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 2024-02-05.

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	8% 7% 3 7% 6% 4 5% 6% 6% 8% 40%	78108
	Night	6% 7% 3 7% 6% 4 5% 5% 6% 9% 42%	80770
Transparency	Day	40% 8% 16% 23% 11% 2	77800
	Night	39% 8% 16% 24% 12% 2	80599
Seeing	Day	11% 12% 32% 28% 14% 3	14840
	Night	10% 11% 35% 32% 11% 1	24537

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 9% 4 8% 9% 6% 9% 8% 6% 7% 30%	2219
	11th to 20th	Day	3 6% 3 8% 11% 6% 9% 12% 10% 9% 22%	2517
	21st to end	Day	4 9% 4 7% 7% 3 6% 5% 6% 9% 40%	2794
February	1st to 10th	Day	7% 9% 6% 12% 9% 5% 6% 6% 7% 9% 26%	1604
	11th to 20th	Day	5% 11% 5% 11% 9% 6% 6% 6% 6% 7% 26%	1575
	21st to end	Day	5% 9% 5% 11% 9% 5% 6% 7% 7% 7% 28%	1422
March	1st to 10th	Day	7% 11% 6% 12% 9% 5% 6% 6% 7% 9% 23%	1800
	11th to 20th	Day	6% 10% 5% 8% 7% 5% 5% 5% 6% 8% 36%	1944
	21st to end	Day	7% 11% 5% 9% 7% 4 4 6% 7% 10% 30%	2301
April	1st to 10th	Day	8% 8% 4 6% 6% 3 5% 6% 6% 9% 37%	2160
	11th to 20th	Day	12% 12% 4 9% 7% 3 5% 6% 7% 7% 27%	2131
	21st to end	Day	13% 9% 4 8% 6% 3 4 4 5% 7% 37%	2460
May	1st to 10th	Day	13% 8% 3 6% 5% 4 6% 5% 7% 7% 36%	2520
	11th to 20th	Day	10% 7% 3 6% 6% 3 5% 5% 7% 8% 38%	2478
	21st to end	Day	19% 6% 3 6% 5% 3 5% 6% 7% 8% 31%	2724
June	1st to 10th	Day	14% 5% 2 5% 6% 4 5% 6% 7% 8% 39%	2592
	11th to 20th	Day	13% 7% 2 6% 7% 3 6% 5% 6% 9% 36%	2700
	21st to end	Day	6% 5% 3 5% 6% 4 6% 6% 6% 8% 46%	2700
July	1st to 10th	Day	5% 4 2 4 5% 3 5% 6% 7% 9% 51%	2700
	11th to 20th	Day	4 3 2 3 5% 3 5% 4 6% 9% 56%	2606
	21st to end	Day	5% 2 2 3 4 3 4 4 6% 7% 60%	2742
August	1st to 10th	Day	7% 5% 2 4 4 3 4 5% 6% 8% 53%	2507
	11th to 20th	Day	6% 4 2 4 4 3 4 4 5% 9% 54%	2382
	21st to end	Day	8% 3 2 4 4 2 4 5% 6% 7% 56%	2444
September	1st to 10th	Day	10% 5% 2 5% 5% 3 4 5% 5% 6% 49%	2160
	11th to 20th	Day	12% 7% 2 4 4 2 4 5% 5% 7% 48%	2160
	21st to end	Day	13% 7% 3 5% 4 3 3 4 5% 8% 47%	1928
October	1st to 10th	Day	14% 9% 3 6% 4 2 3 4 4 6% 44%	1800
	11th to 20th	Day	9% 6% 3 7% 6% 4 5% 5% 5% 8% 43%	1792
	21st to end	Day	6% 11% 4 8% 6% 3 5% 4 6% 7% 40%	1782
November	1st to 10th	Day	10% 12% 5% 7% 6% 3 4 4 6% 5% 38%	1545
	11th to 20th	Day	3 8% 5% 9% 8% 4 5% 6% 6% 8% 38%	1520
	21st to end	Day	7% 7% 5% 8% 7% 3 6% 5% 7% 8% 36%	1520
December	1st to 10th	Day	5% 10% 5% 9% 10% 7% 7% 7% 7% 7% 28%	1520
	11th to 20th	Day	4 10% 4 11% 7% 4 6% 6% 7% 7% 34%	1917
	21st to end	Day	4 7% 3 6% 6% 3 5% 4 6% 9% 47%	2442
January	1st to 10th	Night	3 7% 4 10% 8% 5% 6% 7% 7% 10% 35%	4726
	11th to 20th	Night	2 5% 4 10% 11% 5% 8% 10% 8% 9% 27%	4448
	21st to end	Night	2 9% 4 9% 6% 3 5% 5% 6% 8% 42%	4873
February	1st to 10th	Night	4 11% 6% 9% 7% 4 6% 7% 7% 7% 31%	2465
	11th to 20th	Night	4 9% 5% 10% 8% 5% 6% 5% 7% 10% 29%	2272
	21st to end	Night	4 9% 4 10% 7% 3 6% 6% 7% 7% 36%	1834
