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 2024-01-16.

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% 3 5% 5% 3 5% 5% 6% 8% 39%	74725
	Night	15% 6% 3 5% 5% 3 5% 5% 5% 7% 41%	75069
Transparency	Day	42% 7% 13% 22% 14% 2	74433
	Night	41% 6% 12% 23% 15% 2	74880
Seeing	Day	20% 14% 27% 22% 13% 5%	13755
	Night	18% 11% 30% 29% 11% 1	23279

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 6% 6% 3 5% 5% 5% 7% 39%	1517
	11th to 20th	Day	15% 9% 3 7% 7% 3 6% 5% 5% 6% 33%	1627
	21st to end	Day	14% 10% 4 7% 7% 3 6% 4 5% 5% 36%	1782
February	1st to 10th	Day	15% 9% 3 6% 6% 3 6% 5% 6% 5% 34%	1620
	11th to 20th	Day	15% 7% 3 6% 6% 3 4 4 6% 5% 39%	1618
	21st to end	Day	20% 9% 3 6% 6% 3 3 4 4 5% 35%	1480
March	1st to 10th	Day	13% 6% 2 5% 5% 3 5% 3 5% 6% 45%	1908
	11th to 20th	Day	16% 7% 3 5% 5% 3 5% 5% 6% 8% 36%	1980
	21st to end	Day	20% 7% 3 5% 5% 3 5% 4 4 7% 37%	2231
April	1st to 10th	Day	15% 7% 3 5% 6% 4 5% 5% 5% 6% 39%	2160
	11th to 20th	Day	15% 6% 3 5% 5% 3 4 5% 5% 7% 42%	2134
	21st to end	Day	19% 6% 2 5% 4 3 5% 5% 5% 7% 39%	2160
May	1st to 10th	Day	17% 7% 3 5% 5% 3 5% 6% 6% 7% 36%	2321
	11th to 20th	Day	19% 7% 3 6% 5% 4 5% 5% 5% 7% 34%	2478
	21st to end	Day	15% 6% 2 6% 5% 3 5% 5% 7% 8% 39%	2725
June	1st to 10th	Day	12% 6% 3 5% 4 3 6% 6% 6% 9% 39%	2520
	11th to 20th	Day	13% 6% 2 5% 7% 3 5% 5% 6% 10% 38%	2520
	21st to end	Day	9% 5% 2 4 5% 3 5% 6% 7% 11% 43%	2520
July	1st to 10th	Day	9% 5% 2 4 7% 4 5% 6% 7% 10% 41%	2520
	11th to 20th	Day	7% 4 2 6% 6% 3 5% 6% 8% 10% 43%	2520
	21st to end	Day	9% 7% 3 6% 6% 4 7% 6% 6% 9% 37%	2572
August	1st to 10th	Day	10% 5% 3 5% 6% 4 6% 7% 8% 9% 37%	2327
	11th to 20th	Day	12% 7% 2 5% 6% 3 5% 5% 6% 9% 39%	2218
	21st to end	Day	13% 5% 2 4 5% 3 5% 5% 7% 9% 42%	2376
September	1st to 10th	Day	16% 5% 2 5% 4 3 4 4 5% 7% 45%	2160
	11th to 20th	Day	13% 4 2 4 4 3 3 4 6% 8% 49%	2160
	21st to end	Day	19% 6% 2 6% 4 3 4 4 5% 7% 39%	1903
October	1st to 10th	Day	17% 5% 2 4 5% 3 4 4 6% 7% 42%	1800
	11th to 20th	Day	17% 6% 2 5% 5% 3 3 3 5% 8% 43%	1800
	21st to end	Day	17% 6% 3 5% 5% 3 3 5% 5% 7% 41%	1980
November	1st to 10th	Day	13% 6% 3 4 4 2 3 3 5% 8% 49%	1703
	11th to 20th	Day	17% 5% 2 4 5% 3 4 4 6% 7% 43%	1663
	21st to end	Day	16% 5% 3 4 5% 3 5% 5% 6% 6% 41%	1520
December	1st to 10th	Day	18% 8% 4 5% 6% 4 5% 4 6% 7% 33%	1520
	11th to 20th	Day	19% 6% 3 6% 6% 4 5% 5% 5% 7% 36%	1910
	21st to end	Day	18% 7% 3 8% 7% 4 6% 5% 6% 7% 28%	2772
January	1st to 10th	Night	14% 8% 3 5% 5% 3 5% 5% 5% 8% 39%	2649
	11th to 20th	Night	18% 8% 3 5% 5% 3 4 5% 6% 7% 36%	2441
	21st to end	Night	13% 8% 3 6% 5% 3 5% 5% 5% 7% 40%	2574
