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 2025-02-25.

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% 8% 3 8% 5% 5% 5% 4 6% 8% 41%	112554
	Night	5% 7% 3 9% 6% 5% 5% 4 6% 8% 41%	114880
Transparency	Day	39% 8% 16% 25% 11% 2	112289
	Night	39% 7% 16% 25% 12% 2	114734
Seeing	Day	11% 12% 32% 28% 14% 3	16283
	Night	10% 11% 35% 32% 10% 1	25963

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% 12% 5% 13% 8% 8% 8% 6% 6% 6% 24%	3179
	11th to 20th	Day	2 6% 4 12% 7% 7% 9% 9% 9% 8% 26%	3617
	21st to end	Day	4 8% 4 10% 6% 5% 5% 4 6% 8% 42%	4004
February	1st to 10th	Day	7% 11% 5% 13% 7% 6% 6% 4 7% 8% 25%	1746
	11th to 20th	Day	5% 11% 4 13% 7% 8% 5% 4 7% 7% 29%	1755
	21st to end	Day	4 11% 4 12% 8% 7% 6% 5% 7% 8% 28%	1545
March	1st to 10th	Day	5% 11% 6% 13% 7% 5% 4 3 5% 7% 33%	3130
	11th to 20th	Day	6% 10% 4 8% 5% 4 5% 4 6% 9% 40%	3426
	21st to end	Day	8% 15% 7% 14% 7% 4 5% 4 6% 8% 23%	4038
April	1st to 10th	Day	8% 9% 4 7% 4 5% 5% 5% 7% 9% 37%	2280
	11th to 20th	Day	11% 13% 4 10% 6% 5% 5% 5% 6% 8% 27%	2251
	21st to end	Day	13% 9% 4 8% 5% 4 4 4 5% 7% 37%	2598
May	1st to 10th	Day	16% 11% 3 7% 4 5% 5% 4 7% 6% 31%	4430
	11th to 20th	Day	9% 7% 3 7% 6% 5% 5% 5% 8% 9% 36%	4388
	21st to end	Day	14% 7% 2 7% 5% 5% 6% 5% 7% 9% 34%	4825
June	1st to 10th	Day	14% 5% 2 6% 5% 4 5% 5% 7% 7% 39%	2737
	11th to 20th	Day	13% 7% 2 7% 6% 5% 5% 4 6% 8% 36%	2850
	21st to end	Day	6% 6% 2 5% 5% 5% 5% 4 7% 8% 46%	2850
July	1st to 10th	Day	3 3 1 6% 6% 6% 7% 5% 7% 9% 46%	4740
	11th to 20th	Day	3 2 1 4 4 4 5% 4 7% 9% 58%	4568
	21st to end	Day	3 2 1 4 3 3 4 4 6% 7% 63%	4802
August	1st to 10th	Day	8% 4 2 4 3 4 4 3 6% 7% 54%	4411
	11th to 20th	Day	4 3 2 5% 4 4 5% 4 6% 10% 53%	4164
	21st to end	Day	7% 4 2 5% 3 3 4 3 5% 6% 58%	4287
September	1st to 10th	Day	9% 6% 2 5% 4 4 4 4 5% 6% 51%	2280
	11th to 20th	Day	11% 7% 2 6% 3 3 4 4 6% 7% 47%	2280
	21st to end	Day	12% 7% 2 5% 3 4 3 3 5% 7% 47%	2034
October	1st to 10th	Day	8% 6% 3 5% 3 3 3 2 5% 7% 55%	3170
	11th to 20th	Day	6% 5% 2 5% 4 3 4 4 6% 10% 50%	3129
	21st to end	Day	3 9% 3 7% 4 5% 5% 3 7% 9% 46%	3091
November	1st to 10th	Day	9% 13% 4 8% 5% 4 4 4 6% 6% 38%	1628
	11th to 20th	Day	3 10% 4 11% 7% 5% 4 4 6% 7% 37%	1600
	21st to end	Day	7% 9% 4 9% 7% 5% 6% 4 7% 8% 35%	1600
December	1st to 10th	Day	3 9% 3 13% 8% 8% 6% 5% 6% 7% 31%	2620
	11th to 20th	Day	6% 12% 5% 13% 7% 5% 6% 5% 7% 7% 26%	3003
	21st to end	Day	4 7% 3 8% 7% 5% 5% 4 7% 9% 42%	3498
January	1st to 10th	Night	2 9% 4 14% 8% 6% 6% 5% 6% 8% 30%	6766
	11th to 20th	Night	2 6% 4 13% 8% 7% 7% 7% 8% 9% 28%	6358
	21st to end	Night	2 8% 4 12% 7% 5% 5% 4 6% 8% 40%	6974
February	1st to 10th	Night	5% 13% 4 10% 6% 6% 6% 6% 8% 7% 29%	2670
	11th to 20th	Night	4 9% 4 11% 7% 6% 5% 5% 8% 10% 31%	2532
	21st to end	Night	3 11% 4 9% 6% 5% 6% 5% 8% 7% 37%	2003
