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 2024-11-06.

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	15% 7% 2 6% 5% 4 5% 4 6% 8% 39%	101337
	Night	16% 7% 3 6% 4 4 4 4 5% 7% 40%	97672
Transparency	Day	41% 7% 12% 23% 15% 2	104973
	Night	41% 6% 12% 23% 15% 2	102175
Seeing	Day	20% 13% 27% 23% 13% 4	14898
	Night	18% 11% 30% 29% 11% 1	24283

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	15% 7% 4 7% 5% 3 5% 4 4 5% 41%	2517
	11th to 20th	Day	13% 12% 4 11% 6% 5% 5% 4 5% 6% 29%	2776
	21st to end	Day	18% 12% 3 8% 6% 4 5% 3 5% 5% 31%	3124
February	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 5% 6% 35%	1710
	11th to 20th	Day	14% 8% 3 7% 6% 5% 4 3 6% 5% 39%	1710
	21st to end	Day	20% 10% 3 7% 5% 4 4 3 4 5% 36%	1570
March	1st to 10th	Day	23% 6% 2 6% 4 5% 4 3 5% 6% 35%	3311
	11th to 20th	Day	10% 6% 3 6% 5% 5% 4 4 6% 8% 42%	3480
	21st to end	Day	20% 8% 2 5% 3 3 4 3 4 6% 42%	3909
April	1st to 10th	Day	15% 7% 3 6% 5% 5% 6% 4 6% 6% 37%	2280
	11th to 20th	Day	14% 7% 2 6% 5% 4 4 4 5% 7% 42%	2254
	21st to end	Day	18% 7% 2 6% 4 4 4 4 5% 7% 39%	2280
May	1st to 10th	Day	28% 7% 2 6% 4 4 4 4 5% 7% 30%	4064
	11th to 20th	Day	23% 9% 3 7% 6% 5% 5% 4 5% 6% 28%	4398
	21st to end	Day	12% 5% 2 6% 4 4 5% 4 7% 9% 42%	4823
June	1st to 10th	Day	12% 7% 2 6% 4 5% 5% 5% 7% 9% 39%	2660
	11th to 20th	Day	12% 6% 2 6% 6% 5% 5% 4 6% 11% 38%	2660
	21st to end	Day	9% 5% 2 5% 4 4 5% 5% 7% 11% 43%	2660
July	1st to 10th	Day	6% 4 2 7% 6% 6% 6% 6% 8% 10% 39%	4440
	11th to 20th	Day	8% 5% 3 7% 5% 5% 5% 5% 8% 10% 40%	4440
	21st to end	Day	10% 8% 3 7% 5% 5% 6% 4 6% 8% 37%	4505
August	1st to 10th	Day	18% 5% 2 6% 5% 5% 5% 5% 8% 9% 32%	4104
	11th to 20th	Day	13% 6% 2 6% 4 4 6% 4 7% 8% 40%	3889
	21st to end	Day	13% 5% 2 4 3 3 4 3 5% 8% 49%	4180
September	1st to 10th	Day	16% 5% 2 5% 3 4 4 3 5% 6% 47%	2280
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 6% 8% 49%	2280
	21st to end	Day	22% 7% 2 6% 4 4 4 3 5% 6% 37%	2008
October	1st to 10th	Day	17% 5% 2 5% 3 4 4 3 6% 7% 43%	1900
	11th to 20th	Day	16% 6% 2 5% 4 3 3 3 5% 7% 45%	1900
	21st to end	Day	16% 6% 3 5% 4 3 4 3 5% 7% 43%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 2 6% 8% 50%	1750
	11th to 20th	Day	17% 5% 2 5% 5% 4 4 3 6% 7% 43%	1663
	21st to end	Day	16% 7% 3 5% 4 4 5% 4 6% 6% 41%	1520
December	1st to 10th	Day	18% 8% 3 6% 6% 5% 5% 4 6% 7% 33%	1520
	11th to 20th	Day	19% 7% 2 7% 5% 4 5% 4 5% 7% 35%	1910
	21st to end	Day	18% 8% 3 9% 6% 6% 5% 4 6% 7% 28%	2772
January	1st to 10th	Night	15% 8% 3 8% 5% 5% 4 3 5% 7% 37%	4429
	11th to 20th	Night	17% 8% 3 7% 4 4 4 4 6% 7% 36%	4181
	21st to end	Night	23% 9% 3 8% 4 4 4 3 4 5% 33%	4532
February	1st to 10th	Night	17% 8% 3 6% 5% 4 5% 3 5% 6% 38%	2470
	11th to 20th	Night	16% 9% 3 6% 4 4 4 3 5% 7% 39%	2356
	21st to end	Night	17% 8% 3 8% 5% 4 5% 4 4 6% 38%	1884
