The Bruce M. Bedow Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the The Bruce M. Bedow Memorial Observatory clear sky chart from 2005-11-06 to 2026-09-29.

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	17% 11% 4 9% 5% 5% 5% 4 5% 7% 29%	149215
	Night	17% 12% 5% 10% 5% 4 4 3 4 6% 29%	142180
Transparency	Day	53% 6% 11% 19% 10% 1	152814
	Night	55% 6% 10% 18% 10% 1	146652
Seeing	Day	25% 16% 28% 19% 9% 3	17084
	Night	24% 13% 27% 25% 10% 1	26612

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	18% 20% 8% 25% 6% 4 3 2 2 3 9%	4961
	11th to 20th	Day	10% 19% 10% 22% 10% 8% 3 1 2 3 11%	5280
	21st to end	Day	18% 16% 7% 13% 10% 8% 4 3 4 4 13%	5780
February	1st to 10th	Day	17% 20% 5% 13% 7% 5% 4 3 3 4 20%	1890
	11th to 20th	Day	16% 16% 6% 16% 6% 5% 5% 3 4 4 19%	1843
	21st to end	Day	20% 16% 5% 11% 5% 4 4 3 4 5% 22%	1698
March	1st to 10th	Day	17% 11% 7% 10% 5% 3 4 3 4 6% 30%	5045
	11th to 20th	Day	13% 12% 6% 9% 6% 5% 4 3 4 6% 32%	5090
	21st to end	Day	19% 11% 4 10% 5% 4 4 3 4 5% 29%	5586
April	1st to 10th	Day	17% 15% 4 8% 5% 4 4 3 5% 7% 29%	2452
	11th to 20th	Day	19% 13% 3 7% 4 4 4 3 5% 6% 32%	2517
	21st to end	Day	21% 11% 3 6% 5% 4 3 4 5% 6% 33%	2730
May	1st to 10th	Day	23% 10% 3 8% 5% 5% 5% 4 5% 6% 26%	7616
	11th to 20th	Day	19% 9% 3 7% 5% 5% 5% 4 6% 8% 29%	7761
	21st to end	Day	21% 7% 3 7% 4 4 5% 4 5% 6% 34%	9045
June	1st to 10th	Day	16% 6% 2 7% 6% 4 5% 4 6% 8% 35%	2940
	11th to 20th	Day	15% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	2940
	21st to end	Day	15% 6% 2 5% 5% 5% 5% 4 7% 10% 35%	2940
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 37%	8280
	11th to 20th	Day	11% 6% 2 6% 5% 4 6% 4 8% 10% 38%	7803
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 6% 8% 39%	8340
August	1st to 10th	Day	20% 10% 3 8% 5% 4 5% 4 5% 7% 27%	6014
	11th to 20th	Day	16% 8% 3 7% 5% 6% 6% 5% 7% 9% 30%	5951
	21st to end	Day	10% 6% 3 8% 5% 6% 6% 5% 8% 10% 34%	6249
September	1st to 10th	Day	16% 8% 2 6% 3 4 4 3 5% 6% 42%	2520
	11th to 20th	Day	12% 8% 2 7% 4 4 4 4 6% 7% 42%	2364
	21st to end	Day	20% 9% 4 7% 4 5% 4 4 4 6% 32%	2260
October	1st to 10th	Day	17% 8% 2 6% 4 4 4 4 6% 7% 38%	2000
	11th to 20th	Day	21% 9% 3 7% 5% 5% 5% 3 5% 6% 32%	2000
	21st to end	Day	24% 11% 3 8% 5% 3 4 3 5% 6% 27%	2200
November	1st to 10th	Day	12% 10% 3 7% 5% 5% 5% 3 5% 7% 38%	1962
	11th to 20th	Day	18% 16% 3 9% 5% 5% 3 4 5% 5% 27%	1680
	21st to end	Day	20% 21% 5% 9% 4 4 4 4 4 6% 19%	1680
December	1st to 10th	Day	20% 20% 12% 18% 5% 4 3 2 3 3 11%	2680
	11th to 20th	Day	22% 20% 7% 10% 5% 3 3 2 4 5% 20%	3070
	21st to end	Day	24% 24% 6% 12% 6% 4 3 2 3 3 13%	4048
January	1st to 10th	Night	15% 20% 8% 26% 7% 4 3 3 3 3 9%	7915
	11th to 20th	Night	11% 18% 9% 21% 11% 6% 5% 2 3 3 9%	7680
	21st to end	Night	18% 16% 7% 12% 7% 7% 5% 3 4 4 17%	8401
February	1st to 10th	Night	17% 16% 6% 13% 8% 6% 3 3 4 4 21%	2730