February	1st to 10th	Night	17% 7% 3 6% 5% 3 4 5% 5% 7% 37%	2340
	11th to 20th	Night	16% 7% 3 6% 5% 3 4 5% 5% 8% 38%	2228
	21st to end	Night	17% 8% 3 7% 6% 3 5% 4 4 5% 38%	1776
March	1st to 10th	Night	14% 6% 2 5% 5% 3 4 5% 4 8% 45%	2048
	11th to 20th	Night	17% 7% 2 5% 5% 3 4 4 5% 5% 42%	1980
	21st to end	Night	17% 7% 3 6% 5% 3 4 5% 5% 7% 39%	2124
April	1st to 10th	Night	17% 8% 3 7% 6% 4 4 4 5% 7% 36%	1800
	11th to 20th	Night	14% 5% 2 5% 4 3 5% 5% 6% 8% 42%	1775
	21st to end	Night	20% 5% 2 4 4 2 4 5% 6% 6% 42%	1800
May	1st to 10th	Night	19% 6% 3 6% 5% 4 5% 4 4 7% 37%	1648
	11th to 20th	Night	18% 7% 3 5% 4 3 5% 4 7% 8% 37%	1415
	21st to end	Night	12% 6% 3 6% 5% 3 5% 5% 5% 8% 42%	1550
June	1st to 10th	Night	13% 5% 2 3 4 3 4 4 6% 8% 46%	1440
	11th to 20th	Night	12% 5% 3 5% 6% 3 5% 6% 7% 10% 39%	1440
	21st to end	Night	9% 4 3 5% 5% 4 5% 6% 8% 10% 43%	1440
July	1st to 10th	Night	9% 6% 3 6% 6% 4 7% 5% 6% 10% 38%	1440
	11th to 20th	Night	5% 3 2 4 4 4 5% 6% 8% 11% 47%	1440
	21st to end	Night	10% 4 2 4 5% 3 6% 7% 6% 10% 42%	1743
August	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 6% 7% 10% 42%	1611
	11th to 20th	Night	11% 6% 3 7% 7% 3 6% 5% 5% 8% 40%	1663
	21st to end	Night	11% 5% 2 5% 5% 4 5% 6% 6% 9% 42%	1980
September	1st to 10th	Night	14% 6% 3 5% 4 4 4 3 5% 5% 47%	1800
	11th to 20th	Night	12% 5% 2 3 4 3 3 4 4 6% 52%	1800
	21st to end	Night	17% 6% 2 4 5% 4 5% 4 6% 6% 42%	2051
October	1st to 10th	Night	20% 6% 2 5% 4 2 4 3 4 5% 45%	2160
	11th to 20th	Night	16% 5% 2 4 4 3 4 4 4 7% 47%	2160
	21st to end	Night	18% 5% 2 4 4 2 4 4 5% 7% 44%	2376
November	1st to 10th	Night	11% 5% 2 5% 3 3 4 3 6% 6% 51%	2375
	11th to 20th	Night	15% 6% 3 5% 4 3 4 4 4 7% 44%	2513
	21st to end	Night	15% 5% 2 5% 5% 3 5% 5% 4 8% 43%	2660
December	1st to 10th	Night	17% 8% 3 6% 5% 3 5% 5% 5% 7% 36%	2660
	11th to 20th	Night	18% 5% 2 6% 5% 3 5% 5% 6% 7% 38%	3285
	21st to end	Night	17% 8% 3 6% 6% 4 6% 6% 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% 22% 13% 3	1518
	11th to 20th	Day	50% 5% 14% 19% 11% 2	1627
	21st to end	Day	49% 5% 12% 21% 11% 2	1782
February	1st to 10th	Day	49% 6% 13% 21% 10% 2	1620
	11th to 20th	Day	46% 6% 11% 25% 11% 2	1627
	21st to end	Day	51% 4 10% 21% 10% 4	1480
March	1st to 10th	Day	40% 4 14% 25% 15% 2	1908
	11th to 20th	Day	45% 5% 13% 18% 14% 5%	1980
	21st to end	Day	47% 6% 9% 19% 16% 3	2231
April	1st to 10th	Day	44% 6% 11% 19% 15% 4	2160
	11th to 20th	Day	40% 6% 11% 22% 17% 4	2134
	21st to end	Day	44% 6% 9% 21% 16% 3	2160
May	1st to 10th	Day	45% 7% 10% 21% 15% 2	2321
	11th to 20th	Day	49% 7% 10% 19% 13% 2	2482
