Sunridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sunridge Observatory clear sky chart from 2005-11-06 to 2026-09-14.

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	8% 7% 3 7% 4 4 5% 4 6% 8% 45%	158924
	Night	8% 8% 3 8% 5% 4 5% 4 6% 8% 42%	149520
Transparency	Day	35% 7% 15% 26% 13% 3	153545
	Night	38% 8% 16% 24% 12% 2	145910
Seeing	Day	12% 10% 30% 29% 15% 3	18046
	Night	12% 10% 34% 32% 11% 1	27131

By Month

Percentage Cloud Cover SW (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	5% 11% 5% 15% 7% 7% 5% 4 5% 7% 29%	4124
	11th to 20th	Day	5% 10% 5% 10% 4 5% 8% 6% 5% 7% 34%	4140
	21st to end	Day	3 9% 5% 8% 6% 5% 4 3 5% 6% 47%	4988
February	1st to 10th	Day	8% 13% 5% 9% 7% 7% 5% 4 5% 6% 30%	1869
	11th to 20th	Day	7% 13% 5% 10% 6% 6% 7% 5% 7% 7% 28%	1845
	21st to end	Day	4 12% 5% 10% 6% 6% 6% 5% 5% 7% 33%	1557
March	1st to 10th	Day	5% 9% 3 9% 6% 5% 4 3 5% 7% 43%	4835
	11th to 20th	Day	6% 7% 4 9% 5% 5% 5% 4 6% 9% 39%	5110
	21st to end	Day	9% 15% 5% 11% 4 5% 5% 4 6% 8% 27%	5607
April	1st to 10th	Day	9% 11% 3 7% 6% 5% 5% 4 6% 8% 35%	2544
	11th to 20th	Day	10% 13% 4 9% 5% 5% 5% 4 5% 7% 33%	2697
	21st to end	Day	11% 10% 3 8% 6% 5% 4 4 5% 7% 37%	2730
May	1st to 10th	Day	12% 8% 2 6% 4 4 5% 4 7% 8% 41%	8077
	11th to 20th	Day	12% 8% 3 8% 5% 5% 6% 5% 7% 8% 33%	8393
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 43%	9648
June	1st to 10th	Day	14% 5% 2 7% 4 5% 5% 4 7% 9% 38%	3150
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 5% 7% 9% 34%	3150
	21st to end	Day	9% 4 1 5% 4 4 5% 4 6% 8% 49%	3150
July	1st to 10th	Day	6% 3 1 4 3 3 4 3 5% 7% 60%	8819
	11th to 20th	Day	8% 5% 2 5% 4 3 4 4 6% 8% 52%	8820
	21st to end	Day	5% 3 1 3 2 2 2 2 4 6% 69%	9575
August	1st to 10th	Day	7% 4 2 5% 4 4 4 3 5% 8% 53%	7791
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 9% 50%	7565
	21st to end	Day	6% 3 1 3 2 3 3 3 4 7% 64%	8374
September	1st to 10th	Day	11% 4 2 5% 3 3 3 3 4 7% 55%	2582
	11th to 20th	Day	13% 5% 2 4 3 4 5% 3 5% 7% 49%	2281
	21st to end	Day	8% 5% 2 5% 4 3 3 2 6% 7% 53%	2200
October	1st to 10th	Day	7% 5% 2 6% 4 3 5% 4 7% 9% 48%	3280
	11th to 20th	Day	5% 5% 2 5% 4 4 5% 3 5% 7% 54%	3250
	21st to end	Day	7% 9% 3 6% 4 3 4 3 5% 6% 50%	3174
November	1st to 10th	Day	13% 12% 3 8% 6% 4 6% 4 5% 5% 33%	1648
	11th to 20th	Day	6% 10% 4 7% 6% 6% 5% 3 6% 7% 40%	1680
