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 2025-07-06.

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% 6% 2 6% 4 4 4 4 5% 7% 41%	119967
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 43%	118817
Transparency	Day	41% 5% 11% 24% 17% 3	123572
	Night	41% 4 11% 23% 18% 4	123361
Seeing	Day	21% 15% 29% 21% 10% 3	15809
	Night	19% 13% 30% 26% 10% 1	25166

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 5% 4 4 4 3 5% 5% 48%	3597
	11th to 20th	Day	14% 9% 3 9% 5% 4 4 3 4 4 42%	3815
	21st to end	Day	17% 7% 2 5% 4 4 4 3 4 6% 43%	4466
February	1st to 10th	Day	15% 8% 3 6% 5% 4 4 4 6% 6% 39%	1800
	11th to 20th	Day	17% 8% 2 7% 4 5% 4 3 5% 6% 39%	1872
	21st to end	Day	18% 11% 2 6% 4 3 3 2 4 6% 40%	1650
March	1st to 10th	Day	20% 4 1 4 3 3 4 3 4 6% 47%	4604
	11th to 20th	Day	12% 7% 2 5% 4 4 4 3 5% 6% 46%	4980
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 5% 43%	5729
April	1st to 10th	Day	16% 7% 3 6% 4 4 4 4 5% 6% 40%	2400
	11th to 20th	Day	14% 6% 2 6% 5% 5% 4 4 5% 7% 43%	2374
	21st to end	Day	17% 5% 2 6% 4 4 5% 5% 5% 8% 41%	2400
May	1st to 10th	Day	26% 7% 2 5% 4 4 4 4 6% 8% 31%	5778
	11th to 20th	Day	25% 7% 3 7% 4 4 4 4 5% 7% 31%	6318
	21st to end	Day	19% 5% 2 6% 4 4 5% 4 6% 8% 38%	6949
June	1st to 10th	Day	11% 6% 2 6% 5% 5% 5% 5% 6% 10% 39%	2800
	11th to 20th	Day	13% 7% 2 7% 5% 4 5% 5% 8% 9% 37%	2800
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 6% 10% 43%	2800
July	1st to 10th	Day	7% 4 2 6% 4 6% 7% 5% 9% 11% 39%	4510
	11th to 20th	Day	10% 5% 2 6% 5% 5% 5% 5% 7% 11% 39%	4440
	21st to end	Day	12% 7% 3 8% 6% 6% 5% 5% 7% 8% 33%	4610
August	1st to 10th	Day	19% 5% 2 6% 4 5% 6% 4 7% 8% 33%	4104
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 5% 6% 9% 42%	3772
	21st to end	Day	10% 5% 2 5% 4 4 4 4 6% 8% 47%	4180
September	1st to 10th	Day	14% 6% 2 6% 5% 4 4 3 6% 6% 44%	2280
	11th to 20th	Day	13% 4 2 6% 3 3 4 3 4 8% 50%	2277
	21st to end	Day	19% 8% 3 7% 4 4 3 4 4 7% 37%	2046
October	1st to 10th	Day	16% 7% 2 5% 4 3 4 4 4 7% 44%	1900
	11th to 20th	Day	15% 6% 2 5% 4 3 4 3 4 6% 49%	1900
	21st to end	Day	17% 6% 2 5% 4 3 3 4 4 5% 47%	2069
November	1st to 10th	Day	13% 6% 2 5% 2 3 3 3 5% 7% 52%	1761
	11th to 20th	Day	15% 5% 2 5% 3 3 3 4 5% 7% 48%	1800
	21st to end	Day	15% 6% 2 5% 4 3 5% 3 5% 6% 46%	1640
December	1st to 10th	Day	13% 5% 2 7% 5% 4 5% 3 5% 5% 45%	2600
	11th to 20th	Day	20% 9% 3 4 3 3 3 3 4 5% 43%	2986
	21st to end	Day	16% 6% 3 6% 5% 4 4 5% 6% 7% 39%	3960
January	1st to 10th	Night	13% 7% 2 5% 4 4 4 3 5% 6% 47%	6349
	11th to 20th	Night	16% 7% 2 5% 3 3 3 2 4 5% 50%	6068
	21st to end	Night	17% 8% 2 6% 4 4 4 3 4 5% 45%	6490
February	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 40%	2600
	11th to 20th	Night	18% 8% 2 6% 4 4 4 3 5% 5% 42%	2412
	21st to end	Night	17% 7% 2 5% 5% 4 4 3 5% 6% 41%	1980
