Wayside Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wayside Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	15% 6% 2 6% 4 4 5% 4 6% 8% 41%	149615
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	142449
Transparency	Day	41% 4 11% 24% 17% 3	153178
	Night	41% 4 11% 24% 17% 4	146962
Seeing	Day	21% 15% 29% 21% 10% 3	17568
	Night	20% 13% 30% 26% 10% 1	26851

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% 6% 2 6% 5% 4 4 4 5% 5% 45%	4661
	11th to 20th	Day	15% 8% 2 8% 4 3 3 3 4 4 46%	4965
	21st to end	Day	15% 6% 2 6% 4 5% 6% 4 6% 6% 41%	5780
February	1st to 10th	Day	15% 7% 3 6% 5% 4 4 3 6% 7% 40%	1890
	11th to 20th	Day	17% 8% 3 7% 4 5% 4 3 5% 6% 39%	1968
	21st to end	Day	18% 11% 2 7% 4 3 3 3 4 6% 40%	1730
March	1st to 10th	Day	20% 5% 1 4 3 3 4 3 4 6% 46%	4706
	11th to 20th	Day	12% 7% 2 5% 4 4 4 3 5% 6% 46%	5090
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 6% 43%	5855
April	1st to 10th	Day	15% 7% 3 6% 4 4 4 4 5% 6% 41%	2520
	11th to 20th	Day	13% 6% 2 6% 5% 5% 4 4 5% 7% 43%	2494
	21st to end	Day	17% 5% 2 6% 4 4 5% 5% 5% 7% 40%	2520
May	1st to 10th	Day	23% 7% 2 5% 4 4 4 3 6% 8% 35%	7488
	11th to 20th	Day	20% 6% 2 6% 4 4 4 4 5% 7% 37%	8221
	21st to end	Day	24% 6% 2 5% 3 4 4 4 5% 7% 36%	9060
June	1st to 10th	Day	11% 6% 2 5% 5% 5% 5% 5% 6% 10% 40%	2940
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 4 7% 9% 38%	2940
	21st to end	Day	11% 5% 2 6% 4 5% 4 4 6% 10% 43%	2940
July	1st to 10th	Day	12% 5% 2 6% 4 5% 6% 4 7% 10% 38%	8280
	11th to 20th	Day	10% 5% 2 7% 5% 5% 6% 5% 8% 12% 35%	8280
	21st to end	Day	11% 5% 2 6% 5% 5% 5% 4 7% 10% 40%	8621
August	1st to 10th	Day	15% 5% 2 8% 5% 6% 6% 5% 7% 9% 30%	6014
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 4 6% 10% 40%	5559
	21st to end	Day	10% 5% 2 5% 4 5% 5% 4 7% 9% 44%	6080
September	1st to 10th	Day	14% 5% 2 5% 4 4 4 3 6% 7% 45%	2520
	11th to 20th	Day	13% 4 2 6% 3 3 4 3 5% 7% 50%	2517
	21st to end	Day	19% 7% 3 7% 5% 4 4 4 5% 6% 35%	2261
October	1st to 10th	Day	15% 6% 2 5% 4 3 4 4 4 6% 46%	2100
	11th to 20th	Day	15% 6% 1 5% 4 3 4 3 4 7% 49%	2100
	21st to end	Day	16% 6% 2 5% 3 3 3 4 4 5% 49%	2258
November	1st to 10th	Day	13% 6% 2 5% 2 3 3 3 5% 7% 52%	1851
	11th to 20th	Day	15% 6% 2 5% 3 3 3 4 5% 7% 47%	1890
	21st to end	Day	15% 6% 2 6% 4 3 4 3 5% 6% 45%	1722
December	1st to 10th	Day	13% 5% 2 7% 5% 4 5% 3 5% 5% 45%	2680
	11th to 20th	Day	20% 9% 2 4 3 3 3 3 4 5% 43%	3066
	21st to end	Day	16% 6% 3 6% 5% 4 4 5% 6% 7% 39%	4048
January	1st to 10th	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	8234
	11th to 20th	Night	16% 7% 2 5% 3 3 3 2 4 5% 49%	7904
	21st to end	Night	15% 7% 2 5% 4 4 4 3 5% 5% 47%	8401
February	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 40%	2730
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 5% 41%	2536
	21st to end	Night	17% 7% 2 6% 5% 4 4 4 5% 7% 41%	2076