March	1st to 10th	Night	4 9% 4 10% 8% 6% 5% 4 5% 9% 36%	3734
	11th to 20th	Night	3 7% 3 9% 5% 5% 5% 4 6% 11% 40%	3535
	21st to end	Night	9% 13% 6% 16% 7% 5% 5% 4 6% 6% 24%	3607
April	1st to 10th	Night	6% 9% 2 8% 5% 5% 5% 4 7% 7% 42%	1900
	11th to 20th	Night	11% 10% 3 8% 4 4 5% 4 7% 8% 35%	1877
	21st to end	Night	9% 9% 2 7% 5% 5% 5% 4 6% 8% 41%	1577
May	1st to 10th	Night	15% 10% 2 6% 4 4 4 4 6% 7% 37%	2540
	11th to 20th	Night	9% 6% 2 6% 5% 5% 5% 5% 7% 10% 41%	2517
	21st to end	Night	12% 7% 3 6% 5% 4 5% 4 6% 9% 40%	2762
June	1st to 10th	Night	11% 7% 2 5% 4 4 3 4 6% 9% 47%	1439
	11th to 20th	Night	10% 4 2 6% 6% 4 4 4 7% 7% 47%	1330
	21st to end	Night	6% 4 1 4 3 4 4 3 6% 9% 56%	1330
July	1st to 10th	Night	5% 3 1 3 3 4 4 4 9% 10% 55%	2220
	11th to 20th	Night	4 4 2 5% 4 4 4 4 5% 9% 56%	2396
	21st to end	Night	1 2 1 4 3 3 4 3 6% 9% 64%	2761
August	1st to 10th	Night	7% 5% 2 4 3 3 3 2 4 7% 59%	2528
	11th to 20th	Night	4 3 1 4 3 3 4 4 7% 10% 58%	2673
	21st to end	Night	7% 3 1 5% 3 4 4 4 5% 8% 57%	3380
September	1st to 10th	Night	8% 4 1 4 4 3 4 3 4 7% 58%	1900
	11th to 20th	Night	9% 5% 1 5% 3 3 4 4 5% 10% 51%	1900
	21st to end	Night	9% 6% 2 6% 3 4 3 3 5% 7% 52%	2142
October	1st to 10th	Night	8% 4 2 6% 4 4 4 3 5% 8% 54%	3800
	11th to 20th	Night	5% 4 2 5% 3 3 4 3 6% 10% 54%	3778
	21st to end	Night	5% 7% 3 6% 4 5% 5% 4 7% 8% 44%	4457
November	1st to 10th	Night	10% 11% 4 7% 5% 5% 4 4 6% 9% 36%	2668
	11th to 20th	Night	5% 9% 4 10% 7% 6% 5% 4 7% 8% 35%	2800
	21st to end	Night	5% 9% 5% 10% 6% 6% 4 4 7% 8% 35%	2800
December	1st to 10th	Night	5% 12% 5% 14% 7% 7% 6% 3 5% 8% 28%	4570
	11th to 20th	Night	4 12% 5% 13% 7% 7% 6% 5% 6% 8% 26%	5198
	21st to end	Night	3 8% 3 9% 7% 4 4 3 5% 7% 47%	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	58% 8% 13% 16% 5%	3179
	11th to 20th	Day	48% 9% 15% 16% 11%	3617
	21st to end	Day	40% 7% 24% 26% 4	4004
February	1st to 10th	Day	55% 6% 11% 17% 11%	1746
	11th to 20th	Day	51% 8% 12% 14% 13% 1	1764
	21st to end	Day	50% 9% 16% 14% 10% 1	1545
March	1st to 10th	Day	50% 6% 13% 26% 4	3130
	11th to 20th	Day	39% 9% 23% 22% 7%	3426
	21st to end	Day	57% 10% 15% 13% 6%	4038
April	1st to 10th	Day	41% 8% 18% 22% 10% 1	2280
	11th to 20th	Day	52% 8% 15% 17% 8%	2254
	21st to end	Day	47% 6% 16% 23% 9%	2598
May	1st to 10th	Day	47% 10% 16% 20% 7% 1	4430
	11th to 20th	Day	38% 10% 15% 26% 10%	4392
	21st to end	Day	42% 12% 17% 23% 7%	4825
June	1st to 10th	Day	40% 9% 17% 23% 9% 1	2737
	11th to 20th	Day	43% 8% 17% 21% 11% 1	2850
	21st to end	Day	33% 10% 17% 30% 10% 1	2850
July	1st to 10th	Day	29% 10% 19% 23% 17% 2	4666
	11th to 20th	Day	20% 7% 14% 33% 22% 3	4488
	21st to end	Day	19% 6% 8% 32% 24% 10%	4651
August	1st to 10th	Day	28% 6% 13% 32% 20% 2	4396
	11th to 20th	Day	25% 6% 18% 34% 15% 2	4206
	21st to end	Day	25% 5% 13% 37% 17% 2	4286