March	1st to 10th	Night	21% 9% 3 6% 4 3 4 3 3 6% 37%	3524
	11th to 20th	Night	11% 7% 2 7% 4 4 4 3 5% 6% 48%	3480
	21st to end	Night	15% 9% 2 6% 4 4 3 3 4 5% 45%	3722
April	1st to 10th	Night	17% 9% 3 7% 5% 5% 4 3 5% 7% 35%	1900
	11th to 20th	Night	14% 6% 3 5% 4 4 5% 4 6% 8% 42%	1875
	21st to end	Night	19% 5% 2 4 4 3 4 4 6% 7% 42%	1900
May	1st to 10th	Night	27% 7% 3 7% 4 4 4 3 5% 7% 30%	2854
	11th to 20th	Night	24% 10% 4 7% 4 4 4 4 6% 7% 27%	2495
	21st to end	Night	11% 6% 3 7% 4 5% 5% 4 6% 9% 41%	2732
June	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 7% 8% 45%	1520
	11th to 20th	Night	11% 5% 3 5% 5% 5% 5% 4 8% 9% 40%	1520
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 5% 8% 9% 42%	1520
July	1st to 10th	Night	7% 5% 2 6% 6% 5% 7% 4 7% 11% 41%	2520
	11th to 20th	Night	7% 5% 3 7% 5% 6% 5% 5% 8% 10% 39%	2520
	21st to end	Night	14% 7% 2 6% 5% 5% 5% 4 6% 7% 39%	3045
August	1st to 10th	Night	19% 5% 2 7% 4 5% 5% 4 6% 8% 35%	2825
	11th to 20th	Night	13% 7% 3 7% 5% 4 4 4 6% 8% 39%	2906
	21st to end	Night	13% 5% 2 5% 3 4 5% 4 6% 8% 44%	3476
September	1st to 10th	Night	13% 6% 2 5% 3 5% 3 3 5% 5% 48%	1900
	11th to 20th	Night	12% 6% 2 4 4 4 4 3 4 7% 52%	1900
	21st to end	Night	21% 6% 2 5% 4 5% 4 3 6% 5% 40%	2166
October	1st to 10th	Night	20% 6% 2 6% 4 3 3 2 4 5% 45%	2280
	11th to 20th	Night	16% 5% 2 5% 4 4 3 3 3 7% 48%	2280
	21st to end	Night	17% 5% 2 4 3 3 4 3 5% 7% 45%	2508
November	1st to 10th	Night	10% 6% 2 5% 3 5% 4 3 6% 6% 52%	2450
	11th to 20th	Night	15% 7% 3 5% 4 3 4 3 5% 7% 44%	2513
	21st to end	Night	14% 5% 2 5% 5% 5% 5% 3 5% 7% 43%	2660
December	1st to 10th	Night	17% 9% 3 6% 5% 5% 4 4 5% 7% 36%	2660
	11th to 20th	Night	18% 6% 2 6% 4 4 5% 4 6% 7% 38%	3285
	21st to end	Night	17% 8% 3 7% 6% 6% 5% 5% 6% 8% 30%	4884

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% 5% 14% 25% 9% 3	2518
	11th to 20th	Day	52% 6% 11% 16% 12% 3	2776
	21st to end	Day	53% 6% 11% 21% 7% 1	3124
February	1st to 10th	Day	47% 6% 13% 22% 10% 2	1710
	11th to 20th	Day	45% 6% 11% 24% 12% 2	1719
	21st to end	Day	51% 4 10% 21% 11% 4	1570
March	1st to 10th	Day	48% 5% 11% 25% 10% 2	3311
	11th to 20th	Day	37% 6% 12% 28% 15% 3	3480
	21st to end	Day	44% 5% 10% 22% 16% 3	3909
April	1st to 10th	Day	45% 6% 11% 19% 15% 4	2280
	11th to 20th	Day	40% 6% 11% 22% 16% 4	2254
	21st to end	Day	43% 6% 9% 22% 16% 3	2280
May	1st to 10th	Day	52% 6% 9% 20% 11% 1	4101
	11th to 20th	Day	53% 9% 12% 16% 8% 1	4402
	21st to end	Day	34% 9% 14% 25% 16% 2	4837
June	1st to 10th	Day	39% 7% 14% 23% 16% 1	2660
	11th to 20th	Day	39% 9% 14% 22% 13% 2	2660
	21st to end	Day	34% 8% 14% 28% 15% 1	2660
July	1st to 10th	Day	37% 10% 14% 23% 13% 3	4370
	11th to 20th	Day	36% 8% 16% 26% 14% 1	4365
	21st to end	Day	43% 7% 11% 19% 18% 1	4361
August	1st to 10th	Day	45% 9% 13% 24% 9%	4091
	11th to 20th	Day	39% 6% 12% 27% 15% 1	3928
