Edward L. Naylor Astronomical Center & Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Edward L. Naylor Astronomical Center & Observatory clear sky chart from 2005-11-06 to 2025-11-03.

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% 7% 2 6% 5% 4 5% 4 6% 8% 38%	126874
	Night	15% 7% 2 6% 4 4 4 4 5% 7% 40%	123169
Transparency	Day	41% 6% 12% 23% 15% 2	130466
	Night	41% 6% 12% 23% 16% 3	127646
Seeing	Day	20% 14% 27% 22% 12% 4	16185
	Night	18% 11% 30% 28% 11% 1	25549

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	13% 7% 4 9% 5% 4 5% 4 5% 5% 41%	3597
	11th to 20th	Day	12% 10% 4 10% 6% 5% 5% 3 5% 6% 34%	3968
	21st to end	Day	15% 9% 3 7% 6% 5% 6% 4 5% 6% 35%	4466
February	1st to 10th	Day	14% 9% 3 7% 5% 5% 5% 4 6% 6% 35%	1800
	11th to 20th	Day	15% 9% 3 7% 6% 4 4 3 6% 5% 39%	1803
	21st to end	Day	19% 10% 3 7% 5% 4 4 3 4 6% 36%	1650
March	1st to 10th	Day	19% 5% 2 6% 4 5% 4 3 5% 6% 42%	4755
	11th to 20th	Day	11% 7% 3 6% 4 5% 4 4 6% 8% 42%	4980
	21st to end	Day	16% 7% 2 5% 4 4 5% 3 5% 6% 41%	5601
April	1st to 10th	Day	16% 8% 3 6% 5% 5% 5% 4 5% 6% 36%	2400
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 5% 7% 41%	2374
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 7% 40%	2400
May	1st to 10th	Day	25% 7% 2 6% 4 4 5% 4 6% 8% 30%	5844
	11th to 20th	Day	24% 7% 3 6% 5% 4 4 4 5% 8% 30%	6318
	21st to end	Day	18% 5% 2 6% 4 4 5% 4 7% 9% 35%	6934
June	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 5% 7% 9% 39%	2800
	11th to 20th	Day	14% 6% 2 6% 6% 4 5% 4 6% 10% 38%	2800
	21st to end	Day	9% 5% 2 5% 4 4 5% 5% 7% 11% 43%	2800
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 6% 5% 7% 10% 41%	6360
	11th to 20th	Day	9% 7% 3 7% 6% 6% 6% 6% 9% 11% 31%	6360
	21st to end	Day	9% 7% 3 6% 5% 5% 5% 4 6% 10% 40%	6477
August	1st to 10th	Day	18% 5% 2 6% 5% 5% 5% 5% 8% 9% 33%	4234
	11th to 20th	Day	13% 6% 2 6% 4 4 6% 4 7% 8% 40%	4014
	21st to end	Day	13% 5% 2 4 3 3 4 3 5% 8% 49%	4312
September	1st to 10th	Day	15% 5% 2 5% 3 4 4 3 5% 6% 47%	2400
	11th to 20th	Day	13% 5% 2 5% 3 3 4 3 6% 8% 49%	2400
	21st to end	Day	22% 8% 2 7% 4 4 4 3 4 6% 36%	2113
October	1st to 10th	Day	16% 5% 2 5% 3 4 4 3 6% 7% 44%	2000
	11th to 20th	Day	16% 6% 2 6% 4 3 3 3 6% 7% 45%	2000
	21st to end	Day	16% 6% 2 5% 4 4 4 3 5% 7% 43%	2200
November	1st to 10th	Day	12% 6% 3 4 3 3 3 2 6% 8% 50%	1814
	11th to 20th	Day	16% 5% 2 5% 5% 4 4 3 6% 7% 43%	1750
	21st to end	Day	16% 7% 3 5% 5% 4 4 4 5% 6% 41%	1600
December	1st to 10th	Day	14% 7% 4 7% 5% 4 5% 4 6% 7% 39%	2600
	11th to 20th	Day	20% 9% 2 6% 4 4 4 3 5% 7% 36%	2990
	21st to end	Day	17% 8% 3 8% 5% 5% 6% 4 6% 8% 29%	3960
January	1st to 10th	Night	12% 7% 3 9% 5% 5% 4 4 6% 8% 38%	6349
	11th to 20th	Night	13% 7% 2 6% 5% 4 4 4 6% 8% 41%	5975
	21st to end	Night	17% 7% 2 6% 4 4 4 4 5% 7% 40%	6490
February	1st to 10th	Night	17% 8% 3 6% 5% 4 5% 4 5% 6% 38%	2600
