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 2025-05-26.

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% 12% 4 9% 5% 5% 4 3 5% 6% 29%	114137
	Night	17% 12% 5% 10% 5% 4 4 3 4 6% 29%	115130
Transparency	Day	54% 6% 11% 19% 10% 1	117779
	Night	55% 6% 10% 18% 10% 1	119627
Seeing	Day	25% 16% 28% 19% 9% 3	15289
	Night	24% 13% 27% 25% 10% 1	24971

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	16% 20% 8% 25% 5% 4 3 2 2 3 11%	3827
	11th to 20th	Day	11% 21% 11% 21% 9% 8% 3 1 2 3 12%	4060
	21st to end	Day	21% 18% 8% 13% 8% 5% 4 3 3 4 13%	4466
February	1st to 10th	Day	17% 20% 5% 13% 7% 5% 4 3 3 4 19%	1800
	11th to 20th	Day	16% 16% 6% 15% 7% 5% 5% 3 4 4 19%	1753
	21st to end	Day	21% 16% 4 11% 5% 4 5% 3 4 5% 22%	1620
March	1st to 10th	Day	17% 11% 7% 10% 5% 3 4 3 4 6% 29%	4935
	11th to 20th	Day	13% 12% 6% 8% 6% 5% 4 3 4 6% 33%	4980
	21st to end	Day	19% 11% 4 10% 5% 4 4 3 4 5% 30%	5465
April	1st to 10th	Day	17% 15% 4 8% 5% 4 4 3 4 6% 29%	2335
	11th to 20th	Day	19% 13% 3 7% 4 4 4 3 5% 5% 32%	2396
	21st to end	Day	21% 11% 3 6% 5% 4 3 4 5% 6% 33%	2600
May	1st to 10th	Day	23% 10% 3 7% 5% 4 4 4 5% 6% 28%	4212
	11th to 20th	Day	21% 10% 3 7% 5% 5% 5% 4 6% 7% 27%	4292
	21st to end	Day	13% 6% 3 7% 5% 4 5% 4 6% 8% 39%	4893
June	1st to 10th	Day	17% 7% 3 7% 6% 5% 6% 4 6% 8% 33%	2660
	11th to 20th	Day	14% 8% 3 7% 5% 6% 5% 4 6% 9% 34%	2660
	21st to end	Day	16% 6% 2 5% 5% 5% 5% 4 7% 10% 35%	2660
July	1st to 10th	Day	10% 7% 2 7% 5% 5% 6% 4 7% 10% 37%	4440
	11th to 20th	Day	12% 6% 2 6% 5% 5% 6% 4 7% 8% 39%	4187
	21st to end	Day	11% 6% 2 7% 6% 5% 6% 5% 7% 8% 37%	4466
August	1st to 10th	Day	17% 9% 3 8% 5% 5% 6% 4 6% 8% 29%	4104
	11th to 20th	Day	13% 8% 3 7% 5% 6% 6% 5% 7% 8% 32%	4044
	21st to end	Day	11% 6% 3 8% 4 5% 6% 5% 7% 9% 37%	4292
September	1st to 10th	Day	17% 8% 2 6% 3 4 4 3 5% 6% 42%	2280
	11th to 20th	Day	12% 8% 2 6% 4 4 5% 4 7% 7% 41%	2139
	21st to end	Day	20% 8% 4 7% 4 5% 4 4 4 6% 33%	2064
October	1st to 10th	Day	18% 8% 2 7% 4 4 4 4 6% 7% 35%	1900
	11th to 20th	Day	22% 10% 3 7% 5% 5% 4 3 5% 6% 32%	1900
	21st to end	Day	24% 11% 3 8% 5% 3 4 3 5% 7% 26%	2090
November	1st to 10th	Day	12% 10% 3 7% 5% 4 5% 3 5% 7% 38%	1867
	11th to 20th	Day	18% 16% 3 9% 5% 5% 3 4 6% 5% 27%	1600
	21st to end	Day	20% 20% 5% 9% 4 3 4 4 4 6% 20%	1600
December	1st to 10th	Day	20% 20% 12% 18% 5% 4 3 2 3 3 11%	2600
	11th to 20th	Day	22% 20% 7% 10% 4 3 3 2 4 4 20%	2990
	21st to end	Day	24% 24% 6% 11% 6% 4 3 3 3 3 13%	3960
January	1st to 10th	Night	14% 18% 9% 27% 6% 4 3 3 3 3 11%	6101
	11th to 20th	Night	12% 18% 9% 21% 9% 6% 4 3 4 4 10%	5900
	21st to end	Night	21% 17% 8% 13% 6% 5% 5% 3 4 3 15%	6490
February	1st to 10th	Night	18% 16% 6% 12% 8% 6% 3 3 4 4 20%	2600
	11th to 20th	Night	18% 15% 6% 13% 6% 6% 6% 3 5% 4 20%	2533
	21st to end	Night	21% 12% 5% 14% 6% 4 4 3 4 4 23%	2005
