Gas Line Road
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gas Line Road clear sky chart from 2005-11-06 to 2026-08-15.

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	2 3 1 4 3 3 3 3 5% 6% 67%	152348
	Night	3 4 2 4 3 3 3 3 5% 7% 63%	146906
Transparency	Day	17% 5% 8% 19% 25% 25%	147340
	Night	20% 5% 9% 22% 24% 19%	142920
Seeing	Day	5% 10% 20% 30% 27% 9%	17271
	Night	5% 5% 21% 41% 25% 4	26576

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	2 4 2 6% 5% 4 5% 4 7% 9% 51%	4722
	11th to 20th	Day	1 4 2 6% 3 4 3 3 4 6% 63%	4740
	21st to end	Day	3 6% 2 7% 5% 5% 6% 4 7% 9% 47%	5783
February	1st to 10th	Day	2 5% 2 5% 4 4 5% 4 8% 8% 52%	1937
	11th to 20th	Day	3 5% 2 6% 5% 4 5% 4 7% 9% 51%	2050
	21st to end	Day	2 5% 2 4 3 4 4 5% 6% 7% 57%	1730
March	1st to 10th	Day	3 9% 3 7% 5% 5% 5% 5% 7% 9% 43%	4618
	11th to 20th	Day	1 4 1 4 4 4 4 4 5% 7% 61%	4902
	21st to end	Day	3 6% 2 6% 4 4 5% 4 6% 7% 53%	6049
April	1st to 10th	Day	3 5% 2 5% 5% 4 5% 4 6% 9% 54%	2520
	11th to 20th	Day	3 4 2 5% 4 4 4 4 5% 6% 60%	2489
	21st to end	Day	4 4 1 4 4 3 3 3 4 8% 61%	2520
May	1st to 10th	Day	5% 4 2 3 2 2 3 2 4 5% 67%	7565
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 6% 73%	7894
	21st to end	Day	2 3 1 4 3 3 3 3 4 6% 69%	8992
June	1st to 10th	Day	3 3 1 3 3 3 4 3 5% 7% 65%	2940
	11th to 20th	Day	2 2 2 2 2 3 2 4 6% 75%	2940
	21st to end	Day	2 2 1 2 2 2 3 2 3 4 78%	2940
July	1st to 10th	Day	1 1 1 1 1 2 1 2 2 87%	8219
	11th to 20th	Day	2 2 1 3 2 2 3 3 4 5% 74%	8220
	21st to end	Day	1 1 1 2 2 2 3 3 5% 6% 75%	8922
August	1st to 10th	Day	2 1 2 2 2 2 2 3 4 80%	5761
	11th to 20th	Day	1 1 1 1 1 1 2 3 87%	5413
	21st to end	Day	3 2 1 2 2 3 2 2 3 4 76%	5984
September	1st to 10th	Day	2 1 2 2 2 2 2 2 4 83%	2400
	11th to 20th	Day	2 2 1 3 3 2 3 2 3 5% 76%	2333
	21st to end	Day	2 2 1 2 1 2 3 2 3 5% 78%	2200
October	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 74%	3418
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 5% 75%	3261
	21st to end	Day	2 1 1 3 2 3 4 3 5% 8% 68%	3378
November	1st to 10th	Day	3 4 1 4 4 3 3 3 6% 9% 59%	1854
	11th to 20th	Day	3 4 1 3 3 3 5% 3 6% 9% 59%	1890
	21st to end	Day	3 4 1 5% 3 3 4 4 6% 8% 58%	1864
December	1st to 10th	Day	1 4 1 4 3 3 3 3 5% 9% 64%	2700
	11th to 20th	Day	3 6% 2 8% 5% 7% 6% 5% 8% 10% 40%	3108
	21st to end	Day	3 6% 3 7% 5% 5% 6% 5% 8% 11% 41%	4092
January	1st to 10th	Night	2 5% 2 5% 4 3 4 4 7% 10% 54%	8172
	11th to 20th	Night	1 5% 3 6% 3 3 3 3 4 6% 64%	8220
	21st to end	Night	4 7% 3 6% 4 5% 5% 3 6% 7% 50%	8412
February	1st to 10th	Night	3 6% 2 5% 3 4 5% 4 7% 8% 53%	2682
	11th to 20th	Night	2 5% 2 5% 4 5% 4 4 7% 8% 54%	2458
	21st to end	Night	2 5% 3 5% 5% 4 4 4 5% 8% 54%	2076
March	1st to 10th	Night	3 9% 4 8% 4 4 4 3 5% 7% 50%	5496