	21st to end	Day	34% 6% 11% 29% 18% 2	4180
September	1st to 10th	Day	39% 6% 11% 21% 20% 4	2280
	11th to 20th	Day	33% 6% 10% 26% 21% 4	2280
	21st to end	Day	47% 5% 9% 17% 20% 3	2008
October	1st to 10th	Day	37% 4 8% 22% 23% 4	3160
	11th to 20th	Day	27% 5% 14% 28% 22% 5%	3140
	21st to end	Day	27% 4 12% 29% 26% 2	3428
November	1st to 10th	Day	33% 5% 14% 25% 19% 4	1750
	11th to 20th	Day	39% 5% 12% 22% 18% 3	1663
	21st to end	Day	41% 6% 12% 30% 9% 2	1520
December	1st to 10th	Day	49% 6% 15% 19% 10% 1	1520
	11th to 20th	Day	46% 7% 12% 27% 7% 1	1906
	21st to end	Day	52% 7% 17% 16% 8%	2772
January	1st to 10th	Night	45% 6% 13% 24% 9% 2	4429
	11th to 20th	Night	45% 5% 14% 24% 10% 1	4181
	21st to end	Night	52% 5% 10% 25% 8% 1	4532
February	1st to 10th	Night	46% 5% 13% 24% 11% 1	2470
	11th to 20th	Night	45% 5% 10% 22% 16% 2	2358
	21st to end	Night	47% 5% 10% 20% 14% 4	1884
March	1st to 10th	Night	48% 5% 10% 24% 11% 1	3524
	11th to 20th	Night	35% 7% 11% 31% 14% 2	3480
	21st to end	Night	42% 5% 7% 21% 21% 4	3722
April	1st to 10th	Night	48% 5% 9% 20% 15% 4	1900
	11th to 20th	Night	39% 7% 11% 20% 19% 4	1875
	21st to end	Night	40% 7% 11% 21% 16% 5%	1900
May	1st to 10th	Night	53% 7% 7% 18% 12% 3	2882
	11th to 20th	Night	53% 9% 12% 14% 10% 1	2496
	21st to end	Night	37% 8% 14% 25% 12% 4	2739
June	1st to 10th	Night	35% 8% 12% 24% 19% 2	1520
	11th to 20th	Night	36% 9% 13% 22% 17% 2	1520
	21st to end	Night	35% 8% 14% 26% 16% 2	1520
July	1st to 10th	Night	37% 9% 17% 18% 15% 5%	2480
	11th to 20th	Night	38% 9% 14% 22% 15% 1	2472
	21st to end	Night	41% 7% 14% 17% 19% 1	2948
August	1st to 10th	Night	45% 7% 17% 22% 9% 1	2814
	11th to 20th	Night	40% 7% 12% 24% 16% 1	2933
	21st to end	Night	36% 6% 12% 28% 16% 2	3476
September	1st to 10th	Night	38% 5% 12% 21% 21% 4	1900
	11th to 20th	Night	32% 5% 9% 26% 24% 3	1900
	21st to end	Night	45% 6% 9% 19% 19% 2	2166
October	1st to 10th	Night	42% 5% 10% 21% 21% 2	3800
	11th to 20th	Night	28% 4 10% 28% 25% 4	3779
	21st to end	Night	26% 3 12% 33% 23% 2	4120
November	1st to 10th	Night	33% 5% 11% 26% 21% 4	2450
	11th to 20th	Night	39% 4 11% 24% 17% 4	2513
	21st to end	Night	39% 5% 13% 26% 14% 2	2660
December	1st to 10th	Night	47% 6% 13% 22% 12%	2660
	11th to 20th	Night	44% 6% 13% 22% 13% 2	3288
	21st to end	Night	48% 10% 15% 20% 8%	4884

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	27% 28% 29% 13% 3 1	180
	11th to 20th	Day	30% 25% 30% 12% 2	252
	21st to end	Day	24% 26% 34% 12% 4 1	297
February	1st to 10th	Day	26% 25% 33% 13% 2	270
	11th to 20th	Day	29% 26% 36% 8% 1	317
	21st to end	Day	33% 20% 28% 16% 2	382
March	1st to 10th	Day	24% 22% 31% 18% 4	460
	11th to 20th	Day	25% 22% 34% 14% 4 1	460
	21st to end	Day	26% 22% 31% 16% 5% 1	506
April	1st to 10th	Day	22% 22% 34% 15% 7% 1	460
	11th to 20th	Day	22% 20% 28% 17% 10% 4	452
