Maynard F. Jordan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Maynard F. Jordan Observatory clear sky chart from 2005-11-06 to 2026-07-31.

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	18% 8% 3 7% 4 4 4 4 5% 7% 35%	137601
	Night	18% 9% 3 7% 4 4 4 4 5% 6% 35%	135468
Transparency	Day	49% 3 11% 22% 13% 2	141109
	Night	50% 3 11% 22% 13% 2	140050
Seeing	Day	26% 15% 31% 19% 7% 2	18067
	Night	23% 13% 31% 26% 7%	26281

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	13% 12% 4 9% 5% 5% 5% 3 5% 5% 35%	4639
	11th to 20th	Day	16% 12% 4 10% 5% 4 3 3 4 5% 36%	4650
	21st to end	Day	10% 9% 3 6% 4 5% 5% 4 5% 6% 43%	5216
February	1st to 10th	Day	16% 10% 4 7% 4 5% 4 4 6% 6% 34%	1890
	11th to 20th	Day	13% 11% 3 8% 5% 5% 4 3 5% 6% 35%	1942
	21st to end	Day	13% 11% 3 8% 6% 5% 4 3 5% 6% 36%	1730
March	1st to 10th	Day	26% 11% 3 9% 5% 4 4 2 3 4 28%	4617
	11th to 20th	Day	19% 13% 3 8% 5% 4 3 4 4 6% 31%	5090
	21st to end	Day	28% 10% 3 6% 4 3 3 3 5% 6% 28%	5587
April	1st to 10th	Day	22% 11% 3 8% 5% 4 4 3 4 5% 32%	2493
	11th to 20th	Day	20% 10% 2 6% 4 4 4 3 4 6% 37%	2686
	21st to end	Day	24% 9% 2 6% 4 3 3 3 4 5% 36%	2730
May	1st to 10th	Day	28% 9% 3 7% 4 4 4 3 5% 6% 26%	7639
	11th to 20th	Day	19% 8% 3 7% 5% 5% 5% 4 5% 7% 31%	7625
	21st to end	Day	18% 8% 3 7% 5% 4 5% 4 5% 7% 34%	9393
June	1st to 10th	Day	23% 7% 3 7% 5% 4 4 4 5% 7% 30%	3150
	11th to 20th	Day	19% 6% 2 6% 5% 4 4 4 6% 8% 36%	3150
	21st to end	Day	24% 7% 3 6% 4 4 4 4 6% 7% 29%	3150
July	1st to 10th	Day	14% 7% 3 7% 5% 5% 6% 5% 7% 9% 32%	6790
	11th to 20th	Day	14% 6% 2 6% 4 4 5% 4 6% 9% 39%	6500
	21st to end	Day	14% 6% 2 6% 4 4 4 4 6% 8% 42%	6979
August	1st to 10th	Day	18% 7% 3 8% 5% 5% 4 4 5% 7% 35%	4235
	11th to 20th	Day	17% 8% 3 8% 5% 4 5% 4 7% 8% 33%	4175
	21st to end	Day	12% 6% 3 6% 5% 4 4 3 6% 9% 43%	4675
September	1st to 10th	Day	12% 5% 1 5% 4 4 4 3 5% 8% 49%	2305
	11th to 20th	Day	14% 5% 2 5% 4 3 4 3 4 5% 52%	2200
	21st to end	Day	16% 7% 2 6% 3 3 4 3 5% 7% 43%	2200
October	1st to 10th	Day	17% 6% 2 6% 4 4 4 3 4 6% 42%	2136
	11th to 20th	Day	19% 7% 3 6% 4 3 4 3 5% 6% 40%	1985
	21st to end	Day	22% 10% 3 6% 3 4 4 3 4 6% 34%	1980
November	1st to 10th	Day	18% 6% 3 6% 4 4 5% 4 6% 6% 38%	1851
	11th to 20th	Day	20% 7% 2 6% 4 4 4 4 5% 6% 39%	1822
	21st to end	Day	22% 8% 3 6% 4 3 4 3 4 5% 37%	1680
December	1st to 10th	Day	18% 11% 3 5% 4 3 4 3 5% 6% 37%	2470
	11th to 20th	Day	19% 8% 2 5% 4 3 4 3 4 5% 42%	2688
	21st to end	Day	21% 11% 4 6% 4 4 3 3 5% 5% 34%	3553
January	1st to 10th	Night	12% 12% 5% 10% 5% 4 4 3 5% 6% 34%	8253
	11th to 20th	Night	16% 13% 5% 9% 6% 5% 5% 4 5% 5% 27%	8235
	21st to end	Night	10% 11% 3 8% 4 4 5% 5% 6% 7% 38%	8963
February	1st to 10th	Night	14% 11% 4 8% 5% 5% 4 5% 6% 6% 32%	2730
	11th to 20th	Night	12% 11% 4 8% 5% 5% 5% 3 4 5% 38%	2559