	21st to end	Day	41% 8% 16% 22% 12% 2	2725
June	1st to 10th	Day	40% 7% 14% 23% 15% 1	2520
	11th to 20th	Day	41% 9% 14% 22% 13% 2	2520
	21st to end	Day	34% 8% 15% 27% 15% 1	2520
July	1st to 10th	Day	38% 10% 13% 20% 16% 3	2450
	11th to 20th	Day	32% 9% 14% 26% 17% 3	2445
	21st to end	Day	41% 9% 13% 20% 16% 2	2428
August	1st to 10th	Day	38% 11% 14% 24% 13%	2314
	11th to 20th	Day	39% 8% 14% 22% 15% 2	2218
	21st to end	Day	36% 8% 13% 24% 15% 3	2376
September	1st to 10th	Day	40% 6% 12% 20% 18% 4	2160
	11th to 20th	Day	33% 6% 11% 26% 20% 4	2160
	21st to end	Day	44% 5% 9% 18% 21% 3	1903
October	1st to 10th	Day	41% 6% 12% 24% 16% 2	1800
	11th to 20th	Day	39% 6% 12% 24% 18% 1	1800
	21st to end	Day	42% 5% 10% 25% 16% 2	1980
November	1st to 10th	Day	34% 5% 14% 25% 19% 4	1703
	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	43% 5% 13% 24% 12% 3	2649
	11th to 20th	Night	46% 6% 15% 22% 10% 1	2441
	21st to end	Night	42% 4 12% 26% 13% 3	2574
February	1st to 10th	Night	47% 5% 13% 23% 10% 1	2340
	11th to 20th	Night	45% 4 11% 22% 16% 2	2230
	21st to end	Night	48% 5% 9% 20% 15% 3	1776
March	1st to 10th	Night	38% 5% 12% 26% 17% 2	2048
	11th to 20th	Night	42% 6% 10% 22% 16% 3	1980
	21st to end	Night	45% 5% 10% 20% 16% 4	2124
April	1st to 10th	Night	48% 5% 10% 19% 15% 4	1800
	11th to 20th	Night	38% 7% 11% 21% 19% 4	1775
	21st to end	Night	41% 6% 11% 21% 16% 5%	1800
May	1st to 10th	Night	46% 6% 9% 21% 13% 4	1648
	11th to 20th	Night	44% 7% 12% 17% 18% 2	1416
	21st to end	Night	40% 8% 14% 23% 13% 2	1551
June	1st to 10th	Night	36% 8% 12% 23% 19% 2	1440
	11th to 20th	Night	38% 10% 14% 20% 17% 2	1440
	21st to end	Night	34% 8% 14% 27% 16% 2	1440
July	1st to 10th	Night	41% 10% 14% 18% 14% 3	1400
	11th to 20th	Night	30% 10% 15% 25% 17% 2	1392
	21st to end	Night	33% 9% 17% 21% 17% 2	1646
August	1st to 10th	Night	34% 9% 17% 25% 14% 1	1600
	11th to 20th	Night	41% 9% 14% 19% 17% 1	1663
	21st to end	Night	36% 8% 14% 22% 16% 3	1980
September	1st to 10th	Night	39% 5% 12% 21% 19% 4	1800
	11th to 20th	Night	32% 5% 10% 26% 23% 4	1800
	21st to end	Night	42% 6% 10% 20% 20% 2	2051
October	1st to 10th	Night	43% 5% 11% 24% 16% 2	2160
	11th to 20th	Night	38% 5% 11% 28% 16% 2	2160
	21st to end	Night	39% 4 11% 27% 17% 2	2376
November	1st to 10th	Night	33% 6% 11% 26% 20% 4	2375
	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	29% 26% 30% 13% 2	237
	21st to end	Day	24% 28% 32% 11% 4 1	264
February	1st to 10th	Day	29% 26% 32% 11% 2	240
	11th to 20th	Day	29% 26% 39% 6%	285
	21st to end	Day	33% 21% 29% 15% 2	346
March	1st to 10th	Day	23% 22% 32% 20% 4	420
