Rockglen
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Rockglen clear sky chart from 2005-11-06 to 2026-07-01.

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	7% 7% 3 8% 5% 5% 5% 4 6% 8% 41%	147201
	Night	6% 7% 3 9% 6% 5% 5% 4 6% 8% 41%	142549
Transparency	Day	39% 7% 15% 25% 12% 2	141799
	Night	39% 7% 15% 24% 13% 1	138974
Seeing	Day	12% 12% 32% 28% 14% 3	18220
	Night	10% 11% 35% 32% 10% 1	27684

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	4 11% 4 12% 8% 8% 7% 6% 6% 7% 26%	4124
	11th to 20th	Day	1 5% 3 13% 8% 8% 9% 9% 9% 8% 26%	4717
	21st to end	Day	3 8% 4 11% 7% 6% 5% 5% 7% 7% 36%	5188
February	1st to 10th	Day	7% 11% 5% 12% 8% 6% 6% 4 7% 9% 26%	1834
	11th to 20th	Day	5% 11% 4 12% 7% 8% 5% 4 7% 7% 30%	1845
	21st to end	Day	4 11% 4 11% 8% 6% 6% 5% 7% 8% 30%	1662
March	1st to 10th	Day	4 9% 5% 14% 6% 5% 4 3 5% 7% 37%	4600
	11th to 20th	Day	5% 9% 4 9% 5% 5% 6% 5% 7% 10% 36%	5016
	21st to end	Day	7% 14% 6% 15% 7% 5% 5% 4 7% 8% 23%	5918
April	1st to 10th	Day	8% 10% 4 7% 4 5% 5% 5% 7% 9% 37%	2520
	11th to 20th	Day	11% 12% 4 10% 6% 4 5% 5% 6% 8% 28%	2491
	21st to end	Day	12% 9% 4 9% 5% 4 4 4 5% 7% 37%	2873
May	1st to 10th	Day	10% 8% 2 6% 4 5% 5% 4 7% 7% 41%	8224
	11th to 20th	Day	14% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	8191
	21st to end	Day	11% 5% 2 6% 4 5% 5% 4 6% 8% 43%	9026
June	1st to 10th	Day	13% 5% 3 6% 5% 5% 5% 5% 7% 7% 39%	3025
	11th to 20th	Day	13% 7% 2 7% 6% 5% 5% 4 7% 9% 35%	3150
	21st to end	Day	7% 6% 2 5% 5% 5% 5% 5% 7% 8% 45%	3150
July	1st to 10th	Day	3 3 1 6% 5% 5% 6% 4 7% 9% 49%	6780
	11th to 20th	Day	5% 4 2 5% 4 4 6% 5% 8% 10% 47%	6543
	21st to end	Day	5% 4 2 5% 4 4 4 4 6% 8% 54%	6903
August	1st to 10th	Day	10% 6% 3 6% 4 4 4 4 6% 7% 47%	6232
	11th to 20th	Day	3 3 2 5% 4 4 5% 4 6% 10% 55%	5990
	21st to end	Day	6% 3 2 4 3 3 4 3 5% 6% 62%	6143
September	1st to 10th	Day	9% 6% 2 5% 4 4 4 4 5% 6% 51%	2400
	11th to 20th	Day	11% 7% 2 6% 4 4 4 4 7% 7% 46%	2400
	21st to end	Day	12% 7% 2 5% 3 4 3 3 5% 7% 49%	2140
October	1st to 10th	Day	8% 6% 2 5% 3 3 3 2 5% 7% 55%	3270
	11th to 20th	Day	7% 6% 2 5% 4 3 4 4 6% 10% 49%	3228
	21st to end	Day	3 9% 3 6% 4 5% 5% 3 7% 9% 46%	3190
November	1st to 10th	Day	9% 13% 4 8% 6% 4 4 4 6% 6% 37%	1711
	11th to 20th	Day	4 10% 4 11% 7% 6% 4 4 6% 7% 37%	1680
	21st to end	Day	7% 9% 4 9% 7% 5% 6% 4 7% 8% 33%	1680
December	1st to 10th	Day	3 9% 3 13% 8% 8% 7% 5% 6% 7% 31%	2700
	11th to 20th	Day	6% 12% 5% 13% 6% 5% 6% 5% 7% 7% 27%	3082
	21st to end	Day	4 7% 3 8% 7% 5% 5% 4 7% 9% 42%	3575
January	1st to 10th	Night	2 9% 4 13% 9% 7% 6% 5% 6% 8% 31%	8768
	11th to 20th	Night	2 5% 4 13% 8% 6% 7% 7% 8% 10% 30%	8268
	21st to end	Night	2 9% 4 11% 8% 5% 6% 4 7% 9% 36%	9026
February	1st to 10th	Night	4 13% 4 9% 6% 6% 6% 6% 8% 8% 30%	2803
	11th to 20th	Night	4 9% 4 11% 7% 6% 5% 5% 8% 10% 32%	2662