	11th to 20th	Night	18% 14% 6% 14% 6% 6% 6% 3 5% 4 19%	2663
	21st to end	Night	20% 13% 5% 14% 6% 4 4 3 4 4 23%	2102
March	1st to 10th	Night	20% 14% 6% 8% 6% 3 3 2 3 5% 29%	5028
	11th to 20th	Night	14% 11% 6% 7% 4 3 3 3 4 6% 38%	5090
	21st to end	Night	18% 12% 5% 8% 5% 4 4 3 4 4 33%	5588
April	1st to 10th	Night	19% 13% 4 8% 5% 4 4 3 4 6% 29%	2167
	11th to 20th	Night	18% 11% 3 8% 5% 4 4 3 5% 6% 35%	2070
	21st to end	Night	14% 11% 3 7% 5% 4 4 3 5% 8% 35%	1890
May	1st to 10th	Night	27% 10% 3 7% 4 4 4 3 4 6% 29%	5237
	11th to 20th	Night	20% 10% 3 6% 4 4 4 3 5% 6% 35%	5097
	21st to end	Night	23% 7% 2 5% 4 3 4 3 4 8% 36%	5108
June	1st to 10th	Night	10% 6% 2 6% 5% 4 4 4 6% 8% 44%	1680
	11th to 20th	Night	13% 7% 3 6% 5% 5% 4 4 5% 9% 40%	1680
	21st to end	Night	13% 6% 2 7% 4 4 4 4 6% 9% 40%	1680
July	1st to 10th	Night	13% 8% 3 7% 5% 5% 5% 4 5% 8% 39%	4678
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 4 7% 11% 40%	5149
	21st to end	Night	11% 7% 2 6% 5% 4 4 3 5% 7% 45%	5736
August	1st to 10th	Night	18% 10% 3 9% 4 4 5% 3 5% 7% 32%	4135
	11th to 20th	Night	17% 8% 3 7% 4 4 4 4 5% 7% 37%	4095
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 7% 9% 43%	4828
September	1st to 10th	Night	14% 6% 2 6% 3 4 4 3 5% 6% 47%	2100
	11th to 20th	Night	10% 5% 2 5% 3 4 4 3 5% 8% 49%	2252
	21st to end	Night	20% 7% 2 7% 5% 5% 4 3 5% 6% 35%	2315
October	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 5% 7% 44%	2400
	11th to 20th	Night	18% 8% 2 7% 4 4 4 4 5% 7% 37%	2400
	21st to end	Night	24% 11% 3 6% 4 4 3 3 5% 6% 32%	2640
November	1st to 10th	Night	13% 10% 3 7% 4 4 4 3 5% 7% 39%	2551
	11th to 20th	Night	19% 17% 3 8% 5% 4 4 3 4 6% 26%	2940
	21st to end	Night	21% 17% 6% 8% 5% 4 4 3 5% 6% 23%	2940
December	1st to 10th	Night	17% 18% 10% 16% 6% 5% 4 3 4 4 13%	4720
	11th to 20th	Night	21% 18% 7% 12% 5% 4 3 2 3 5% 19%	5345
	21st to end	Night	22% 21% 6% 11% 5% 4 4 3 4 5% 15%	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	82% 3 6% 6% 3	4962
	11th to 20th	Day	81% 3 6% 7% 3 1	5280
	21st to end	Day	75% 4 8% 10% 3	5780
February	1st to 10th	Day	69% 4 8% 13% 6%	1890
	11th to 20th	Day	69% 5% 8% 13% 6%	1852
	21st to end	Day	65% 5% 10% 14% 5% 1	1698
March	1st to 10th	Day	56% 5% 11% 21% 7%	5045
	11th to 20th	Day	53% 5% 16% 19% 6% 2	5090
	21st to end	Day	56% 5% 7% 19% 12% 1	5586
April	1st to 10th	Day	56% 5% 11% 15% 12% 1	2452
	11th to 20th	Day	53% 5% 10% 18% 12% 2	2517
	21st to end	Day	52% 6% 10% 20% 11% 2	2730
May	1st to 10th	Day	56% 7% 9% 18% 7% 3	7651
	11th to 20th	Day	51% 7% 10% 21% 10% 1	7765
	21st to end	Day	48% 8% 10% 18% 15% 2	9061
June	1st to 10th	Day	45% 8% 10% 22% 14% 1	2940
	11th to 20th	Day	47% 7% 13% 19% 12% 1	2940
	21st to end	Day	42% 9% 12% 25% 12% 1	2940
July	1st to 10th	Day	41% 8% 12% 24% 14% 1	8210
	11th to 20th	Day	37% 8% 13% 30% 12%	7731
	21st to end	Day	40% 8% 10% 23% 18% 1	8156