March	1st to 10th	Night	20% 13% 6% 8% 6% 3 3 2 3 5% 29%	4918
	11th to 20th	Night	14% 11% 6% 7% 4 3 3 3 4 6% 38%	4980
	21st to end	Night	18% 12% 5% 8% 5% 4 4 3 4 4 33%	5467
April	1st to 10th	Night	18% 14% 4 8% 5% 4 4 3 4 6% 30%	2064
	11th to 20th	Night	18% 10% 3 7% 5% 4 4 3 5% 6% 35%	1970
	21st to end	Night	15% 11% 3 7% 5% 4 4 3 5% 8% 35%	1800
May	1st to 10th	Night	28% 9% 2 6% 5% 4 4 3 4 7% 29%	2903
	11th to 20th	Night	19% 10% 3 7% 5% 4 4 3 5% 6% 33%	2819
	21st to end	Night	13% 6% 2 5% 4 3 4 4 6% 10% 42%	2778
June	1st to 10th	Night	9% 6% 2 6% 6% 5% 4 5% 6% 9% 42%	1520
	11th to 20th	Night	12% 6% 3 6% 5% 5% 4 4 6% 9% 41%	1520
	21st to end	Night	13% 6% 2 7% 4 4 4 4 6% 10% 40%	1520
July	1st to 10th	Night	12% 8% 3 7% 5% 4 5% 4 5% 7% 40%	2520
	11th to 20th	Night	11% 6% 2 6% 4 4 5% 3 7% 9% 44%	2768
	21st to end	Night	11% 7% 2 6% 5% 4 5% 4 6% 7% 43%	3087
August	1st to 10th	Night	15% 9% 3 8% 4 4 5% 4 5% 7% 36%	2825
	11th to 20th	Night	15% 7% 3 7% 4 5% 5% 4 5% 8% 38%	2785
	21st to end	Night	10% 4 2 6% 4 4 6% 4 6% 9% 45%	3330
September	1st to 10th	Night	14% 6% 2 6% 3 4 4 3 5% 6% 47%	1900
	11th to 20th	Night	11% 5% 2 5% 3 4 3 4 5% 8% 50%	2038
	21st to end	Night	20% 7% 3 7% 5% 4 4 3 4 6% 37%	2114
October	1st to 10th	Night	15% 7% 2 6% 4 3 4 3 5% 7% 42%	2280
	11th to 20th	Night	19% 8% 2 7% 5% 4 4 4 5% 6% 36%	2280
	21st to end	Night	24% 11% 3 6% 4 4 4 3 5% 6% 31%	2508
November	1st to 10th	Night	14% 10% 3 7% 4 4 4 3 5% 7% 40%	2426
	11th to 20th	Night	19% 16% 3 8% 5% 4 4 3 4 6% 26%	2800
	21st to end	Night	21% 17% 5% 8% 5% 4 4 3 5% 6% 24%	2800
December	1st to 10th	Night	17% 18% 10% 16% 6% 5% 4 3 4 4 13%	4580
	11th to 20th	Night	22% 18% 8% 12% 5% 4 3 2 3 5% 19%	5205
	21st to end	Night	22% 21% 6% 11% 5% 4 4 3 4 5% 15%	6996

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	80% 3 5% 7% 4	3828
	11th to 20th	Day	81% 2 6% 8% 2 1	4060
	21st to end	Day	77% 3 7% 9% 3	4466
February	1st to 10th	Day	70% 4 8% 12% 6%	1800
	11th to 20th	Day	68% 5% 8% 13% 6%	1762
	21st to end	Day	65% 5% 10% 14% 5% 1	1620
March	1st to 10th	Day	56% 5% 11% 21% 7%	4935
	11th to 20th	Day	52% 5% 16% 19% 6% 2	4980
	21st to end	Day	56% 5% 7% 19% 12% 1	5465
April	1st to 10th	Day	56% 5% 11% 16% 12% 1	2335
	11th to 20th	Day	53% 5% 10% 18% 12% 2	2396
	21st to end	Day	52% 6% 9% 19% 12% 2	2600
May	1st to 10th	Day	55% 6% 9% 22% 7% 1	4250
	11th to 20th	Day	54% 8% 10% 20% 8% 1	4296
	21st to end	Day	41% 9% 13% 22% 14% 2	4907
June	1st to 10th	Day	47% 8% 11% 21% 12% 1	2660
	11th to 20th	Day	46% 7% 13% 19% 13% 1	2660
	21st to end	Day	42% 8% 11% 26% 12% 1	2660
July	1st to 10th	Day	41% 8% 12% 23% 15% 2	4370
	11th to 20th	Day	39% 8% 12% 25% 16%	4115
	21st to end	Day	41% 9% 12% 21% 17% 1	4324
August	1st to 10th	Day	51% 8% 13% 21% 7%	4091
	11th to 20th	Day	44% 10% 14% 24% 8% 1	4083
