Rockridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rockridge Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	12% 9% 3 8% 5% 4 4 4 6% 7% 38%	126977
	Night	11% 9% 4 9% 5% 4 4 4 5% 6% 39%	122684
Transparency	Day	44% 5% 13% 26% 12% 1	130555
	Night	45% 4 12% 25% 13% 2	127182
Seeing	Day	17% 8% 31% 27% 14% 4	17308
	Night	16% 7% 32% 33% 11% 1	27099

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	7% 14% 6% 18% 9% 6% 5% 4 5% 5% 22%	3617
	11th to 20th	Day	5% 11% 6% 13% 8% 6% 8% 8% 11% 5% 19%	3727
	21st to end	Day	6% 16% 6% 14% 6% 4 5% 3 5% 6% 30%	4488
February	1st to 10th	Day	9% 12% 5% 10% 7% 6% 5% 5% 5% 6% 30%	1800
	11th to 20th	Day	9% 11% 4 10% 6% 6% 4 5% 5% 6% 33%	1754
	21st to end	Day	12% 11% 4 9% 6% 5% 5% 4 5% 7% 33%	1542
March	1st to 10th	Day	13% 8% 3 5% 4 3 4 3 5% 7% 45%	4864
	11th to 20th	Day	10% 11% 3 7% 4 4 4 4 5% 8% 41%	4980
	21st to end	Day	23% 11% 5% 9% 6% 4 4 3 4 6% 25%	5502
April	1st to 10th	Day	17% 11% 4 8% 4 4 4 3 5% 7% 34%	2400
	11th to 20th	Day	19% 13% 4 7% 4 4 4 3 5% 5% 32%	2386
	21st to end	Day	20% 12% 4 6% 5% 4 4 4 4 6% 30%	2600
May	1st to 10th	Day	13% 6% 2 5% 3 4 4 3 5% 6% 49%	6064
	11th to 20th	Day	15% 9% 3 5% 3 3 3 3 5% 7% 43%	6319
	21st to end	Day	17% 8% 3 7% 5% 5% 4 4 5% 7% 37%	6935
June	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 9% 46%	2800
	11th to 20th	Day	14% 6% 3 7% 4 5% 5% 4 6% 9% 37%	2800
	21st to end	Day	10% 6% 2 6% 5% 5% 6% 5% 6% 9% 39%	2800
July	1st to 10th	Day	8% 5% 2 6% 4 4 5% 4 8% 10% 44%	6360
	11th to 20th	Day	8% 4 1 4 3 3 4 3 6% 8% 56%	6360
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 7% 9% 48%	6772
August	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 5% 7% 8% 41%	4234
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 4 7% 8% 42%	4171
	21st to end	Day	9% 6% 3 7% 4 4 4 3 6% 7% 48%	4477
September	1st to 10th	Day	12% 5% 2 6% 4 4 4 4 5% 7% 48%	2400
	11th to 20th	Day	11% 6% 2 6% 3 4 4 4 6% 7% 47%	2357
	21st to end	Day	13% 6% 2 6% 4 4 4 3 6% 6% 46%	2110
October	1st to 10th	Day	15% 7% 2 7% 4 4 3 3 5% 6% 44%	1930
	11th to 20th	Day	14% 9% 2 6% 5% 4 4 3 4 7% 42%	1900
	21st to end	Day	18% 11% 4 6% 4 3 4 3 5% 6% 37%	2085
November	1st to 10th	Day	12% 10% 4 8% 5% 4 4 3 5% 6% 39%	1643
	11th to 20th	Day	11% 14% 7% 10% 6% 4 4 4 5% 6% 30%	1600
	21st to end	Day	12% 12% 4 8% 6% 4 4 3 5% 7% 37%	1600
December	1st to 10th	Day	7% 10% 4 12% 11% 7% 6% 5% 5% 6% 27%	2610
	11th to 20th	Day	11% 15% 5% 11% 8% 6% 5% 4 5% 6% 25%	3008
	21st to end	Day	19% 17% 8% 20% 5% 4 3 2 4 4 15%	3982
January	1st to 10th	Night	6% 12% 5% 16% 8% 7% 5% 4 5% 5% 27%	6348
	11th to 20th	Night	6% 10% 5% 13% 7% 7% 9% 8% 8% 5% 21%	6250
	21st to end	Night	7% 15% 7% 11% 6% 5% 4 3 5% 5% 33%	6490
February	1st to 10th	Night	8% 12% 5% 11% 6% 5% 5% 4 4 7% 34%	2600
	11th to 20th	Night	7% 10% 3 9% 6% 6% 5% 4 5% 7% 39%	2533
