Heaven's Gate Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heaven's Gate Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	20% 9% 2 5% 3 3 3 3 4 5% 42%	112796
	Night	24% 10% 2 5% 3 3 3 3 4 5% 38%	114563
Transparency	Day	45% 5% 11% 19% 14% 6%	112542
	Night	49% 5% 11% 17% 12% 5%	114406
Seeing	Day	24% 10% 24% 26% 12% 3	15016
	Night	27% 6% 14% 30% 21% 2	24159

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	39% 16% 3 8% 4 4 3 2 3 4 14%	3639
	11th to 20th	Day	29% 11% 3 5% 3 3 3 2 3 4 34%	3663
	21st to end	Day	30% 9% 2 5% 3 3 3 3 4 5% 34%	4510
February	1st to 10th	Day	30% 14% 3 7% 5% 4 6% 3 5% 4 19%	1800
	11th to 20th	Day	32% 14% 3 7% 4 3 3 3 3 5% 22%	1755
	21st to end	Day	31% 19% 2 8% 4 4 3 3 3 5% 19%	1504
March	1st to 10th	Day	24% 19% 4 7% 4 3 3 4 4 5% 23%	3320
	11th to 20th	Day	22% 12% 2 5% 4 4 3 2 3 4 39%	3490
	21st to end	Day	28% 16% 4 8% 6% 5% 5% 3 4 4 16%	3886
April	1st to 10th	Day	31% 17% 4 8% 5% 3 4 2 4 4 17%	2280
	11th to 20th	Day	26% 15% 4 7% 5% 3 3 3 4 5% 24%	2249
	21st to end	Day	27% 11% 3 6% 4 4 3 3 5% 6% 27%	2433
May	1st to 10th	Day	24% 10% 2 7% 5% 3 4 3 4 4 34%	4281
	11th to 20th	Day	13% 6% 2 5% 3 2 3 3 5% 7% 51%	4377
	21st to end	Day	19% 9% 3 8% 5% 4 5% 4 5% 6% 31%	4813
June	1st to 10th	Day	25% 9% 2 6% 4 3 4 3 4 5% 33%	2660
	11th to 20th	Day	19% 8% 3 6% 4 4 4 4 5% 7% 35%	2660
	21st to end	Day	10% 5% 2 4 3 3 3 3 4 5% 58%	2660
July	1st to 10th	Day	4 2 1 2 1 1 1 2 2 3 81%	4420
	11th to 20th	Day	4 2 1 3 2 2 2 2 3 4 77%	4420
	21st to end	Day	3 2 1 2 2 2 2 2 2 4 78%	4712
August	1st to 10th	Day	3 3 1 3 2 2 3 2 4 5% 72%	4091
	11th to 20th	Day	4 4 1 3 3 2 4 3 5% 6% 65%	4026
	21st to end	Day	7% 4 1 4 2 2 3 2 4 6% 65%	4273
September	1st to 10th	Day	6% 3 1 3 2 2 2 1 3 4 71%	2280
	11th to 20th	Day	12% 7% 3 5% 3 4 3 3 5% 6% 50%	2275
	21st to end	Day	15% 6% 2 6% 4 3 3 3 4 6% 49%	2000
October	1st to 10th	Day	14% 6% 2 4 3 2 3 2 3 5% 56%	3180
	11th to 20th	Day	19% 7% 2 7% 4 3 4 3 5% 7% 38%	3161
	21st to end	Day	29% 9% 2 6% 4 4 5% 3 5% 7% 26%	3430
November	1st to 10th	Day	35% 10% 3 8% 4 4 3 3 3 5% 22%	1696
	11th to 20th	Day	35% 10% 3 7% 4 4 4 3 5% 5% 22%	1600
	21st to end	Day	30% 10% 3 6% 4 3 4 3 4 6% 27%	1600
December	1st to 10th	Day	22% 8% 2 4 3 3 4 4 4 5% 41%	2620
	11th to 20th	Day	38% 12% 4 8% 4 4 3 3 4 5% 14%	3028
	21st to end	Day	43% 15% 4 6% 3 3 3 2 3 3 14%	4004
January	1st to 10th	Night	43% 15% 3 6% 3 3 3 2 3 3 14%	6306
	11th to 20th	Night	29% 13% 3 6% 3 3 3 2 4 4 31%	6293
	21st to end	Night	30% 11% 3 5% 3 3 2 2 3 4 36%	6468
February	1st to 10th	Night	30% 17% 4 7% 4 4 3 3 4 5% 19%	2600
	11th to 20th	Night	31% 14% 3 7% 4 4 3 3 3 4 25%	2532
	21st to end	Night	30% 17% 3 6% 4 3 3 3 5% 5% 22%	1951
March	1st to 10th	Night	25% 22% 4 6% 4 3 3 3 4 5% 21%	3550