	11th to 20th	Night	2 5% 2 5% 3 3 3 3 6% 8% 61%	5302
	21st to end	Night	4 6% 2 6% 4 5% 4 4 6% 8% 50%	5167
April	1st to 10th	Night	3 5% 1 5% 4 4 5% 4 6% 8% 55%	2100
	11th to 20th	Night	3 5% 1 4 4 4 3 2 5% 7% 62%	2079
	21st to end	Night	6% 6% 2 5% 3 3 2 2 4 6% 61%	2100
May	1st to 10th	Night	5% 4 2 4 3 3 3 3 3 4 68%	5382
	11th to 20th	Night	3 2 1 3 2 2 3 2 3 5% 75%	4962
	21st to end	Night	3 3 1 4 3 3 3 3 4 6% 66%	5181
June	1st to 10th	Night	3 4 1 4 3 4 3 4 5% 6% 62%	1680
	11th to 20th	Night	2 2 1 2 2 3 3 2 3 4 76%	1680
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 78%	1680
July	1st to 10th	Night	1 1 2 1 1 2 1 2 3 85%	4740
	11th to 20th	Night	4 2 1 4 3 3 3 3 4 5% 68%	4740
	21st to end	Night	2 2 1 4 3 3 4 4 5% 7% 65%	5153
August	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 77%	4248
	11th to 20th	Night	2 1 1 2 2 2 2 2 3 4 80%	4546
	21st to end	Night	4 2 1 3 2 2 3 2 4 4 72%	5015
September	1st to 10th	Night	1 1 1 2 1 2 2 2 3 5% 80%	2000
	11th to 20th	Night	3 2 1 3 2 2 3 3 5% 5% 72%	2066
	21st to end	Night	3 1 1 2 2 2 2 2 4 5% 76%	2200
October	1st to 10th	Night	3 2 1 3 3 3 3 2 3 5% 72%	3778
	11th to 20th	Night	2 1 2 2 2 2 2 4 5% 79%	3897
	21st to end	Night	2 2 1 3 2 3 3 3 4 6% 71%	4426
November	1st to 10th	Night	4 4 1 5% 3 2 3 3 4 7% 64%	2665
	11th to 20th	Night	3 4 2 4 3 3 4 3 5% 8% 62%	2730
	21st to end	Night	3 5% 1 5% 3 3 3 3 5% 7% 60%	2751
December	1st to 10th	Night	2 4 2 4 3 3 3 3 4 6% 66%	4700
	11th to 20th	Night	3 7% 2 6% 5% 5% 5% 5% 8% 12% 43%	5316
	21st to end	Night	3 7% 3 9% 6% 6% 6% 5% 8% 9% 37%	7106

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	24% 8% 23% 36% 9%	4723
	11th to 20th	Day	21% 6% 11% 26% 32% 5%	4740
	21st to end	Day	30% 8% 11% 24% 23% 5%	5783
February	1st to 10th	Day	26% 8% 15% 30% 16% 6%	1937
	11th to 20th	Day	27% 6% 14% 31% 19% 3	2060
	21st to end	Day	23% 6% 18% 29% 18% 6%	1730
March	1st to 10th	Day	32% 8% 11% 26% 17% 5%	4618
	11th to 20th	Day	19% 6% 11% 31% 28% 4	4902
	21st to end	Day	25% 8% 13% 25% 25% 5%	6049
April	1st to 10th	Day	26% 6% 13% 25% 23% 7%	2520
	11th to 20th	Day	23% 7% 9% 25% 26% 10%	2492
	21st to end	Day	21% 6% 8% 29% 29% 6%	2520
May	1st to 10th	Day	18% 5% 6% 19% 38% 14%	7565
	11th to 20th	Day	12% 4 5% 18% 38% 23%	7897
	21st to end	Day	16% 4 5% 17% 41% 17%	8994
June	1st to 10th	Day	18% 6% 8% 18% 33% 18%	2940
	11th to 20th	Day	11% 3 5% 15% 29% 36%	2940
	21st to end	Day	11% 3 3 8% 27% 48%	2940
July	1st to 10th	Day	6% 1 1 4 15% 72%	8151
	11th to 20th	Day	12% 3 3 7% 12% 63%	8145
	21st to end	Day	10% 3 3 9% 23% 51%	8723
August	1st to 10th	Day	13% 3 3 7% 19% 56%	4233
	11th to 20th	Day	6% 2 3 5% 17% 68%	3929
	21st to end	Day	9% 2 2 7% 29% 51%	4311
September	1st to 10th	Day	8% 2 2 7% 27% 53%	2400
	11th to 20th	Day	13% 4 5% 15% 32% 32%	2333
	21st to end	Day	11% 3 4 16% 34% 32%	2200