	21st to end	Night	11% 10% 4 9% 5% 5% 5% 4 4 6% 38%	2087
March	1st to 10th	Night	10% 6% 2 7% 4 3 4 3 5% 7% 48%	5019
	11th to 20th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 9% 42%	4980
	21st to end	Night	25% 12% 4 9% 4 3 3 3 4 5% 29%	5427
April	1st to 10th	Night	13% 12% 3 7% 4 4 5% 3 6% 7% 37%	2000
	11th to 20th	Night	19% 9% 4 10% 3 4 4 3 4 6% 35%	1960
	21st to end	Night	19% 9% 4 6% 6% 4 4 3 4 6% 36%	1800
May	1st to 10th	Night	11% 6% 2 6% 3 4 4 3 5% 5% 51%	3851
	11th to 20th	Night	14% 6% 2 4 3 3 3 3 5% 6% 51%	3596
	21st to end	Night	17% 8% 3 6% 4 4 4 3 4 6% 41%	3942
June	1st to 10th	Night	11% 5% 2 4 4 3 4 4 6% 9% 49%	1600
	11th to 20th	Night	9% 6% 2 6% 4 6% 5% 4 7% 7% 45%	1600
	21st to end	Night	8% 4 2 5% 3 4 4 4 6% 8% 53%	1600
July	1st to 10th	Night	8% 4 1 5% 4 4 5% 4 8% 10% 48%	3618
	11th to 20th	Night	5% 4 2 4 3 4 5% 4 5% 8% 56%	3617
	21st to end	Night	9% 4 2 5% 4 5% 4 3 6% 8% 50%	4082
August	1st to 10th	Night	12% 7% 2 5% 5% 4 5% 3 5% 6% 47%	2926
	11th to 20th	Night	13% 5% 2 5% 2 3 4 3 4 8% 51%	2888
	21st to end	Night	8% 7% 2 6% 4 4 4 3 5% 7% 50%	3426
September	1st to 10th	Night	10% 4 2 5% 3 3 4 3 5% 8% 54%	2000
	11th to 20th	Night	9% 5% 2 4 3 4 3 4 5% 8% 53%	2040
	21st to end	Night	11% 6% 2 5% 3 3 4 3 4 5% 53%	2269
October	1st to 10th	Night	13% 6% 2 6% 3 3 3 3 4 6% 51%	2317
	11th to 20th	Night	14% 9% 2 6% 4 4 4 3 5% 6% 43%	2280
	21st to end	Night	18% 11% 3 6% 4 3 3 3 4 6% 37%	2513
November	1st to 10th	Night	13% 9% 3 6% 4 4 4 4 4 6% 42%	2650
	11th to 20th	Night	12% 12% 5% 10% 5% 4 4 3 4 5% 35%	2800
	21st to end	Night	13% 10% 4 9% 6% 4 4 3 4 6% 37%	2800
December	1st to 10th	Night	6% 8% 4 14% 7% 7% 6% 4 6% 7% 32%	4580
	11th to 20th	Night	10% 14% 7% 14% 8% 5% 4 4 5% 5% 25%	5199
	21st to end	Night	21% 15% 7% 17% 5% 4 4 3 4 4 16%	6996

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	64% 4 10% 17% 5%	3618
	11th to 20th	Day	58% 5% 8% 17% 11%	3727
	21st to end	Day	56% 4 16% 16% 7%	4488
February	1st to 10th	Day	54% 4 14% 18% 9% 1	1800
	11th to 20th	Day	50% 6% 11% 24% 8% 1	1763
	21st to end	Day	51% 5% 13% 20% 12%	1542
March	1st to 10th	Day	39% 3 16% 28% 11% 2	4864
	11th to 20th	Day	41% 5% 13% 29% 11% 2	4980
	21st to end	Day	61% 4 9% 17% 8%	5504
April	1st to 10th	Day	50% 4 11% 22% 12% 1	2400
	11th to 20th	Day	55% 4 10% 18% 11% 2	2386
	21st to end	Day	55% 4 11% 20% 9% 1	2600
May	1st to 10th	Day	35% 5% 11% 30% 18% 2	6102
	11th to 20th	Day	40% 5% 9% 25% 17% 5%	6323
	21st to end	Day	46% 5% 9% 30% 10% 1	6949
June	1st to 10th	Day	34% 6% 13% 31% 14% 2	2800
	11th to 20th	Day	43% 6% 13% 23% 14% 1	2800
	21st to end	Day	38% 7% 16% 25% 11% 2	2800
July	1st to 10th	Day	34% 6% 18% 33% 9% 1	6291
	11th to 20th	Day	26% 5% 14% 37% 16% 1	6285
	21st to end	Day	31% 7% 14% 31% 16% 1	6579
August	1st to 10th	Day	39% 6% 11% 29% 13% 1	4220
	11th to 20th	Day	38% 6% 13% 33% 10%	4210
