Starlight Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starlight Ridge Observatory clear sky chart from 2005-11-06 to 2026-04-02.

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% 7% 2 6% 4 4 4 3 5% 7% 49%	139869
	Night	8% 7% 2 7% 4 4 4 4 5% 7% 48%	139369
Transparency	Day	35% 5% 13% 26% 18% 3	134632
	Night	35% 5% 13% 26% 18% 3	135618
Seeing	Day	13% 10% 33% 30% 13% 2	17712
	Night	12% 10% 38% 32% 8%	27806

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	7% 15% 5% 10% 6% 4 3 2 3 5% 40%	4720
	11th to 20th	Day	4 10% 4 11% 6% 7% 7% 4 6% 6% 36%	4740
	21st to end	Day	4 6% 4 12% 5% 4 5% 3 5% 6% 47%	5315
February	1st to 10th	Day	9% 10% 3 7% 5% 4 5% 4 5% 8% 40%	1916
	11th to 20th	Day	6% 10% 3 9% 4 4 3 3 4 6% 47%	2049
	21st to end	Day	8% 8% 2 7% 4 4 4 3 5% 5% 50%	1730
March	1st to 10th	Day	9% 8% 2 6% 3 3 3 2 3 5% 54%	4600
	11th to 20th	Day	6% 7% 2 4 3 3 3 3 5% 7% 56%	5040
	21st to end	Day	16% 11% 4 7% 4 4 4 3 5% 7% 35%	5621
April	1st to 10th	Day	14% 7% 3 5% 4 4 4 3 6% 7% 44%	2413
	11th to 20th	Day	15% 9% 2 6% 5% 3 4 3 5% 7% 42%	2466
	21st to end	Day	16% 7% 2 6% 4 4 4 3 6% 7% 42%	2600
May	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 6% 8% 49%	5900
	11th to 20th	Day	13% 6% 2 5% 4 4 4 3 5% 6% 47%	5862
	21st to end	Day	20% 7% 3 6% 4 3 3 3 4 6% 41%	6764
June	1st to 10th	Day	9% 4 2 5% 3 4 4 4 6% 8% 51%	2800
	11th to 20th	Day	11% 4 2 5% 3 4 4 3 6% 8% 50%	2800
	21st to end	Day	5% 4 1 5% 4 5% 3 4 7% 8% 55%	2800
July	1st to 10th	Day	8% 4 2 6% 4 4 4 4 6% 8% 50%	6339
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 6% 7% 51%	6340
	21st to end	Day	6% 4 2 5% 3 4 5% 4 7% 9% 52%	6817
August	1st to 10th	Day	7% 5% 2 5% 4 4 4 4 6% 9% 49%	5801
	11th to 20th	Day	8% 5% 3 6% 3 4 4 4 5% 7% 51%	5821
	21st to end	Day	10% 4 2 5% 4 3 4 3 5% 8% 51%	6178
September	1st to 10th	Day	8% 4 2 4 3 2 2 3 4 7% 61%	2350
	11th to 20th	Day	7% 4 2 5% 3 3 4 3 5% 7% 59%	2200
	21st to end	Day	10% 4 1 4 2 2 3 3 4 6% 60%	2200
October	1st to 10th	Day	8% 3 1 2 2 2 2 2 3 4 72%	3590
	11th to 20th	Day	8% 5% 1 4 3 2 3 3 4 8% 59%	3171
	21st to end	Day	8% 6% 3 5% 4 5% 4 4 6% 6% 49%	3190
November	1st to 10th	Day	7% 6% 3 6% 3 4 3 4 5% 7% 52%	1853
	11th to 20th	Day	5% 9% 2 7% 4 3 3 3 4 6% 55%	1890
	21st to end	Day	7% 10% 3 5% 3 3 4 3 5% 7% 49%	1890
December	1st to 10th	Day	4 8% 4 9% 6% 5% 4 3 6% 5% 47%	2905
	11th to 20th	Day	4 6% 3 8% 5% 5% 5% 4 6% 8% 48%	3106
	21st to end	Day	10% 11% 3 9% 4 3 3 3 4 5% 43%	4092
January	1st to 10th	Night	7% 10% 4 11% 8% 5% 4 3 5% 6% 37%	8201
	11th to 20th	Night	2 8% 4 10% 6% 6% 7% 5% 6% 7% 38%	8250
	21st to end	Night	2 6% 4 10% 5% 4 4 3 5% 5% 50%	8897
February	1st to 10th	Night	10% 10% 4 7% 5% 4 3 4 5% 7% 40%	2723
	11th to 20th	Night	5% 9% 3 9% 5% 4 5% 4 6% 7% 43%	2458
	21st to end	Night	7% 7% 2 6% 4 4 5% 3 5% 6% 50%	2076