September	1st to 10th	Day	33% 6% 12% 27% 17% 4	2280
	11th to 20th	Day	35% 7% 15% 26% 13% 3	2280
	21st to end	Day	35% 5% 13% 27% 17% 3	2034
October	1st to 10th	Day	31% 4 10% 34% 17% 4	3170
	11th to 20th	Day	29% 4 14% 38% 13% 2	3129
	21st to end	Day	35% 6% 16% 33% 10%	3091
November	1st to 10th	Day	47% 6% 14% 21% 9% 2	1628
	11th to 20th	Day	44% 8% 19% 20% 8% 1	1600
	21st to end	Day	46% 8% 21% 19% 5% 1	1600
December	1st to 10th	Day	50% 8% 21% 17% 4	2620
	11th to 20th	Day	54% 9% 22% 12% 3	3001
	21st to end	Day	38% 6% 10% 28% 12% 6%	3498
January	1st to 10th	Night	49% 8% 17% 18% 7% 1	6766
	11th to 20th	Night	48% 7% 15% 16% 13% 1	6358
	21st to end	Night	41% 8% 21% 26% 4	6974
February	1st to 10th	Night	49% 8% 15% 18% 9% 1	2670
	11th to 20th	Night	46% 9% 15% 17% 12% 2	2533
	21st to end	Night	43% 8% 14% 22% 13% 1	2003
March	1st to 10th	Night	43% 7% 17% 23% 9%	3734
	11th to 20th	Night	36% 9% 24% 24% 7% 1	3535
	21st to end	Night	58% 8% 14% 14% 5%	3607
April	1st to 10th	Night	38% 8% 17% 25% 12% 1	1900
	11th to 20th	Night	45% 8% 14% 20% 12% 1	1877
	21st to end	Night	40% 8% 14% 28% 11%	1577
May	1st to 10th	Night	42% 9% 13% 27% 10% 1	2540
	11th to 20th	Night	33% 11% 16% 28% 12%	2518
	21st to end	Night	36% 10% 17% 29% 7% 1	2762
June	1st to 10th	Night	34% 7% 18% 25% 13% 3	1439
	11th to 20th	Night	35% 7% 17% 25% 14% 2	1330
	21st to end	Night	25% 9% 20% 31% 13% 2	1330
July	1st to 10th	Night	21% 10% 26% 27% 14% 2	2185
	11th to 20th	Night	25% 5% 15% 28% 24% 3	2353
	21st to end	Night	17% 4 12% 35% 23% 9%	2674
August	1st to 10th	Night	26% 4 13% 33% 22% 2	2520
	11th to 20th	Night	21% 5% 15% 40% 16% 2	2697
	21st to end	Night	26% 5% 13% 35% 18% 3	3380
September	1st to 10th	Night	26% 5% 12% 31% 21% 6%	1900
	11th to 20th	Night	29% 8% 15% 26% 20% 3	1900
	21st to end	Night	32% 5% 13% 29% 19% 2	2142
October	1st to 10th	Night	30% 5% 10% 32% 21% 3	3800
	11th to 20th	Night	26% 5% 13% 41% 15% 1	3778
	21st to end	Night	36% 5% 11% 35% 12% 1	4457
November	1st to 10th	Night	46% 8% 15% 21% 9% 2	2668
	11th to 20th	Night	45% 8% 18% 20% 8% 1	2800
	21st to end	Night	46% 8% 20% 19% 8%	2800
December	1st to 10th	Night	55% 6% 19% 14% 5% 1	4570
	11th to 20th	Night	54% 9% 18% 14% 4	5199
	21st to end	Night	38% 5% 12% 24% 17% 5%	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	9% 18% 35% 26% 11% 2	200
	11th to 20th	Day	8% 15% 41% 23% 10% 1	295
	21st to end	Day	9% 17% 38% 26% 8%	330
February	1st to 10th	Day	14% 16% 41% 24% 4 1	300
	11th to 20th	Day	8% 10% 44% 28% 9% 1	300
	21st to end	Day	8% 10% 40% 29% 12% 2	237
March	1st to 10th	Day	13% 14% 35% 30% 7% 1	270
	11th to 20th	Day	11% 15% 37% 23% 12% 2	422
	21st to end	Day	12% 13% 37% 23% 12% 3	506
April	1st to 10th	Day	10% 17% 39% 28% 5%	460
	11th to 20th	Day	16% 18% 36% 23% 6% 1	452
	21st to end	Day	18% 13% 27% 30% 12% 1	598