August	1st to 10th	Day	54% 8% 11% 17% 7% 3	6001
	11th to 20th	Day	46% 9% 14% 23% 6%	5990
	21st to end	Day	39% 10% 16% 25% 10% 1	6249
September	1st to 10th	Day	42% 7% 10% 23% 16% 2	2520
	11th to 20th	Day	40% 6% 13% 24% 16% 1	2364
	21st to end	Day	51% 6% 11% 19% 13%	2260
October	1st to 10th	Day	36% 7% 11% 27% 18% 1	3260
	11th to 20th	Day	45% 7% 12% 19% 13% 4	3240
	21st to end	Day	43% 5% 13% 25% 12% 1	3538
November	1st to 10th	Day	45% 7% 12% 19% 15% 2	1962
	11th to 20th	Day	58% 5% 11% 18% 7% 2	1680
	21st to end	Day	65% 6% 11% 13% 4 1	1680
December	1st to 10th	Day	80% 4 8% 6% 2	2680
	11th to 20th	Day	68% 4 9% 14% 4	3066
	21st to end	Day	77% 4 8% 8% 3	4048
January	1st to 10th	Night	81% 5% 6% 7% 2	7916
	11th to 20th	Night	80% 4 7% 6% 2	7680
	21st to end	Night	71% 5% 9% 11% 4 1	8401
February	1st to 10th	Night	68% 4 8% 15% 6%	2730
	11th to 20th	Night	68% 5% 9% 13% 5%	2665
	21st to end	Night	65% 4 9% 14% 8% 1	2102
March	1st to 10th	Night	59% 4 10% 19% 7% 1	5028
	11th to 20th	Night	47% 6% 14% 22% 8% 3	5090
	21st to end	Night	53% 5% 8% 18% 15% 1	5588
April	1st to 10th	Night	56% 5% 9% 17% 12% 1	2167
	11th to 20th	Night	49% 6% 10% 19% 13% 2	2070
	21st to end	Night	47% 6% 11% 23% 11% 2	1890
May	1st to 10th	Night	56% 6% 8% 15% 13% 3	5264
	11th to 20th	Night	49% 6% 10% 24% 10% 2	5098
	21st to end	Night	47% 6% 10% 21% 13% 2	5115
June	1st to 10th	Night	38% 8% 12% 25% 16% 2	1680
	11th to 20th	Night	41% 8% 13% 23% 14% 2	1680
	21st to end	Night	39% 8% 14% 22% 16%	1680
July	1st to 10th	Night	42% 8% 10% 23% 16% 1	4640
	11th to 20th	Night	35% 8% 16% 29% 11% 1	5101
	21st to end	Night	38% 7% 10% 19% 26% 1	5606
August	1st to 10th	Night	51% 7% 11% 19% 11% 1	4124
	11th to 20th	Night	44% 8% 15% 25% 7% 1	4122
	21st to end	Night	35% 8% 16% 28% 12% 1	4828
September	1st to 10th	Night	37% 7% 11% 25% 17% 3	2100
	11th to 20th	Night	32% 6% 13% 28% 20% 1	2252
	21st to end	Night	49% 6% 11% 18% 14% 2	2315
October	1st to 10th	Night	37% 5% 10% 24% 23% 1	3920
	11th to 20th	Night	40% 7% 12% 20% 15% 6%	3899
	21st to end	Night	40% 4 14% 23% 18% 1	4252
November	1st to 10th	Night	44% 6% 12% 23% 13% 3	2551
	11th to 20th	Night	58% 5% 9% 17% 8% 3	2940
	21st to end	Night	63% 5% 10% 18% 4 1	2940
December	1st to 10th	Night	74% 6% 10% 7% 3	4720
	11th to 20th	Night	69% 4 10% 14% 3	5348
	21st to end	Night	72% 5% 9% 11% 3	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	34% 30% 26% 8% 2	275
	11th to 20th	Day	37% 31% 24% 5% 2 1	330
	21st to end	Day	33% 28% 31% 6% 2	363
February	1st to 10th	Day	32% 27% 32% 8% 1	330
	11th to 20th	Day	39% 26% 27% 7% 1	330
	21st to end	Day	37% 23% 33% 7%	320
March	1st to 10th	Day	31% 24% 31% 13% 2	540
	11th to 20th	Day	35% 20% 31% 10% 3 1	540
	21st to end	Day	35% 21% 28% 12% 3 1	594
April	1st to 10th	Day	33% 21% 26% 14% 4 1	540
	11th to 20th	Day	29% 22% 26% 14% 7% 2	532