	21st to end	Night	3 10% 4 9% 6% 5% 6% 5% 8% 8% 37%	2144
March	1st to 10th	Night	5% 8% 3 9% 6% 5% 4 4 6% 10% 40%	5494
	11th to 20th	Night	4 7% 3 10% 6% 5% 6% 5% 6% 10% 38%	5183
	21st to end	Night	8% 12% 7% 16% 7% 6% 5% 4 6% 7% 23%	5284
April	1st to 10th	Night	7% 9% 3 8% 5% 6% 5% 4 6% 7% 40%	2100
	11th to 20th	Night	11% 9% 3 8% 5% 4 5% 4 7% 8% 35%	2077
	21st to end	Night	8% 9% 2 7% 6% 5% 5% 4 6% 7% 41%	1742
May	1st to 10th	Night	8% 6% 2 5% 4 4 5% 5% 7% 9% 46%	4723
	11th to 20th	Night	14% 9% 2 7% 5% 5% 4 4 6% 8% 36%	4701
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 45%	5181
June	1st to 10th	Night	10% 7% 2 5% 4 4 3 4 6% 9% 46%	1590
	11th to 20th	Night	10% 4 2 7% 6% 4 4 4 6% 8% 44%	1470
	21st to end	Night	7% 5% 1 4 3 3 4 3 6% 9% 54%	1470
July	1st to 10th	Night	6% 3 1 4 3 4 4 4 8% 9% 53%	3180
	11th to 20th	Night	8% 5% 2 5% 4 3 4 3 5% 9% 51%	3426
	21st to end	Night	5% 3 1 4 3 4 4 4 6% 8% 57%	3970
August	1st to 10th	Night	9% 6% 2 5% 4 4 4 3 5% 8% 49%	3580
	11th to 20th	Night	3 2 1 4 3 4 4 4 7% 10% 57%	3843
	21st to end	Night	7% 3 1 5% 3 4 4 3 5% 7% 58%	4834
September	1st to 10th	Night	7% 4 1 4 4 3 4 3 4 7% 58%	2000
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 5% 10% 51%	2000
	21st to end	Night	9% 5% 2 5% 3 4 3 3 5% 6% 54%	2256
October	1st to 10th	Night	8% 4 2 6% 4 3 4 3 5% 8% 54%	3920
	11th to 20th	Night	5% 4 2 5% 3 3 4 3 6% 10% 53%	3898
	21st to end	Night	5% 7% 3 6% 4 5% 5% 4 7% 8% 45%	4600
November	1st to 10th	Night	10% 11% 4 7% 5% 5% 4 4 6% 8% 36%	2805
	11th to 20th	Night	4 9% 4 10% 7% 6% 5% 4 7% 8% 35%	2940
	21st to end	Night	5% 9% 5% 10% 6% 6% 4 4 7% 8% 34%	2940
December	1st to 10th	Night	5% 12% 5% 14% 7% 7% 6% 3 5% 8% 28%	4710
	11th to 20th	Night	4 12% 6% 13% 7% 7% 6% 5% 6% 8% 26%	5338
	21st to end	Night	3 8% 3 10% 7% 4 4 3 5% 7% 47%	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	54% 7% 13% 18% 7%	4125
	11th to 20th	Day	49% 8% 17% 16% 9% 1	4717
	21st to end	Day	44% 7% 20% 23% 6%	5188
February	1st to 10th	Day	54% 6% 11% 18% 11%	1834
	11th to 20th	Day	51% 8% 12% 15% 13% 1	1854
	21st to end	Day	49% 8% 16% 17% 10% 1	1662
March	1st to 10th	Day	47% 6% 15% 24% 8%	4600
	11th to 20th	Day	41% 8% 18% 25% 8%	5016
	21st to end	Day	57% 9% 14% 14% 6%	5918
April	1st to 10th	Day	42% 7% 17% 23% 10% 1	2520
	11th to 20th	Day	52% 7% 16% 18% 7%	2494
	21st to end	Day	47% 5% 15% 23% 9%	2873
May	1st to 10th	Day	38% 7% 11% 28% 15% 1	8224
	11th to 20th	Day	47% 7% 13% 21% 12%	8195
	21st to end	Day	36% 8% 12% 28% 16% 1	9027
June	1st to 10th	Day	40% 9% 16% 23% 10% 2	3025
	11th to 20th	Day	43% 8% 16% 21% 10% 1	3150
	21st to end	Day	34% 10% 17% 30% 10% 1	3150
July	1st to 10th	Day	28% 9% 19% 24% 18% 3	6707
	11th to 20th	Day	30% 8% 16% 29% 16% 2	6463
	21st to end	Day	27% 7% 11% 29% 19% 7%	6705
August	1st to 10th	Day	29% 6% 13% 31% 20% 2	4536
	11th to 20th	Day	25% 6% 18% 34% 15% 2	4340