March	1st to 10th	Night	17% 6% 3 6% 3 3 3 2 4 6% 46%	5233
	11th to 20th	Night	11% 6% 3 6% 4 3 3 3 4 6% 51%	4979
	21st to end	Night	15% 8% 2 6% 4 4 4 3 5% 6% 45%	5202
April	1st to 10th	Night	16% 8% 2 8% 5% 4 4 3 5% 6% 40%	2000
	11th to 20th	Night	16% 6% 2 6% 4 4 4 3 6% 7% 43%	1975
	21st to end	Night	18% 6% 2 5% 4 3 4 3 5% 7% 44%	2000
May	1st to 10th	Night	24% 8% 3 7% 5% 5% 4 3 6% 7% 29%	4201
	11th to 20th	Night	25% 8% 2 6% 3 4 4 4 5% 8% 31%	3575
	21st to end	Night	21% 7% 2 6% 4 3 4 3 5% 8% 38%	3926
June	1st to 10th	Night	13% 6% 2 5% 5% 4 4 4 6% 7% 44%	1600
	11th to 20th	Night	13% 6% 3 8% 4 4 5% 3 6% 10% 39%	1600
	21st to end	Night	10% 4 2 5% 5% 4 4 5% 6% 11% 41%	1600
July	1st to 10th	Night	7% 6% 3 7% 6% 5% 5% 5% 7% 10% 39%	2560
	11th to 20th	Night	11% 7% 2 6% 4 5% 6% 5% 8% 9% 37%	2520
	21st to end	Night	15% 6% 3 7% 5% 5% 5% 4 7% 8% 37%	2925
August	1st to 10th	Night	21% 5% 2 7% 4 5% 4 4 6% 8% 34%	2825
	11th to 20th	Night	14% 6% 2 6% 5% 5% 4 3 5% 7% 42%	3024
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 50%	3476
September	1st to 10th	Night	14% 6% 2 6% 4 3 4 2 5% 6% 47%	1900
	11th to 20th	Night	13% 6% 2 4 3 2 3 3 5% 7% 53%	1902
	21st to end	Night	20% 6% 2 6% 5% 3 3 3 5% 7% 39%	2132
October	1st to 10th	Night	19% 7% 2 6% 5% 3 4 2 4 5% 43%	2280
	11th to 20th	Night	16% 4 2 4 3 3 3 3 5% 6% 51%	2280
	21st to end	Night	17% 5% 2 5% 3 3 3 2 5% 6% 50%	2527
November	1st to 10th	Night	11% 5% 2 6% 3 3 3 4 6% 6% 53%	2537
	11th to 20th	Night	16% 6% 2 6% 3 3 3 3 4 6% 47%	2600
	21st to end	Night	13% 5% 1 5% 4 4 5% 4 6% 7% 46%	2755
December	1st to 10th	Night	12% 7% 2 6% 4 3 4 3 5% 7% 47%	4580
	11th to 20th	Night	22% 6% 2 4 3 3 3 3 5% 7% 40%	5208
	21st to end	Night	18% 7% 3 6% 4 4 5% 4 5% 6% 39%	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	36% 4 8% 24% 24% 4	3598
	11th to 20th	Day	45% 3 7% 22% 17% 7%	3848
	21st to end	Day	42% 4 11% 26% 14% 2	4466
February	1st to 10th	Day	44% 4 13% 25% 12% 2	1800
	11th to 20th	Day	45% 4 8% 25% 15% 3	1881
	21st to end	Day	46% 4 9% 23% 14% 4	1650
March	1st to 10th	Day	39% 3 8% 25% 18% 7%	4607
	11th to 20th	Day	37% 4 8% 28% 17% 6%	4980
	21st to end	Day	44% 3 8% 18% 21% 6%	5729
April	1st to 10th	Day	42% 5% 11% 19% 17% 6%	2400
	11th to 20th	Day	39% 6% 11% 22% 17% 5%	2374
	21st to end	Day	40% 5% 10% 20% 19% 4	2400
May	1st to 10th	Day	51% 5% 13% 19% 11% 2	5787
	11th to 20th	Day	50% 6% 10% 23% 11% 1	6322
	21st to end	Day	43% 5% 10% 23% 16% 2	6949
June	1st to 10th	Day	38% 7% 12% 24% 16% 2	2800
	11th to 20th	Day	40% 7% 12% 25% 13% 3	2800
	21st to end	Day	34% 8% 14% 24% 19% 1	2800
July	1st to 10th	Day	36% 9% 16% 22% 13% 4	4440
	11th to 20th	Day	38% 5% 17% 26% 12% 2	4365
	21st to end	Day	46% 6% 11% 20% 16% 2	4463
August	1st to 10th	Day	47% 6% 15% 23% 9%	4091
	11th to 20th	Day	38% 5% 11% 31% 14% 1	3809
	21st to end	Day	34% 4 11% 30% 19% 2	4180