	21st to end	Day	7% 13% 3 9% 7% 6% 5% 5% 5% 8% 33%	1621
December	1st to 10th	Day	8% 16% 4 9% 6% 4 4 3 5% 8% 32%	2360
	11th to 20th	Day	7% 10% 5% 11% 7% 6% 6% 4 4 7% 32%	2715
	21st to end	Day	3 9% 4 10% 6% 4 4 4 6% 7% 43%	3575
January	1st to 10th	Night	4 10% 5% 12% 6% 5% 5% 4 6% 8% 34%	8768
	11th to 20th	Night	6% 12% 4 9% 5% 4 5% 4 6% 8% 38%	8820
	21st to end	Night	4 8% 5% 9% 5% 5% 5% 4 6% 8% 39%	9252
February	1st to 10th	Night	9% 12% 4 9% 7% 5% 5% 4 6% 7% 32%	2750
	11th to 20th	Night	6% 12% 4 9% 7% 6% 6% 5% 6% 7% 33%	2662
	21st to end	Night	5% 11% 4 9% 6% 5% 6% 4 6% 7% 35%	2249
March	1st to 10th	Night	4 8% 4 11% 6% 5% 5% 4 6% 9% 38%	5276
	11th to 20th	Night	7% 7% 3 7% 4 5% 6% 4 6% 9% 41%	5110
	21st to end	Night	11% 16% 5% 11% 6% 5% 5% 4 6% 7% 25%	5610
April	1st to 10th	Night	9% 13% 3 9% 5% 5% 4 5% 5% 7% 35%	2072
	11th to 20th	Night	12% 11% 3 7% 6% 4 5% 3 6% 8% 35%	1871
	21st to end	Night	12% 10% 3 8% 6% 4 4 3 5% 8% 37%	1890
May	1st to 10th	Night	16% 8% 2 6% 4 3 4 2 4 6% 44%	4868
	11th to 20th	Night	12% 8% 3 8% 6% 5% 6% 5% 7% 10% 31%	4470
	21st to end	Night	13% 5% 2 7% 5% 5% 5% 4 7% 9% 40%	4525
June	1st to 10th	Night	17% 5% 2 5% 3 5% 5% 4 6% 8% 39%	1470
	11th to 20th	Night	12% 4 2 6% 5% 6% 6% 4 8% 8% 40%	1470
	21st to end	Night	8% 4 2 4 4 4 4 4 6% 8% 53%	1470
July	1st to 10th	Night	5% 3 1 4 3 4 4 4 5% 8% 60%	4140
	11th to 20th	Night	7% 5% 2 5% 4 4 4 3 7% 10% 49%	4140
	21st to end	Night	6% 2 1 3 3 3 4 3 4 7% 65%	4500
August	1st to 10th	Night	9% 5% 2 5% 3 4 4 4 6% 9% 51%	5020
	11th to 20th	Night	7% 4 2 5% 4 4 5% 5% 8% 11% 44%	5291
	21st to end	Night	8% 3 1 3 3 3 3 3 5% 7% 61%	5842
September	1st to 10th	Night	9% 4 2 5% 3 4 3 4 6% 8% 52%	2037
	11th to 20th	Night	12% 5% 2 5% 3 3 4 4 6% 7% 50%	2182
	21st to end	Night	7% 5% 1 4 4 4 4 4 6% 7% 54%	2200
October	1st to 10th	Night	8% 5% 2 5% 3 4 4 4 5% 7% 52%	3910
	11th to 20th	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	3898
	21st to end	Night	7% 8% 3 6% 4 4 4 4 5% 7% 48%	4590
November	1st to 10th	Night	12% 12% 3 7% 6% 4 4 3 5% 7% 36%	2870
	11th to 20th	Night	7% 10% 3 9% 6% 5% 4 4 6% 7% 39%	2940
	21st to end	Night	6% 12% 4 9% 5% 5% 5% 3 6% 8% 37%	2998
December	1st to 10th	Night	6% 14% 4 10% 6% 6% 6% 4 5% 6% 32%	5040
	11th to 20th	Night	6% 11% 5% 11% 7% 6% 5% 4 6% 8% 29%	5696
	21st to end	Night	4 9% 3 9% 4 4 5% 4 6% 7% 46%	7623