	11th to 20th	Night	21% 12% 2 6% 3 3 3 2 3 4 41%	3490
	21st to end	Night	29% 21% 4 9% 5% 4 3 3 3 4 16%	3767
April	1st to 10th	Night	31% 17% 3 6% 4 3 3 3 3 5% 22%	1900
	11th to 20th	Night	26% 14% 3 5% 4 3 3 3 4 4 29%	1879
	21st to end	Night	26% 12% 3 6% 4 4 4 3 4 6% 28%	1743
May	1st to 10th	Night	26% 14% 3 6% 3 3 2 2 3 4 34%	2682
	11th to 20th	Night	12% 7% 2 5% 3 3 4 2 4 5% 53%	2516
	21st to end	Night	21% 8% 3 7% 3 4 4 3 4 6% 36%	2762
June	1st to 10th	Night	22% 10% 2 5% 3 4 3 4 6% 6% 35%	1520
	11th to 20th	Night	16% 8% 2 6% 3 3 4 3 5% 6% 43%	1520
	21st to end	Night	11% 4 1 3 3 3 3 2 3 5% 62%	1520
July	1st to 10th	Night	4 1 2 1 1 1 1 2 3 82%	2540
	11th to 20th	Night	3 2 1 2 2 1 2 2 3 2 80%	2540
	21st to end	Night	6% 1 1 2 1 1 1 1 2 4 82%	2839
August	1st to 10th	Night	2 3 1 2 2 2 3 2 4 5% 75%	2848
	11th to 20th	Night	4 3 1 4 3 3 4 3 5% 6% 64%	2812
	21st to end	Night	8% 3 1 4 2 2 3 2 3 4 69%	3402
September	1st to 10th	Night	6% 3 1 3 1 2 2 2 4 4 71%	1900
	11th to 20th	Night	12% 5% 2 3 3 3 4 3 5% 6% 55%	1901
	21st to end	Night	13% 6% 2 5% 3 2 4 2 4 6% 52%	2180
October	1st to 10th	Night	13% 6% 2 4 3 2 2 2 3 4 59%	3800
	11th to 20th	Night	16% 7% 2 6% 3 3 4 3 5% 8% 43%	3777
	21st to end	Night	31% 11% 3 6% 4 4 4 2 4 4 27%	4137
November	1st to 10th	Night	32% 9% 3 7% 4 3 4 3 5% 5% 25%	2600
	11th to 20th	Night	37% 10% 3 6% 3 3 3 3 4 5% 21%	2800
	21st to end	Night	27% 11% 3 6% 4 3 3 4 4 5% 31%	2800
December	1st to 10th	Night	19% 9% 2 5% 3 3 3 3 4 6% 43%	4560
	11th to 20th	Night	39% 13% 3 7% 4 3 4 3 4 5% 14%	5176
	21st to end	Night	38% 14% 3 6% 4 3 4 3 3 4 17%	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	75% 5% 7% 8% 5% 1	3639
	11th to 20th	Day	56% 2 7% 22% 10% 3	3663
	21st to end	Day	51% 6% 7% 14% 14% 8%	4510
February	1st to 10th	Day	66% 7% 10% 10% 6% 1	1800
	11th to 20th	Day	65% 8% 10% 9% 5% 3	1764
	21st to end	Day	69% 6% 8% 9% 5% 3	1504
March	1st to 10th	Day	60% 7% 11% 17% 5%	3320
	11th to 20th	Day	49% 7% 8% 23% 12% 2	3490
	21st to end	Day	66% 11% 10% 8% 4	3886
April	1st to 10th	Day	70% 6% 10% 10% 4	2280
	11th to 20th	Day	61% 8% 10% 11% 7% 3	2252
	21st to end	Day	56% 8% 14% 14% 6% 1	2433
May	1st to 10th	Day	51% 9% 12% 18% 10%	4281
	11th to 20th	Day	31% 8% 15% 31% 14% 1	4380
	21st to end	Day	49% 11% 23% 16% 2	4814
June	1st to 10th	Day	53% 6% 15% 20% 5%	2660
	11th to 20th	Day	47% 8% 17% 21% 7%	2660
	21st to end	Day	28% 5% 14% 34% 14% 4	2660
July	1st to 10th	Day	11% 3 8% 16% 26% 37%	4351
	11th to 20th	Day	14% 2 5% 16% 30% 32%	4345
	21st to end	Day	12% 2 8% 26% 40% 12%	4563
August	1st to 10th	Day	17% 3 7% 26% 34% 13%	4076
	11th to 20th	Day	21% 3 12% 39% 20% 5%	4065
	21st to end	Day	23% 2 15% 25% 24% 11%	4272
September	1st to 10th	Day	19% 3 8% 30% 25% 15%	2280
	11th to 20th	Day	35% 6% 11% 24% 17% 8%	2275