	21st to end	Day	29% 18% 30% 15% 6% 1	540
May	1st to 10th	Day	30% 21% 26% 15% 6% 2	543
	11th to 20th	Day	25% 14% 31% 19% 9% 2	567
	21st to end	Day	20% 13% 26% 24% 13% 5%	617
June	1st to 10th	Day	20% 13% 31% 25% 9% 2	570
	11th to 20th	Day	20% 17% 25% 25% 11% 2	570
	21st to end	Day	17% 15% 30% 24% 11% 3	570
July	1st to 10th	Day	12% 7% 24% 29% 20% 7%	565
	11th to 20th	Day	11% 8% 29% 27% 18% 7%	564
	21st to end	Day	13% 7% 30% 28% 17% 5%	620
August	1st to 10th	Day	15% 6% 25% 28% 20% 7%	566
	11th to 20th	Day	16% 6% 26% 28% 14% 9%	559
	21st to end	Day	13% 7% 26% 30% 16% 7%	627
September	1st to 10th	Day	19% 9% 21% 26% 17% 8%	570
	11th to 20th	Day	12% 5% 24% 28% 20% 10%	570
	21st to end	Day	26% 11% 25% 23% 12% 3	562
October	1st to 10th	Day	19% 9% 28% 23% 17% 4	530
	11th to 20th	Day	26% 17% 31% 17% 7% 2	530
	21st to end	Day	33% 16% 28% 18% 5%	583
November	1st to 10th	Day	23% 20% 31% 17% 8% 2	445
	11th to 20th	Day	34% 22% 27% 12% 4 1	220
	21st to end	Day	30% 27% 34% 8% 1	220
December	1st to 10th	Day	38% 25% 27% 6% 2 2	220
	11th to 20th	Day	38% 19% 33% 7% 3	220
	21st to end	Day	34% 26% 27% 11% 2	242
January	1st to 10th	Night	30% 23% 33% 11% 3	840
	11th to 20th	Night	33% 24% 28% 14% 2	840
	21st to end	Night	27% 24% 32% 16% 2	924
February	1st to 10th	Night	31% 18% 29% 20% 1	840
	11th to 20th	Night	29% 22% 34% 14% 1	823
	21st to end	Night	29% 22% 30% 16% 2	692
March	1st to 10th	Night	26% 18% 35% 18% 3	840
	11th to 20th	Night	29% 19% 28% 19% 5%	840
	21st to end	Night	27% 17% 34% 17% 4 1	924
April	1st to 10th	Night	29% 16% 30% 20% 5%	840
	11th to 20th	Night	26% 14% 31% 21% 8% 1	826
	21st to end	Night	21% 11% 35% 27% 5% 1	630
May	1st to 10th	Night	26% 10% 22% 32% 10%	630
	11th to 20th	Night	21% 10% 27% 33% 8%	575
	21st to end	Night	15% 5% 28% 32% 18% 2	455
June	1st to 10th	Night	10% 6% 30% 38% 15%	420
	11th to 20th	Night	15% 7% 26% 38% 14%	420
	21st to end	Night	14% 5% 29% 35% 17% 1	420
July	1st to 10th	Night	11% 4 18% 43% 20% 3	410
	11th to 20th	Night	11% 4 19% 38% 25% 3	565
	21st to end	Night	12% 3 16% 44% 24% 1	690
August	1st to 10th	Night	12% 1 17% 43% 25% 2	627
	11th to 20th	Night	14% 4 13% 39% 27% 3	624
	21st to end	Night	12% 4 20% 34% 26% 4	820
September	1st to 10th	Night	16% 5% 22% 32% 22% 3	840
	11th to 20th	Night	13% 5% 18% 34% 26% 4	840
	21st to end	Night	22% 9% 23% 26% 17% 3	833
October	1st to 10th	Night	19% 7% 24% 28% 19% 2	800
	11th to 20th	Night	23% 11% 33% 24% 9% 1	800
	21st to end	Night	30% 13% 27% 24% 5%	880
November	1st to 10th	Night	21% 12% 31% 26% 10%	820
	11th to 20th	Night	30% 16% 27% 20% 6% 1	840
	21st to end	Night	32% 17% 29% 18% 4	840
December	1st to 10th	Night	33% 21% 28% 15% 3	840
	11th to 20th	Night	30% 23% 28% 16% 4	840
	21st to end	Night	31% 19% 30% 17% 2	924

Page generated at 2026-09-26 04:18:42. Version ac07

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