October	1st to 10th	Day	12% 3 5% 13% 33% 34%	3418
	11th to 20th	Day	14% 2 5% 21% 44% 14%	3261
	21st to end	Day	14% 5% 7% 30% 33% 11%	3378
November	1st to 10th	Day	21% 7% 13% 28% 26% 6%	1854
	11th to 20th	Day	20% 7% 17% 32% 21% 3	1890
	21st to end	Day	23% 5% 13% 32% 23% 3	1864
December	1st to 10th	Day	19% 4 12% 36% 26% 3	2700
	11th to 20th	Day	33% 11% 17% 27% 10% 1	3108
	21st to end	Day	30% 11% 26% 25% 7%	4092
January	1st to 10th	Night	23% 7% 24% 35% 11% 1	8172
	11th to 20th	Night	22% 5% 10% 24% 32% 8%	8220
	21st to end	Night	31% 7% 10% 24% 24% 4	8412
February	1st to 10th	Night	26% 8% 13% 29% 19% 6%	2682
	11th to 20th	Night	26% 6% 12% 33% 21% 2	2459
	21st to end	Night	26% 7% 15% 30% 16% 7%	2076
March	1st to 10th	Night	30% 8% 8% 27% 21% 6%	5496
	11th to 20th	Night	20% 6% 9% 40% 22% 3	5302
	21st to end	Night	28% 8% 16% 28% 17% 3	5167
April	1st to 10th	Night	25% 6% 13% 28% 19% 7%	2100
	11th to 20th	Night	22% 6% 11% 29% 23% 10%	2079
	21st to end	Night	24% 5% 10% 27% 28% 6%	2100
May	1st to 10th	Night	19% 5% 5% 19% 36% 15%	5380
	11th to 20th	Night	13% 4 5% 16% 44% 19%	4962
	21st to end	Night	17% 5% 6% 19% 37% 16%	5182
June	1st to 10th	Night	20% 6% 7% 19% 28% 20%	1680
	11th to 20th	Night	11% 3 5% 14% 31% 35%	1680
	21st to end	Night	10% 2 4 11% 29% 44%	1680
July	1st to 10th	Night	7% 2 2 4 17% 68%	4700
	11th to 20th	Night	17% 3 3 7% 11% 60%	4692
	21st to end	Night	16% 5% 4 12% 18% 46%	5040
August	1st to 10th	Night	14% 3 3 7% 20% 53%	3120
	11th to 20th	Night	8% 2 2 6% 17% 64%	3296
	21st to end	Night	10% 1 2 7% 27% 53%	3608
September	1st to 10th	Night	9% 2 4 8% 28% 50%	2000
	11th to 20th	Night	13% 4 5% 16% 29% 33%	2066
	21st to end	Night	11% 3 4 14% 32% 36%	2200
October	1st to 10th	Night	15% 4 4 12% 29% 36%	3778
	11th to 20th	Night	9% 3 5% 18% 47% 18%	3897
	21st to end	Night	13% 4 6% 25% 38% 14%	4426
November	1st to 10th	Night	19% 5% 13% 29% 23% 10%	2665
	11th to 20th	Night	20% 5% 14% 30% 26% 4	2730
	21st to end	Night	23% 5% 12% 28% 27% 4	2751
December	1st to 10th	Night	20% 5% 10% 30% 27% 8%	4700
	11th to 20th	Night	30% 9% 17% 31% 12% 1	5316
	21st to end	Night	37% 10% 19% 22% 12%	7106

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	5% 9% 28% 35% 19% 4	220
	11th to 20th	Day	6% 6% 20% 29% 29% 9%	220
	21st to end	Day	6% 5% 18% 39% 24% 8%	452
February	1st to 10th	Day	6% 13% 22% 31% 24% 4	455
	11th to 20th	Day	6% 15% 24% 34% 18% 3	535
	21st to end	Day	6% 15% 26% 34% 18% 1	446
March	1st to 10th	Day	9% 16% 26% 31% 14% 3	540
	11th to 20th	Day	6% 18% 26% 27% 18% 6%	540
	21st to end	Day	9% 18% 26% 27% 18% 2	594
April	1st to 10th	Day	7% 17% 25% 27% 18% 6%	540
	11th to 20th	Day	7% 19% 23% 27% 17% 6%	532
	21st to end	Day	6% 16% 25% 24% 21% 7%	540
May	1st to 10th	Day	8% 16% 26% 26% 19% 6%	543