March	1st to 10th	Night	18% 7% 3 6% 3 3 3 2 4 6% 45%	5350
	11th to 20th	Night	11% 6% 3 6% 4 3 3 3 4 6% 51%	5089
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 6% 44%	5318
April	1st to 10th	Night	16% 7% 2 8% 5% 4 4 3 6% 6% 40%	2100
	11th to 20th	Night	15% 6% 2 6% 4 5% 4 3 5% 8% 43%	2075
	21st to end	Night	18% 6% 2 5% 4 3 4 3 5% 7% 44%	2100
May	1st to 10th	Night	21% 8% 2 6% 4 4 4 4 6% 8% 33%	5463
	11th to 20th	Night	22% 7% 2 6% 3 4 4 4 5% 9% 34%	4639
	21st to end	Night	26% 7% 3 6% 3 3 4 3 4 7% 35%	5114
June	1st to 10th	Night	13% 6% 2 5% 5% 4 3 4 6% 7% 45%	1680
	11th to 20th	Night	12% 6% 3 8% 4 4 5% 3 6% 9% 39%	1680
	21st to end	Night	11% 4 2 5% 5% 4 4 5% 6% 11% 41%	1680
July	1st to 10th	Night	13% 5% 3 6% 6% 4 5% 4 6% 9% 38%	4680
	11th to 20th	Night	12% 7% 3 7% 5% 5% 6% 5% 7% 11% 32%	4680
	21st to end	Night	12% 5% 2 6% 4 5% 5% 4 6% 9% 41%	5438
August	1st to 10th	Night	18% 6% 3 8% 5% 5% 5% 4 7% 8% 31%	4135
	11th to 20th	Night	15% 6% 2 6% 5% 5% 4 3 6% 8% 40%	4439
	21st to end	Night	12% 6% 2 6% 4 4 4 4 5% 7% 47%	5045
September	1st to 10th	Night	14% 6% 2 6% 4 3 4 2 5% 6% 47%	2100
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 7% 52%	2102
	21st to end	Night	21% 6% 3 6% 4 4 4 3 5% 7% 37%	2356
October	1st to 10th	Night	18% 6% 2 6% 5% 4 4 2 4 5% 45%	2520
	11th to 20th	Night	17% 4 2 4 3 3 3 3 4 6% 50%	2520
	21st to end	Night	16% 5% 2 4 3 3 3 2 4 6% 52%	2757
November	1st to 10th	Night	11% 5% 2 6% 3 3 3 4 6% 6% 53%	2667
	11th to 20th	Night	16% 6% 2 6% 4 3 3 3 4 6% 47%	2730
	21st to end	Night	13% 5% 2 5% 4 4 5% 4 5% 7% 46%	2893
December	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 7% 47%	4720
	11th to 20th	Night	22% 6% 2 4 3 3 3 3 5% 7% 41%	5348
	21st to end	Night	18% 7% 3 6% 4 4 5% 4 5% 6% 39%	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	40% 3 9% 25% 20% 3	4662
	11th to 20th	Day	43% 2 6% 22% 20% 8%	4998
	21st to end	Day	42% 3 11% 28% 13% 2	5780
February	1st to 10th	Day	43% 4 13% 25% 13% 2	1890
	11th to 20th	Day	46% 3 8% 25% 15% 3	1977
	21st to end	Day	46% 4 9% 24% 14% 4	1730
March	1st to 10th	Day	39% 3 8% 25% 18% 7%	4709
	11th to 20th	Day	37% 4 8% 28% 17% 6%	5090
	21st to end	Day	43% 3 9% 18% 21% 6%	5855
April	1st to 10th	Day	42% 5% 12% 19% 17% 5%	2520
	11th to 20th	Day	39% 5% 12% 22% 17% 5%	2494
	21st to end	Day	41% 5% 10% 21% 18% 4	2520
May	1st to 10th	Day	48% 4 12% 20% 15% 1	7497
	11th to 20th	Day	45% 4 10% 26% 13% 1	8225
	21st to end	Day	48% 4 9% 21% 16% 2	9061
June	1st to 10th	Day	37% 7% 12% 24% 17% 2	2940
	11th to 20th	Day	40% 7% 13% 25% 13% 3	2940
	21st to end	Day	34% 8% 14% 24% 19% 1	2940
July	1st to 10th	Day	41% 6% 14% 21% 15% 2	8210
	11th to 20th	Day	42% 4 15% 27% 11% 1	8205
	21st to end	Day	39% 4 9% 26% 21% 1	8431
August	1st to 10th	Day	49% 6% 17% 21% 7%	6001
	11th to 20th	Day	40% 4 14% 30% 12% 1	5596