March	1st to 10th	Night	9% 8% 2 6% 4 4 4 3 4 5% 50%	5494
	11th to 20th	Night	10% 6% 2 4 3 3 3 4 5% 7% 54%	5142
	21st to end	Night	14% 10% 4 8% 4 5% 4 4 6% 8% 35%	5549
April	1st to 10th	Night	13% 7% 2 6% 4 4 4 3 5% 8% 43%	2010
	11th to 20th	Night	13% 7% 2 6% 4 3 4 4 5% 7% 45%	1880
	21st to end	Night	16% 7% 3 6% 5% 3 4 4 6% 7% 41%	1800
May	1st to 10th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 50%	4080
	11th to 20th	Night	10% 6% 2 4 4 3 4 3 6% 7% 50%	4053
	21st to end	Night	18% 6% 2 6% 4 3 4 4 6% 7% 40%	4129
June	1st to 10th	Night	8% 4 1 4 2 3 4 3 5% 10% 54%	1600
	11th to 20th	Night	12% 4 3 4 4 3 4 4 5% 7% 50%	1600
	21st to end	Night	6% 4 1 4 4 5% 5% 4 7% 9% 52%	1600
July	1st to 10th	Night	6% 3 1 3 3 3 3 4 7% 10% 57%	3640
	11th to 20th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 48%	3640
	21st to end	Night	5% 4 1 6% 4 5% 5% 4 6% 9% 51%	4052
August	1st to 10th	Night	6% 4 2 6% 5% 5% 5% 5% 8% 11% 43%	4011
	11th to 20th	Night	7% 4 2 5% 4 5% 5% 4 7% 9% 48%	4028
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 49%	4796
September	1st to 10th	Night	9% 3 1 4 3 3 3 3 5% 5% 60%	2050
	11th to 20th	Night	7% 4 1 4 3 2 3 2 5% 8% 61%	2200
	21st to end	Night	9% 4 1 3 3 2 3 3 4 6% 61%	2200
October	1st to 10th	Night	8% 3 1 2 2 1 2 2 3 4 71%	3590
	11th to 20th	Night	10% 4 1 3 2 2 2 2 4 7% 63%	3934
	21st to end	Night	8% 6% 2 6% 4 4 3 3 5% 8% 52%	4610
November	1st to 10th	Night	8% 6% 2 5% 3 4 3 3 6% 7% 53%	2665
	11th to 20th	Night	6% 6% 2 6% 4 3 4 4 5% 7% 53%	2730
	21st to end	Night	6% 8% 3 6% 4 4 4 4 5% 7% 51%	2730
December	1st to 10th	Night	4 5% 3 6% 4 4 4 4 4 6% 56%	4502
	11th to 20th	Night	5% 8% 3 7% 4 3 4 4 5% 8% 48%	5321
	21st to end	Night	13% 10% 3 10% 4 3 3 3 4 6% 41%	7128

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	50% 2 10% 25% 12%	4723
	11th to 20th	Day	48% 5% 12% 19% 14% 2	4740
	21st to end	Day	39% 4 16% 24% 15% 2	5315
February	1st to 10th	Day	44% 5% 16% 20% 14% 2	1916
	11th to 20th	Day	39% 4 13% 23% 15% 5%	2059
	21st to end	Day	37% 4 14% 25% 16% 5%	1730
March	1st to 10th	Day	34% 3 7% 26% 26% 3	4600
	11th to 20th	Day	28% 4 10% 27% 29% 2	5040
	21st to end	Day	49% 5% 15% 20% 9% 1	5621
April	1st to 10th	Day	39% 5% 14% 25% 14% 2	2413
	11th to 20th	Day	42% 5% 12% 23% 16% 2	2469
	21st to end	Day	41% 5% 13% 27% 13%	2600
May	1st to 10th	Day	32% 6% 10% 27% 24% 1	5900
	11th to 20th	Day	37% 6% 10% 24% 19% 4	5866
	21st to end	Day	44% 5% 10% 26% 13% 1	6765
June	1st to 10th	Day	30% 8% 17% 30% 13% 2	2800
	11th to 20th	Day	31% 6% 16% 26% 17% 3	2800
	21st to end	Day	25% 7% 14% 31% 19% 4	2800
July	1st to 10th	Day	32% 5% 12% 29% 20% 2	6271
	11th to 20th	Day	32% 5% 11% 29% 19% 3	6265
	21st to end	Day	27% 8% 15% 25% 19% 5%	6621
August	1st to 10th	Day	31% 7% 16% 24% 18% 5%	4220
	11th to 20th	Day	37% 7% 12% 27% 15% 2	4210
	21st to end	Day	24% 6% 19% 24% 22% 6%	4453