March	1st to 10th	Night	6% 9% 5% 9% 9% 4 5% 5% 6% 10% 31%	2160
	11th to 20th	Night	6% 9% 4 7% 7% 4 5% 6% 6% 9% 37%	2012
	21st to end	Night	8% 7% 5% 11% 8% 4 5% 5% 6% 7% 33%	2054
April	1st to 10th	Night	7% 8% 4 7% 6% 4 5% 5% 6% 7% 41%	1800
	11th to 20th	Night	12% 10% 3 7% 5% 3 5% 5% 6% 9% 35%	1777
	21st to end	Night	9% 9% 3 6% 7% 3 5% 5% 6% 8% 39%	1495
May	1st to 10th	Night	9% 7% 3 5% 5% 4 4 5% 6% 9% 42%	1440
	11th to 20th	Night	10% 6% 2 4 5% 3 5% 5% 6% 10% 44%	1417
	21st to end	Night	14% 6% 3 5% 6% 3 5% 6% 6% 11% 35%	1552
June	1st to 10th	Night	11% 6% 3 4 5% 2 3 4 6% 9% 46%	1364
	11th to 20th	Night	10% 4 2 5% 6% 3 3 5% 6% 8% 46%	1260
	21st to end	Night	6% 3 2 3 3 3 4 4 5% 10% 56%	1260
July	1st to 10th	Night	6% 4 1 3 3 3 4 4 7% 8% 57%	1260
	11th to 20th	Night	4 3 2 4 4 3 4 5% 6% 10% 55%	1352
	21st to end	Night	2 2 1 3 3 3 4 5% 5% 8% 64%	1576
August	1st to 10th	Night	5% 3 2 4 4 2 5% 4 4 9% 59%	1432
	11th to 20th	Night	5% 3 1 3 3 2 3 5% 6% 9% 61%	1517
	21st to end	Night	8% 3 1 4 3 2 4 4 4 7% 60%	1912
September	1st to 10th	Night	8% 4 2 3 5% 2 4 4 4 8% 56%	1800
	11th to 20th	Night	9% 5% 2 4 4 2 4 5% 5% 10% 52%	1800
	21st to end	Night	10% 5% 2 5% 4 3 3 4 5% 7% 52%	2028
October	1st to 10th	Night	12% 5% 2 6% 5% 2 4 4 5% 8% 47%	2160
	11th to 20th	Night	8% 6% 2 6% 5% 3 4 4 5% 7% 49%	2160
	21st to end	Night	8% 10% 4 7% 6% 4 5% 5% 6% 8% 38%	2564
November	1st to 10th	Night	11% 10% 4 6% 6% 4 5% 5% 6% 8% 36%	2531
	11th to 20th	Night	4 7% 5% 8% 9% 5% 5% 5% 7% 8% 36%	2660
	21st to end	Night	5% 8% 5% 9% 7% 4 4 5% 7% 9% 37%	2660
December	1st to 10th	Night	5% 9% 5% 9% 9% 5% 8% 5% 7% 9% 31%	2660
	11th to 20th	Night	4 9% 5% 10% 7% 4 6% 8% 7% 9% 32%	3275
	21st to end	Night	4 7% 2 6% 6% 3 4 3 5% 8% 53%	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	50% 9% 16% 19% 5%	2219
	11th to 20th	Day	49% 11% 18% 17% 5% 1	2517
	21st to end	Day	40% 8% 17% 29% 5%	2794
February	1st to 10th	Day	54% 7% 12% 17% 10%	1604
	11th to 20th	Day	55% 9% 12% 14% 9% 1	1584
	21st to end	Day	51% 8% 15% 14% 10% 1	1422
March	1st to 10th	Day	55% 9% 15% 15% 5%	1800
	11th to 20th	Day	45% 8% 18% 21% 7% 1	1944
	21st to end	Day	48% 10% 18% 17% 6% 1	2301
April	1st to 10th	Day	41% 7% 18% 22% 11% 1	2160
	11th to 20th	Day	53% 8% 15% 17% 8%	2134
	21st to end	Day	48% 5% 16% 22% 8%	2460
May	1st to 10th	Day	43% 8% 13% 24% 10% 1	2520
	11th to 20th	Day	42% 7% 15% 27% 9%	2482
	21st to end	Day	49% 9% 14% 19% 7%	2724
June	1st to 10th	Day	40% 9% 17% 23% 9% 1	2592
	11th to 20th	Day	43% 8% 17% 20% 10% 1	2700
	21st to end	Day	33% 10% 17% 29% 10% 1	2700
July	1st to 10th	Day	25% 10% 18% 30% 15% 3	2626
	11th to 20th	Day	24% 7% 16% 27% 22% 4	2526
	21st to end	Day	21% 7% 12% 32% 24% 3	2590
August	1st to 10th	Day	28% 7% 16% 29% 16% 3	2491
	11th to 20th	Day	27% 6% 18% 29% 16% 3	2382
	21st to end	Day	26% 6% 13% 31% 21% 3	2444