	21st to end	Day	35% 5% 16% 29% 14% 1	4476
September	1st to 10th	Day	36% 5% 14% 31% 14%	2400
	11th to 20th	Day	35% 5% 18% 32% 10%	2357
	21st to end	Day	38% 4 11% 33% 13% 1	2110
October	1st to 10th	Day	31% 3 7% 36% 23% 1	3190
	11th to 20th	Day	39% 4 11% 25% 20% 1	3140
	21st to end	Day	43% 3 14% 29% 10%	3411
November	1st to 10th	Day	46% 4 13% 20% 14% 3	1643
	11th to 20th	Day	54% 4 12% 19% 10% 1	1600
	21st to end	Day	48% 5% 17% 23% 7% 1	1600
December	1st to 10th	Day	57% 6% 16% 15% 6% 1	2610
	11th to 20th	Day	60% 4 15% 16% 5%	3005
	21st to end	Day	74% 4 8% 9% 4	3982
January	1st to 10th	Night	59% 4 10% 20% 7%	6348
	11th to 20th	Night	59% 5% 9% 15% 10%	6250
	21st to end	Night	54% 4 11% 22% 8%	6490
February	1st to 10th	Night	51% 3 13% 22% 10% 1	2600
	11th to 20th	Night	46% 5% 13% 26% 9% 2	2534
	21st to end	Night	48% 3 11% 23% 13% 2	2087
March	1st to 10th	Night	36% 4 12% 29% 16% 2	5019
	11th to 20th	Night	38% 5% 12% 29% 15% 1	4980
	21st to end	Night	59% 4 8% 18% 10% 1	5427
April	1st to 10th	Night	46% 4 9% 25% 14% 1	2000
	11th to 20th	Night	51% 3 8% 19% 16% 3	1960
	21st to end	Night	50% 4 8% 22% 14% 2	1800
May	1st to 10th	Night	34% 5% 10% 27% 21% 3	3878
	11th to 20th	Night	34% 4 10% 23% 21% 8%	3597
	21st to end	Night	43% 5% 10% 28% 12% 1	3950
June	1st to 10th	Night	31% 5% 13% 31% 18% 2	1600
	11th to 20th	Night	37% 4 15% 22% 18% 3	1600
	21st to end	Night	29% 5% 15% 31% 16% 3	1600
July	1st to 10th	Night	29% 6% 20% 30% 14% 1	3580
	11th to 20th	Night	25% 6% 14% 39% 14% 2	3572
	21st to end	Night	32% 6% 15% 29% 16% 2	3964
August	1st to 10th	Night	37% 5% 12% 28% 13% 4	2916
	11th to 20th	Night	33% 4 14% 33% 16% 1	2915
	21st to end	Night	33% 6% 17% 28% 14% 2	3426
September	1st to 10th	Night	28% 5% 13% 35% 19% 1	2000
	11th to 20th	Night	30% 5% 16% 32% 17% 1	2040
	21st to end	Night	33% 4 11% 31% 19% 2	2269
October	1st to 10th	Night	24% 2 6% 36% 28% 4	3837
	11th to 20th	Night	32% 5% 12% 25% 22% 4	3779
	21st to end	Night	43% 4 11% 29% 12% 1	4137
November	1st to 10th	Night	42% 5% 12% 24% 15% 2	2650
	11th to 20th	Night	52% 4 11% 20% 12% 1	2800
	21st to end	Night	49% 4 15% 23% 8% 1	2800
December	1st to 10th	Night	51% 5% 19% 20% 5%	4580
	11th to 20th	Night	61% 5% 11% 18% 5% 1	5201
	21st to end	Night	71% 4 8% 13% 4	6996

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	13% 10% 39% 27% 8% 3	300
	11th to 20th	Day	15% 12% 43% 24% 5%	300
	21st to end	Day	18% 10% 37% 24% 8% 3	330
February	1st to 10th	Day	16% 15% 44% 20% 4 1	300
	11th to 20th	Day	16% 15% 39% 23% 7% 1	300
	21st to end	Day	19% 12% 40% 26% 3	249
March	1st to 10th	Day	21% 11% 43% 18% 6%	300
	11th to 20th	Day	23% 11% 37% 21% 8% 1	300
	21st to end	Day	30% 10% 29% 21% 8% 1	345
April	1st to 10th	Day	24% 12% 36% 21% 6%	500
	11th to 20th	Day	29% 14% 25% 21% 9% 1	504
	21st to end	Day	28% 9% 30% 22% 9% 2	700