	11th to 20th	Night	17% 8% 3 6% 4 4 4 3 5% 7% 38%	2483
	21st to end	Night	16% 8% 3 8% 5% 4 5% 3 4 6% 38%	1980
March	1st to 10th	Night	17% 8% 3 6% 4 3 4 3 4 6% 43%	5080
	11th to 20th	Night	10% 6% 2 7% 4 3 4 3 4 7% 49%	4980
	21st to end	Night	13% 8% 2 6% 4 4 4 4 5% 6% 43%	5330
April	1st to 10th	Night	18% 9% 3 8% 5% 4 4 3 5% 7% 35%	2000
	11th to 20th	Night	14% 6% 3 6% 4 4 5% 4 6% 8% 41%	1975
	21st to end	Night	19% 5% 2 4 4 3 4 4 6% 7% 43%	2000
May	1st to 10th	Night	27% 7% 3 7% 4 5% 4 3 5% 7% 28%	4088
	11th to 20th	Night	24% 8% 3 6% 3 3 3 3 6% 9% 31%	3575
	21st to end	Night	19% 6% 3 7% 4 4 4 4 5% 7% 37%	3920
June	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 7% 8% 45%	1600
	11th to 20th	Night	13% 5% 3 5% 5% 4 5% 4 8% 9% 39%	1600
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 5% 8% 9% 42%	1600
July	1st to 10th	Night	7% 6% 3 7% 5% 5% 6% 4 7% 11% 40%	3598
	11th to 20th	Night	10% 7% 3 9% 6% 6% 6% 5% 7% 10% 31%	3597
	21st to end	Night	11% 6% 2 6% 4 5% 5% 4 6% 8% 43%	4371
August	1st to 10th	Night	19% 5% 3 7% 4 5% 4 4 6% 8% 36%	2915
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 8% 39%	3000
	21st to end	Night	13% 5% 2 5% 4 4 5% 4 6% 8% 44%	3586
September	1st to 10th	Night	13% 6% 2 5% 3 5% 4 3 5% 6% 48%	2000
	11th to 20th	Night	11% 6% 2 4 3 4 4 3 4 7% 51%	2000
	21st to end	Night	20% 6% 3 5% 4 5% 4 3 6% 5% 38%	2281
October	1st to 10th	Night	19% 6% 2 6% 3 3 3 2 4 5% 46%	2400
	11th to 20th	Night	16% 5% 2 5% 4 4 3 3 3 7% 48%	2400
	21st to end	Night	16% 5% 2 4 4 3 4 3 5% 7% 46%	2640
November	1st to 10th	Night	10% 5% 2 5% 3 5% 4 3 6% 6% 52%	2529
	11th to 20th	Night	15% 7% 3 5% 4 3 4 3 5% 7% 44%	2646
	21st to end	Night	14% 5% 2 5% 5% 5% 5% 3 5% 7% 43%	2800
December	1st to 10th	Night	12% 7% 3 6% 4 4 4 4 5% 7% 41%	4580
	11th to 20th	Night	22% 6% 2 5% 4 4 4 4 6% 7% 37%	5205
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 4 6% 7% 30%	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	43% 4 12% 23% 15% 2	3598
	11th to 20th	Day	49% 5% 10% 17% 15% 4	3968
	21st to end	Day	47% 7% 11% 21% 12% 1	4466
February	1st to 10th	Day	48% 6% 12% 23% 10% 2	1800
	11th to 20th	Day	46% 6% 10% 24% 12% 2	1812
	21st to end	Day	49% 4 11% 21% 11% 4	1650
March	1st to 10th	Day	43% 4 10% 23% 14% 6%	4755
	11th to 20th	Day	38% 5% 12% 27% 12% 5%	4980
	21st to end	Day	43% 5% 11% 19% 18% 4	5601
April	1st to 10th	Day	46% 6% 11% 19% 14% 4	2400
	11th to 20th	Day	41% 6% 11% 22% 17% 4	2374
	21st to end	Day	42% 6% 9% 23% 17% 3	2400
May	1st to 10th	Day	50% 7% 12% 19% 11% 2	5878
	11th to 20th	Day	51% 7% 10% 20% 10% 1	6322
	21st to end	Day	42% 8% 13% 22% 15% 1	6949
June	1st to 10th	Day	39% 7% 13% 23% 17% 1	2800
	11th to 20th	Day	40% 9% 14% 22% 13% 2	2800
	21st to end	Day	34% 8% 14% 27% 16% 1	2800
July	1st to 10th	Day	36% 10% 13% 22% 17% 2	6290
	11th to 20th	Day	42% 9% 16% 22% 10% 1	6285
	21st to end	Day	38% 7% 11% 21% 22% 1	6291