May	1st to 10th	Day	15% 7% 25% 33% 17% 3	653
	11th to 20th	Day	13% 10% 24% 31% 16% 6%	675
	21st to end	Day	16% 10% 27% 26% 16% 5%	734
June	1st to 10th	Day	14% 14% 25% 28% 16% 3	680
	11th to 20th	Day	16% 15% 34% 23% 10% 3	680
	21st to end	Day	8% 11% 26% 32% 21% 3	680
July	1st to 10th	Day	5% 7% 26% 37% 21% 4	671
	11th to 20th	Day	6% 7% 31% 35% 17% 4	668
	21st to end	Day	5% 6% 27% 32% 24% 5%	739
August	1st to 10th	Day	9% 7% 24% 33% 20% 6%	676
	11th to 20th	Day	10% 8% 21% 33% 22% 6%	564
	21st to end	Day	7% 11% 26% 36% 17% 4	539
September	1st to 10th	Day	9% 10% 33% 32% 12% 3	490
	11th to 20th	Day	14% 11% 29% 30% 15% 1	490
	21st to end	Day	18% 12% 31% 25% 10% 4	329
October	1st to 10th	Day	19% 12% 26% 25% 16% 2	300
	11th to 20th	Day	10% 14% 38% 22% 14% 3	300
	21st to end	Day	11% 17% 44% 18% 8% 2	330
November	1st to 10th	Day	16% 16% 36% 24% 8% 1	300
	11th to 20th	Day	14% 16% 40% 21% 8% 1	300
	21st to end	Day	10% 16% 40% 25% 8%	300
December	1st to 10th	Day	9% 18% 44% 21% 6% 3	300
	11th to 20th	Day	10% 19% 44% 18% 7% 1	295
	21st to end	Day	9% 9% 42% 28% 8% 4	220
January	1st to 10th	Night	8% 15% 43% 26% 8% 1	1000
	11th to 20th	Night	9% 18% 40% 26% 6% 1	1000
	21st to end	Night	7% 17% 43% 24% 7% 1	1100
February	1st to 10th	Night	10% 14% 36% 29% 11% 1	870
	11th to 20th	Night	9% 12% 44% 28% 8%	784
	21st to end	Night	6% 8% 36% 32% 14% 3	646
March	1st to 10th	Night	9% 12% 38% 30% 8% 2	760
	11th to 20th	Night	10% 12% 39% 29% 9% 1	760
	21st to end	Night	11% 9% 31% 37% 11% 1	705
April	1st to 10th	Night	10% 12% 33% 37% 8% 1	570
	11th to 20th	Night	16% 9% 37% 30% 7% 1	563
	21st to end	Night	12% 9% 34% 36% 8%	570
May	1st to 10th	Night	11% 8% 26% 39% 14% 1	570
	11th to 20th	Night	11% 7% 28% 35% 17% 2	561
	21st to end	Night	16% 8% 26% 33% 16% 1	617
June	1st to 10th	Night	13% 8% 29% 35% 14% 2	489
	11th to 20th	Night	14% 7% 29% 37% 13%	380
	21st to end	Night	9% 7% 28% 38% 17% 2	380
July	1st to 10th	Night	7% 4 24% 46% 18% 1	370
	11th to 20th	Night	6% 6% 29% 46% 12%	465
	21st to end	Night	3 4 30% 46% 16% 2	624
August	1st to 10th	Night	5% 5% 24% 50% 14% 2	567
	11th to 20th	Night	6% 4 25% 44% 19% 2	564
	21st to end	Night	10% 5% 26% 41% 17% 1	627
September	1st to 10th	Night	9% 5% 24% 44% 16% 2	570
	11th to 20th	Night	11% 5% 28% 39% 16% 1	570
	21st to end	Night	12% 8% 31% 39% 10% 1	651
October	1st to 10th	Night	15% 9% 29% 32% 15% 1	760
	11th to 20th	Night	11% 11% 34% 31% 11% 2	760
	21st to end	Night	12% 14% 41% 27% 6%	1035
November	1st to 10th	Night	15% 13% 39% 27% 5% 1	975
	11th to 20th	Night	9% 16% 43% 24% 8% 1	1000
	21st to end	Night	9% 16% 44% 24% 6%	1000
December	1st to 10th	Night	9% 14% 40% 26% 9% 1	1000
	11th to 20th	Night	9% 17% 42% 26% 6% 1	1000
	21st to end	Night	11% 12% 41% 27% 9% 1	1100

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