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 2024-11-02.

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	9% 8% 3 7% 4 4 5% 4 6% 7% 44%	101830
	Night	8% 8% 3 8% 5% 4 5% 4 6% 8% 41%	97298
Transparency	Day	37% 8% 16% 25% 12% 2	101550
	Night	39% 8% 17% 23% 11% 2	97168
Seeing	Day	12% 9% 29% 30% 16% 3	15540
	Night	12% 9% 34% 32% 11% 1	24720

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% 13% 4 12% 5% 5% 5% 3 6% 9% 32%	2219
	11th to 20th	Day	6% 8% 4 13% 5% 6% 9% 7% 5% 6% 31%	2220
	21st to end	Day	5% 9% 4 9% 5% 5% 5% 3 5% 5% 46%	2679
February	1st to 10th	Day	9% 14% 6% 9% 7% 7% 5% 4 5% 6% 29%	1691
	11th to 20th	Day	7% 14% 5% 10% 6% 6% 7% 5% 7% 8% 25%	1665
	21st to end	Day	5% 12% 5% 11% 7% 7% 6% 5% 5% 6% 33%	1413
March	1st to 10th	Day	4 10% 4 10% 7% 6% 4 4 5% 8% 38%	3285
	11th to 20th	Day	8% 8% 3 7% 5% 4 4 3 5% 9% 44%	3490
	21st to end	Day	12% 15% 5% 12% 4 4 4 4 6% 7% 28%	3825
April	1st to 10th	Day	9% 11% 3 7% 7% 6% 6% 4 7% 8% 34%	2302
	11th to 20th	Day	10% 13% 4 8% 5% 5% 6% 4 5% 7% 33%	2437
	21st to end	Day	12% 10% 3 8% 6% 5% 4 4 5% 7% 36%	2470
May	1st to 10th	Day	20% 12% 3 7% 4 4 4 4 5% 6% 31%	4353
	11th to 20th	Day	11% 6% 2 6% 5% 4 6% 6% 7% 8% 38%	4499
	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	11% 8% 2 6% 4 3 5% 3 6% 8% 44%	1910
	11th to 20th	Day	8% 7% 3 7% 5% 5% 5% 4 6% 6% 44%	1900
	21st to end	Day	10% 12% 3 6% 4 4 4 3 5% 7% 42%	1869
November	1st to 10th	Day	14% 12% 3 8% 6% 4 5% 4 5% 5% 32%	1496
	11th to 20th	Day	6% 11% 4 8% 7% 6% 4 3 6% 6% 39%	1520
	21st to end	Day	6% 13% 3 8% 6% 6% 5% 4 5% 9% 35%	1467
December	1st to 10th	Day	7% 15% 4 9% 6% 6% 5% 4 5% 7% 32%	1330
	11th to 20th	Day	6% 9% 5% 11% 7% 5% 6% 5% 5% 7% 34%	1685
	21st to end	Day	3 10% 3 7% 5% 3 4 4 5% 7% 49%	2442
January	1st to 10th	Night	4 10% 3 11% 6% 5% 5% 5% 7% 9% 35%	4726
	11th to 20th	Night	5% 9% 5% 10% 7% 5% 7% 5% 6% 8% 32%	4740
	21st to end	Night	5% 9% 4 10% 5% 5% 5% 3 6% 8% 41%	5008
February	1st to 10th	Night	9% 13% 4 9% 7% 5% 5% 4 6% 6% 33%	2489
	11th to 20th	Night	7% 13% 4 9% 7% 6% 5% 5% 6% 7% 31%	2402
	21st to end	Night	5% 11% 5% 10% 7% 5% 6% 4 6% 7% 34%	2041
March	1st to 10th	Night	5% 9% 5% 12% 7% 6% 5% 4 6% 7% 34%	3586
	11th to 20th	Night	10% 8% 2 6% 4 4 4 4 5% 8% 44%	3490
	21st to end	Night	11% 16% 5% 11% 6% 4 5% 5% 6% 8% 23%	3828
April	1st to 10th	Night	9% 13% 3 9% 5% 5% 4 5% 6% 7% 34%	1875
	11th to 20th	Night	12% 11% 3 7% 6% 4 5% 3 6% 8% 35%	1691
	21st to end	Night	12% 9% 3 8% 6% 4 5% 3 6% 8% 37%	1710
May	1st to 10th	Night	21% 11% 3 6% 5% 4 4 3 4 5% 34%	2615
	11th to 20th	Night	9% 6% 1 6% 5% 5% 6% 6% 7% 11% 39%	2398
	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	12% 7% 2 6% 4 4 5% 4 5% 8% 46%	2270
	11th to 20th	Night	9% 7% 2 7% 4 4 4 4 6% 8% 44%	2280
	21st to end	Night	10% 10% 3 7% 5% 4 5% 4 6% 7% 39%	2706
November	1st to 10th	Night	12% 12% 3 7% 6% 4 4 3 5% 7% 36%	2605
	11th to 20th	Night	7% 10% 3 9% 6% 5% 4 4 6% 7% 40%	2660
	21st to end	Night	6% 11% 4 8% 5% 5% 5% 3 6% 8% 39%	2712
December	1st to 10th	Night	7% 13% 4 10% 6% 5% 5% 4 6% 6% 34%	2850
	11th to 20th	Night	5% 9% 4 10% 6% 5% 5% 4 6% 8% 37%	3506
	21st to end	Night	5% 8% 3 6% 4 4 5% 4 6% 7% 49%	5214