	21st to end	Day	27% 13% 32% 19% 7% 2	460
May	1st to 10th	Day	27% 17% 27% 18% 9% 3	463
	11th to 20th	Day	21% 14% 30% 21% 10% 4	487
	21st to end	Day	17% 8% 24% 25% 19% 6%	529
June	1st to 10th	Day	15% 13% 27% 24% 16% 5%	490
	11th to 20th	Day	16% 8% 26% 28% 18% 4	490
	21st to end	Day	11% 9% 29% 30% 17% 4	490
July	1st to 10th	Day	12% 5% 20% 32% 20% 11%	485
	11th to 20th	Day	6% 4 22% 33% 26% 9%	484
	21st to end	Day	11% 5% 25% 34% 19% 5%	532
August	1st to 10th	Day	13% 3 20% 28% 27% 8%	486
	11th to 20th	Day	14% 4 20% 33% 18% 10%	479
	21st to end	Day	13% 4 20% 31% 20% 11%	539
September	1st to 10th	Day	18% 7% 20% 23% 21% 10%	490
	11th to 20th	Day	15% 5% 19% 30% 22% 10%	490
	21st to end	Day	27% 11% 22% 23% 12% 5%	490
October	1st to 10th	Day	22% 6% 26% 22% 17% 7%	490
	11th to 20th	Day	19% 14% 26% 26% 13% 2	490
	21st to end	Day	17% 14% 35% 22% 10% 2	539
November	1st to 10th	Day	17% 13% 30% 24% 13% 2	398
	11th to 20th	Day	22% 18% 29% 21% 9% 1	323
	21st to end	Day	21% 24% 31% 18% 7%	180
December	1st to 10th	Day	26% 25% 29% 13% 4 2	180
	11th to 20th	Day	27% 22% 31% 16% 4 1	180
	21st to end	Day	24% 23% 27% 19% 7% 1	198
January	1st to 10th	Night	18% 18% 41% 20% 3	760
	11th to 20th	Night	24% 16% 31% 25% 3	760
	21st to end	Night	19% 17% 41% 22% 2	836
February	1st to 10th	Night	21% 18% 32% 27% 2	760
	11th to 20th	Night	22% 22% 38% 17% 1	743
	21st to end	Night	24% 19% 38% 17% 3	628
March	1st to 10th	Night	20% 16% 38% 22% 4	760
	11th to 20th	Night	22% 19% 34% 20% 5%	760
	21st to end	Night	22% 13% 39% 20% 6%	836
April	1st to 10th	Night	22% 18% 35% 21% 5%	760
	11th to 20th	Night	17% 13% 36% 27% 7%	751
	21st to end	Night	22% 11% 31% 28% 8%	760
May	1st to 10th	Night	20% 11% 29% 29% 11%	599
	11th to 20th	Night	20% 7% 33% 29% 11%	374
	21st to end	Night	13% 7% 21% 40% 18% 1	411
June	1st to 10th	Night	13% 4 26% 38% 19%	380
	11th to 20th	Night	12% 6% 27% 38% 17%	380
	21st to end	Night	9% 2 29% 46% 13%	380
July	1st to 10th	Night	12% 4 21% 38% 23% 2	370
	11th to 20th	Night	6% 2 17% 44% 27% 3	368
	21st to end	Night	12% 2 17% 42% 26% 1	595
August	1st to 10th	Night	13% 1 14% 43% 28% 2	567
	11th to 20th	Night	12% 4 14% 42% 25% 3	633
	21st to end	Night	13% 3 16% 37% 28% 3	836
September	1st to 10th	Night	16% 4 23% 33% 23% 2	760
	11th to 20th	Night	13% 4 17% 36% 27% 3	760
	21st to end	Night	24% 5% 20% 29% 20% 2	760
October	1st to 10th	Night	21% 6% 24% 27% 18% 3	760
	11th to 20th	Night	18% 10% 28% 32% 12% 1	760
	21st to end	Night	19% 11% 31% 28% 10%	836
November	1st to 10th	Night	14% 9% 35% 30% 12% 1	764
	11th to 20th	Night	18% 15% 33% 26% 8% 1	760
	21st to end	Night	17% 16% 34% 26% 7%	760
December	1st to 10th	Night	23% 16% 35% 21% 5%	760
	11th to 20th	Night	19% 13% 38% 26% 3	760
	21st to end	Night	19% 14% 36% 29% 2	836

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