	21st to end	Day	36% 3 13% 28% 18% 2	6080
September	1st to 10th	Day	38% 5% 12% 21% 20% 3	2520
	11th to 20th	Day	34% 5% 9% 27% 22% 4	2517
	21st to end	Day	50% 4 8% 18% 18% 2	2261
October	1st to 10th	Day	36% 3 7% 19% 29% 5%	3360
	11th to 20th	Day	27% 3 10% 23% 28% 9%	3340
	21st to end	Day	24% 2 9% 30% 28% 6%	3572
November	1st to 10th	Day	33% 3 13% 25% 22% 4	1851
	11th to 20th	Day	37% 4 10% 24% 22% 3	1890
	21st to end	Day	39% 5% 9% 31% 13% 2	1722
December	1st to 10th	Day	41% 3 15% 26% 13% 3	2680
	11th to 20th	Day	45% 3 12% 29% 10% 2	3066
	21st to end	Day	43% 5% 12% 21% 17% 2	4048
January	1st to 10th	Night	41% 2 11% 25% 17% 4	8234
	11th to 20th	Night	39% 2 8% 23% 20% 8%	7946
	21st to end	Night	40% 2 7% 29% 20% 3	8401
February	1st to 10th	Night	45% 3 12% 24% 14% 2	2730
	11th to 20th	Night	45% 3 9% 21% 19% 4	2538
	21st to end	Night	44% 3 10% 21% 16% 6%	2076
March	1st to 10th	Night	41% 4 8% 25% 13% 9%	5360
	11th to 20th	Night	35% 4 12% 28% 15% 6%	5090
	21st to end	Night	42% 3 8% 18% 25% 4	5318
April	1st to 10th	Night	43% 4 10% 19% 20% 4	2100
	11th to 20th	Night	40% 5% 11% 20% 19% 5%	2075
	21st to end	Night	40% 5% 10% 21% 18% 6%	2100
May	1st to 10th	Night	50% 4 10% 19% 14% 2	5470
	11th to 20th	Night	47% 5% 11% 25% 11% 1	4640
	21st to end	Night	50% 4 9% 20% 13% 2	5115
June	1st to 10th	Night	37% 7% 11% 24% 20% 1	1680
	11th to 20th	Night	41% 6% 13% 24% 15% 1	1680
	21st to end	Night	36% 6% 13% 26% 16% 3	1680
July	1st to 10th	Night	44% 5% 15% 21% 13% 2	4640
	11th to 20th	Night	46% 4 17% 23% 10% 1	4632
	21st to end	Night	40% 5% 13% 22% 18% 1	5314
August	1st to 10th	Night	50% 6% 16% 18% 9%	4124
	11th to 20th	Night	43% 4 14% 27% 11% 1	4468
	21st to end	Night	38% 3 10% 30% 17% 1	5045
September	1st to 10th	Night	38% 5% 10% 22% 21% 4	2100
	11th to 20th	Night	33% 5% 9% 25% 24% 4	2102
	21st to end	Night	48% 4 8% 20% 18% 3	2356
October	1st to 10th	Night	45% 2 8% 22% 21% 3	4040
	11th to 20th	Night	26% 2 10% 26% 29% 6%	4019
	21st to end	Night	25% 3 11% 32% 23% 7%	4381
November	1st to 10th	Night	32% 4 10% 27% 21% 6%	2667
	11th to 20th	Night	39% 4 8% 23% 20% 6%	2730
	21st to end	Night	38% 4 11% 27% 18% 2	2893
December	1st to 10th	Night	39% 2 12% 30% 15% 2	4720
	11th to 20th	Night	44% 3 12% 21% 15% 5%	5348
	21st to end	Night	45% 5% 11% 19% 18% 1	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	28% 25% 33% 10% 2 1	220
	11th to 20th	Day	37% 27% 26% 9% 1	278
	21st to end	Day	24% 35% 26% 12% 4	363
February	1st to 10th	Day	27% 30% 30% 11% 3	330
	11th to 20th	Day	37% 23% 32% 7% 1	456
	21st to end	Day	30% 25% 31% 12% 2	446
March	1st to 10th	Day	29% 24% 30% 14% 3	540
	11th to 20th	Day	30% 24% 30% 11% 4 1	540
	21st to end	Day	27% 26% 32% 11% 3 1	594
April	1st to 10th	Day	30% 21% 34% 9% 5% 1	540
	11th to 20th	Day	24% 23% 30% 16% 6% 2	532
	21st to end	Day	24% 19% 37% 14% 6%	540
