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 2025-04-11.

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 4 1 4 3 3 3 3 5% 6% 65%	117314
	Night	3 4 2 5% 3 3 3 3 5% 7% 62%	119798
Transparency	Day	19% 5% 9% 20% 24% 23%	117055
	Night	21% 6% 10% 23% 23% 17%	119646
Seeing	Day	5% 10% 21% 30% 26% 8%	15465
	Night	5% 5% 21% 41% 25% 4	24967

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% 5% 7% 9% 51%	3639
	11th to 20th	Day	2 5% 2 7% 4 4 4 3 5% 6% 58%	3640
	21st to end	Day	4 7% 3 7% 5% 5% 6% 4 5% 7% 46%	4464
February	1st to 10th	Day	2 6% 2 5% 4 4 5% 4 8% 8% 52%	1845
	11th to 20th	Day	3 5% 2 6% 5% 4 5% 4 7% 9% 52%	1950
	21st to end	Day	2 5% 2 4 3 4 4 5% 6% 7% 57%	1650
March	1st to 10th	Day	3 9% 3 7% 5% 5% 5% 5% 7% 9% 42%	4518
	11th to 20th	Day	1 4 1 4 4 4 4 4 5% 8% 61%	4797
	21st to end	Day	3 6% 2 6% 4 4 5% 4 6% 7% 54%	5918
April	1st to 10th	Day	3 6% 2 5% 5% 4 5% 4 6% 9% 54%	2400
	11th to 20th	Day	3 4 2 5% 4 4 4 4 5% 6% 59%	2251
	21st to end	Day	4 4 1 4 4 3 3 3 4 7% 63%	2280
May	1st to 10th	Day	3 4 1 4 2 2 3 2 4 6% 68%	4075
	11th to 20th	Day	3 3 1 3 2 2 3 3 4 5% 71%	4239
	21st to end	Day	3 3 2 4 3 3 3 3 4 5% 67%	4813
June	1st to 10th	Day	3 3 1 3 3 3 4 4 5% 7% 64%	2660
	11th to 20th	Day	2 2 2 2 2 3 2 4 6% 73%	2660
	21st to end	Day	2 2 1 2 2 2 3 2 3 4 77%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 1 2 3 87%	4420
	11th to 20th	Day	2 2 1 3 3 2 3 3 5% 5% 72%	4420
	21st to end	Day	2 2 1 2 2 2 3 3 4 5% 74%	4790
August	1st to 10th	Day	2 2 1 3 2 3 3 2 4 5% 74%	3995
	11th to 20th	Day	1 1 1 1 2 2 2 3 3 85%	3724
	21st to end	Day	2 1 1 1 1 1 1 1 2 2 85%	4180
September	1st to 10th	Day	2 1 1 1 1 2 1 2 4 84%	2280
	11th to 20th	Day	2 2 1 2 3 2 3 2 3 5% 76%	2217
	21st to end	Day	2 1 1 2 1 2 2 2 3 5% 78%	2090
October	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 74%	3314
	11th to 20th	Day	3 3 1 3 2 2 2 2 3 5% 76%	3161
	21st to end	Day	2 1 1 3 2 3 4 3 5% 8% 68%	3273
November	1st to 10th	Day	3 4 1 4 4 3 3 3 6% 9% 59%	1764
	11th to 20th	Day	2 3 1 3 3 3 4 3 6% 9% 62%	1800
	21st to end	Day	3 4 1 6% 4 3 4 4 6% 8% 57%	1775
December	1st to 10th	Day	1 4 1 4 3 3 3 3 5% 9% 64%	2620
	11th to 20th	Day	3 7% 2 8% 5% 7% 6% 5% 8% 10% 40%	3028
	21st to end	Day	3 6% 3 7% 5% 5% 6% 5% 8% 11% 41%	4004
January	1st to 10th	Night	2 4 2 5% 4 4 4 4 7% 10% 54%	6306
	11th to 20th	Night	2 6% 3 7% 4 3 4 3 4 6% 57%	6320
	21st to end	Night	5% 8% 3 6% 4 4 4 3 5% 6% 51%	6496
February	1st to 10th	Night	3 6% 2 5% 3 4 5% 4 7% 8% 53%	2555
	11th to 20th	Night	2 5% 2 5% 4 5% 4 4 7% 8% 55%	2338
	21st to end	Night	2 6% 3 5% 5% 4 4 4 5% 8% 55%	1980
March	1st to 10th	Night	3 9% 4 8% 4 4 4 3 5% 7% 50%	5376
	11th to 20th	Night	2 5% 2 5% 3 3 3 3 6% 8% 60%	5187
	21st to end	Night	4 6% 2 6% 4 5% 5% 4 6% 8% 50%	5056