	21st to end	Day	36% 7% 12% 22% 16% 8%	2000
October	1st to 10th	Day	33% 4 8% 20% 21% 15%	3180
	11th to 20th	Day	46% 3 15% 21% 12% 2	3161
	21st to end	Day	57% 4 8% 17% 12% 2	3430
November	1st to 10th	Day	65% 6% 10% 13% 6% 1	1696
	11th to 20th	Day	63% 6% 11% 9% 7% 4	1600
	21st to end	Day	59% 5% 11% 10% 11% 4	1600
December	1st to 10th	Day	46% 4 8% 15% 22% 5%	2620
	11th to 20th	Day	72% 6% 8% 10% 3	3028
	21st to end	Day	77% 4 9% 7% 2 1	4004
January	1st to 10th	Night	76% 5% 6% 8% 4 1	6306
	11th to 20th	Night	57% 4 8% 19% 9% 3	6293
	21st to end	Night	55% 3 5% 13% 14% 10%	6468
February	1st to 10th	Night	68% 6% 9% 10% 6% 1	2600
	11th to 20th	Night	64% 6% 9% 12% 6% 3	2534
	21st to end	Night	63% 7% 10% 12% 6% 2	1951
March	1st to 10th	Night	62% 8% 12% 13% 3 1	3550
	11th to 20th	Night	46% 8% 12% 21% 12% 1	3490
	21st to end	Night	69% 9% 11% 9% 1	3767
April	1st to 10th	Night	66% 7% 12% 10% 5%	1900
	11th to 20th	Night	58% 8% 14% 11% 6% 3	1879
	21st to end	Night	56% 9% 13% 13% 5% 3	1743
May	1st to 10th	Night	53% 9% 11% 16% 11% 1	2682
	11th to 20th	Night	33% 8% 11% 37% 9% 1	2516
	21st to end	Night	45% 12% 24% 16% 2	2762
June	1st to 10th	Night	48% 10% 13% 25% 4	1520
	11th to 20th	Night	40% 10% 18% 24% 7% 1	1520
	21st to end	Night	26% 5% 18% 33% 14% 5%	1520
July	1st to 10th	Night	11% 3 7% 19% 27% 34%	2500
	11th to 20th	Night	13% 2 5% 18% 33% 29%	2492
	21st to end	Night	11% 1 9% 28% 42% 9%	2750
August	1st to 10th	Night	14% 3 8% 34% 29% 12%	2839
	11th to 20th	Night	21% 2 17% 37% 19% 4	2839
	21st to end	Night	22% 3 12% 27% 28% 8%	3402
September	1st to 10th	Night	17% 4 8% 31% 26% 14%	1900
	11th to 20th	Night	30% 6% 12% 27% 15% 9%	1901
	21st to end	Night	34% 5% 12% 23% 16% 10%	2180
October	1st to 10th	Night	31% 3 7% 22% 27% 9%	3800
	11th to 20th	Night	40% 4 22% 21% 10% 2	3777
	21st to end	Night	61% 4 8% 14% 10% 3	4137
November	1st to 10th	Night	60% 7% 10% 13% 9% 1	2600
	11th to 20th	Night	65% 7% 9% 10% 6% 4	2800
	21st to end	Night	56% 6% 9% 12% 11% 6%	2800
December	1st to 10th	Night	43% 4 8% 18% 23% 3	4560
	11th to 20th	Night	73% 6% 8% 8% 4	5176
	21st to end	Night	69% 6% 14% 8% 2 1	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	51% 10% 14% 16% 10% 1	200
	11th to 20th	Day	42% 6% 13% 23% 12% 4	210
	21st to end	Day	34% 10% 20% 19% 15% 3	440
February	1st to 10th	Day	37% 10% 22% 21% 9% 2	400
	11th to 20th	Day	36% 14% 24% 18% 6% 2	395
	21st to end	Day	40% 15% 17% 21% 5% 1	351
March	1st to 10th	Day	40% 14% 21% 17% 8%	460
	11th to 20th	Day	31% 12% 25% 20% 10% 2	460
	21st to end	Day	37% 19% 21% 19% 5% 1	506
April	1st to 10th	Day	36% 15% 21% 19% 8% 1	460
	11th to 20th	Day	34% 17% 22% 17% 7% 2	452
	21st to end	Day	31% 17% 22% 22% 8% 1	460
May	1st to 10th	Day	23% 15% 26% 25% 8% 3	463
