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 2025-05-11.

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% 8% 3 7% 5% 4 5% 4 6% 7% 44%	117238
	Night	7% 9% 3 8% 5% 4 5% 4 6% 8% 41%	120489
Transparency	Day	37% 8% 16% 26% 12% 2	116958
	Night	39% 8% 16% 23% 11% 2	120359
Seeing	Day	12% 10% 29% 30% 16% 3	16124
	Night	12% 9% 34% 32% 11% 1	25527

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	6% 14% 6% 15% 6% 6% 5% 3 5% 6% 28%	3179
	11th to 20th	Day	7% 10% 5% 10% 4 5% 9% 6% 5% 7% 33%	3180
	21st to end	Day	4 10% 6% 8% 4 4 4 2 4 6% 47%	3847
February	1st to 10th	Day	8% 14% 5% 9% 8% 7% 5% 4 5% 6% 29%	1780
	11th to 20th	Day	7% 13% 5% 10% 6% 6% 7% 5% 7% 8% 26%	1755
	21st to end	Day	5% 11% 5% 11% 6% 6% 6% 5% 5% 7% 33%	1485
March	1st to 10th	Day	5% 9% 3 9% 6% 5% 5% 3 5% 7% 42%	4730
	11th to 20th	Day	6% 7% 4 9% 5% 5% 5% 4 6% 9% 40%	5000
	21st to end	Day	9% 15% 5% 11% 4 5% 5% 4 6% 8% 27%	5486
April	1st to 10th	Day	8% 11% 3 7% 6% 6% 5% 4 6% 8% 35%	2423
	11th to 20th	Day	10% 14% 4 8% 5% 5% 6% 4 5% 7% 33%	2567
	21st to end	Day	12% 10% 3 8% 6% 5% 4 4 5% 7% 37%	2600
May	1st to 10th	Day	19% 12% 3 7% 4 4 4 4 5% 6% 32%	4491
	11th to 20th	Day	11% 6% 2 6% 5% 4 6% 6% 7% 8% 38%	4501
	21st to end	Day	15% 7% 2 8% 5% 5% 5% 4 7% 9% 33%	5161
June	1st to 10th	Day	13% 5% 2 7% 4 4 5% 4 7% 9% 39%	2850
	11th to 20th	Day	17% 6% 2 6% 5% 5% 5% 5% 7% 9% 33%	2850
	21st to end	Day	8% 4 1 5% 4 4 4 4 7% 8% 51%	2850
July	1st to 10th	Day	6% 4 2 5% 3 4 5% 4 6% 8% 53%	4740
	11th to 20th	Day	5% 3 1 4 3 3 4 3 6% 7% 61%	4740
	21st to end	Day	3 1 1 2 2 2 2 3 5% 6% 73%	5138
August	1st to 10th	Day	6% 4 1 5% 4 4 4 3 5% 9% 54%	4226
	11th to 20th	Day	5% 3 1 4 3 4 5% 4 6% 9% 55%	4028
	21st to end	Day	7% 2 1 3 2 3 4 3 5% 8% 62%	4521
September	1st to 10th	Day	11% 4 2 5% 2 3 3 3 4 7% 56%	2336
	11th to 20th	Day	14% 5% 2 3 3 4 5% 3 5% 7% 49%	2133
	21st to end	Day	9% 5% 2 5% 4 3 3 2 7% 7% 52%	2090
October	1st to 10th	Day	7% 5% 2 6% 5% 3 5% 4 7% 9% 48%	3180
	11th to 20th	Day	5% 5% 2 5% 3 4 5% 3 5% 7% 55%	3150
	21st to end	Day	7% 9% 3 6% 4 3 4 3 5% 6% 50%	3076
November	1st to 10th	Day	13% 13% 3 8% 6% 5% 5% 4 5% 5% 32%	1568
	11th to 20th	Day	6% 10% 4 7% 6% 6% 5% 3 6% 7% 40%	1600
	21st to end	Day	7% 14% 3 9% 6% 5% 5% 4 5% 8% 33%	1544
December	1st to 10th	Day	8% 16% 4 9% 6% 4 4 3 5% 8% 33%	2290
	11th to 20th	Day	7% 10% 5% 11% 7% 6% 6% 4 4 7% 32%	2645
	21st to end	Day	3 9% 4 11% 6% 4 4 4 6% 7% 43%	3498
January	1st to 10th	Night	4 12% 6% 14% 6% 5% 4 4 6% 7% 33%	6766
	11th to 20th	Night	7% 12% 4 10% 6% 5% 5% 4 6% 7% 34%	6780
	21st to end	Night	4 9% 5% 10% 5% 5% 5% 3 6% 8% 41%	7154
February	1st to 10th	Night	9% 12% 4 9% 7% 5% 5% 4 6% 6% 32%	2619
	11th to 20th	Night	7% 12% 4 9% 7% 6% 6% 5% 7% 7% 32%	2532
	21st to end	Night	5% 11% 4 9% 7% 5% 6% 4 6% 7% 35%	2145