April	1st to 10th	Night	2 5% 1 5% 4 4 5% 4 6% 7% 56%	2000
	11th to 20th	Night	3 5% 1 4 4 4 3 2 5% 7% 63%	1883
	21st to end	Night	6% 6% 2 4 3 3 2 2 4 6% 62%	1900
May	1st to 10th	Night	4 3 1 3 2 3 3 3 4 5% 70%	2893
	11th to 20th	Night	4 2 1 3 3 2 3 2 4 6% 71%	2654
	21st to end	Night	4 4 1 4 3 3 3 2 4 6% 66%	2762
June	1st to 10th	Night	4 4 1 5% 4 4 3 4 4 6% 62%	1520
	11th to 20th	Night	3 2 1 2 2 3 3 2 4 5% 75%	1520
	21st to end	Night	2 2 1 2 2 2 2 2 3 4 77%	1520
July	1st to 10th	Night	1 1 1 1 1 2 1 2 2 87%	2540
	11th to 20th	Night	3 2 1 4 3 3 3 3 4 5% 68%	2540
	21st to end	Night	4 2 1 4 3 3 4 3 4 6% 67%	2761
August	1st to 10th	Night	3 3 1 3 2 3 2 2 4 6% 71%	2944
	11th to 20th	Night	2 1 1 1 1 1 2 1 3 3 84%	3126
	21st to end	Night	4 1 1 1 1 1 1 1 2 3 84%	3498
September	1st to 10th	Night	1 1 1 2 1 2 2 2 3 4 81%	1900
	11th to 20th	Night	3 2 1 3 2 2 3 3 5% 5% 73%	1962
	21st to end	Night	3 1 1 2 2 1 2 2 4 5% 77%	2090
October	1st to 10th	Night	3 2 1 3 3 3 3 2 3 5% 71%	3662
	11th to 20th	Night	2 1 2 2 2 2 2 4 5% 79%	3777
	21st to end	Night	2 2 1 3 2 3 3 3 4 7% 71%	4289
November	1st to 10th	Night	4 4 1 5% 3 2 3 3 4 7% 63%	2535
	11th to 20th	Night	3 4 2 4 3 3 3 3 4 7% 64%	2600
	21st to end	Night	4 5% 1 5% 4 4 3 3 5% 7% 59%	2620
December	1st to 10th	Night	2 4 2 4 3 3 3 3 5% 6% 65%	4560
	11th to 20th	Night	3 7% 2 6% 5% 5% 5% 5% 8% 12% 42%	5176
	21st to end	Night	3 7% 3 9% 6% 6% 6% 5% 8% 10% 37%	6952

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	23% 9% 25% 32% 11%	3639
	11th to 20th	Day	25% 7% 12% 31% 21% 4	3640
	21st to end	Day	34% 9% 12% 19% 22% 4	4464
February	1st to 10th	Day	26% 8% 15% 30% 16% 6%	1845
	11th to 20th	Day	26% 6% 14% 32% 19% 3	1960
	21st to end	Day	23% 6% 17% 29% 18% 7%	1650
March	1st to 10th	Day	32% 8% 11% 26% 17% 5%	4518
	11th to 20th	Day	19% 6% 12% 31% 28% 4	4797
	21st to end	Day	26% 8% 13% 25% 25% 4	5918
April	1st to 10th	Day	26% 6% 13% 26% 23% 7%	2400
	11th to 20th	Day	24% 7% 9% 26% 26% 8%	2254
	21st to end	Day	21% 6% 9% 30% 29% 6%	2280
May	1st to 10th	Day	16% 6% 7% 27% 36% 8%	4075
	11th to 20th	Day	14% 4 6% 19% 37% 21%	4242
	21st to end	Day	19% 5% 7% 21% 38% 10%	4814
June	1st to 10th	Day	18% 6% 8% 18% 32% 17%	2660
	11th to 20th	Day	12% 3 5% 17% 31% 32%	2660
	21st to end	Day	12% 3 3 8% 26% 48%	2660
July	1st to 10th	Day	5% 1 2 5% 11% 75%	4351
	11th to 20th	Day	13% 3 4 6% 14% 61%	4345
	21st to end	Day	12% 3 4 6% 21% 54%	4639
August	1st to 10th	Day	14% 3 3 7% 18% 56%	3979
	11th to 20th	Day	6% 2 3 4 16% 69%	3760
	21st to end	Day	8% 2 2 7% 29% 53%	4179
September	1st to 10th	Day	7% 2 2 6% 26% 56%	2280
	11th to 20th	Day	12% 4 5% 15% 31% 33%	2217
	21st to end	Day	10% 3 4 16% 34% 32%	2090
October	1st to 10th	Day	12% 3 5% 12% 33% 34%	3314