September	1st to 10th	Day	24% 4 13% 28% 23% 8%	2350
	11th to 20th	Day	25% 5% 13% 31% 22% 4	2200
	21st to end	Day	26% 4 9% 31% 27% 3	2200
October	1st to 10th	Day	19% 2 7% 21% 38% 12%	3590
	11th to 20th	Day	25% 3 16% 31% 20% 4	3171
	21st to end	Day	35% 4 9% 32% 15% 4	3190
November	1st to 10th	Day	31% 5% 12% 28% 20% 4	1853
	11th to 20th	Day	33% 3 12% 27% 19% 6%	1890
	21st to end	Day	35% 4 14% 27% 17% 4	1890
December	1st to 10th	Day	40% 3 14% 27% 14% 2	2905
	11th to 20th	Day	34% 4 19% 24% 18% 1	3104
	21st to end	Day	44% 4 7% 33% 10% 1	4092
January	1st to 10th	Night	48% 3 11% 23% 13% 1	8202
	11th to 20th	Night	43% 5% 16% 22% 13% 1	8250
	21st to end	Night	36% 4 14% 26% 19% 1	8897
February	1st to 10th	Night	43% 5% 15% 22% 13% 2	2723
	11th to 20th	Night	39% 6% 12% 25% 14% 4	2458
	21st to end	Night	34% 4 12% 28% 16% 5%	2076
March	1st to 10th	Night	35% 4 9% 27% 22% 3	5494
	11th to 20th	Night	30% 4 10% 27% 27% 2	5142
	21st to end	Night	47% 7% 12% 24% 10% 1	5549
April	1st to 10th	Night	39% 6% 14% 23% 16% 3	2010
	11th to 20th	Night	39% 6% 10% 26% 17% 2	1880
	21st to end	Night	40% 8% 13% 25% 14% 1	1800
May	1st to 10th	Night	32% 6% 12% 25% 22% 3	4080
	11th to 20th	Night	31% 7% 13% 27% 18% 5%	4054
	21st to end	Night	42% 7% 14% 23% 13% 1	4130
June	1st to 10th	Night	26% 7% 18% 30% 17% 3	1600
	11th to 20th	Night	33% 7% 14% 26% 16% 4	1600
	21st to end	Night	26% 9% 16% 30% 17% 3	1600
July	1st to 10th	Night	22% 6% 21% 34% 16% 2	3600
	11th to 20th	Night	33% 6% 13% 29% 16% 4	3592
	21st to end	Night	29% 7% 18% 24% 19% 4	3937
August	1st to 10th	Night	30% 8% 19% 26% 12% 5%	2916
	11th to 20th	Night	35% 6% 15% 28% 14% 2	2915
	21st to end	Night	25% 7% 15% 23% 23% 8%	3452
September	1st to 10th	Night	25% 5% 12% 26% 25% 6%	2050
	11th to 20th	Night	23% 4 15% 31% 24% 4	2200
	21st to end	Night	25% 5% 7% 30% 29% 4	2200
October	1st to 10th	Night	20% 3 6% 18% 48% 5%	3590
	11th to 20th	Night	24% 3 12% 30% 25% 7%	3934
	21st to end	Night	32% 4 14% 26% 22% 3	4610
November	1st to 10th	Night	30% 5% 13% 26% 21% 6%	2665
	11th to 20th	Night	30% 4 12% 26% 21% 6%	2730
	21st to end	Night	33% 4 12% 29% 19% 3	2730
December	1st to 10th	Night	30% 4 17% 31% 17% 2	4502
	11th to 20th	Night	34% 4 15% 33% 13% 1	5322
	21st to end	Night	46% 3 10% 28% 12% 1	7128

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	14% 14% 32% 25% 12% 3	330
	11th to 20th	Day	12% 19% 37% 21% 9% 1	330
	21st to end	Day	16% 15% 36% 22% 8% 4	363
February	1st to 10th	Day	13% 15% 45% 22% 4	330
	11th to 20th	Day	13% 14% 43% 21% 8% 1	330
	21st to end	Day	15% 16% 48% 18% 4	273
March	1st to 10th	Day	15% 11% 37% 27% 8% 2	330
	11th to 20th	Day	13% 14% 37% 25% 9% 1	330
	21st to end	Day	21% 15% 32% 21% 9% 2	374
April	1st to 10th	Day	21% 16% 40% 20% 3 1	505
	11th to 20th	Day	24% 17% 34% 22% 4	587