August	1st to 10th	Day	45% 9% 13% 24% 9%	4221
	11th to 20th	Day	39% 7% 12% 27% 14% 1	4053
	21st to end	Day	33% 6% 11% 29% 19% 2	4312
September	1st to 10th	Day	38% 6% 11% 21% 20% 4	2400
	11th to 20th	Day	33% 6% 10% 27% 21% 4	2400
	21st to end	Day	48% 5% 8% 17% 19% 3	2113
October	1st to 10th	Day	36% 4 8% 23% 23% 5%	3260
	11th to 20th	Day	28% 5% 14% 28% 22% 5%	3240
	21st to end	Day	27% 4 12% 29% 27% 2	3538
November	1st to 10th	Day	33% 5% 14% 25% 19% 4	1814
	11th to 20th	Day	39% 5% 12% 23% 19% 3	1750
	21st to end	Day	41% 6% 12% 29% 9% 2	1600
December	1st to 10th	Day	42% 5% 17% 25% 9% 2	2600
	11th to 20th	Day	47% 5% 14% 25% 8% 1	2986
	21st to end	Day	50% 6% 17% 15% 10% 2	3960
January	1st to 10th	Night	41% 5% 13% 24% 15% 2	6349
	11th to 20th	Night	40% 4 13% 23% 15% 5%	5975
	21st to end	Night	43% 5% 12% 24% 14% 2	6490
February	1st to 10th	Night	46% 5% 13% 25% 10% 1	2600
	11th to 20th	Night	45% 4 10% 22% 16% 2	2485
	21st to end	Night	47% 5% 10% 20% 15% 4	1980
March	1st to 10th	Night	42% 4 10% 25% 12% 7%	5080
	11th to 20th	Night	34% 6% 13% 30% 12% 5%	4980
	21st to end	Night	41% 5% 9% 19% 24% 3	5330
April	1st to 10th	Night	49% 5% 9% 19% 15% 4	2000
	11th to 20th	Night	39% 6% 12% 20% 19% 4	1975
	21st to end	Night	39% 7% 11% 21% 17% 5%	2000
May	1st to 10th	Night	55% 6% 9% 16% 11% 4	4116
	11th to 20th	Night	49% 8% 12% 20% 11% 1	3576
	21st to end	Night	46% 6% 11% 23% 12% 3	3927
June	1st to 10th	Night	36% 8% 12% 23% 19% 2	1600
	11th to 20th	Night	37% 9% 13% 22% 17% 2	1600
	21st to end	Night	35% 8% 13% 26% 16% 2	1600
July	1st to 10th	Night	39% 9% 14% 19% 16% 3	3560
	11th to 20th	Night	46% 9% 14% 18% 11% 1	3552
	21st to end	Night	37% 7% 13% 21% 21% 1	4243
August	1st to 10th	Night	45% 7% 17% 22% 10% 1	2904
	11th to 20th	Night	40% 7% 12% 24% 16% 1	3027
	21st to end	Night	36% 6% 11% 28% 16% 2	3586
September	1st to 10th	Night	37% 5% 12% 21% 21% 4	2000
	11th to 20th	Night	33% 5% 10% 26% 23% 3	2000
	21st to end	Night	46% 6% 9% 18% 19% 2	2281
October	1st to 10th	Night	41% 5% 10% 21% 22% 2	3920
	11th to 20th	Night	29% 4 10% 28% 25% 4	3899
	21st to end	Night	26% 3 12% 33% 24% 2	4252
November	1st to 10th	Night	32% 5% 12% 26% 21% 4	2529
	11th to 20th	Night	39% 4 11% 25% 17% 4	2646
	21st to end	Night	40% 5% 13% 26% 14% 2	2800
December	1st to 10th	Night	41% 5% 14% 28% 10% 1	4580
	11th to 20th	Night	45% 5% 14% 22% 11% 2	5208
	21st to end	Night	49% 8% 12% 17% 13%	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	28% 30% 28% 12% 3 1	200
	11th to 20th	Day	32% 25% 30% 11% 2	280
	21st to end	Day	24% 27% 34% 12% 3 1	330
February	1st to 10th	Day	27% 25% 34% 12% 2	300
	11th to 20th	Day	33% 25% 33% 7% 1	350
	21st to end	Day	32% 21% 28% 16% 2	414
March	1st to 10th	Day	25% 21% 31% 18% 4	500
	11th to 20th	Day	25% 22% 33% 15% 4 1	500
	21st to end	Day	26% 22% 31% 15% 4 1	550
April	1st to 10th	Day	25% 21% 33% 14% 6% 1	500