	11th to 20th	Day	14% 2 5% 21% 44% 14%	3161
	21st to end	Day	14% 5% 7% 29% 33% 12%	3273
November	1st to 10th	Day	21% 7% 13% 28% 26% 6%	1764
	11th to 20th	Day	18% 6% 18% 33% 23% 3	1800
	21st to end	Day	24% 5% 12% 31% 23% 3	1775
December	1st to 10th	Day	20% 4 11% 36% 26% 3	2620
	11th to 20th	Day	34% 11% 17% 27% 10% 1	3028
	21st to end	Day	30% 11% 27% 25% 7%	4004
January	1st to 10th	Night	23% 8% 25% 31% 12% 1	6306
	11th to 20th	Night	27% 6% 13% 27% 22% 4	6320
	21st to end	Night	31% 8% 12% 20% 25% 4	6496
February	1st to 10th	Night	26% 8% 13% 29% 18% 6%	2555
	11th to 20th	Night	25% 6% 12% 33% 21% 2	2339
	21st to end	Night	26% 7% 14% 30% 16% 7%	1980
March	1st to 10th	Night	31% 8% 8% 27% 21% 6%	5376
	11th to 20th	Night	20% 6% 9% 40% 22% 3	5187
	21st to end	Night	28% 8% 16% 28% 17% 3	5056
April	1st to 10th	Night	25% 7% 13% 29% 19% 7%	2000
	11th to 20th	Night	21% 6% 12% 30% 22% 9%	1883
	21st to end	Night	24% 5% 11% 27% 28% 6%	1900
May	1st to 10th	Night	14% 6% 8% 28% 35% 9%	2893
	11th to 20th	Night	14% 4 7% 18% 38% 19%	2654
	21st to end	Night	19% 5% 9% 25% 36% 7%	2762
June	1st to 10th	Night	21% 6% 7% 20% 27% 19%	1520
	11th to 20th	Night	12% 3 5% 16% 32% 31%	1520
	21st to end	Night	11% 3 4 11% 28% 44%	1520
July	1st to 10th	Night	6% 2 2 5% 14% 72%	2500
	11th to 20th	Night	16% 3 3 7% 12% 58%	2492
	21st to end	Night	16% 4 4 7% 18% 51%	2674
August	1st to 10th	Night	15% 3 4 7% 20% 52%	2936
	11th to 20th	Night	8% 2 2 5% 17% 65%	3156
	21st to end	Night	10% 1 2 6% 27% 54%	3498
September	1st to 10th	Night	9% 2 3 7% 27% 53%	1900
	11th to 20th	Night	13% 4 5% 16% 29% 34%	1962
	21st to end	Night	11% 2 4 14% 32% 36%	2090
October	1st to 10th	Night	16% 4 4 11% 29% 37%	3662
	11th to 20th	Night	9% 3 5% 18% 47% 19%	3777
	21st to end	Night	13% 4 6% 25% 38% 14%	4289
November	1st to 10th	Night	20% 6% 12% 28% 23% 11%	2535
	11th to 20th	Night	19% 5% 13% 30% 27% 5%	2600
	21st to end	Night	23% 5% 13% 27% 27% 5%	2620
December	1st to 10th	Night	20% 5% 9% 30% 28% 8%	4560
	11th to 20th	Night	30% 9% 17% 31% 12% 1	5176
	21st to end	Night	37% 10% 19% 22% 12%	6952

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	6% 9% 27% 36% 19% 4	200
	11th to 20th	Day	7% 7% 23% 31% 26% 7%	200
	21st to end	Day	7% 5% 18% 38% 24% 8%	420
February	1st to 10th	Day	7% 14% 22% 30% 23% 4	423
	11th to 20th	Day	6% 16% 23% 33% 19% 3	495
	21st to end	Day	6% 16% 26% 33% 18% 1	414
March	1st to 10th	Day	9% 17% 26% 31% 14% 3	500
	11th to 20th	Day	6% 19% 27% 26% 16% 5%	500
	21st to end	Day	9% 19% 26% 27% 17% 2	550
April	1st to 10th	Day	7% 18% 26% 28% 17% 5%	500
	11th to 20th	Day	8% 20% 24% 26% 16% 6%	453
	21st to end	Day	5% 18% 27% 23% 20% 7%	460
May	1st to 10th	Day	8% 17% 28% 26% 16% 4	463
	11th to 20th	Day	7% 16% 22% 24% 21% 9%	488
