Clegg Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Clegg Observatory clear sky chart from 2005-11-06 to 2024-01-15.

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	13% 6% 3 6% 6% 3 5% 5% 6% 7% 40%	74638
	Night	13% 6% 2 5% 5% 3 5% 5% 6% 7% 43%	75135
Transparency	Day	42% 7% 11% 22% 15% 3	74356
	Night	39% 6% 12% 23% 16% 3	74936
Seeing	Day	20% 15% 26% 22% 13% 5%	13792
	Night	17% 12% 25% 30% 13% 2	23572

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	11% 7% 4 7% 6% 4 5% 5% 5% 7% 40%	1682
	11th to 20th	Day	14% 6% 3 7% 7% 4 5% 5% 4 7% 38%	1656
	21st to end	Day	13% 8% 4 7% 7% 3 5% 4 4 6% 40%	1782
February	1st to 10th	Day	11% 9% 4 7% 7% 3 5% 5% 5% 7% 37%	1620
	11th to 20th	Day	14% 8% 3 6% 6% 4 5% 5% 5% 6% 38%	1573
	21st to end	Day	17% 8% 3 6% 4 3 5% 4 5% 6% 40%	1512
March	1st to 10th	Day	12% 6% 3 6% 5% 3 4 4 4 5% 49%	1980
	11th to 20th	Day	13% 7% 3 6% 6% 3 6% 5% 4 6% 41%	1980
	21st to end	Day	16% 7% 4 7% 6% 3 5% 5% 5% 6% 36%	2178
April	1st to 10th	Day	9% 6% 3 6% 7% 4 5% 5% 6% 7% 42%	2128
	11th to 20th	Day	13% 6% 3 5% 5% 3 5% 4 5% 6% 46%	2134
	21st to end	Day	17% 5% 3 5% 6% 4 5% 5% 6% 6% 39%	2221
May	1st to 10th	Day	15% 7% 2 6% 6% 3 6% 5% 6% 8% 37%	2340
	11th to 20th	Day	19% 6% 3 6% 6% 3 5% 5% 5% 7% 35%	2301
	21st to end	Day	13% 5% 2 6% 6% 4 5% 6% 6% 8% 38%	2708
June	1st to 10th	Day	12% 5% 2 5% 5% 4 6% 6% 7% 9% 40%	2520
	11th to 20th	Day	10% 5% 2 6% 7% 4 6% 6% 7% 11% 36%	2520
	21st to end	Day	10% 4 2 4 5% 4 7% 6% 7% 10% 41%	2520
July	1st to 10th	Day	9% 5% 3 6% 7% 4 6% 5% 7% 9% 39%	2473
	11th to 20th	Day	6% 5% 2 5% 6% 4 6% 8% 9% 11% 38%	2340
	21st to end	Day	10% 6% 3 7% 6% 4 7% 7% 7% 9% 34%	2547
August	1st to 10th	Day	10% 6% 3 6% 8% 6% 7% 7% 7% 9% 32%	2327
	11th to 20th	Day	12% 6% 2 6% 7% 4 7% 6% 7% 8% 36%	2282
	21st to end	Day	11% 5% 3 5% 6% 4 6% 6% 6% 9% 38%	2376
September	1st to 10th	Day	14% 6% 3 5% 5% 3 5% 5% 5% 7% 43%	2160
	11th to 20th	Day	12% 5% 2 5% 5% 3 5% 5% 5% 7% 46%	2084
	21st to end	Day	18% 7% 2 5% 6% 4 4 4 6% 7% 38%	1921
October	1st to 10th	Day	15% 7% 2 5% 4 2 5% 4 6% 7% 44%	1800
	11th to 20th	Day	16% 5% 2 5% 5% 3 4 4 5% 6% 45%	1800
	21st to end	Day	14% 5% 2 5% 4 3 4 4 6% 6% 47%	1980
November	1st to 10th	Day	10% 5% 2 4 5% 3 3 3 5% 7% 53%	1857
	11th to 20th	Day	14% 6% 2 4 4 3 4 4 6% 6% 46%	1614
	21st to end	Day	14% 5% 2 4 5% 3 5% 4 5% 6% 47%	1520
December	1st to 10th	Day	15% 8% 3 5% 4 3 5% 5% 6% 7% 38%	1520
	11th to 20th	Day	14% 6% 2 5% 6% 2 4 5% 5% 6% 44%	1910
	21st to end	Day	16% 6% 3 5% 6% 3 5% 6% 7% 8% 34%	2772
January	1st to 10th	Night	11% 6% 3 5% 5% 3 5% 5% 5% 7% 45%	2478
	11th to 20th	Night	14% 7% 3 6% 5% 3 5% 5% 5% 7% 40%	2399
	21st to end	Night	13% 8% 3 5% 4 2 5% 5% 6% 7% 42%	2574
February	1st to 10th	Night	12% 7% 3 6% 6% 3 5% 5% 6% 6% 40%	2340
	11th to 20th	Night	15% 7% 3 6% 6% 3 5% 6% 6% 6% 38%	2273