March	1st to 10th	Night	4 8% 4 11% 6% 5% 5% 4 6% 9% 38%	5161
	11th to 20th	Night	7% 7% 3 7% 4 5% 6% 4 6% 9% 41%	5000
	21st to end	Night	11% 16% 5% 11% 6% 5% 5% 4 6% 7% 25%	5489
April	1st to 10th	Night	9% 13% 3 9% 5% 5% 4 5% 5% 7% 35%	1973
	11th to 20th	Night	12% 10% 3 6% 6% 4 5% 3 6% 8% 36%	1781
	21st to end	Night	12% 9% 3 8% 6% 4 5% 3 5% 8% 38%	1800
May	1st to 10th	Night	21% 11% 3 6% 5% 4 4 3 4 5% 35%	2697
	11th to 20th	Night	9% 6% 1 6% 5% 5% 6% 5% 7% 11% 39%	2402
	21st to end	Night	15% 7% 3 9% 6% 6% 6% 4 6% 8% 30%	2414
June	1st to 10th	Night	16% 6% 2 6% 3 5% 5% 4 6% 8% 38%	1330
	11th to 20th	Night	12% 4 2 6% 5% 5% 5% 5% 8% 8% 40%	1330
	21st to end	Night	7% 4 2 4 4 4 4 4 6% 8% 54%	1330
July	1st to 10th	Night	6% 3 1 5% 4 4 4 4 5% 8% 57%	2220
	11th to 20th	Night	5% 2 1 3 3 3 4 2 7% 11% 58%	2220
	21st to end	Night	3 1 1 2 2 2 4 3 5% 8% 69%	2413
August	1st to 10th	Night	8% 4 2 4 3 3 3 4 7% 9% 52%	2706
	11th to 20th	Night	5% 3 2 6% 3 4 5% 4 7% 12% 50%	2811
	21st to end	Night	10% 3 1 3 3 3 3 3 5% 7% 59%	3146
September	1st to 10th	Night	9% 4 2 5% 3 4 3 4 6% 8% 52%	1843
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 7% 49%	2043
	21st to end	Night	8% 5% 1 5% 4 4 4 4 6% 7% 53%	2090
October	1st to 10th	Night	9% 5% 2 5% 3 4 4 4 5% 7% 52%	3790
	11th to 20th	Night	6% 5% 2 6% 4 4 4 4 6% 9% 50%	3778
	21st to end	Night	7% 8% 3 6% 4 4 4 4 5% 6% 47%	4447
November	1st to 10th	Night	12% 12% 3 7% 6% 4 4 3 4 7% 36%	2730
	11th to 20th	Night	7% 10% 4 9% 6% 5% 4 4 6% 7% 39%	2800
	21st to end	Night	6% 12% 4 9% 5% 5% 5% 3 6% 8% 37%	2855
December	1st to 10th	Night	6% 14% 4 10% 6% 6% 6% 4 5% 6% 32%	4890
	11th to 20th	Night	6% 11% 5% 11% 7% 6% 5% 4 6% 8% 30%	5546
	21st to end	Night	4 9% 3 9% 4 4 5% 4 6% 7% 46%	7458

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	55% 8% 15% 18% 3	3179
	11th to 20th	Day	48% 8% 14% 16% 13% 1	3180
	21st to end	Day	38% 7% 22% 26% 6%	3847
February	1st to 10th	Day	55% 8% 14% 15% 8%	1780
	11th to 20th	Day	51% 10% 15% 17% 7%	1764
	21st to end	Day	49% 9% 13% 19% 8% 1	1485
March	1st to 10th	Day	38% 8% 16% 25% 13%	4730
	11th to 20th	Day	38% 9% 17% 25% 10%	5000
	21st to end	Day	50% 10% 20% 15% 5%	5486
April	1st to 10th	Day	44% 9% 16% 22% 9%	2423
	11th to 20th	Day	50% 6% 12% 23% 8% 1	2570
	21st to end	Day	45% 7% 14% 24% 9% 1	2600
May	1st to 10th	Day	51% 8% 12% 22% 7% 1	4491
	11th to 20th	Day	34% 12% 16% 27% 9% 1	4505
	21st to end	Day	42% 12% 14% 25% 7%	5162
June	1st to 10th	Day	39% 10% 17% 22% 10% 2	2850
	11th to 20th	Day	44% 9% 16% 19% 10% 1	2850
	21st to end	Day	28% 8% 19% 29% 13% 3	2850
July	1st to 10th	Day	26% 9% 17% 25% 19% 4	4666
	11th to 20th	Day	21% 7% 15% 32% 20% 5%	4659
	21st to end	Day	13% 4 10% 29% 29% 16%	4976
August	1st to 10th	Day	26% 7% 12% 32% 21% 3	4209
	11th to 20th	Day	24% 8% 16% 38% 13% 2	4067
	21st to end	Day	19% 5% 14% 39% 19% 2	4520