September	1st to 10th	Day	34% 6% 13% 27% 16% 4	2160
	11th to 20th	Day	35% 7% 15% 27% 13% 4	2160
	21st to end	Day	36% 5% 13% 28% 16% 2	1928
October	1st to 10th	Day	42% 6% 14% 25% 11% 1	1800
	11th to 20th	Day	39% 6% 16% 26% 12% 1	1792
	21st to end	Day	43% 7% 18% 23% 9%	1782
November	1st to 10th	Day	48% 6% 15% 20% 10% 2	1545
	11th to 20th	Day	43% 8% 20% 20% 8% 1	1520
	21st to end	Day	45% 8% 22% 20% 5% 1	1520
December	1st to 10th	Day	53% 10% 15% 16% 6% 1	1520
	11th to 20th	Day	46% 11% 22% 16% 5%	1915
	21st to end	Day	34% 6% 11% 25% 15% 9%	2442
January	1st to 10th	Night	41% 10% 21% 20% 7% 1	4726
	11th to 20th	Night	48% 8% 16% 20% 7% 1	4448
	21st to end	Night	40% 9% 19% 26% 6% 1	4873
February	1st to 10th	Night	48% 8% 16% 19% 8% 1	2465
	11th to 20th	Night	49% 9% 16% 16% 10% 1	2273
	21st to end	Night	44% 7% 13% 21% 14% 1	1834
March	1st to 10th	Night	47% 7% 17% 21% 7%	2160
	11th to 20th	Night	43% 8% 15% 25% 9% 1	2012
	21st to end	Night	50% 7% 16% 18% 8% 1	2054
April	1st to 10th	Night	40% 7% 17% 24% 12% 1	1800
	11th to 20th	Night	45% 8% 14% 20% 12% 1	1777
	21st to end	Night	41% 8% 15% 27% 10%	1495
May	1st to 10th	Night	36% 7% 15% 27% 14% 1	1440
	11th to 20th	Night	35% 9% 15% 27% 14% 1	1418
	21st to end	Night	43% 9% 17% 22% 9% 1	1552
June	1st to 10th	Night	34% 7% 19% 26% 12% 2	1364
	11th to 20th	Night	36% 7% 18% 23% 13% 2	1260
	21st to end	Night	25% 9% 20% 31% 13% 2	1260
July	1st to 10th	Night	22% 9% 15% 33% 17% 3	1225
	11th to 20th	Night	23% 7% 17% 28% 21% 3	1309
	21st to end	Night	17% 5% 15% 31% 25% 5%	1488
August	1st to 10th	Night	23% 6% 15% 33% 19% 4	1424
	11th to 20th	Night	20% 6% 15% 36% 20% 3	1517
	21st to end	Night	24% 6% 10% 28% 27% 5%	1912
September	1st to 10th	Night	28% 5% 12% 30% 20% 6%	1800
	11th to 20th	Night	28% 8% 15% 25% 21% 3	1800
	21st to end	Night	33% 5% 13% 29% 19% 1	2028
October	1st to 10th	Night	36% 7% 14% 25% 16% 2	2160
	11th to 20th	Night	35% 6% 15% 29% 13% 2	2160
	21st to end	Night	42% 8% 15% 23% 10% 2	2564
November	1st to 10th	Night	46% 8% 15% 20% 9% 2	2531
	11th to 20th	Night	44% 8% 18% 20% 8% 1	2660
	21st to end	Night	43% 8% 21% 20% 8%	2660
December	1st to 10th	Night	50% 7% 16% 19% 8% 1	2660
	11th to 20th	Night	45% 12% 18% 18% 7% 1	3276
	21st to end	Night	30% 5% 13% 26% 18% 7%	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	9% 16% 34% 26% 12% 2	180
	11th to 20th	Day	9% 15% 39% 25% 11% 2	265
	21st to end	Day	9% 16% 37% 28% 9%	297
February	1st to 10th	Day	12% 15% 42% 26% 4 1	252
	11th to 20th	Day	10% 11% 43% 29% 6% 1	240
	21st to end	Day	8% 10% 39% 28% 14% 2	198
March	1st to 10th	Day	14% 14% 35% 31% 6%	240
	11th to 20th	Day	10% 14% 37% 24% 13% 2	384
	21st to end	Day	12% 13% 38% 22% 12% 3	462
April	1st to 10th	Day	10% 17% 39% 28% 5%	420
	11th to 20th	Day	17% 18% 35% 22% 7% 1	412
	21st to end	Day	18% 13% 27% 29% 12% 1	550