September	1st to 10th	Day	39% 5% 11% 20% 21% 4	2280
	11th to 20th	Day	33% 5% 9% 27% 23% 4	2277
	21st to end	Day	47% 4 8% 19% 19% 2	2046
October	1st to 10th	Day	37% 4 7% 19% 29% 5%	3160
	11th to 20th	Day	27% 3 10% 23% 28% 9%	3140
	21st to end	Day	25% 2 9% 30% 28% 5%	3383
November	1st to 10th	Day	34% 3 13% 25% 21% 4	1761
	11th to 20th	Day	37% 4 10% 24% 22% 3	1800
	21st to end	Day	38% 5% 10% 31% 13% 2	1640
December	1st to 10th	Day	41% 3 16% 25% 13% 3	2600
	11th to 20th	Day	45% 3 12% 29% 9% 2	2986
	21st to end	Day	43% 5% 12% 21% 17% 2	3960
January	1st to 10th	Night	39% 3 8% 25% 21% 5%	6349
	11th to 20th	Night	38% 2 9% 25% 19% 7%	6110
	21st to end	Night	42% 2 8% 27% 18% 2	6490
February	1st to 10th	Night	45% 3 13% 25% 13% 2	2600
	11th to 20th	Night	45% 3 8% 21% 19% 4	2414
	21st to end	Night	45% 3 10% 20% 16% 6%	1980
March	1st to 10th	Night	40% 4 8% 25% 13% 10%	5243
	11th to 20th	Night	35% 4 13% 28% 15% 6%	4980
	21st to end	Night	42% 3 7% 18% 26% 4	5202
April	1st to 10th	Night	44% 4 9% 19% 20% 4	2000
	11th to 20th	Night	40% 5% 11% 20% 19% 5%	1975
	21st to end	Night	40% 5% 10% 20% 19% 6%	2000
May	1st to 10th	Night	54% 6% 10% 16% 11% 3	4208
	11th to 20th	Night	51% 6% 10% 20% 11% 1	3576
	21st to end	Night	45% 6% 12% 22% 13% 3	3927
June	1st to 10th	Night	38% 7% 11% 24% 19% 2	1600
	11th to 20th	Night	40% 7% 13% 24% 15% 1	1600
	21st to end	Night	35% 7% 14% 26% 17% 3	1600
July	1st to 10th	Night	41% 7% 18% 19% 13% 4	2520
	11th to 20th	Night	42% 5% 18% 24% 10% 1	2472
	21st to end	Night	44% 7% 13% 19% 17% 1	2832
August	1st to 10th	Night	47% 7% 18% 19% 9% 1	2814
	11th to 20th	Night	42% 5% 11% 28% 14% 1	3053
	21st to end	Night	35% 4 10% 30% 19% 2	3476
September	1st to 10th	Night	38% 5% 10% 21% 21% 4	1900
	11th to 20th	Night	31% 5% 9% 25% 25% 5%	1902
	21st to end	Night	45% 5% 7% 21% 19% 3	2132
October	1st to 10th	Night	45% 2 8% 21% 21% 3	3800
	11th to 20th	Night	25% 3 10% 26% 30% 6%	3779
	21st to end	Night	25% 3 11% 32% 23% 6%	4151
November	1st to 10th	Night	32% 4 10% 26% 21% 6%	2537
	11th to 20th	Night	38% 4 9% 23% 20% 5%	2600
	21st to end	Night	38% 4 11% 27% 18% 2	2755
December	1st to 10th	Night	39% 2 12% 30% 15% 2	4580
	11th to 20th	Night	45% 3 12% 21% 15% 4	5208
	21st to end	Night	45% 5% 11% 19% 18% 1	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	29% 26% 32% 11% 2 1	200
	11th to 20th	Day	35% 26% 28% 10% 1	253
	21st to end	Day	22% 33% 28% 13% 4	330
February	1st to 10th	Day	28% 29% 29% 11% 3	300
	11th to 20th	Day	36% 24% 31% 7% 1	420
	21st to end	Day	31% 24% 30% 13% 2	414
March	1st to 10th	Day	27% 25% 31% 14% 3	500
	11th to 20th	Day	28% 25% 30% 11% 4 1	500
	21st to end	Day	26% 25% 33% 12% 3 1	550
April	1st to 10th	Day	29% 23% 33% 9% 5% 1	500
	11th to 20th	Day	24% 22% 29% 16% 5% 2	492
	21st to end	Day	24% 20% 36% 14% 6%	500