	21st to end	Day	25% 5% 13% 37% 17% 2	4422
September	1st to 10th	Day	33% 6% 12% 28% 17% 4	2400
	11th to 20th	Day	36% 7% 16% 26% 12% 3	2400
	21st to end	Day	34% 5% 12% 28% 18% 3	2140
October	1st to 10th	Day	31% 4 10% 34% 17% 4	3270
	11th to 20th	Day	30% 4 14% 37% 13% 2	3228
	21st to end	Day	35% 6% 16% 34% 10%	3190
November	1st to 10th	Day	47% 6% 14% 22% 9% 2	1711
	11th to 20th	Day	44% 8% 19% 21% 7% 1	1680
	21st to end	Day	47% 8% 21% 19% 5% 1	1680
December	1st to 10th	Day	50% 8% 21% 17% 4	2700
	11th to 20th	Day	53% 9% 22% 12% 3	3080
	21st to end	Day	38% 6% 10% 28% 12% 6%	3575
January	1st to 10th	Night	49% 7% 15% 19% 10%	8769
	11th to 20th	Night	46% 6% 17% 17% 12% 1	8268
	21st to end	Night	44% 7% 19% 21% 9%	9026
February	1st to 10th	Night	48% 8% 15% 19% 9% 1	2803
	11th to 20th	Night	45% 8% 14% 18% 13% 2	2663
	21st to end	Night	41% 7% 15% 24% 12% 1	2144
March	1st to 10th	Night	38% 6% 18% 24% 13%	5494
	11th to 20th	Night	40% 8% 19% 25% 9%	5183
	21st to end	Night	59% 7% 13% 15% 6%	5284
April	1st to 10th	Night	39% 7% 16% 24% 12% 1	2100
	11th to 20th	Night	44% 8% 14% 21% 12% 1	2077
	21st to end	Night	40% 7% 13% 28% 11%	1742
May	1st to 10th	Night	32% 6% 12% 31% 19% 1	4724
	11th to 20th	Night	44% 8% 13% 21% 13% 1	4702
	21st to end	Night	33% 8% 13% 31% 14% 2	5182
June	1st to 10th	Night	34% 7% 17% 25% 14% 3	1590
	11th to 20th	Night	37% 7% 16% 25% 13% 2	1470
	21st to end	Night	26% 8% 20% 31% 12% 2	1470
July	1st to 10th	Night	25% 8% 23% 26% 17% 2	3145
	11th to 20th	Night	30% 5% 18% 26% 18% 2	3383
	21st to end	Night	24% 5% 15% 32% 18% 6%	3856
August	1st to 10th	Night	27% 5% 13% 32% 21% 2	2600
	11th to 20th	Night	21% 5% 16% 40% 16% 2	2782
	21st to end	Night	26% 5% 13% 35% 18% 3	3486
September	1st to 10th	Night	26% 5% 11% 32% 20% 6%	2000
	11th to 20th	Night	29% 8% 16% 26% 19% 3	2000
	21st to end	Night	30% 5% 13% 30% 21% 2	2256
October	1st to 10th	Night	30% 5% 10% 32% 21% 2	3920
	11th to 20th	Night	27% 5% 13% 40% 15% 1	3898
	21st to end	Night	36% 5% 11% 35% 12% 1	4600
November	1st to 10th	Night	46% 8% 14% 20% 10% 2	2805
	11th to 20th	Night	45% 8% 18% 20% 8% 1	2940
	21st to end	Night	46% 7% 20% 19% 8%	2940
December	1st to 10th	Night	56% 6% 19% 14% 5% 1	4710
	11th to 20th	Night	54% 9% 18% 14% 4	5339
	21st to end	Night	38% 5% 12% 24% 17% 5%	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	8% 20% 36% 25% 10% 1	220
	11th to 20th	Day	11% 16% 41% 22% 9% 1	325
	21st to end	Day	10% 17% 38% 26% 9%	363
February	1st to 10th	Day	13% 17% 42% 24% 4 1	330
	11th to 20th	Day	9% 10% 42% 28% 9% 1	330
	21st to end	Day	8% 12% 41% 27% 10% 1	273
March	1st to 10th	Day	13% 14% 35% 30% 7% 1	330
	11th to 20th	Day	11% 16% 38% 22% 10% 2	498
	21st to end	Day	12% 14% 38% 22% 11% 3	594
April	1st to 10th	Day	11% 16% 39% 27% 6%	540
	11th to 20th	Day	16% 18% 38% 22% 6% 1	532
	21st to end	Day	17% 14% 26% 30% 12% 2	693