	21st to end	Night	15% 12% 3 8% 5% 4 4 4 5% 5% 36%	2076
March	1st to 10th	Night	27% 12% 4 7% 4 3 3 3 4 4 28%	5442
	11th to 20th	Night	18% 12% 3 6% 5% 5% 5% 4 5% 7% 30%	5089
	21st to end	Night	26% 9% 3 7% 4 4 4 4 5% 6% 27%	5587
April	1st to 10th	Night	24% 11% 3 7% 4 4 3 3 4 5% 33%	2125
	11th to 20th	Night	21% 8% 3 6% 5% 5% 4 4 5% 6% 33%	1883
	21st to end	Night	25% 9% 2 6% 4 4 4 3 6% 6% 32%	1890
May	1st to 10th	Night	29% 9% 3 6% 3 3 3 3 5% 6% 29%	5259
	11th to 20th	Night	21% 7% 3 7% 4 4 5% 4 6% 8% 31%	5235
	21st to end	Night	15% 8% 3 7% 5% 4 4 3 6% 7% 36%	4840
June	1st to 10th	Night	22% 7% 3 8% 4 4 4 4 7% 7% 29%	1470
	11th to 20th	Night	16% 7% 3 6% 4 3 4 4 7% 9% 35%	1470
	21st to end	Night	23% 8% 3 7% 4 4 3 4 7% 7% 30%	1470
July	1st to 10th	Night	11% 7% 3 8% 6% 6% 6% 5% 7% 9% 33%	3397
	11th to 20th	Night	14% 8% 4 8% 6% 5% 5% 4 7% 8% 33%	3680
	21st to end	Night	11% 5% 2 5% 4 4 5% 4 7% 9% 43%	4047
August	1st to 10th	Night	19% 8% 3 7% 6% 5% 4 4 6% 6% 32%	2915
	11th to 20th	Night	16% 7% 3 7% 5% 6% 4 5% 7% 8% 32%	2872
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 5% 8% 44%	3223
September	1st to 10th	Night	14% 5% 2 4 3 3 4 4 6% 7% 49%	2090
	11th to 20th	Night	15% 5% 2 5% 3 3 3 3 4 7% 51%	2200
	21st to end	Night	17% 5% 2 5% 4 3 4 3 6% 7% 43%	2200
October	1st to 10th	Night	18% 6% 1 6% 3 4 4 4 5% 6% 43%	2263
	11th to 20th	Night	18% 7% 2 6% 4 4 4 4 5% 7% 39%	2410
	21st to end	Night	24% 10% 2 6% 4 3 3 3 4 6% 34%	2860
November	1st to 10th	Night	19% 7% 3 6% 5% 3 4 3 4 6% 41%	2667
	11th to 20th	Night	18% 8% 3 5% 4 4 4 4 5% 7% 38%	2796
	21st to end	Night	19% 9% 3 6% 4 3 4 4 6% 7% 36%	2940
December	1st to 10th	Night	17% 9% 3 7% 4 4 5% 4 6% 6% 34%	4924
	11th to 20th	Night	23% 8% 2 5% 3 3 3 3 4 5% 40%	5741
	21st to end	Night	22% 10% 3 6% 4 4 4 3 5% 5% 34%	7667

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	52% 2 11% 24% 10% 1	4640
	11th to 20th	Day	53% 2 9% 23% 11% 2	4680
	21st to end	Day	43% 2 9% 22% 19% 4	5216
February	1st to 10th	Day	50% 3 10% 21% 14% 3	1890
	11th to 20th	Day	50% 3 10% 22% 13% 2	1950
	21st to end	Day	51% 3 10% 21% 12% 4	1730
March	1st to 10th	Day	63% 2 6% 10% 15% 5%	4621
	11th to 20th	Day	56% 2 13% 17% 11% 2	5090
	21st to end	Day	58% 2 4 18% 16% 2	5587
April	1st to 10th	Day	56% 3 7% 16% 14% 4	2493
	11th to 20th	Day	50% 3 8% 23% 14% 2	2686
	21st to end	Day	52% 3 7% 19% 16% 4	2730
May	1st to 10th	Day	60% 3 10% 18% 8% 1	7651
	11th to 20th	Day	53% 3 12% 21% 8% 2	7629
	21st to end	Day	50% 3 10% 21% 14% 1	9394
June	1st to 10th	Day	54% 3 11% 22% 10% 1	3150
	11th to 20th	Day	46% 4 11% 21% 16% 2	3150
	21st to end	Day	54% 4 11% 20% 10% 2	3150
July	1st to 10th	Day	47% 5% 16% 23% 9%	6718
	11th to 20th	Day	42% 5% 14% 24% 14% 1	6425
	21st to end	Day	40% 4 12% 28% 15% 1	6782
August	1st to 10th	Day	49% 4 13% 27% 7%	4222
	11th to 20th	Day	48% 5% 15% 23% 9%	4214