	21st to end	Day	20% 16% 30% 25% 7% 2	700
May	1st to 10th	Day	16% 12% 31% 30% 10% 2	703
	11th to 20th	Day	20% 11% 28% 27% 12% 2	725
	21st to end	Day	18% 10% 31% 29% 11% 1	789
June	1st to 10th	Day	8% 8% 30% 31% 16% 6%	730
	11th to 20th	Day	11% 7% 35% 31% 14% 2	730
	21st to end	Day	7% 7% 31% 36% 16% 2	730
July	1st to 10th	Day	7% 3 27% 38% 19% 5%	721
	11th to 20th	Day	7% 4 30% 39% 18% 2	718
	21st to end	Day	6% 3 26% 45% 17% 3	794
August	1st to 10th	Day	7% 3 24% 42% 19% 4	726
	11th to 20th	Day	9% 4 27% 38% 19% 3	715
	21st to end	Day	5% 3 30% 38% 21% 3	673
September	1st to 10th	Day	6% 4 29% 39% 18% 4	480
	11th to 20th	Day	9% 5% 31% 28% 23% 3	330
	21st to end	Day	14% 10% 28% 29% 16% 3	330
October	1st to 10th	Day	16% 8% 25% 30% 16% 5%	330
	11th to 20th	Day	15% 16% 40% 21% 6% 2	330
	21st to end	Day	15% 18% 32% 24% 10% 2	363
November	1st to 10th	Day	16% 9% 38% 25% 8% 4	330
	11th to 20th	Day	14% 12% 44% 21% 9%	330
	21st to end	Day	16% 13% 39% 21% 10% 2	330
December	1st to 10th	Day	11% 15% 44% 22% 8% 2	330
	11th to 20th	Day	14% 14% 42% 21% 8% 1	330
	21st to end	Day	15% 17% 39% 21% 8% 1	363
January	1st to 10th	Night	10% 14% 42% 27% 6%	1050
	11th to 20th	Night	9% 18% 42% 26% 5%	1050
	21st to end	Night	9% 12% 45% 26% 7%	1155
February	1st to 10th	Night	15% 13% 42% 26% 4	1043
	11th to 20th	Night	9% 12% 46% 28% 5%	824
	21st to end	Night	10% 11% 45% 28% 5% 1	692
March	1st to 10th	Night	11% 12% 40% 33% 4	840
	11th to 20th	Night	16% 9% 41% 28% 5% 1	656
	21st to end	Night	17% 11% 38% 26% 7%	693
April	1st to 10th	Night	17% 13% 42% 26% 2	603
	11th to 20th	Night	18% 12% 41% 26% 3	593
	21st to end	Night	19% 9% 34% 30% 7% 1	600
May	1st to 10th	Night	15% 9% 35% 32% 9%	600
	11th to 20th	Night	19% 10% 31% 30% 9% 1	591
	21st to end	Night	16% 8% 35% 34% 6%	650
June	1st to 10th	Night	10% 5% 30% 39% 15% 2	600
	11th to 20th	Night	12% 5% 37% 38% 8%	600
	21st to end	Night	7% 4 31% 47% 11%	600
July	1st to 10th	Night	7% 3 30% 44% 16%	585
	11th to 20th	Night	5% 3 31% 48% 12%	582
	21st to end	Night	5% 4 29% 50% 11%	657
August	1st to 10th	Night	6% 2 27% 49% 15% 1	597
	11th to 20th	Night	9% 3 29% 45% 14%	594
	21st to end	Night	7% 4 32% 42% 14% 1	660
September	1st to 10th	Night	9% 6% 30% 44% 11%	600
	11th to 20th	Night	8% 5% 33% 42% 12% 1	600
	21st to end	Night	11% 8% 35% 32% 12% 1	600
October	1st to 10th	Night	17% 8% 32% 32% 12%	600
	11th to 20th	Night	14% 11% 37% 30% 8%	811
	21st to end	Night	14% 13% 35% 29% 8%	1100
November	1st to 10th	Night	11% 10% 42% 30% 7%	1025
	11th to 20th	Night	10% 13% 40% 30% 7%	1050
	21st to end	Night	10% 10% 41% 30% 7%	1050
December	1st to 10th	Night	10% 14% 45% 26% 6%	1050
	11th to 20th	Night	12% 11% 42% 29% 6%	1050
	21st to end	Night	17% 13% 37% 27% 6%	1155

Page generated at 2026-03-30 01:18:45. Version ac07

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