May	1st to 10th	Day	31% 17% 30% 15% 6% 1	543
	11th to 20th	Day	22% 16% 34% 19% 8% 1	567
	21st to end	Day	24% 11% 26% 22% 14% 3	617
June	1st to 10th	Day	13% 13% 32% 25% 14% 3	570
	11th to 20th	Day	18% 12% 30% 26% 11% 3	570
	21st to end	Day	13% 9% 30% 31% 12% 4	570
July	1st to 10th	Day	14% 5% 22% 34% 18% 7%	565
	11th to 20th	Day	7% 2 26% 36% 23% 5%	564
	21st to end	Day	12% 5% 25% 35% 19% 3	620
August	1st to 10th	Day	14% 3 24% 34% 18% 7%	566
	11th to 20th	Day	14% 5% 21% 34% 19% 8%	559
	21st to end	Day	11% 5% 31% 29% 16% 8%	627
September	1st to 10th	Day	17% 8% 26% 24% 18% 7%	570
	11th to 20th	Day	15% 5% 26% 31% 16% 7%	570
	21st to end	Day	28% 9% 26% 27% 9% 2	570
October	1st to 10th	Day	20% 8% 29% 22% 14% 6%	570
	11th to 20th	Day	20% 16% 28% 22% 11% 2	570
	21st to end	Day	18% 16% 35% 20% 9% 2	602
November	1st to 10th	Day	16% 18% 33% 20% 10% 2	425
	11th to 20th	Day	23% 22% 30% 19% 6%	430
	21st to end	Day	25% 23% 29% 19% 4	262
December	1st to 10th	Day	27% 25% 32% 13% 4	220
	11th to 20th	Day	34% 22% 31% 11% 3	220
	21st to end	Day	26% 24% 29% 16% 5%	242
January	1st to 10th	Night	22% 22% 36% 17% 3	840
	11th to 20th	Night	27% 19% 31% 20% 3	840
	21st to end	Night	19% 22% 39% 19% 2	924
February	1st to 10th	Night	22% 19% 36% 20% 2	840
	11th to 20th	Night	28% 24% 32% 15% 1	823
	21st to end	Night	23% 22% 36% 17% 2	692
March	1st to 10th	Night	22% 21% 34% 20% 4	840
	11th to 20th	Night	23% 20% 34% 18% 5% 1	840
	21st to end	Night	22% 15% 40% 18% 5%	924
April	1st to 10th	Night	23% 20% 34% 18% 5%	840
	11th to 20th	Night	19% 15% 35% 25% 6%	831
	21st to end	Night	23% 14% 31% 25% 7%	840
May	1st to 10th	Night	21% 12% 35% 26% 5% 1	704
	11th to 20th	Night	22% 11% 31% 29% 8%	414
	21st to end	Night	20% 7% 22% 35% 15% 1	455
June	1st to 10th	Night	16% 6% 28% 34% 16%	420
	11th to 20th	Night	15% 5% 26% 42% 11%	420
	21st to end	Night	11% 3 27% 42% 15%	420
July	1st to 10th	Night	13% 4 21% 39% 21% 2	410
	11th to 20th	Night	7% 5% 16% 44% 26% 2	408
	21st to end	Night	11% 4 18% 41% 24% 2	583
August	1st to 10th	Night	12% 2 17% 42% 25% 2	627
	11th to 20th	Night	15% 3 15% 39% 25% 3	775
	21st to end	Night	14% 4 20% 37% 22% 3	924
September	1st to 10th	Night	16% 4 23% 34% 19% 3	840
	11th to 20th	Night	14% 4 20% 35% 24% 4	840
	21st to end	Night	25% 4 23% 28% 18% 2	840
October	1st to 10th	Night	22% 6% 25% 26% 18% 4	840
	11th to 20th	Night	20% 12% 27% 30% 11%	840
	21st to end	Night	19% 14% 32% 24% 10% 1	913
November	1st to 10th	Night	15% 12% 35% 26% 10% 1	820
	11th to 20th	Night	21% 18% 29% 25% 6% 1	840
	21st to end	Night	18% 20% 30% 25% 7%	840
December	1st to 10th	Night	23% 19% 33% 18% 6% 1	840
	11th to 20th	Night	21% 18% 36% 21% 4	840
	21st to end	Night	22% 18% 32% 26% 2	924

Page generated at 2026-10-26 04:37:48. Version ac07

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