	21st to end	Day	39% 8% 13% 26% 11% 1	4292
September	1st to 10th	Day	43% 7% 10% 22% 16% 2	2280
	11th to 20th	Day	41% 6% 12% 24% 16% 1	2139
	21st to end	Day	51% 6% 11% 20% 12%	2064
October	1st to 10th	Day	37% 7% 12% 26% 17% 1	3160
	11th to 20th	Day	45% 7% 12% 19% 13% 4	3140
	21st to end	Day	43% 5% 13% 25% 12%	3428
November	1st to 10th	Day	45% 7% 12% 19% 15% 2	1867
	11th to 20th	Day	57% 5% 11% 18% 8% 2	1600
	21st to end	Day	65% 5% 11% 13% 4 1	1600
December	1st to 10th	Day	81% 4 8% 6% 2	2600
	11th to 20th	Day	69% 4 9% 14% 4	2986
	21st to end	Day	77% 4 8% 8% 3	3960
January	1st to 10th	Night	79% 4 5% 9% 2	6101
	11th to 20th	Night	78% 5% 8% 7% 2	5900
	21st to end	Night	74% 5% 8% 9% 4 1	6490
February	1st to 10th	Night	69% 4 8% 14% 5%	2600
	11th to 20th	Night	68% 5% 8% 14% 5%	2535
	21st to end	Night	65% 4 9% 14% 8% 1	2005
March	1st to 10th	Night	59% 4 10% 19% 7% 1	4918
	11th to 20th	Night	47% 6% 14% 22% 8% 3	4980
	21st to end	Night	53% 5% 8% 18% 15% 1	5467
April	1st to 10th	Night	56% 5% 9% 17% 12% 1	2064
	11th to 20th	Night	49% 6% 10% 19% 14% 2	1970
	21st to end	Night	47% 6% 11% 23% 11% 2	1800
May	1st to 10th	Night	56% 5% 9% 18% 11% 2	2930
	11th to 20th	Night	50% 7% 11% 22% 10% 1	2820
	21st to end	Night	36% 8% 14% 24% 15% 4	2785
June	1st to 10th	Night	38% 8% 13% 26% 14% 1	1520
	11th to 20th	Night	40% 8% 13% 24% 14% 2	1520
	21st to end	Night	39% 8% 13% 22% 17%	1520
July	1st to 10th	Night	43% 8% 10% 20% 18% 1	2480
	11th to 20th	Night	34% 7% 16% 26% 16% 1	2717
	21st to end	Night	40% 7% 10% 18% 23% 1	2988
August	1st to 10th	Night	46% 8% 13% 23% 11% 1	2814
	11th to 20th	Night	42% 8% 12% 29% 9% 1	2812
	21st to end	Night	33% 8% 16% 29% 12% 2	3330
September	1st to 10th	Night	38% 7% 11% 25% 17% 3	1900
	11th to 20th	Night	31% 6% 13% 27% 21% 1	2038
	21st to end	Night	48% 6% 11% 19% 15% 1	2114
October	1st to 10th	Night	38% 5% 10% 24% 22% 1	3800
	11th to 20th	Night	40% 7% 12% 20% 14% 7%	3779
	21st to end	Night	39% 4 14% 23% 18% 1	4120
November	1st to 10th	Night	44% 6% 11% 23% 13% 3	2426
	11th to 20th	Night	58% 5% 9% 17% 8% 3	2800
	21st to end	Night	62% 5% 10% 18% 4 1	2800
December	1st to 10th	Night	74% 6% 10% 7% 2	4580
	11th to 20th	Night	69% 4 11% 13% 3	5208
	21st to end	Night	72% 5% 9% 11% 3	6996

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	32% 32% 25% 8% 2	250
	11th to 20th	Day	36% 30% 25% 6% 2 1	300
	21st to end	Day	32% 28% 32% 6% 2	330
February	1st to 10th	Day	34% 27% 32% 7%	300
	11th to 20th	Day	39% 27% 27% 7% 1	300
	21st to end	Day	40% 23% 30% 7%	294
March	1st to 10th	Day	30% 23% 30% 14% 2	500
	11th to 20th	Day	32% 20% 33% 10% 3 2	500
	21st to end	Day	35% 21% 28% 13% 3 1	550
April	1st to 10th	Day	32% 21% 26% 15% 4 1	500
	11th to 20th	Day	29% 21% 25% 14% 7% 3	492
	21st to end	Day	29% 19% 30% 15% 6% 1	500