	11th to 20th	Day	24% 22% 35% 14% 4 1	420
	21st to end	Day	26% 22% 30% 16% 5% 2	462
April	1st to 10th	Day	21% 22% 34% 15% 7% 1	420
	11th to 20th	Day	21% 19% 28% 17% 11% 4	412
	21st to end	Day	28% 13% 32% 19% 7% 2	420
May	1st to 10th	Day	25% 18% 26% 19% 9% 3	423
	11th to 20th	Day	21% 15% 30% 21% 10% 3	447
	21st to end	Day	18% 8% 23% 25% 19% 7%	485
June	1st to 10th	Day	15% 11% 28% 24% 17% 5%	450
	11th to 20th	Day	17% 8% 27% 26% 17% 5%	450
	21st to end	Day	11% 8% 28% 30% 18% 5%	450
July	1st to 10th	Day	13% 5% 18% 31% 21% 12%	445
	11th to 20th	Day	6% 4 21% 32% 27% 10%	444
	21st to end	Day	12% 5% 25% 33% 19% 6%	488
August	1st to 10th	Day	12% 3 20% 28% 28% 9%	446
	11th to 20th	Day	14% 4 19% 32% 20% 11%	439
	21st to end	Day	13% 4 20% 31% 20% 11%	495
September	1st to 10th	Day	18% 6% 20% 23% 22% 11%	450
	11th to 20th	Day	15% 5% 19% 29% 21% 10%	450
	21st to end	Day	22% 11% 24% 24% 13% 5%	450
October	1st to 10th	Day	22% 6% 24% 23% 18% 7%	450
	11th to 20th	Day	19% 14% 26% 26% 13% 2	450
	21st to end	Day	19% 14% 36% 21% 8% 2	495
November	1st to 10th	Day	18% 13% 30% 24% 14% 2	381
	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% 4	741
	21st to end	Night	18% 17% 41% 23% 2	792
February	1st to 10th	Night	22% 19% 32% 26% 2	720
	11th to 20th	Night	22% 21% 39% 17% 1	703
	21st to end	Night	24% 20% 38% 16% 2	592
March	1st to 10th	Night	20% 16% 39% 22% 3	720
	11th to 20th	Night	22% 18% 34% 20% 5%	720
	21st to end	Night	22% 12% 39% 20% 6% 1	792
April	1st to 10th	Night	22% 18% 34% 20% 5%	720
	11th to 20th	Night	17% 12% 36% 27% 8%	711
	21st to end	Night	23% 10% 32% 28% 7%	720
May	1st to 10th	Night	20% 10% 30% 29% 11%	568
	11th to 20th	Night	20% 7% 33% 30% 10%	354
	21st to end	Night	13% 6% 21% 40% 18% 1	389
June	1st to 10th	Night	14% 3 26% 37% 19%	360
	11th to 20th	Night	13% 6% 26% 37% 18%	360
	21st to end	Night	10% 1 27% 48% 14%	360
July	1st to 10th	Night	13% 4 20% 37% 24% 2	350
	11th to 20th	Night	6% 2 16% 46% 27% 3	348
	21st to end	Night	11% 2 17% 42% 26% 1	563
August	1st to 10th	Night	12% 1 14% 43% 27% 2	537
	11th to 20th	Night	12% 4 13% 42% 27% 3	599
	21st to end	Night	13% 3 17% 36% 28% 3	792
September	1st to 10th	Night	16% 3 21% 33% 24% 2	720
	11th to 20th	Night	13% 4 18% 35% 26% 3	720
	21st to end	Night	21% 5% 21% 30% 21% 2	720
October	1st to 10th	Night	21% 6% 24% 28% 18% 3	720
	11th to 20th	Night	18% 10% 28% 31% 12% 1	720
	21st to end	Night	20% 11% 32% 27% 10%	792
November	1st to 10th	Night	14% 9% 35% 30% 12% 1	740
	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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