	21st to end	Day	37% 5% 12% 27% 18% 1	4675
September	1st to 10th	Day	34% 4 12% 29% 19% 1	2305
	11th to 20th	Day	36% 3 8% 29% 22% 2	2200
	21st to end	Day	41% 3 9% 28% 18% 1	2200
October	1st to 10th	Day	53% 4 7% 22% 13% 1	3474
	11th to 20th	Day	39% 2 10% 33% 16% 1	3213
	21st to end	Day	39% 3 15% 24% 19% 1	3180
November	1st to 10th	Day	46% 3 13% 22% 14% 1	1851
	11th to 20th	Day	47% 4 10% 23% 14% 3	1822
	21st to end	Day	50% 4 14% 23% 8% 1	1680
December	1st to 10th	Day	48% 3 17% 18% 13% 1	2470
	11th to 20th	Day	46% 2 13% 19% 14% 6%	2688
	21st to end	Day	53% 2 10% 20% 12% 3	3553
January	1st to 10th	Night	53% 3 10% 22% 12% 1	8253
	11th to 20th	Night	59% 3 10% 16% 10% 2	8280
	21st to end	Night	46% 3 8% 23% 17% 4	8963
February	1st to 10th	Night	51% 3 10% 21% 12% 2	2730
	11th to 20th	Night	50% 2 9% 22% 15% 2	2562
	21st to end	Night	51% 3 7% 22% 14% 3	2076
March	1st to 10th	Night	61% 1 5% 14% 13% 5%	5451
	11th to 20th	Night	55% 2 17% 17% 8% 1	5090
	21st to end	Night	58% 2 4 19% 16% 1	5587
April	1st to 10th	Night	54% 2 7% 16% 17% 3	2125
	11th to 20th	Night	51% 4 7% 22% 14% 2	1883
	21st to end	Night	52% 3 10% 18% 14% 3	1890
May	1st to 10th	Night	57% 2 14% 16% 11% 1	5265
	11th to 20th	Night	52% 3 12% 24% 9% 1	5236
	21st to end	Night	48% 3 13% 22% 13%	4841
June	1st to 10th	Night	53% 4 11% 23% 9% 1	1470
	11th to 20th	Night	44% 4 16% 23% 11% 2	1470
	21st to end	Night	52% 4 13% 19% 9% 2	1470
July	1st to 10th	Night	46% 5% 15% 25% 8%	3359
	11th to 20th	Night	48% 4 13% 26% 9%	3632
	21st to end	Night	37% 5% 18% 29% 11% 1	3930
August	1st to 10th	Night	53% 5% 13% 21% 8%	2903
	11th to 20th	Night	48% 5% 16% 21% 9%	2899
	21st to end	Night	37% 7% 13% 25% 17% 1	3223
September	1st to 10th	Night	34% 4 13% 32% 15% 1	2090
	11th to 20th	Night	35% 3 11% 29% 20% 2	2200
	21st to end	Night	41% 4 9% 30% 15% 1	2200
October	1st to 10th	Night	43% 4 12% 26% 14% 1	3705
	11th to 20th	Night	38% 3 10% 31% 17% 1	3921
	21st to end	Night	42% 3 12% 28% 13% 2	4610
November	1st to 10th	Night	46% 3 11% 25% 14% 1	2667
	11th to 20th	Night	47% 3 10% 23% 16% 2	2796
	21st to end	Night	48% 5% 14% 19% 12% 1	2940
December	1st to 10th	Night	50% 4 10% 26% 10%	4924
	11th to 20th	Night	48% 2 10% 21% 18% 1	5742
	21st to end	Night	54% 2 9% 18% 15% 1	7667

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	33% 13% 35% 16% 1 1	220
	11th to 20th	Day	28% 21% 35% 11% 3 1	220
	21st to end	Day	22% 19% 38% 17% 4	269
February	1st to 10th	Day	29% 15% 28% 23% 4	430
	11th to 20th	Day	27% 23% 36% 11% 2 1	478
	21st to end	Day	24% 22% 32% 19% 3	446
March	1st to 10th	Day	32% 19% 30% 16% 2	540
	11th to 20th	Day	29% 22% 30% 13% 5% 1	540
	21st to end	Day	35% 21% 25% 12% 5% 3	594
April	1st to 10th	Day	37% 21% 26% 10% 5% 2	540
	11th to 20th	Day	34% 20% 29% 11% 5% 1	532
	21st to end	Day	34% 16% 30% 15% 4 2	540