May	1st to 10th	Day	30% 19% 27% 16% 6% 2	503
	11th to 20th	Day	25% 14% 29% 20% 9% 3	527
	21st to end	Day	18% 12% 26% 25% 14% 5%	549
June	1st to 10th	Day	20% 14% 30% 25% 10% 2	490
	11th to 20th	Day	18% 13% 25% 28% 13% 2	490
	21st to end	Day	18% 15% 30% 23% 10% 2	490
July	1st to 10th	Day	12% 6% 25% 29% 20% 7%	485
	11th to 20th	Day	12% 7% 29% 27% 18% 7%	484
	21st to end	Day	13% 7% 30% 27% 17% 6%	532
August	1st to 10th	Day	15% 6% 25% 27% 21% 7%	486
	11th to 20th	Day	16% 5% 26% 28% 14% 10%	479
	21st to end	Day	15% 7% 24% 30% 17% 8%	539
September	1st to 10th	Day	21% 7% 20% 26% 18% 8%	490
	11th to 20th	Day	13% 6% 23% 29% 19% 10%	490
	21st to end	Day	26% 12% 24% 22% 12% 4	490
October	1st to 10th	Day	20% 9% 29% 22% 16% 4	490
	11th to 20th	Day	26% 17% 30% 17% 8% 2	490
	21st to end	Day	34% 15% 28% 18% 5%	539
November	1st to 10th	Day	23% 18% 31% 18% 9% 2	410
	11th to 20th	Day	33% 23% 28% 13% 4 1	200
	21st to end	Day	28% 29% 35% 9% 1	200
December	1st to 10th	Day	38% 26% 26% 7% 2 2	200
	11th to 20th	Day	38% 18% 35% 7% 3	200
	21st to end	Day	33% 25% 28% 11% 2	220
January	1st to 10th	Night	30% 23% 33% 11% 3	800
	11th to 20th	Night	32% 23% 28% 15% 2	800
	21st to end	Night	26% 23% 32% 16% 2	880
February	1st to 10th	Night	33% 19% 29% 19% 1	800
	11th to 20th	Night	30% 22% 33% 13% 1	783
	21st to end	Night	29% 22% 31% 16% 2	660
March	1st to 10th	Night	26% 18% 35% 19% 3	800
	11th to 20th	Night	28% 19% 28% 20% 6%	800
	21st to end	Night	27% 16% 34% 18% 4 1	880
April	1st to 10th	Night	29% 16% 30% 20% 6%	800
	11th to 20th	Night	26% 14% 30% 21% 8% 1	786
	21st to end	Night	21% 11% 36% 26% 5% 1	600
May	1st to 10th	Night	26% 10% 21% 32% 10% 1	600
	11th to 20th	Night	21% 10% 27% 33% 8%	547
	21st to end	Night	13% 4 29% 33% 19% 2	423
June	1st to 10th	Night	10% 6% 30% 39% 15%	380
	11th to 20th	Night	13% 6% 25% 40% 16%	380
	21st to end	Night	14% 4 29% 36% 17% 1	380
July	1st to 10th	Night	11% 3 19% 44% 19% 4	370
	11th to 20th	Night	11% 4 19% 39% 25% 3	509
	21st to end	Night	13% 3 16% 43% 24% 1	624
August	1st to 10th	Night	12% 1 17% 43% 25% 2	567
	11th to 20th	Night	14% 4 13% 38% 27% 4	564
	21st to end	Night	12% 4 21% 33% 26% 4	742
September	1st to 10th	Night	16% 5% 22% 31% 23% 3	760
	11th to 20th	Night	13% 5% 18% 34% 27% 4	760
	21st to end	Night	22% 9% 23% 26% 17% 3	760
October	1st to 10th	Night	20% 7% 25% 27% 19% 2	760
	11th to 20th	Night	23% 11% 31% 24% 9% 1	760
	21st to end	Night	30% 13% 27% 25% 5%	836
November	1st to 10th	Night	21% 12% 30% 27% 10%	780
	11th to 20th	Night	30% 16% 28% 20% 6% 1	800
	21st to end	Night	31% 18% 29% 18% 4	800
December	1st to 10th	Night	34% 21% 28% 15% 3	800
	11th to 20th	Night	30% 23% 27% 16% 4	800
	21st to end	Night	31% 19% 30% 18% 2	880

Page generated at 2025-05-23 09:21:02. Version ac07

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