	21st to end	Day	6% 12% 19% 26% 25% 12%	529
June	1st to 10th	Day	3 14% 20% 23% 31% 10%	490
	11th to 20th	Day	6% 11% 24% 29% 22% 9%	490
	21st to end	Day	2 8% 18% 32% 30% 9%	490
July	1st to 10th	Day	1 4 17% 34% 33% 11%	486
	11th to 20th	Day	1 5% 11% 30% 39% 15%	484
	21st to end	Day	2 3 14% 30% 39% 12%	531
August	1st to 10th	Day	3 5% 16% 27% 34% 15%	487
	11th to 20th	Day	2 5% 15% 31% 31% 15%	477
	21st to end	Day	2 4 16% 30% 37% 11%	539
September	1st to 10th	Day	2 5% 15% 29% 38% 12%	490
	11th to 20th	Day	3 8% 17% 34% 29% 8%	490
	21st to end	Day	2 6% 19% 30% 36% 7%	490
October	1st to 10th	Day	3 8% 19% 29% 32% 9%	490
	11th to 20th	Day	4 8% 17% 28% 35% 8%	490
	21st to end	Day	4 8% 18% 33% 27% 10%	439
November	1st to 10th	Day	5% 7% 22% 31% 25% 11%	300
	11th to 20th	Day	1 5% 21% 39% 22% 11%	300
	21st to end	Day	4 8% 20% 32% 29% 7%	287
December	1st to 10th	Day	5% 6% 20% 30% 27% 14%	200
	11th to 20th	Day	5% 8% 21% 36% 24% 7%	200
	21st to end	Day	10% 8% 24% 34% 20% 4	220
January	1st to 10th	Night	7% 3 24% 38% 22% 6%	800
	11th to 20th	Night	3 4 20% 44% 25% 4	800
	21st to end	Night	8% 3 17% 44% 23% 5%	880
February	1st to 10th	Night	5% 5% 22% 43% 22% 3	800
	11th to 20th	Night	3 5% 24% 43% 21% 4	784
	21st to end	Night	4 5% 29% 43% 17% 3	660
March	1st to 10th	Night	7% 6% 28% 40% 17% 3	800
	11th to 20th	Night	5% 8% 26% 35% 23% 3	800
	21st to end	Night	6% 10% 29% 38% 16% 1	880
April	1st to 10th	Night	4 9% 29% 35% 22% 1	800
	11th to 20th	Night	5% 10% 26% 37% 17% 5%	753
	21st to end	Night	6% 10% 28% 31% 21% 3	760
May	1st to 10th	Night	7% 10% 29% 35% 17% 3	590
	11th to 20th	Night	7% 5% 24% 39% 22% 2	460
	21st to end	Night	7% 6% 21% 41% 23% 2	411
June	1st to 10th	Night	6% 6% 22% 41% 24% 2	380
	11th to 20th	Night	3 5% 23% 44% 24% 1	380
	21st to end	Night	1 2 24% 49% 22% 1	380
July	1st to 10th	Night	2 2 20% 50% 25% 1	370
	11th to 20th	Night	2 1 21% 53% 23%	368
	21st to end	Night	2 2 15% 45% 34% 1	416
August	1st to 10th	Night	4 2 18% 43% 31% 2	611
	11th to 20th	Night	2 3 16% 46% 31% 2	752
	21st to end	Night	3 1 16% 46% 32% 2	836
September	1st to 10th	Night	2 2 14% 39% 38% 5%	760
	11th to 20th	Night	3 3 17% 35% 35% 7%	760
	21st to end	Night	3 3 14% 37% 34% 8%	760
October	1st to 10th	Night	5% 3 15% 42% 28% 6%	760
	11th to 20th	Night	4 4 15% 38% 34% 5%	760
	21st to end	Night	3 3 14% 38% 31% 11%	836
November	1st to 10th	Night	5% 3 16% 37% 31% 7%	780
	11th to 20th	Night	5% 4 15% 46% 26% 5%	800
	21st to end	Night	5% 4 21% 38% 27% 6%	800
December	1st to 10th	Night	6% 3 20% 41% 25% 4	800
	11th to 20th	Night	6% 4 21% 42% 23% 3	800
	21st to end	Night	6% 7% 27% 44% 16% 1	880

Page generated at 2025-04-08 06:17:31. Version ac07

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