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	46% 11% 19% 20% 4 1	2219
	11th to 20th	Day	50% 9% 12% 15% 13% 1	2220
	21st to end	Day	38% 9% 24% 25% 4	2679
February	1st to 10th	Day	54% 8% 15% 16% 7%	1691
	11th to 20th	Day	52% 10% 15% 17% 7%	1674
	21st to end	Day	50% 9% 12% 18% 8% 1	1413
March	1st to 10th	Day	41% 10% 16% 26% 7%	3285
	11th to 20th	Day	35% 10% 21% 25% 8%	3490
	21st to end	Day	52% 10% 18% 15% 5%	3825
April	1st to 10th	Day	45% 9% 16% 21% 9%	2302
	11th to 20th	Day	50% 7% 12% 23% 7% 1	2440
	21st to end	Day	46% 7% 14% 24% 9% 1	2470
May	1st to 10th	Day	52% 8% 13% 22% 6%	4353
	11th to 20th	Day	34% 12% 16% 27% 9% 1	4503
	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	37% 8% 18% 24% 11% 1	1910
	11th to 20th	Day	39% 7% 17% 25% 11% 1	1900
	21st to end	Day	42% 7% 15% 25% 11%	1869
November	1st to 10th	Day	52% 9% 14% 16% 8% 1	1496
	11th to 20th	Day	44% 8% 19% 21% 7%	1520
	21st to end	Day	46% 9% 18% 19% 7% 1	1467
December	1st to 10th	Day	50% 8% 18% 17% 6% 1	1330
	11th to 20th	Day	47% 10% 18% 21% 4 1	1684
	21st to end	Day	34% 6% 18% 24% 16% 2	2442
January	1st to 10th	Night	42% 10% 22% 21% 4 1	4726
	11th to 20th	Night	47% 9% 14% 17% 11% 2	4740
	21st to end	Night	40% 9% 20% 24% 6% 1	5008
February	1st to 10th	Night	49% 8% 15% 17% 10% 1	2489
	11th to 20th	Night	49% 9% 16% 17% 9% 1	2403
	21st to end	Night	46% 9% 14% 21% 9% 2	2041
March	1st to 10th	Night	44% 11% 18% 20% 7%	3586
	11th to 20th	Night	35% 8% 22% 26% 8%	3490
	21st to end	Night	55% 12% 16% 13% 3 1	3828
April	1st to 10th	Night	47% 9% 13% 22% 9%	1875
	11th to 20th	Night	46% 7% 15% 20% 11% 1	1691
	21st to end	Night	44% 7% 16% 23% 9% 1	1710
May	1st to 10th	Night	52% 7% 10% 22% 9% 1	2615
	11th to 20th	Night	31% 14% 17% 27% 11% 1	2399
	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	36% 8% 16% 25% 14% 2	2270
	11th to 20th	Night	36% 7% 15% 27% 12% 2	2280
	21st to end	Night	42% 8% 16% 25% 9%	2706
November	1st to 10th	Night	48% 8% 16% 19% 8% 1	2605
	11th to 20th	Night	43% 8% 19% 19% 10%	2660
	21st to end	Night	42% 8% 24% 18% 8% 1	2712
December	1st to 10th	Night	48% 9% 17% 17% 8% 1	2850
	11th to 20th	Night	42% 9% 25% 19% 3 1	3507
	21st to end	Night	34% 7% 16% 24% 16% 4	5214