	21st to end	Night	15% 9% 2 6% 7% 3 4 5% 5% 7% 38%	1738
March	1st to 10th	Night	11% 4 2 5% 4 3 5% 4 5% 6% 50%	1980
	11th to 20th	Night	15% 6% 4 5% 5% 3 4 4 5% 7% 42%	1980
	21st to end	Night	15% 6% 2 6% 5% 2 5% 5% 5% 7% 41%	2178
April	1st to 10th	Night	13% 7% 3 5% 5% 3 6% 6% 6% 5% 41%	1832
	11th to 20th	Night	12% 5% 2 5% 6% 2 4 5% 5% 8% 45%	1775
	21st to end	Night	15% 6% 2 4 5% 3 5% 4 4 7% 45%	1754
May	1st to 10th	Night	15% 6% 3 5% 5% 3 6% 4 5% 8% 40%	1620
	11th to 20th	Night	16% 6% 3 6% 6% 3 5% 4 5% 6% 40%	1592
	21st to end	Night	13% 5% 2 4 5% 3 5% 5% 5% 9% 43%	1563
June	1st to 10th	Night	10% 4 1 4 6% 4 4 5% 6% 8% 47%	1440
	11th to 20th	Night	11% 6% 2 6% 5% 3 5% 6% 7% 8% 41%	1440
	21st to end	Night	9% 3 2 5% 4 3 6% 6% 9% 10% 44%	1440
July	1st to 10th	Night	10% 5% 2 5% 6% 4 5% 5% 7% 9% 41%	1482
	11th to 20th	Night	7% 3 2 4 6% 3 5% 6% 7% 10% 47%	1620
	21st to end	Night	7% 4 2 5% 5% 3 6% 6% 9% 12% 41%	1771
August	1st to 10th	Night	8% 4 2 6% 6% 4 6% 7% 7% 8% 42%	1611
	11th to 20th	Night	10% 6% 2 6% 6% 3 6% 4 6% 9% 41%	1599
	21st to end	Night	10% 5% 2 4 7% 4 6% 5% 6% 11% 40%	1980
September	1st to 10th	Night	13% 6% 3 5% 5% 3 5% 4 6% 7% 45%	1800
	11th to 20th	Night	11% 5% 2 4 5% 3 3 4 6% 7% 50%	1872
	21st to end	Night	17% 6% 2 5% 5% 3 4 3 5% 5% 44%	2038
October	1st to 10th	Night	17% 6% 2 5% 4 2 4 4 5% 6% 45%	2160
	11th to 20th	Night	13% 5% 2 4 4 3 4 4 5% 6% 50%	2160
	21st to end	Night	14% 5% 2 4 4 3 4 5% 4 6% 50%	2376
November	1st to 10th	Night	9% 6% 3 5% 5% 3 3 4 4 7% 52%	2221
	11th to 20th	Night	13% 6% 2 5% 4 2 4 4 5% 7% 49%	2560
	21st to end	Night	13% 4 2 5% 4 4 5% 4 5% 7% 46%	2660
December	1st to 10th	Night	13% 7% 3 7% 5% 3 4 5% 5% 6% 40%	2660
	11th to 20th	Night	14% 5% 3 5% 5% 3 5% 5% 5% 7% 43%	3285
	21st to end	Night	15% 6% 3 6% 6% 4 5% 6% 6% 8% 35%	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	44% 5% 8% 22% 17% 3	1683
	11th to 20th	Day	47% 5% 11% 21% 15% 1	1656
	21st to end	Day	46% 4 10% 20% 16% 3	1782
February	1st to 10th	Day	48% 5% 12% 19% 12% 3	1620
	11th to 20th	Day	46% 5% 9% 21% 16% 2	1582
	21st to end	Day	45% 4 11% 20% 16% 4	1512
March	1st to 10th	Day	39% 4 10% 24% 20% 3	1980
	11th to 20th	Day	45% 5% 9% 19% 17% 5%	1980
	21st to end	Day	48% 6% 10% 18% 14% 4	2178
April	1st to 10th	Day	40% 8% 10% 20% 17% 6%	2128
	11th to 20th	Day	38% 5% 10% 21% 20% 6%	2134
	21st to end	Day	44% 7% 11% 20% 14% 4	2221
May	1st to 10th	Day	45% 7% 13% 20% 13% 2	2340
	11th to 20th	Day	49% 7% 12% 19% 10% 2	2305
	21st to end	Day	43% 8% 14% 24% 10% 1	2708
June	1st to 10th	Day	39% 8% 12% 24% 15% 1	2520
	11th to 20th	Day	41% 10% 14% 19% 14% 2	2520
	21st to end	Day	37% 9% 15% 24% 13% 2	2520
July	1st to 10th	Day	41% 10% 12% 21% 14% 2	2405
	11th to 20th	Day	35% 12% 15% 23% 13% 2	2271
	21st to end	Day	42% 10% 12% 19% 15% 2	2405
August	1st to 10th	Day	45% 11% 13% 21% 10%	2314
	11th to 20th	Day	44% 8% 13% 21% 12% 1	2282