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	52% 9% 16% 18% 5%	4125
	11th to 20th	Day	47% 8% 15% 17% 12% 2	4140
	21st to end	Day	39% 7% 22% 25% 7%	4988
February	1st to 10th	Day	53% 8% 14% 15% 9% 1	1869
	11th to 20th	Day	50% 9% 14% 18% 8% 1	1854
	21st to end	Day	48% 9% 13% 20% 8% 1	1557
March	1st to 10th	Day	38% 8% 16% 25% 13%	4835
	11th to 20th	Day	39% 9% 17% 25% 10%	5110
	21st to end	Day	50% 10% 19% 15% 5%	5607
April	1st to 10th	Day	44% 9% 17% 22% 8%	2544
	11th to 20th	Day	49% 6% 13% 23% 7% 1	2700
	21st to end	Day	45% 7% 13% 25% 9% 1	2730
May	1st to 10th	Day	37% 7% 12% 25% 14% 4	8077
	11th to 20th	Day	41% 10% 13% 25% 10%	8397
	21st to end	Day	33% 9% 13% 31% 14%	9650
June	1st to 10th	Day	39% 10% 16% 22% 11% 2	3150
	11th to 20th	Day	43% 9% 16% 21% 11% 1	3150
	21st to end	Day	29% 9% 19% 29% 12% 2	3150
July	1st to 10th	Day	22% 7% 17% 28% 22% 5%	8746
	11th to 20th	Day	28% 7% 15% 27% 17% 6%	8739
	21st to end	Day	17% 5% 12% 28% 23% 16%	9362
August	1st to 10th	Day	22% 6% 14% 31% 22% 5%	6167
	11th to 20th	Day	28% 8% 19% 33% 10% 1	5964
	21st to end	Day	20% 5% 13% 40% 19% 3	6569
September	1st to 10th	Day	30% 5% 14% 28% 17% 7%	2582
	11th to 20th	Day	34% 8% 13% 29% 14% 3	2281
	21st to end	Day	30% 6% 16% 30% 15% 3	2200
October	1st to 10th	Day	30% 7% 15% 31% 14% 2	3280
	11th to 20th	Day	29% 6% 17% 33% 15% 1	3250
	21st to end	Day	35% 5% 10% 37% 13%	3174
November	1st to 10th	Day	51% 9% 14% 17% 7% 2	1648
	11th to 20th	Day	42% 8% 20% 22% 8%	1680
	21st to end	Day	48% 8% 18% 18% 7% 1	1621
December	1st to 10th	Day	50% 7% 22% 17% 3 1	2360
	11th to 20th	Day	51% 8% 19% 18% 3	2714
	21st to end	Day	39% 6% 15% 25% 13% 2	3575
January	1st to 10th	Night	45% 8% 18% 20% 8% 1	8769
	11th to 20th	Night	43% 7% 19% 19% 12% 1	8820
	21st to end	Night	40% 8% 21% 23% 7%	9252
February	1st to 10th	Night	49% 8% 15% 18% 10% 1	2750
	11th to 20th	Night	47% 9% 15% 18% 11% 1	2663
	21st to end	Night	45% 8% 15% 22% 8% 2	2249
March	1st to 10th	Night	39% 9% 21% 20% 11%	5276
	11th to 20th	Night	36% 8% 18% 30% 9%	5110
	21st to end	Night	54% 10% 17% 14% 4	5610
April	1st to 10th	Night	46% 9% 14% 22% 9% 1	2072
	11th to 20th	Night	45% 7% 14% 21% 11% 2	1871
	21st to end	Night	44% 7% 15% 23% 9% 1	1890
May	1st to 10th	Night	41% 6% 7% 28% 14% 5%	4867
	11th to 20th	Night	41% 11% 15% 20% 13%	4471
	21st to end	Night	35% 11% 15% 27% 11% 1	4526
June	1st to 10th	Night	41% 7% 18% 22% 11% 2	1470
	11th to 20th	Night	37% 10% 14% 24% 13% 1	1470
	21st to end	Night	27% 7% 18% 33% 13% 2	1470
July	1st to 10th	Night	21% 6% 16% 31% 20% 6%	4105
	11th to 20th	Night	30% 7% 13% 26% 17% 7%	4098
	21st to end	Night	19% 5% 12% 27% 22% 16%	4401
August	1st to 10th	Night	23% 7% 12% 30% 21% 6%	3969
	11th to 20th	Night	32% 9% 21% 25% 11% 2	4168
	21st to end	Night	23% 5% 12% 36% 22% 3	4578
September	1st to 10th	Night	28% 8% 13% 26% 18% 8%	2037
	11th to 20th	Night	32% 7% 13% 28% 15% 5%	2182
	21st to end	Night	28% 6% 15% 30% 18% 3	2200
October	1st to 10th	Night	30% 6% 12% 34% 17% 2	3910
	11th to 20th	Night	30% 6% 16% 31% 16% 2	3898
	21st to end	Night	35% 7% 12% 35% 11%	4590
November	1st to 10th	Night	48% 8% 15% 19% 9% 1	2870
	11th to 20th	Night	44% 8% 19% 19% 10%	2940
	21st to end	Night	44% 7% 23% 17% 8% 1	2998
December	1st to 10th	Night	51% 8% 20% 16% 5%	5040
	11th to 20th	Night	50% 9% 25% 14% 2	5697
	21st to end	Night	36% 6% 14% 26% 15% 3	7623