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% 11% 34% 30% 14% 3	180
	11th to 20th	Day	11% 11% 38% 29% 11% 1	180
	21st to end	Day	10% 11% 43% 26% 8% 2	198
February	1st to 10th	Day	22% 12% 36% 23% 7% 1	261
	11th to 20th	Day	11% 9% 39% 29% 11% 1	270
	21st to end	Day	8% 10% 34% 34% 12% 2	225
March	1st to 10th	Day	12% 14% 36% 26% 11% 2	457
	11th to 20th	Day	11% 11% 38% 27% 10% 3	460
	21st to end	Day	14% 13% 36% 23% 14% 2	506
April	1st to 10th	Day	13% 16% 34% 29% 7% 1	460
	11th to 20th	Day	14% 14% 34% 28% 10% 1	452
	21st to end	Day	17% 12% 26% 30% 13% 2	460
May	1st to 10th	Day	16% 9% 24% 30% 17% 5%	602
	11th to 20th	Day	13% 7% 26% 32% 19% 4	675
	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	25% 10% 37% 20% 9%	273
	11th to 20th	Day	11% 19% 40% 24% 6%	270
	21st to end	Day	10% 11% 42% 23% 8% 5%	245
December	1st to 10th	Day	12% 13% 43% 24% 8% 1	180
	11th to 20th	Day	12% 14% 43% 24% 6% 1	180
	21st to end	Day	15% 9% 33% 32% 10% 2	198
January	1st to 10th	Night	11% 13% 42% 26% 7% 1	950
	11th to 20th	Night	12% 12% 39% 28% 8% 1	950
	21st to end	Night	12% 12% 39% 27% 9% 1	917
February	1st to 10th	Night	13% 12% 31% 29% 12% 2	760
	11th to 20th	Night	10% 11% 38% 31% 9%	744
	21st to end	Night	9% 8% 32% 36% 13% 2	628
March	1st to 10th	Night	8% 10% 38% 33% 10% 2	760
	11th to 20th	Night	11% 12% 36% 30% 9% 1	760
	21st to end	Night	12% 11% 34% 32% 10% 1	836
April	1st to 10th	Night	15% 11% 35% 28% 10% 1	609
	11th to 20th	Night	17% 7% 37% 29% 10% 1	563
	21st to end	Night	17% 9% 27% 36% 10%	570
May	1st to 10th	Night	16% 8% 22% 36% 16% 1	570
	11th to 20th	Night	11% 4 28% 37% 19% 1	495
	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% 40% 24% 6%	931
	11th to 20th	Night	14% 15% 41% 24% 5% 1	950
	21st to end	Night	11% 11% 44% 27% 7%	950
December	1st to 10th	Night	13% 9% 40% 27% 10% 1	950
	11th to 20th	Night	10% 13% 42% 27% 8% 1	950
	21st to end	Night	14% 9% 40% 29% 6% 2	1045

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