	21st to end	Day	40% 9% 14% 23% 12% 2	2376
September	1st to 10th	Day	41% 8% 13% 17% 18% 3	2160
	11th to 20th	Day	37% 6% 8% 27% 18% 4	2084
	21st to end	Day	46% 6% 9% 21% 17% 2	1921
October	1st to 10th	Day	39% 7% 10% 24% 16% 4	1800
	11th to 20th	Day	41% 4 9% 23% 20% 3	1800
	21st to end	Day	38% 5% 8% 24% 20% 4	1980
November	1st to 10th	Day	32% 5% 11% 25% 23% 5%	1857
	11th to 20th	Day	37% 5% 10% 20% 22% 5%	1614
	21st to end	Day	38% 5% 11% 25% 19% 2	1520
December	1st to 10th	Day	46% 6% 12% 22% 12% 2	1520
	11th to 20th	Day	39% 5% 9% 31% 12% 4	1906
	21st to end	Day	44% 7% 13% 23% 11% 1	2772
January	1st to 10th	Night	39% 6% 10% 21% 19% 4	2478
	11th to 20th	Night	44% 5% 11% 22% 15% 2	2399
	21st to end	Night	41% 6% 9% 24% 17% 3	2574
February	1st to 10th	Night	43% 6% 12% 24% 14% 2	2340
	11th to 20th	Night	45% 6% 9% 21% 16% 4	2275
	21st to end	Night	46% 5% 11% 21% 12% 4	1738
March	1st to 10th	Night	35% 5% 11% 29% 17% 4	1980
	11th to 20th	Night	43% 5% 11% 22% 17% 3	1980
	21st to end	Night	42% 6% 12% 20% 15% 5%	2178
April	1st to 10th	Night	42% 7% 10% 21% 17% 3	1832
	11th to 20th	Night	37% 7% 11% 22% 18% 5%	1775
	21st to end	Night	40% 7% 11% 26% 13% 3	1754
May	1st to 10th	Night	42% 6% 13% 22% 13% 4	1620
	11th to 20th	Night	45% 7% 13% 20% 14% 1	1593
	21st to end	Night	38% 8% 19% 24% 10% 1	1564
June	1st to 10th	Night	35% 8% 13% 28% 14% 1	1440
	11th to 20th	Night	38% 10% 14% 23% 13% 1	1440
	21st to end	Night	33% 9% 16% 24% 16% 2	1440
July	1st to 10th	Night	38% 9% 17% 20% 13% 3	1440
	11th to 20th	Night	31% 10% 17% 26% 13% 2	1566
	21st to end	Night	31% 11% 19% 24% 15%	1672
August	1st to 10th	Night	36% 10% 17% 23% 13% 1	1600
	11th to 20th	Night	39% 7% 15% 25% 13%	1599
	21st to end	Night	37% 9% 14% 25% 12% 2	1980
September	1st to 10th	Night	39% 7% 13% 20% 17% 4	1800
	11th to 20th	Night	33% 6% 11% 30% 19% 2	1872
	21st to end	Night	42% 6% 10% 22% 18% 2	2038
October	1st to 10th	Night	40% 5% 11% 24% 18% 2	2160
	11th to 20th	Night	34% 5% 11% 27% 21% 2	2160
	21st to end	Night	35% 5% 9% 24% 23% 2	2376
November	1st to 10th	Night	33% 5% 10% 24% 22% 6%	2221
	11th to 20th	Night	36% 5% 8% 25% 21% 5%	2560
	21st to end	Night	38% 5% 11% 23% 19% 3	2660
December	1st to 10th	Night	44% 5% 11% 25% 12% 3	2660
	11th to 20th	Night	41% 4 10% 23% 18% 3	3288
	21st to end	Night	43% 8% 14% 21% 14% 1	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	33% 28% 26% 9% 3 1	258
	11th to 20th	Day	31% 25% 31% 11% 2	252
	21st to end	Day	30% 30% 28% 9% 2	264
February	1st to 10th	Day	33% 29% 29% 8% 1	240
	11th to 20th	Day	40% 27% 30% 3	240
	21st to end	Day	37% 22% 25% 14% 2	320
March	1st to 10th	Day	28% 25% 25% 19% 3	420
	11th to 20th	Day	26% 25% 25% 17% 5% 2	420
	21st to end	Day	26% 21% 30% 17% 6% 1	462
April	1st to 10th	Day	21% 25% 26% 20% 6% 1	420
	11th to 20th	Day	21% 18% 29% 19% 10% 4	412