May	1st to 10th	Day	20% 10% 32% 26% 10% 2	703
	11th to 20th	Day	22% 8% 26% 26% 14% 3	725
	21st to end	Day	18% 7% 25% 32% 14% 3	789
June	1st to 10th	Day	11% 7% 27% 31% 19% 5%	730
	11th to 20th	Day	16% 5% 35% 30% 12% 3	730
	21st to end	Day	14% 7% 30% 29% 16% 4	730
July	1st to 10th	Day	10% 4 20% 32% 24% 10%	721
	11th to 20th	Day	9% 5% 26% 30% 21% 7%	718
	21st to end	Day	7% 3 23% 40% 21% 6%	794
August	1st to 10th	Day	12% 3 22% 32% 24% 7%	726
	11th to 20th	Day	14% 3 20% 33% 24% 6%	715
	21st to end	Day	11% 4 25% 35% 19% 7%	683
September	1st to 10th	Day	15% 6% 28% 29% 17% 5%	530
	11th to 20th	Day	14% 4 27% 31% 21% 4	487
	21st to end	Day	16% 8% 29% 22% 17% 8%	330
October	1st to 10th	Day	16% 7% 29% 27% 15% 6%	309
	11th to 20th	Day	17% 16% 37% 22% 7% 1	300
	21st to end	Day	25% 13% 35% 19% 7% 1	330
November	1st to 10th	Day	21% 11% 38% 20% 10% 1	300
	11th to 20th	Day	17% 16% 43% 20% 4	300
	21st to end	Day	21% 12% 43% 19% 5%	300
December	1st to 10th	Day	15% 10% 44% 25% 5% 1	300
	11th to 20th	Day	19% 13% 40% 23% 6%	300
	21st to end	Day	27% 10% 42% 16% 4	330
January	1st to 10th	Night	12% 10% 39% 31% 8% 1	1000
	11th to 20th	Night	17% 10% 38% 30% 6% 1	1000
	21st to end	Night	18% 9% 36% 31% 6%	1100
February	1st to 10th	Night	14% 11% 39% 29% 7% 1	1000
	11th to 20th	Night	13% 10% 42% 28% 6% 1	979
	21st to end	Night	16% 10% 37% 28% 8% 1	767
March	1st to 10th	Night	17% 9% 38% 27% 8% 1	652
	11th to 20th	Night	18% 9% 40% 27% 6% 1	600
	21st to end	Night	23% 9% 29% 26% 11% 2	660
April	1st to 10th	Night	20% 10% 34% 29% 7%	600
	11th to 20th	Night	23% 10% 30% 30% 7%	593
	21st to end	Night	23% 7% 33% 28% 9% 1	600
May	1st to 10th	Night	20% 6% 31% 33% 9% 1	600
	11th to 20th	Night	18% 7% 27% 36% 12% 1	591
	21st to end	Night	17% 5% 26% 39% 13% 1	650
June	1st to 10th	Night	13% 4 27% 40% 16%	600
	11th to 20th	Night	13% 3 31% 44% 9%	600
	21st to end	Night	10% 3 23% 47% 16% 1	600
July	1st to 10th	Night	6% 2 20% 43% 26% 3	585
	11th to 20th	Night	5% 3 22% 49% 19% 3	582
	21st to end	Night	6% 2 18% 54% 19% 2	657
August	1st to 10th	Night	9% 2 17% 47% 23% 2	597
	11th to 20th	Night	14% 2 15% 41% 26% 3	594
	21st to end	Night	9% 2 24% 38% 25% 2	660
September	1st to 10th	Night	12% 3 23% 41% 18% 4	600
	11th to 20th	Night	12% 4 30% 35% 16% 3	600
	21st to end	Night	14% 7% 29% 33% 16% 1	669
October	1st to 10th	Night	17% 7% 29% 32% 15% 1	773
	11th to 20th	Night	19% 9% 35% 30% 7% 1	760
	21st to end	Night	25% 9% 27% 31% 7%	836
November	1st to 10th	Night	17% 10% 34% 30% 7% 1	894
	11th to 20th	Night	18% 11% 38% 27% 6% 1	1000
	21st to end	Night	18% 11% 39% 25% 8%	1000
December	1st to 10th	Night	16% 8% 39% 29% 7% 1	1000
	11th to 20th	Night	19% 8% 38% 27% 6% 1	1000
	21st to end	Night	25% 10% 31% 28% 5%	1100

Page generated at 2025-10-01 00:39:05. Version ac07

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