May	1st to 10th	Day	13% 8% 26% 33% 16% 3	753
	11th to 20th	Day	16% 11% 25% 28% 14% 5%	775
	21st to end	Day	15% 9% 28% 27% 16% 5%	844
June	1st to 10th	Day	15% 15% 27% 26% 15% 3	780
	11th to 20th	Day	16% 16% 34% 23% 9% 2	780
	21st to end	Day	9% 10% 27% 32% 19% 3	780
July	1st to 10th	Day	5% 7% 27% 36% 20% 4	722
	11th to 20th	Day	7% 8% 30% 35% 17% 4	718
	21st to end	Day	6% 6% 26% 32% 24% 5%	794
August	1st to 10th	Day	9% 8% 24% 33% 21% 6%	726
	11th to 20th	Day	10% 8% 22% 33% 22% 5%	608
	21st to end	Day	7% 10% 26% 35% 18% 4	583
September	1st to 10th	Day	9% 10% 32% 32% 14% 3	530
	11th to 20th	Day	13% 11% 29% 29% 16% 1	530
	21st to end	Day	17% 11% 29% 28% 11% 4	360
October	1st to 10th	Day	18% 12% 28% 26% 15% 2	330
	11th to 20th	Day	14% 13% 37% 20% 13% 2	330
	21st to end	Day	11% 17% 44% 19% 8% 1	363
November	1st to 10th	Day	17% 15% 38% 22% 7% 1	330
	11th to 20th	Day	13% 15% 38% 23% 9% 1	330
	21st to end	Day	10% 16% 40% 25% 9%	330
December	1st to 10th	Day	9% 18% 45% 20% 5% 2	330
	11th to 20th	Day	12% 18% 44% 18% 7% 1	324
	21st to end	Day	9% 8% 45% 27% 7% 4	242
January	1st to 10th	Night	7% 15% 43% 26% 8%	1050
	11th to 20th	Night	9% 19% 40% 25% 6% 1	1050
	21st to end	Night	7% 18% 42% 24% 7% 1	1155
February	1st to 10th	Night	10% 14% 36% 30% 11% 1	913
	11th to 20th	Night	8% 12% 44% 28% 8%	824
	21st to end	Night	6% 10% 36% 32% 13% 3	692
March	1st to 10th	Night	9% 13% 38% 30% 8% 2	840
	11th to 20th	Night	10% 11% 39% 30% 8% 1	840
	21st to end	Night	11% 10% 32% 36% 10% 1	779
April	1st to 10th	Night	10% 12% 34% 36% 8% 1	630
	11th to 20th	Night	16% 9% 38% 30% 7%	623
	21st to end	Night	12% 10% 34% 36% 8%	630
May	1st to 10th	Night	10% 8% 26% 41% 13% 1	630
	11th to 20th	Night	13% 9% 27% 34% 16% 1	621
	21st to end	Night	15% 8% 27% 33% 16% 1	683
June	1st to 10th	Night	13% 8% 30% 34% 13% 2	540
	11th to 20th	Night	14% 6% 29% 39% 12%	420
	21st to end	Night	9% 7% 29% 38% 15% 1	420
July	1st to 10th	Night	7% 4 23% 47% 18% 1	390
	11th to 20th	Night	7% 7% 29% 46% 12%	490
	21st to end	Night	3 4 29% 46% 16% 2	657
August	1st to 10th	Night	5% 5% 24% 50% 14% 2	597
	11th to 20th	Night	6% 4 26% 45% 18% 2	594
	21st to end	Night	10% 5% 26% 41% 17% 1	660
September	1st to 10th	Night	9% 5% 24% 45% 16% 2	600
	11th to 20th	Night	11% 5% 27% 40% 16% 1	600
	21st to end	Night	11% 7% 30% 40% 10% 1	686
October	1st to 10th	Night	15% 9% 28% 34% 15% 1	800
	11th to 20th	Night	12% 11% 35% 31% 10% 2	800
	21st to end	Night	11% 14% 41% 27% 6%	1090
November	1st to 10th	Night	15% 14% 39% 26% 5% 1	1025
	11th to 20th	Night	8% 16% 43% 24% 8% 1	1050
	21st to end	Night	9% 16% 43% 25% 6%	1050
December	1st to 10th	Night	9% 14% 41% 26% 9% 1	1050
	11th to 20th	Night	10% 17% 42% 25% 6%	1050
	21st to end	Night	10% 12% 41% 27% 8% 1	1155

Page generated at 2026-06-28 00:10:46. Version ac07

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