	21st to end	Day	25% 17% 31% 17% 8% 2	420
May	1st to 10th	Day	25% 17% 30% 16% 9% 4	423
	11th to 20th	Day	23% 15% 30% 21% 9% 3	447
	21st to end	Day	19% 8% 27% 21% 18% 7%	485
June	1st to 10th	Day	15% 11% 26% 29% 14% 5%	450
	11th to 20th	Day	16% 10% 28% 25% 14% 6%	450
	21st to end	Day	11% 9% 32% 30% 15% 3	450
July	1st to 10th	Day	11% 4 22% 30% 24% 9%	445
	11th to 20th	Day	5% 4 22% 33% 25% 11%	444
	21st to end	Day	11% 4 22% 31% 23% 8%	488
August	1st to 10th	Day	12% 7% 22% 25% 24% 11%	446
	11th to 20th	Day	12% 5% 20% 28% 24% 11%	439
	21st to end	Day	10% 7% 20% 27% 22% 15%	495
September	1st to 10th	Day	17% 8% 18% 23% 23% 11%	450
	11th to 20th	Day	14% 4 23% 25% 24% 10%	450
	21st to end	Day	25% 10% 22% 24% 15% 4	450
October	1st to 10th	Day	20% 7% 21% 30% 18% 4	450
	11th to 20th	Day	20% 16% 24% 26% 11% 2	450
	21st to end	Day	19% 19% 31% 20% 8% 3	495
November	1st to 10th	Day	18% 14% 30% 24% 10% 4	455
	11th to 20th	Day	21% 19% 30% 20% 8% 1	264
	21st to end	Day	26% 18% 36% 16% 4	180
December	1st to 10th	Day	27% 22% 37% 10% 3 1	180
	11th to 20th	Day	26% 23% 29% 16% 6%	180
	21st to end	Day	20% 24% 32% 19% 5% 1	198
January	1st to 10th	Night	20% 20% 35% 22% 3	760
	11th to 20th	Night	24% 16% 28% 30% 3	739
	21st to end	Night	19% 18% 38% 24% 1	792
February	1st to 10th	Night	18% 22% 30% 28% 2	720
	11th to 20th	Night	26% 25% 30% 18% 1	703
	21st to end	Night	27% 16% 34% 20% 2	592
March	1st to 10th	Night	20% 18% 33% 25% 4	720
	11th to 20th	Night	25% 21% 27% 22% 5% 1	720
	21st to end	Night	20% 18% 33% 22% 6% 1	792
April	1st to 10th	Night	19% 21% 31% 23% 6%	720
	11th to 20th	Night	16% 16% 31% 26% 10% 1	711
	21st to end	Night	19% 14% 27% 31% 8%	674
May	1st to 10th	Night	20% 9% 26% 29% 15% 1	540
	11th to 20th	Night	18% 6% 28% 32% 15% 1	531
	21st to end	Night	14% 4 20% 36% 23% 3	403
June	1st to 10th	Night	11% 4 23% 39% 22% 1	360
	11th to 20th	Night	13% 6% 14% 44% 21% 3	360
	21st to end	Night	7% 4 19% 48% 21% 1	360
July	1st to 10th	Night	12% 2 13% 41% 26% 6%	390
	11th to 20th	Night	7% 1 12% 38% 38% 4	522
	21st to end	Night	8% 2 14% 41% 29% 6%	591
August	1st to 10th	Night	10% 1 12% 41% 33% 3	537
	11th to 20th	Night	10% 4 13% 40% 29% 4	535
	21st to end	Night	11% 5% 13% 36% 32% 4	792
September	1st to 10th	Night	16% 4 13% 30% 31% 5%	720
	11th to 20th	Night	11% 5% 17% 33% 29% 5%	720
	21st to end	Night	21% 5% 21% 31% 21% 1	720
October	1st to 10th	Night	20% 9% 17% 30% 22% 3	720
	11th to 20th	Night	17% 13% 25% 32% 14% 1	720
	21st to end	Night	19% 14% 26% 30% 11% 1	792
November	1st to 10th	Night	13% 12% 29% 32% 13% 1	740
	11th to 20th	Night	18% 17% 26% 28% 9% 1	760
	21st to end	Night	18% 16% 27% 30% 7% 1	760
December	1st to 10th	Night	21% 17% 30% 25% 6% 1	760
	11th to 20th	Night	18% 13% 34% 31% 4	760
	21st to end	Night	19% 16% 28% 35% 2	836

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