% 5% 4 4 3 4 5% 35%	6090
April	1st to 10th	Day	21% 10% 3 8% 5% 4 3 3 4 6% 32%	2520
	11th to 20th	Day	20% 8% 2 7% 5% 4 4 3 4 6% 36%	2494
	21st to end	Day	20% 9% 3 8% 5% 4 4 3 4 6% 33%	2676
May	1st to 10th	Day	24% 10% 3 9% 5% 5% 5% 4 5% 7% 24%	6428
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 3 5% 7% 32%	6402
	21st to end	Day	22% 7% 3 6% 4 4 4 4 6% 7% 32%	6949
June	1st to 10th	Day	19% 8% 3 7% 6% 4 5% 4 6% 7% 32%	2800
	11th to 20th	Day	14% 5% 3 6% 5% 4 5% 4 5% 9% 39%	2800
	21st to end	Day	19% 7% 3 7% 5% 6% 5% 4 6% 7% 33%	2800
July	1st to 10th	Day	11% 6% 3 8% 6% 6% 6% 4 6% 8% 35%	6360
	11th to 20th	Day	10% 6% 2 7% 4 5% 5% 4 6% 8% 43%	6360
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	6926
August	1st to 10th	Day	14% 6% 3 7% 5% 5% 5% 5% 7% 9% 35%	4563
	11th to 20th	Day	13% 5% 3 7% 5% 5% 4 3 6% 8% 40%	4112
	21st to end	Day	16% 4 2 5% 5% 4 5% 4 6% 9% 40%	4312
September	1st to 10th	Day	11% 6% 2 6% 4 4 5% 5% 6% 9% 43%	2400
	11th to 20th	Day	13% 5% 2 5% 4 4 4 4 5% 7% 50%	2325
	21st to end	Day	17% 5% 2 5% 5% 5% 4 3 5% 7% 42%	2200
October	1st to 10th	Day	19% 7% 3 6% 4 5% 4 3 6% 6% 38%	2051
	11th to 20th	Day	17% 8% 2 7% 7% 6% 5% 4 5% 6% 32%	1968
	21st to end	Day	24% 10% 3 6% 5% 3 4 4 6% 6% 30%	1980
November	1st to 10th	Day	17% 10% 3 7% 5% 4 5% 4 6% 7% 33%	1851
	11th to 20th	Day	17% 11% 3 9% 5% 5% 4 4 6% 6% 29%	1796
	21st to end	Day	23% 13% 4 7% 5% 6% 5% 4 5% 7% 23%	1680
December	1st to 10th	Day	21% 19% 6% 10% 5% 4 4 4 5% 4 17%	2680
	11th to 20th	Day	17% 13% 4 10% 6% 4 5% 4 5% 4 27%	3066
	21st to end	Day	24% 12% 4 10% 5% 4 6% 4 5% 5% 22%	4048
January	1st to 10th	Night	13% 19% 7% 13% 5% 5% 4 3 4 5% 21%	8234
	11th to 20th	Night	12% 16% 6% 17% 8% 6% 5% 4 5% 5% 17%	8235
	21st to end	Night	11% 14% 5% 12% 8% 6% 4 3 5% 6% 26%	9057
February	1st to 10th	Night	13% 15% 5% 12% 6% 5% 4 4 5% 5% 24%	2756
	11th to 20th	Night	14% 11% 4 10% 5% 6% 6% 4 6% 6% 30%	2561
	21st to end	Night	15% 15% 4 9% 5% 5% 4 4 3 5% 32%	2076
March	1st to 10th	Night	20% 13% 4 9% 5% 4 4 3 4 4 30%	5484
	11th to 20th	Night	16% 13% 3 7% 5% 5% 4 4 6% 8% 30%	5337
	21st to end	Night	16% 11% 3 9% 6% 5% 5% 3 5% 5% 32%	5083
April	1st to 10th	Night	24% 9% 3 6% 4 4 5% 3 5% 6% 31%	2100
	11th to 20th	Night	22% 9% 3 7% 5% 4 4 3 4 5% 34%	2075
	21st to end	Night	22% 8% 3 6% 4 4 3 3 5% 6% 35%	1943
May	1st to 10th	Night	26% 9% 3 9% 5% 5% 4 4 5% 7% 24%	3776
	11th to 20th	Night	23% 8% 3 7% 5% 4 4 3 5% 6% 32%	3623
	21st to end	Night	19% 8% 3 9% 5% 4 5% 3 4 7% 32%	3926
June	1st to 10th	Night	20% 8% 2 7% 5% 5% 4 3 7% 7% 32%	1600
	11th to 20th	Night	15% 7% 3 6% 4 3 4 4 6% 7% 40%	1600