	11th to 20th	Day	24% 21% 27% 16% 9% 3	492
	21st to end	Day	26% 14% 31% 20% 7% 2	500
May	1st to 10th	Day	27% 16% 27% 18% 8% 2	503
	11th to 20th	Day	21% 15% 30% 21% 9% 4	527
	21st to end	Day	20% 8% 24% 24% 18% 6%	573
June	1st to 10th	Day	15% 13% 27% 23% 17% 5%	530
	11th to 20th	Day	18% 9% 25% 27% 17% 4	530
	21st to end	Day	12% 8% 28% 30% 17% 5%	530
July	1st to 10th	Day	12% 5% 20% 32% 21% 10%	525
	11th to 20th	Day	6% 4 23% 32% 26% 9%	524
	21st to end	Day	12% 5% 25% 34% 19% 5%	576
August	1st to 10th	Day	13% 4 21% 29% 26% 8%	526
	11th to 20th	Day	14% 5% 19% 33% 19% 10%	519
	21st to end	Day	13% 4 21% 31% 19% 11%	583
September	1st to 10th	Day	17% 8% 20% 25% 21% 10%	530
	11th to 20th	Day	15% 5% 18% 30% 23% 9%	530
	21st to end	Day	27% 11% 23% 23% 12% 4	530
October	1st to 10th	Day	20% 6% 25% 23% 17% 8%	530
	11th to 20th	Day	20% 15% 25% 25% 12% 2	530
	21st to end	Day	17% 15% 35% 22% 9% 2	583
November	1st to 10th	Day	17% 14% 31% 24% 13% 2	420
	11th to 20th	Day	21% 18% 30% 21% 9% 1	350
	21st to end	Day	24% 25% 29% 17% 6%	200
December	1st to 10th	Day	26% 25% 32% 12% 4 2	200
	11th to 20th	Day	28% 22% 29% 17% 4 1	200
	21st to end	Day	24% 24% 27% 18% 7%	220
January	1st to 10th	Night	19% 20% 40% 19% 3	800
	11th to 20th	Night	24% 17% 32% 24% 3	800
	21st to end	Night	19% 16% 41% 22% 2	880
February	1st to 10th	Night	22% 18% 33% 26% 2	800
	11th to 20th	Night	24% 21% 37% 17% 1	783
	21st to end	Night	23% 19% 38% 17% 3	660
March	1st to 10th	Night	20% 17% 38% 22% 3	800
	11th to 20th	Night	21% 19% 34% 21% 5%	800
	21st to end	Night	22% 14% 39% 20% 5%	880
April	1st to 10th	Night	23% 18% 34% 20% 4	800
	11th to 20th	Night	18% 13% 35% 26% 7%	791
	21st to end	Night	22% 11% 31% 28% 7%	800
May	1st to 10th	Night	21% 10% 29% 29% 10%	630
	11th to 20th	Night	20% 8% 33% 29% 10%	394
	21st to end	Night	14% 7% 21% 39% 18% 1	433
June	1st to 10th	Night	13% 4 27% 37% 19%	400
	11th to 20th	Night	14% 6% 27% 38% 16%	400
	21st to end	Night	10% 3 28% 45% 15%	400
July	1st to 10th	Night	12% 4 21% 38% 23% 2	390
	11th to 20th	Night	6% 2 17% 45% 27% 3	388
	21st to end	Night	12% 2 17% 42% 26% 1	627
August	1st to 10th	Night	14% 1 14% 42% 28% 2	597
	11th to 20th	Night	12% 4 13% 43% 25% 3	667
	21st to end	Night	13% 3 17% 37% 28% 3	880
September	1st to 10th	Night	15% 4 23% 33% 24% 2	800
	11th to 20th	Night	13% 4 17% 37% 26% 3	800
	21st to end	Night	23% 5% 20% 30% 20% 2	800
October	1st to 10th	Night	21% 6% 23% 28% 19% 3	800
	11th to 20th	Night	19% 10% 28% 31% 11% 1	800
	21st to end	Night	19% 11% 32% 28% 10%	880
November	1st to 10th	Night	14% 9% 35% 31% 11% 1	789
	11th to 20th	Night	19% 14% 33% 25% 8% 1	800
	21st to end	Night	18% 17% 34% 25% 7%	800
December	1st to 10th	Night	23% 16% 35% 21% 5%	800
	11th to 20th	Night	20% 13% 37% 26% 4	800
	21st to end	Night	19% 14% 35% 29% 2	880

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