September	1st to 10th	Day	29% 5% 14% 26% 18% 8%	2336
	11th to 20th	Day	34% 8% 13% 29% 13% 3	2133
	21st to end	Day	31% 6% 16% 30% 15% 3	2090
October	1st to 10th	Day	31% 7% 15% 30% 14% 2	3180
	11th to 20th	Day	28% 6% 17% 33% 15% 1	3150
	21st to end	Day	35% 5% 11% 37% 12%	3076
November	1st to 10th	Day	52% 9% 13% 17% 7% 1	1568
	11th to 20th	Day	43% 9% 19% 22% 8%	1600
	21st to end	Day	47% 8% 18% 18% 7% 1	1544
December	1st to 10th	Day	49% 7% 22% 18% 3 1	2290
	11th to 20th	Day	51% 8% 20% 18% 3	2644
	21st to end	Day	39% 6% 15% 25% 13% 2	3498
January	1st to 10th	Night	48% 8% 20% 19% 5% 1	6766
	11th to 20th	Night	48% 7% 15% 15% 14% 1	6780
	21st to end	Night	40% 8% 19% 26% 6% 1	7154
February	1st to 10th	Night	49% 8% 14% 17% 10% 1	2619
	11th to 20th	Night	48% 9% 15% 18% 9% 1	2533
	21st to end	Night	45% 9% 15% 21% 9% 2	2145
March	1st to 10th	Night	39% 9% 21% 20% 11%	5161
	11th to 20th	Night	36% 8% 18% 30% 9%	5000
	21st to end	Night	54% 11% 17% 13% 4	5489
April	1st to 10th	Night	46% 9% 13% 22% 9% 1	1973
	11th to 20th	Night	45% 7% 14% 21% 11% 2	1781
	21st to end	Night	44% 7% 16% 23% 9% 1	1800
May	1st to 10th	Night	51% 7% 10% 23% 9% 1	2697
	11th to 20th	Night	31% 14% 17% 27% 11% 1	2403
	21st to end	Night	44% 14% 16% 20% 6% 1	2414
June	1st to 10th	Night	42% 7% 18% 22% 10% 1	1330
	11th to 20th	Night	38% 10% 14% 24% 12% 2	1330
	21st to end	Night	26% 7% 19% 31% 14% 2	1330
July	1st to 10th	Night	24% 7% 17% 27% 22% 3	2185
	11th to 20th	Night	19% 7% 14% 34% 20% 6%	2178
	21st to end	Night	13% 5% 9% 30% 29% 14%	2337
August	1st to 10th	Night	25% 9% 14% 30% 19% 3	2699
	11th to 20th	Night	25% 8% 18% 31% 16% 3	2838
	21st to end	Night	24% 5% 13% 34% 20% 3	3146
September	1st to 10th	Night	28% 8% 12% 25% 18% 9%	1843
	11th to 20th	Night	33% 7% 13% 27% 15% 5%	2043
	21st to end	Night	29% 6% 15% 30% 17% 3	2090
October	1st to 10th	Night	30% 6% 12% 34% 17% 2	3790
	11th to 20th	Night	30% 6% 15% 31% 16% 2	3778
	21st to end	Night	35% 6% 12% 35% 11%	4447
November	1st to 10th	Night	48% 8% 15% 19% 9% 1	2730
	11th to 20th	Night	43% 8% 19% 19% 10%	2800
	21st to end	Night	44% 7% 23% 17% 8% 1	2855
December	1st to 10th	Night	51% 8% 20% 16% 5%	4890
	11th to 20th	Night	50% 9% 25% 14% 2	5547
	21st to end	Night	36% 6% 14% 26% 15% 3	7458

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	8% 12% 35% 30% 14% 3	200
	11th to 20th	Day	11% 11% 40% 28% 10% 1	200
	21st to end	Day	10% 11% 45% 24% 7% 2	220
February	1st to 10th	Day	21% 13% 36% 23% 6% 1	290
	11th to 20th	Day	10% 9% 37% 33% 10% 1	300
	21st to end	Day	8% 10% 37% 32% 11% 2	249
March	1st to 10th	Day	12% 14% 34% 26% 11% 3	497
	11th to 20th	Day	11% 12% 39% 26% 9% 3	500
	21st to end	Day	13% 13% 35% 23% 13% 2	550
April	1st to 10th	Day	12% 17% 33% 29% 7% 2	500
	11th to 20th	Day	15% 15% 35% 26% 9% 1	492
	21st to end	Day	16% 11% 26% 31% 14% 2	500