	21st to end	Night	18% 6% 2 8% 5% 5% 4 4 6% 7% 35%	1600
July	1st to 10th	Night	13% 7% 2 7% 5% 5% 6% 4 8% 8% 34%	3600
	11th to 20th	Night	13% 5% 2 7% 5% 4 5% 4 5% 8% 41%	3600
	21st to end	Night	10% 6% 2 6% 4 5% 5% 3 6% 8% 45%	3926
August	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 8% 39%	2586
	11th to 20th	Night	15% 7% 3 7% 4 5% 5% 3 5% 7% 40%	2899
	21st to end	Night	16% 6% 3 7% 4 4 4 4 6% 7% 40%	3586
September	1st to 10th	Night	11% 6% 3 6% 5% 4 5% 4 6% 7% 44%	2000
	11th to 20th	Night	15% 6% 2 6% 3 3 2 3 5% 6% 51%	2075
	21st to end	Night	16% 6% 3 6% 4 4 4 4 5% 7% 42%	2200
October	1st to 10th	Night	18% 6% 2 7% 4 4 5% 3 5% 6% 39%	2345
	11th to 20th	Night	19% 7% 3 7% 5% 4 5% 3 4 5% 37%	2430
	21st to end	Night	22% 8% 3 7% 5% 5% 5% 3 5% 6% 31%	2860
November	1st to 10th	Night	20% 9% 4 8% 5% 4 5% 3 5% 7% 32%	2667
	11th to 20th	Night	18% 11% 3 8% 5% 4 5% 4 5% 5% 32%	2819
	21st to end	Night	22% 14% 4 8% 6% 5% 4 3 4 5% 25%	2940
December	1st to 10th	Night	18% 18% 6% 11% 6% 5% 5% 4 5% 4 18%	4720
	11th to 20th	Night	22% 14% 5% 9% 5% 4 4 4 6% 5% 23%	5348
	21st to end	Night	25% 16% 4 9% 4 3 3 3 6% 5% 22%	7150

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	51% 22% 10% 13% 3	4662
	11th to 20th	Day	50% 22% 8% 10% 8% 1	4680
	21st to end	Day	50% 24% 8% 9% 8% 1	5124
February	1st to 10th	Day	24% 59% 4 10% 3 1	1859
	11th to 20th	Day	24% 58% 4 9% 5%	1952
	21st to end	Day	24% 59% 4 5% 6% 1	1730
March	1st to 10th	Day	45% 29% 5% 10% 8% 3	4579
	11th to 20th	Day	38% 29% 12% 10% 9% 1	4826
	21st to end	Day	40% 28% 4 13% 14% 1	6090
April	1st to 10th	Day	23% 59% 4 7% 6% 1	2520
	11th to 20th	Day	22% 59% 3 9% 6% 1	2494
	21st to end	Day	22% 59% 4 8% 6%	2676
May	1st to 10th	Day	48% 30% 7% 13% 3	6439
	11th to 20th	Day	41% 29% 12% 13% 5% 1	6406
	21st to end	Day	39% 29% 9% 14% 7%	6949
June	1st to 10th	Day	20% 62% 3 8% 6% 1	2800
	11th to 20th	Day	17% 62% 5% 9% 6% 1	2800
	21st to end	Day	19% 61% 4 10% 6% 1	2800
July	1st to 10th	Day	37% 27% 10% 20% 6%	6290
	11th to 20th	Day	28% 27% 10% 24% 11%	6285
	21st to end	Day	25% 26% 10% 31% 7%	6728
August	1st to 10th	Day	30% 37% 12% 15% 6%	4549
	11th to 20th	Day	30% 37% 11% 18% 3	4154
	21st to end	Day	27% 37% 12% 14% 10%	4312
September	1st to 10th	Day	13% 59% 7% 15% 6%	2400
	11th to 20th	Day	13% 57% 6% 16% 7% 1	2325
	21st to end	Day	16% 57% 6% 13% 8% 1	2200
October	1st to 10th	Day	27% 37% 9% 21% 6%	3337
	11th to 20th	Day	24% 36% 14% 22% 4	3184
	21st to end	Day	27% 37% 15% 16% 5%	3180
November	1st to 10th	Day	27% 55% 6% 9% 3	1851
	11th to 20th	Day	22% 55% 5% 11% 6% 1	1796