May	1st to 10th	Day	14% 7% 23% 34% 18% 4	603
	11th to 20th	Day	14% 10% 24% 31% 16% 6%	625
	21st to end	Day	17% 10% 26% 25% 16% 6%	679
June	1st to 10th	Day	13% 13% 27% 28% 17% 3	630
	11th to 20th	Day	15% 15% 34% 24% 10% 3	630
	21st to end	Day	8% 10% 24% 33% 21% 4	630
July	1st to 10th	Day	6% 7% 27% 37% 20% 4	621
	11th to 20th	Day	7% 8% 32% 35% 16% 3	618
	21st to end	Day	5% 7% 27% 32% 24% 4	684
August	1st to 10th	Day	9% 7% 24% 34% 20% 5%	626
	11th to 20th	Day	11% 8% 21% 31% 23% 6%	520
	21st to end	Day	7% 10% 25% 37% 18% 4	495
September	1st to 10th	Day	10% 11% 35% 31% 11% 3	450
	11th to 20th	Day	15% 10% 28% 30% 15% 1	450
	21st to end	Day	19% 11% 31% 24% 11% 4	298
October	1st to 10th	Day	20% 10% 27% 26% 15% 2	270
	11th to 20th	Day	10% 14% 39% 21% 14% 3	270
	21st to end	Day	12% 17% 44% 16% 8% 2	297
November	1st to 10th	Day	17% 16% 36% 23% 8% 1	270
	11th to 20th	Day	11% 16% 41% 23% 9% 1	270
	21st to end	Day	9% 17% 40% 25% 9%	270
December	1st to 10th	Day	9% 16% 43% 23% 6% 3	270
	11th to 20th	Day	9% 19% 46% 17% 8% 2	266
	21st to end	Day	10% 9% 43% 27% 8% 5%	198
January	1st to 10th	Night	7% 15% 43% 26% 8% 1	950
	11th to 20th	Night	9% 18% 40% 26% 7% 1	950
	21st to end	Night	7% 17% 43% 25% 7% 1	1045
February	1st to 10th	Night	9% 13% 36% 30% 11% 1	805
	11th to 20th	Night	9% 12% 45% 25% 7%	704
	21st to end	Night	7% 7% 35% 33% 14% 4	592
March	1st to 10th	Night	9% 12% 38% 30% 8% 2	720
	11th to 20th	Night	10% 12% 38% 29% 9% 1	720
	21st to end	Night	11% 9% 32% 37% 11% 1	668
April	1st to 10th	Night	11% 12% 32% 36% 7% 1	540
	11th to 20th	Night	17% 9% 36% 30% 7% 1	533
	21st to end	Night	12% 9% 35% 35% 8%	540
May	1st to 10th	Night	11% 8% 26% 40% 15% 1	540
	11th to 20th	Night	11% 7% 28% 34% 18% 2	531
	21st to end	Night	17% 8% 26% 31% 17% 1	584
June	1st to 10th	Night	13% 8% 28% 35% 14% 2	464
	11th to 20th	Night	14% 6% 29% 37% 13%	360
	21st to end	Night	9% 6% 27% 38% 18% 2	360
July	1st to 10th	Night	7% 4 25% 47% 16% 1	350
	11th to 20th	Night	6% 6% 30% 46% 11%	439
	21st to end	Night	3 4 30% 46% 15% 2	591
August	1st to 10th	Night	5% 4 24% 50% 14% 3	537
	11th to 20th	Night	6% 4 24% 44% 19% 2	534
	21st to end	Night	10% 5% 25% 41% 18% 1	594
September	1st to 10th	Night	10% 5% 25% 44% 15% 2	540
	11th to 20th	Night	11% 5% 28% 39% 17% 1	540
	21st to end	Night	12% 8% 30% 39% 10% 1	616
October	1st to 10th	Night	16% 9% 28% 32% 14% 1	720
	11th to 20th	Night	12% 11% 34% 31% 11% 2	720
	21st to end	Night	12% 15% 41% 26% 6%	980
November	1st to 10th	Night	16% 13% 39% 26% 5% 1	925
	11th to 20th	Night	8% 16% 43% 24% 8% 1	950
	21st to end	Night	9% 16% 45% 23% 6%	950
December	1st to 10th	Night	9% 13% 41% 27% 10% 1	950
	11th to 20th	Night	9% 17% 41% 25% 7% 1	950
	21st to end	Night	11% 12% 42% 26% 9% 1	1045

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