May	1st to 10th	Day	32% 15% 30% 16% 6% 1	503
	11th to 20th	Day	22% 16% 33% 19% 9% 1	527
	21st to end	Day	21% 11% 26% 23% 15% 3	573
June	1st to 10th	Day	13% 13% 33% 24% 15% 3	530
	11th to 20th	Day	17% 11% 30% 27% 12% 4	530
	21st to end	Day	12% 8% 31% 32% 12% 5%	530
July	1st to 10th	Day	12% 5% 23% 34% 18% 8%	505
	11th to 20th	Day	7% 2 24% 37% 23% 6%	484
	21st to end	Day	13% 5% 25% 34% 20% 3	532
August	1st to 10th	Day	14% 3 22% 35% 19% 7%	486
	11th to 20th	Day	12% 5% 19% 35% 20% 9%	479
	21st to end	Day	12% 5% 28% 29% 18% 9%	539
September	1st to 10th	Day	18% 9% 21% 26% 18% 8%	490
	11th to 20th	Day	14% 6% 26% 30% 17% 8%	490
	21st to end	Day	27% 9% 25% 27% 10% 2	490
October	1st to 10th	Day	21% 8% 28% 22% 15% 6%	490
	11th to 20th	Day	17% 16% 28% 24% 13% 2	490
	21st to end	Day	19% 15% 35% 20% 9% 2	527
November	1st to 10th	Day	15% 18% 34% 21% 11% 2	395
	11th to 20th	Day	21% 23% 30% 20% 6% 1	400
	21st to end	Day	24% 23% 29% 20% 4	240
December	1st to 10th	Day	25% 25% 33% 14% 4	200
	11th to 20th	Day	34% 21% 31% 12% 3	200
	21st to end	Day	24% 23% 30% 17% 6%	220
January	1st to 10th	Night	23% 23% 35% 17% 3	800
	11th to 20th	Night	26% 18% 31% 21% 3	800
	21st to end	Night	19% 21% 39% 19% 2	880
February	1st to 10th	Night	22% 19% 36% 21% 2	800
	11th to 20th	Night	27% 25% 32% 14% 1	783
	21st to end	Night	23% 22% 35% 17% 2	660
March	1st to 10th	Night	21% 22% 34% 20% 4	800
	11th to 20th	Night	22% 20% 34% 18% 5% 1	800
	21st to end	Night	22% 14% 41% 18% 5%	880
April	1st to 10th	Night	23% 20% 34% 18% 5%	800
	11th to 20th	Night	20% 15% 34% 25% 6%	791
	21st to end	Night	23% 14% 32% 24% 7%	800
May	1st to 10th	Night	21% 11% 34% 27% 5% 1	670
	11th to 20th	Night	23% 11% 30% 28% 8%	394
	21st to end	Night	19% 6% 22% 37% 15% 1	433
June	1st to 10th	Night	16% 5% 28% 35% 16%	400
	11th to 20th	Night	15% 5% 26% 43% 12% 1	400
	21st to end	Night	11% 3 28% 43% 16% 1	400
July	1st to 10th	Night	12% 5% 22% 39% 19% 2	380
	11th to 20th	Night	6% 5% 16% 45% 26% 2	368
	21st to end	Night	11% 4 17% 42% 24% 2	527
August	1st to 10th	Night	11% 2 17% 42% 25% 2	567
	11th to 20th	Night	14% 4 15% 39% 25% 3	701
	21st to end	Night	14% 4 19% 36% 24% 3	836
September	1st to 10th	Night	16% 5% 22% 34% 20% 3	760
	11th to 20th	Night	13% 4 19% 35% 25% 4	760
	21st to end	Night	23% 4 24% 28% 18% 2	760
October	1st to 10th	Night	23% 6% 24% 25% 17% 4	760
	11th to 20th	Night	18% 11% 28% 31% 12% 1	760
	21st to end	Night	20% 13% 31% 24% 10% 1	836
November	1st to 10th	Night	14% 12% 36% 26% 11% 1	780
	11th to 20th	Night	20% 18% 29% 26% 7% 1	800
	21st to end	Night	17% 20% 31% 25% 7%	800
December	1st to 10th	Night	23% 19% 33% 18% 6% 1	800
	11th to 20th	Night	20% 18% 36% 21% 5%	800
	21st to end	Night	21% 18% 32% 26% 3	880

Page generated at 2025-07-03 01:30:12. Version ac07

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