	21st to end	Day	29% 56% 6% 5% 4	1680
December	1st to 10th	Day	47% 35% 6% 11% 1	2680
	11th to 20th	Day	43% 33% 6% 11% 7%	3066
	21st to end	Day	48% 28% 8% 12% 5%	4048
January	1st to 10th	Night	55% 21% 9% 10% 4	8234
	11th to 20th	Night	57% 23% 7% 9% 5%	8280
	21st to end	Night	49% 22% 7% 11% 11%	9057
February	1st to 10th	Night	25% 59% 5% 8% 3	2756
	11th to 20th	Night	22% 58% 4 10% 5%	2563
	21st to end	Night	21% 58% 4 8% 8% 1	2076
March	1st to 10th	Night	42% 28% 5% 11% 10% 4	5494
	11th to 20th	Night	39% 29% 11% 12% 9% 2	5338
	21st to end	Night	42% 28% 3 17% 9% 1	5083
April	1st to 10th	Night	24% 58% 3 8% 7%	2100
	11th to 20th	Night	25% 59% 2 8% 5% 1	2075
	21st to end	Night	22% 59% 3 7% 8% 1	1943
May	1st to 10th	Night	47% 30% 8% 13% 3	3782
	11th to 20th	Night	41% 29% 9% 16% 6%	3624
	21st to end	Night	41% 30% 7% 12% 9%	3927
June	1st to 10th	Night	19% 62% 3 8% 7% 1	1600
	11th to 20th	Night	17% 62% 4 9% 8% 1	1600
	21st to end	Night	20% 61% 4 9% 6% 1	1600
July	1st to 10th	Night	36% 27% 15% 15% 7%	3560
	11th to 20th	Night	31% 27% 13% 22% 8%	3552
	21st to end	Night	28% 27% 12% 23% 11%	3809
August	1st to 10th	Night	31% 37% 8% 19% 5%	2576
	11th to 20th	Night	33% 37% 13% 14% 4	2923
	21st to end	Night	29% 37% 10% 14% 9% 1	3586
September	1st to 10th	Night	13% 58% 6% 14% 8%	2000
	11th to 20th	Night	15% 57% 7% 15% 6% 1	2075
	21st to end	Night	17% 58% 5% 13% 9%	2200
October	1st to 10th	Night	27% 36% 10% 19% 6% 1	3839
	11th to 20th	Night	25% 36% 9% 24% 7%	3953
	21st to end	Night	29% 36% 12% 13% 9% 1	4610
November	1st to 10th	Night	27% 54% 6% 9% 3 1	2667
	11th to 20th	Night	25% 55% 5% 9% 5% 1	2819
	21st to end	Night	29% 55% 5% 7% 4	2940
December	1st to 10th	Night	47% 35% 8% 8% 2	4720
	11th to 20th	Night	45% 32% 5% 9% 9%	5348
	21st to end	Night	49% 26% 5% 11% 9%	7150

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% 14% 31% 21% 10%	220
	11th to 20th	Day	27% 20% 28% 20% 6%	220
	21st to end	Day	19% 16% 33% 24% 7% 1	242
February	1st to 10th	Day	25% 17% 35% 18% 4	314
	11th to 20th	Day	23% 15% 40% 20% 3	430
	21st to end	Day	25% 13% 31% 28% 2	446
March	1st to 10th	Day	23% 15% 34% 21% 7%	540
	11th to 20th	Day	21% 15% 35% 22% 6% 1	540
	21st to end	Day	26% 15% 30% 18% 9% 2	594
April	1st to 10th	Day	28% 16% 27% 19% 8% 2	540
	11th to 20th	Day	25% 17% 26% 20% 10% 2	532
	21st to end	Day	25% 15% 29% 20% 9% 2	540
May	1st to 10th	Day	23% 10% 30% 25% 10% 2	543
	11th to 20th	Day	17% 9% 31% 26% 13% 4	551
	21st to end	Day	19% 10% 29% 26% 13% 3	573
June	1st to 10th	Day	21% 11% 26% 28% 11% 3	530
	11th to 20th	Day	16% 9% 26% 31% 15% 2	530
	21st to end	Day	21% 9% 28% 23% 15% 4	530