	11th to 20th	Day	30% 12% 21% 23% 11% 3	488
	21st to end	Day	22% 13% 24% 25% 12% 4	529
June	1st to 10th	Day	26% 10% 22% 29% 11% 3	490
	11th to 20th	Day	20% 15% 30% 25% 9% 1	490
	21st to end	Day	8% 7% 30% 37% 13% 5%	490
July	1st to 10th	Day	7% 6% 32% 36% 14% 5%	486
	11th to 20th	Day	4 7% 33% 36% 15% 6%	484
	21st to end	Day	3 5% 30% 37% 19% 7%	531
August	1st to 10th	Day	3 3 29% 43% 15% 7%	487
	11th to 20th	Day	4 5% 24% 40% 19% 9%	477
	21st to end	Day	7% 8% 30% 34% 16% 5%	539
September	1st to 10th	Day	7% 7% 29% 32% 20% 5%	490
	11th to 20th	Day	17% 6% 24% 34% 17% 2	490
	21st to end	Day	17% 11% 27% 27% 16% 2	490
October	1st to 10th	Day	20% 8% 27% 24% 17% 4	490
	11th to 20th	Day	25% 7% 20% 27% 18% 4	490
	21st to end	Day	30% 12% 21% 21% 12% 3	533
November	1st to 10th	Day	35% 10% 21% 20% 12% 2	265
	11th to 20th	Day	37% 9% 18% 21% 12% 5%	200
	21st to end	Day	35% 9% 16% 23% 16% 4	200
December	1st to 10th	Day	33% 11% 17% 21% 11% 9%	200
	11th to 20th	Day	45% 8% 17% 21% 8% 2	200
	21st to end	Day	45% 10% 18% 19% 7% 2	220
January	1st to 10th	Night	46% 8% 15% 20% 10% 2	800
	11th to 20th	Night	36% 7% 16% 22% 16% 3	800
	21st to end	Night	34% 5% 15% 29% 14% 2	880
February	1st to 10th	Night	38% 7% 15% 29% 11% 1	800
	11th to 20th	Night	37% 7% 15% 29% 12% 1	784
	21st to end	Night	36% 9% 15% 28% 11%	628
March	1st to 10th	Night	36% 10% 14% 28% 11% 1	760
	11th to 20th	Night	29% 8% 16% 31% 15% 1	760
	21st to end	Night	37% 10% 15% 24% 14% 1	836
April	1st to 10th	Night	38% 8% 15% 24% 15% 1	760
	11th to 20th	Night	30% 10% 17% 25% 17% 2	751
	21st to end	Night	30% 7% 16% 29% 16% 2	603
May	1st to 10th	Night	25% 8% 15% 29% 22% 3	470
	11th to 20th	Night	23% 4 11% 32% 28% 2	374
	21st to end	Night	25% 4 16% 26% 27% 2	411
June	1st to 10th	Night	22% 6% 11% 32% 28% 2	380
	11th to 20th	Night	17% 7% 17% 34% 24% 1	380
	21st to end	Night	14% 1 11% 38% 37% 1	380
July	1st to 10th	Night	6% 2 8% 40% 43% 1	370
	11th to 20th	Night	6% 1 15% 46% 32% 1	368
	21st to end	Night	3 9% 40% 44% 3	458
August	1st to 10th	Night	4 2 5% 34% 47% 8%	567
	11th to 20th	Night	5% 1 5% 36% 47% 7%	564
	21st to end	Night	7% 2 12% 39% 36% 5%	779
September	1st to 10th	Night	8% 2 12% 34% 40% 4	760
	11th to 20th	Night	14% 2 10% 30% 39% 6%	760
	21st to end	Night	15% 5% 11% 35% 31% 4	760
October	1st to 10th	Night	19% 2 14% 35% 25% 4	760
	11th to 20th	Night	22% 4 10% 34% 26% 4	760
	21st to end	Night	31% 5% 14% 30% 16% 4	836
November	1st to 10th	Night	34% 7% 14% 30% 14% 1	780
	11th to 20th	Night	40% 7% 14% 27% 10% 2	800
	21st to end	Night	32% 6% 14% 32% 14% 2	800
December	1st to 10th	Night	32% 6% 17% 28% 16% 2	800
	11th to 20th	Night	43% 6% 15% 23% 13% 1	800
	21st to end	Night	46% 8% 16% 23% 6% 1	880

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