	11th to 20th	Day	8% 16% 22% 24% 21% 9%	568
	21st to end	Day	6% 11% 18% 28% 25% 12%	617
June	1st to 10th	Day	2 13% 20% 25% 30% 10%	570
	11th to 20th	Day	6% 10% 23% 28% 24% 9%	570
	21st to end	Day	2 9% 17% 32% 30% 9%	570
July	1st to 10th	Day	1 5% 17% 34% 34% 10%	566
	11th to 20th	Day	2 4 10% 28% 41% 15%	564
	21st to end	Day	2 3 13% 29% 40% 13%	619
August	1st to 10th	Day	3 4 16% 27% 35% 15%	567
	11th to 20th	Day	2 5% 14% 30% 33% 16%	534
	21st to end	Day	2 4 15% 30% 37% 12%	583
September	1st to 10th	Day	2 5% 14% 30% 37% 12%	530
	11th to 20th	Day	3 7% 16% 33% 31% 9%	530
	21st to end	Day	2 6% 18% 31% 36% 8%	530
October	1st to 10th	Day	3 8% 18% 29% 33% 9%	530
	11th to 20th	Day	5% 8% 17% 27% 35% 8%	530
	21st to end	Day	5% 7% 18% 31% 28% 11%	478
November	1st to 10th	Day	5% 6% 21% 31% 26% 12%	330
	11th to 20th	Day	3 5% 21% 39% 22% 10%	330
	21st to end	Day	3 8% 20% 32% 30% 8%	316
December	1st to 10th	Day	4 5% 18% 31% 28% 13%	220
	11th to 20th	Day	5% 7% 20% 35% 23% 10%	220
	21st to end	Day	11% 9% 23% 33% 20% 4	242
January	1st to 10th	Night	6% 3 25% 38% 22% 6%	840
	11th to 20th	Night	3 3 19% 44% 26% 4	840
	21st to end	Night	7% 3 17% 44% 24% 5%	924
February	1st to 10th	Night	5% 5% 22% 43% 22% 3	840
	11th to 20th	Night	3 5% 25% 44% 20% 4	824
	21st to end	Night	3 5% 30% 43% 16% 3	692
March	1st to 10th	Night	6% 6% 28% 40% 17% 3	840
	11th to 20th	Night	5% 7% 26% 36% 23% 3	840
	21st to end	Night	6% 9% 29% 40% 15% 1	924
April	1st to 10th	Night	4 9% 28% 36% 22% 1	840
	11th to 20th	Night	5% 10% 28% 37% 17% 4	831
	21st to end	Night	7% 10% 28% 33% 20% 3	840
May	1st to 10th	Night	6% 10% 29% 36% 17% 2	652
	11th to 20th	Night	7% 6% 26% 39% 20% 2	509
	21st to end	Night	6% 5% 22% 43% 22% 2	455
June	1st to 10th	Night	5% 5% 22% 44% 22% 2	420
	11th to 20th	Night	3 5% 24% 45% 22% 1	420
	21st to end	Night	1 2 24% 50% 21% 1	420
July	1st to 10th	Night	2 2 20% 52% 23% 1	410
	11th to 20th	Night	2 1 21% 54% 21%	408
	21st to end	Night	2 2 16% 47% 31% 1	460
August	1st to 10th	Night	4 2 18% 46% 29% 2	675
	11th to 20th	Night	2 3 16% 46% 31% 2	808
	21st to end	Night	3 2 16% 46% 31% 2	880
September	1st to 10th	Night	2 2 14% 40% 37% 5%	800
	11th to 20th	Night	3 3 17% 36% 35% 6%	800
	21st to end	Night	3 3 15% 38% 34% 8%	800
October	1st to 10th	Night	6% 3 16% 42% 27% 6%	800
	11th to 20th	Night	4 4 16% 39% 33% 5%	800
	21st to end	Night	3 3 15% 38% 31% 10%	880
November	1st to 10th	Night	5% 3 17% 36% 32% 7%	820
	11th to 20th	Night	6% 4 15% 46% 25% 5%	840
	21st to end	Night	5% 4 20% 39% 27% 5%	840
December	1st to 10th	Night	6% 3 20% 42% 25% 4	840
	11th to 20th	Night	6% 4 22% 41% 24% 4	840
	21st to end	Night	6% 7% 27% 43% 16% 1	924

Page generated at 2026-08-12 00:37:15. Version ac07

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