July	1st to 10th	Day	10% 7% 28% 29% 19% 6%	525
	11th to 20th	Day	12% 6% 29% 30% 19% 5%	524
	21st to end	Day	12% 6% 24% 33% 20% 4	576
August	1st to 10th	Day	13% 5% 22% 32% 21% 7%	526
	11th to 20th	Day	11% 6% 20% 30% 22% 10%	519
	21st to end	Day	14% 7% 26% 25% 19% 9%	583
September	1st to 10th	Day	14% 7% 24% 28% 18% 9%	530
	11th to 20th	Day	12% 7% 19% 25% 25% 12%	530
	21st to end	Day	18% 8% 25% 25% 18% 6%	530
October	1st to 10th	Day	19% 10% 27% 24% 15% 5%	530
	11th to 20th	Day	20% 12% 32% 23% 12% 2	511
	21st to end	Day	32% 11% 26% 22% 9%	462
November	1st to 10th	Day	28% 14% 24% 24% 9% 1	425
	11th to 20th	Day	23% 15% 34% 19% 8% 1	336
	21st to end	Day	31% 15% 28% 19% 6%	220
December	1st to 10th	Day	31% 10% 26% 23% 8% 2	220
	11th to 20th	Day	24% 13% 41% 18% 4 1	220
	21st to end	Day	29% 14% 32% 17% 6% 2	242
January	1st to 10th	Night	26% 10% 32% 26% 5%	840
	11th to 20th	Night	27% 10% 33% 24% 5%	840
	21st to end	Night	18% 10% 36% 29% 8% 1	924
February	1st to 10th	Night	20% 9% 36% 28% 6% 1	840
	11th to 20th	Night	22% 10% 36% 27% 5%	823
	21st to end	Night	23% 9% 33% 32% 4	692
March	1st to 10th	Night	24% 13% 31% 26% 5%	840
	11th to 20th	Night	26% 11% 32% 24% 7%	840
	21st to end	Night	23% 11% 29% 29% 8%	924
April	1st to 10th	Night	28% 11% 28% 28% 5%	840
	11th to 20th	Night	26% 11% 30% 27% 6%	831
	21st to end	Night	26% 9% 30% 26% 8%	683
May	1st to 10th	Night	25% 8% 30% 27% 8% 1	470
	11th to 20th	Night	21% 7% 26% 36% 9% 1	406
	21st to end	Night	18% 6% 32% 34% 11%	433
June	1st to 10th	Night	24% 6% 26% 34% 11% 1	400
	11th to 20th	Night	18% 6% 26% 37% 13% 1	400
	21st to end	Night	19% 4 29% 32% 15% 1	400
July	1st to 10th	Night	13% 3 25% 39% 19% 1	390
	11th to 20th	Night	12% 2 24% 44% 16% 2	388
	21st to end	Night	12% 4 20% 46% 17% 1	438
August	1st to 10th	Night	16% 3 15% 45% 21% 1	398
	11th to 20th	Night	13% 3 21% 35% 25% 3	602
	21st to end	Night	15% 3 23% 37% 21% 2	880
September	1st to 10th	Night	13% 4 23% 41% 18% 1	800
	11th to 20th	Night	17% 5% 17% 35% 23% 3	800
	21st to end	Night	18% 5% 25% 31% 18% 2	800
October	1st to 10th	Night	20% 6% 29% 30% 15% 1	800
	11th to 20th	Night	22% 7% 30% 29% 11% 1	800
	21st to end	Night	27% 9% 29% 29% 6%	880
November	1st to 10th	Night	25% 11% 28% 27% 8%	820
	11th to 20th	Night	24% 12% 33% 23% 9% 1	840
	21st to end	Night	30% 12% 29% 22% 7% 1	840
December	1st to 10th	Night	29% 12% 33% 21% 6%	840
	11th to 20th	Night	28% 15% 31% 21% 5% 1	840
	21st to end	Night	35% 12% 28% 21% 4 1	924

Page generated at 2026-05-14 01:58:56. Version ac07

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