May	1st to 10th	Day	15% 10% 24% 30% 16% 4	650
	11th to 20th	Day	13% 7% 26% 32% 19% 4	676
	21st to end	Day	19% 7% 20% 26% 22% 6%	734
June	1st to 10th	Day	14% 12% 23% 32% 16% 2	680
	11th to 20th	Day	18% 13% 30% 25% 14% 1	680
	21st to end	Day	10% 6% 25% 34% 19% 5%	680
July	1st to 10th	Day	5% 4 24% 37% 24% 5%	671
	11th to 20th	Day	8% 5% 25% 39% 20% 4	668
	21st to end	Day	5% 6% 21% 34% 28% 6%	739
August	1st to 10th	Day	6% 4 22% 38% 25% 6%	530
	11th to 20th	Day	6% 3 22% 37% 25% 6%	477
	21st to end	Day	7% 7% 26% 32% 23% 5%	539
September	1st to 10th	Day	11% 9% 26% 30% 23% 1	490
	11th to 20th	Day	14% 7% 26% 32% 17% 3	490
	21st to end	Day	13% 11% 33% 29% 11% 3	490
October	1st to 10th	Day	15% 7% 32% 28% 16% 3	310
	11th to 20th	Day	9% 14% 33% 29% 14% 1	300
	21st to end	Day	17% 16% 33% 22% 10% 1	330
November	1st to 10th	Day	23% 10% 38% 20% 9%	300
	11th to 20th	Day	11% 22% 39% 23% 6%	300
	21st to end	Day	12% 12% 40% 24% 8% 5%	272
December	1st to 10th	Day	13% 14% 43% 24% 7% 1	200
	11th to 20th	Day	13% 14% 42% 26% 6% 1	200
	21st to end	Day	14% 10% 34% 32% 9% 2	220
January	1st to 10th	Night	11% 13% 42% 26% 7% 1	1000
	11th to 20th	Night	12% 13% 39% 28% 7% 1	1000
	21st to end	Night	12% 12% 40% 26% 9% 1	965
February	1st to 10th	Night	13% 12% 32% 28% 12% 2	800
	11th to 20th	Night	10% 11% 37% 32% 9%	784
	21st to end	Night	9% 8% 34% 34% 12% 2	660
March	1st to 10th	Night	9% 10% 37% 33% 10% 2	800
	11th to 20th	Night	11% 12% 37% 30% 9% 1	800
	21st to end	Night	12% 11% 34% 32% 10% 1	880
April	1st to 10th	Night	14% 11% 34% 29% 10% 2	641
	11th to 20th	Night	16% 7% 38% 29% 10% 1	593
	21st to end	Night	16% 10% 27% 36% 11%	600
May	1st to 10th	Night	16% 8% 23% 36% 15% 1	600
	11th to 20th	Night	11% 5% 28% 36% 19% 1	497
	21st to end	Night	20% 7% 24% 32% 16% 2	411
June	1st to 10th	Night	18% 9% 26% 31% 14% 1	380
	11th to 20th	Night	15% 9% 31% 34% 10%	380
	21st to end	Night	7% 6% 28% 43% 17% 1	380
July	1st to 10th	Night	6% 5% 23% 47% 19%	370
	11th to 20th	Night	7% 4 30% 42% 17%	368
	21st to end	Night	4 5% 25% 43% 22% 1	416
August	1st to 10th	Night	8% 4 23% 40% 24% 1	524
	11th to 20th	Night	7% 3 22% 43% 25% 1	564
	21st to end	Night	9% 4 27% 38% 21%	627
September	1st to 10th	Night	10% 4 31% 41% 14% 1	570
	11th to 20th	Night	15% 4 25% 41% 14%	713
	21st to end	Night	11% 6% 34% 35% 12% 2	760
October	1st to 10th	Night	15% 7% 29% 36% 12% 1	760
	11th to 20th	Night	14% 9% 35% 32% 9% 2	760
	21st to end	Night	14% 14% 35% 29% 7%	849
November	1st to 10th	Night	18% 11% 41% 24% 6%	975
	11th to 20th	Night	14% 16% 41% 24% 5% 1	1000
	21st to end	Night	11% 11% 44% 28% 6%	1000
December	1st to 10th	Night	13% 9% 41% 27% 10% 1	1000
	11th to 20th	Night	11% 13% 42% 26% 8% 1	1000
	21st to end	Night	14% 10% 40% 29% 6% 2	1100

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