May	1st to 10th	Day	33% 16% 31% 14% 4 1	543
	11th to 20th	Day	28% 14% 34% 17% 6% 1	567
	21st to end	Day	26% 17% 32% 18% 7%	749
June	1st to 10th	Day	28% 11% 31% 20% 9% 1	780
	11th to 20th	Day	23% 14% 27% 24% 11% 1	780
	21st to end	Day	28% 10% 29% 24% 9% 1	780
July	1st to 10th	Day	18% 11% 36% 23% 10% 2	697
	11th to 20th	Day	20% 12% 36% 21% 9% 2	564
	21st to end	Day	17% 10% 36% 25% 10% 3	616
August	1st to 10th	Day	16% 9% 35% 28% 10% 2	526
	11th to 20th	Day	18% 8% 29% 28% 12% 4	519
	21st to end	Day	17% 11% 31% 26% 13% 3	583
September	1st to 10th	Day	16% 12% 33% 24% 12% 3	530
	11th to 20th	Day	20% 7% 29% 22% 16% 6%	530
	21st to end	Day	18% 15% 29% 22% 11% 6%	530
October	1st to 10th	Day	21% 15% 24% 21% 15% 3	494
	11th to 20th	Day	24% 16% 28% 20% 10% 2	420
	21st to end	Day	29% 16% 28% 19% 6% 1	462
November	1st to 10th	Day	27% 15% 27% 23% 7% 2	425
	11th to 20th	Day	28% 16% 34% 15% 6%	430
	21st to end	Day	31% 17% 27% 21% 4	430
December	1st to 10th	Day	28% 18% 34% 16% 4	301
	11th to 20th	Day	34% 23% 29% 12% 1	220
	21st to end	Day	34% 18% 27% 14% 5% 2	242
January	1st to 10th	Night	23% 18% 31% 25% 4	843
	11th to 20th	Night	27% 16% 30% 22% 5%	840
	21st to end	Night	19% 18% 35% 24% 4	924
February	1st to 10th	Night	22% 18% 34% 23% 4	840
	11th to 20th	Night	21% 20% 36% 20% 3 1	823
	21st to end	Night	24% 15% 36% 23% 3	692
March	1st to 10th	Night	28% 15% 33% 23% 2	840
	11th to 20th	Night	25% 17% 33% 20% 5%	840
	21st to end	Night	26% 15% 31% 21% 6%	924
April	1st to 10th	Night	32% 17% 30% 18% 3	840
	11th to 20th	Night	25% 16% 35% 20% 5%	639
	21st to end	Night	31% 12% 30% 21% 5%	630
May	1st to 10th	Night	27% 13% 30% 26% 4	630
	11th to 20th	Night	25% 12% 36% 23% 5%	622
	21st to end	Night	20% 9% 36% 27% 7%	491
June	1st to 10th	Night	24% 7% 32% 31% 7%	420
	11th to 20th	Night	19% 8% 32% 32% 9%	420
	21st to end	Night	27% 5% 26% 36% 6%	420
July	1st to 10th	Night	16% 4 36% 36% 9%	410
	11th to 20th	Night	17% 3 26% 43% 11%	408
	21st to end	Night	18% 4 24% 41% 11%	474
August	1st to 10th	Night	17% 5% 30% 37% 11%	598
	11th to 20th	Night	16% 7% 28% 36% 12% 1	592
	21st to end	Night	15% 7% 27% 38% 12% 1	660
September	1st to 10th	Night	17% 6% 30% 34% 14% 1	790
	11th to 20th	Night	17% 8% 27% 30% 16% 3	800
	21st to end	Night	19% 8% 29% 31% 12% 1	800
October	1st to 10th	Night	22% 8% 25% 30% 14% 1	800
	11th to 20th	Night	22% 10% 30% 28% 9% 1	800
	21st to end	Night	30% 14% 28% 23% 5%	880
November	1st to 10th	Night	25% 12% 30% 25% 7%	820
	11th to 20th	Night	25% 15% 30% 22% 8% 1	876
	21st to end	Night	26% 16% 29% 23% 5% 1	950
December	1st to 10th	Night	28% 11% 33% 23% 5%	950
	11th to 20th	Night	26% 18% 32% 21% 3	950
	21st to end	Night	28% 16% 32% 20% 5%	1045

Page generated at 2026-07-28 03:25:23. Version ac07

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