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	7% 14% 37% 27% 12% 3	220
	11th to 20th	Day	13% 11% 40% 25% 9% 1	220
	21st to end	Day	10% 12% 46% 24% 7% 2	242
February	1st to 10th	Day	20% 13% 37% 23% 6% 1	319
	11th to 20th	Day	10% 9% 37% 32% 11% 1	330
	21st to end	Day	12% 10% 35% 30% 11% 2	273
March	1st to 10th	Day	13% 14% 35% 26% 10% 3	537
	11th to 20th	Day	13% 14% 38% 24% 9% 3	540
	21st to end	Day	13% 15% 35% 23% 12% 2	594
April	1st to 10th	Day	13% 17% 33% 29% 7% 2	540
	11th to 20th	Day	14% 15% 35% 26% 9% 1	532
	21st to end	Day	16% 11% 25% 31% 14% 2	540
May	1st to 10th	Day	15% 11% 24% 30% 16% 4	697
	11th to 20th	Day	15% 9% 26% 30% 18% 3	775
	21st to end	Day	18% 7% 21% 27% 21% 6%	844
June	1st to 10th	Day	16% 12% 25% 30% 15% 2	780
	11th to 20th	Day	16% 13% 31% 26% 13% 1	780
	21st to end	Day	11% 7% 27% 32% 18% 5%	780
July	1st to 10th	Day	5% 5% 26% 36% 23% 4	771
	11th to 20th	Day	9% 6% 27% 37% 19% 3	768
	21st to end	Day	6% 6% 23% 34% 27% 6%	849
August	1st to 10th	Day	6% 4 24% 38% 23% 6%	614
	11th to 20th	Day	7% 4 24% 35% 24% 6%	557
	21st to end	Day	7% 8% 25% 32% 23% 5%	627
September	1st to 10th	Day	12% 9% 25% 29% 22% 1	570
	11th to 20th	Day	13% 8% 25% 33% 17% 4	543
	21st to end	Day	12% 11% 34% 28% 11% 3	530
October	1st to 10th	Day	14% 7% 34% 27% 15% 3	340
	11th to 20th	Day	11% 14% 33% 28% 13% 1	330
	21st to end	Day	17% 17% 33% 22% 9% 1	363
November	1st to 10th	Day	25% 12% 37% 19% 8%	330
	11th to 20th	Day	11% 21% 39% 23% 6%	330
	21st to end	Day	12% 13% 39% 23% 8% 4	299
December	1st to 10th	Day	13% 16% 41% 23% 6%	220
	11th to 20th	Day	14% 16% 40% 24% 5%	220
	21st to end	Day	14% 10% 35% 31% 9% 2	242
January	1st to 10th	Night	11% 13% 43% 26% 7% 1	1050
	11th to 20th	Night	13% 13% 39% 27% 7% 1	1050
	21st to end	Night	12% 13% 39% 27% 9% 1	1013
February	1st to 10th	Night	13% 12% 33% 29% 11% 2	840
	11th to 20th	Night	10% 11% 38% 32% 9%	824
	21st to end	Night	10% 9% 33% 34% 12% 2	692
March	1st to 10th	Night	9% 10% 37% 33% 10% 1	840
	11th to 20th	Night	12% 13% 36% 29% 8% 1	840
	21st to end	Night	12% 12% 34% 31% 10% 1	924
April	1st to 10th	Night	15% 11% 34% 29% 10% 1	673
	11th to 20th	Night	17% 8% 37% 29% 9%	623
	21st to end	Night	17% 10% 27% 36% 11%	630
May	1st to 10th	Night	16% 8% 24% 36% 15% 1	630
	11th to 20th	Night	13% 6% 29% 35% 17% 1	547
	21st to end	Night	19% 7% 24% 33% 16% 2	455
June	1st to 10th	Night	19% 10% 26% 30% 13% 1	420
	11th to 20th	Night	14% 10% 32% 34% 10%	420
	21st to end	Night	8% 7% 27% 42% 16%	420
July	1st to 10th	Night	6% 5% 24% 47% 19%	410
	11th to 20th	Night	7% 4 32% 40% 16%	408
	21st to end	Night	4 5% 26% 43% 20% 1	460
August	1st to 10th	Night	9% 4 23% 40% 23% 1	580
	11th to 20th	Night	7% 4 21% 44% 23% 1	624
	21st to end	Night	10% 5% 27% 38% 20%	693
September	1st to 10th	Night	10% 5% 30% 40% 13% 1	630
	11th to 20th	Night	14% 4 24% 41% 16%	761
	21st to end	Night	11% 7% 34% 35% 12% 2	800
October	1st to 10th	Night	15% 8% 30% 35% 12% 1	800
	11th to 20th	Night	14% 9% 34% 32% 9% 2	800
	21st to end	Night	14% 16% 36% 28% 7%	894
November	1st to 10th	Night	18% 11% 40% 24% 6%	1025
	11th to 20th	Night	15% 15% 41% 24% 5%	1050
	21st to end	Night	11% 11% 43% 28% 6%	1050
December	1st to 10th	Night	13% 10% 40% 26% 9% 1	1050
	11th to 20th	Night	12% 14% 41% 25% 7% 1	1050
	21st to end	Night	14% 10% 40% 28% 6% 2	1155

Page generated at 2026-09-11 02:11:53. Version ac07

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