Gold Mountain Observing Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gold Mountain Observing Center clear sky chart from 2005-11-06 to 2026-10-14.

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	15% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	154153
	Night	17% 10% 3 8% 5% 5% 4 4 5% 6% 32%	146320
Transparency	Day	49% 6% 15% 19% 9% 1	153867
	Night	51% 6% 15% 18% 8% 1	146188
Seeing	Day	22% 12% 21% 24% 16% 5%	17714
	Night	23% 10% 16% 28% 20% 3	25943

By Month

Percentage Cloud Cover NW (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	30% 15% 4 9% 6% 6% 5% 4 5% 5% 11%	4125
	11th to 20th	Day	16% 9% 5% 11% 7% 6% 5% 4 6% 6% 25%	4614
	21st to end	Day	28% 12% 4 8% 5% 4 4 3 4 5% 22%	5425
February	1st to 10th	Day	21% 14% 3 10% 7% 6% 5% 5% 6% 6% 16%	1890
	11th to 20th	Day	21% 15% 4 9% 6% 4 4 4 4 5% 22%	1845
	21st to end	Day	19% 14% 4 10% 6% 5% 5% 4 5% 6% 23%	1607
March	1st to 10th	Day	17% 15% 5% 12% 7% 6% 6% 4 6% 6% 17%	4790
	11th to 20th	Day	18% 13% 4 9% 6% 5% 5% 3 5% 6% 27%	5110
	21st to end	Day	19% 12% 5% 11% 7% 7% 7% 5% 6% 7% 15%	5741
April	1st to 10th	Day	21% 11% 4 8% 6% 5% 6% 5% 6% 7% 21%	2520
	11th to 20th	Day	18% 11% 3 8% 6% 5% 5% 4 6% 6% 27%	2542
	21st to end	Day	16% 11% 3 9% 6% 6% 6% 5% 5% 7% 26%	2751
May	1st to 10th	Day	11% 7% 3 8% 5% 5% 5% 4 6% 7% 40%	8195
	11th to 20th	Day	15% 11% 3 9% 5% 6% 6% 4 6% 9% 25%	8160
	21st to end	Day	14% 8% 3 8% 6% 5% 6% 4 7% 9% 30%	8993
June	1st to 10th	Day	15% 9% 4 9% 6% 5% 5% 4 6% 9% 28%	3029
	11th to 20th	Day	14% 11% 4 10% 7% 7% 6% 5% 7% 6% 22%	3150
	21st to end	Day	12% 8% 3 8% 5% 5% 5% 4 6% 7% 36%	3150
July	1st to 10th	Day	7% 8% 3 7% 4 4 4 3 4 6% 51%	8820
	11th to 20th	Day	4 5% 2 5% 3 3 3 3 4 6% 62%	8544
	21st to end	Day	7% 8% 2 6% 4 4 4 3 5% 8% 48%	8916
August	1st to 10th	Day	3 4 2 5% 4 4 4 3 5% 7% 59%	6262
	11th to 20th	Day	9% 7% 2 7% 5% 5% 5% 4 6% 9% 41%	5923
	21st to end	Day	11% 9% 3 7% 4 4 4 4 6% 7% 40%	6344
September	1st to 10th	Day	10% 6% 2 7% 4 4 4 3 5% 8% 46%	2520
	11th to 20th	Day	11% 8% 3 8% 5% 4 5% 4 5% 7% 40%	2520
	21st to end	Day	15% 8% 2 8% 5% 5% 4 3 5% 8% 38%	2210
October	1st to 10th	Day	11% 7% 2 6% 5% 4 5% 5% 8% 12% 35%	3380
	11th to 20th	Day	23% 10% 4 8% 5% 4 4 4 6% 8% 26%	3292
	21st to end	Day	23% 12% 4 10% 6% 4 4 3 6% 6% 21%	3399
November	1st to 10th	Day	26% 13% 3 9% 5% 5% 4 5% 5% 6% 19%	1670
	11th to 20th	Day	27% 11% 4 9% 6% 5% 4 3 5% 5% 21%	1680
	21st to end	Day	25% 13% 4 8% 7% 5% 4 3 5% 7% 20%	1680
December	1st to 10th	Day	18% 9% 3 11% 8% 7% 6% 5% 8% 7% 18%	2700
	11th to 20th	Day	33% 12% 4 12% 7% 5% 5% 3 4 4 10%	3081
	21st to end	Day	27% 15% 6% 11% 7% 6% 5% 4 4 5% 10%	3575
January	1st to 10th	Night	30% 16% 4 10% 6% 6% 4 4 4 4 13%	8770
	11th to 20th	Night	17% 11% 5% 9% 5% 4 3 3 4 6% 34%	8267
	21st to end	Night	24% 13% 4 9% 5% 4 5% 3 4 5% 26%	8753
February	1st to 10th	Night	20% 16% 4 9% 6% 5% 4 4 5% 7% 20%	2730
	11th to 20th	Night	19% 14% 4 8% 6% 5% 4 3 4 6% 26%	2662
	21st to end	Night	19% 13% 3 9% 6% 6% 5% 4 6% 6% 24%	2198
March	1st to 10th	Night	15% 15% 4 10% 7% 6% 5% 4 6% 8% 19%	5321
	11th to 20th	Night	20% 13% 3 8% 5% 4 4 3 5% 6% 30%	5109
	21st to end	Night	23% 16% 4 10% 5% 5% 5% 4 6% 6% 16%	5463
April	1st to 10th	Night	17% 11% 3 9% 7% 5% 5% 5% 6% 6% 25%	2100
	11th to 20th	Night	14% 10% 3 9% 5% 4 6% 4 7% 7% 32%	2026
	21st to end	Night	14% 9% 3 9% 5% 5% 5% 4 7% 6% 33%	1869
May	1st to 10th	Night	11% 7% 3 6% 5% 5% 5% 4 5% 5% 46%	4724
	11th to 20th	Night	12% 8% 3 9% 6% 5% 5% 5% 7% 8% 32%	4700
	21st to end	Night	8% 7% 2 7% 5% 7% 7% 6% 8% 10% 33%	5178
June	1st to 10th	Night	14% 6% 3 7% 4 5% 5% 5% 8% 8% 35%	1586
	11th to 20th	Night	11% 8% 3 7% 6% 6% 6% 4 8% 8% 32%	1470
	21st to end	Night	7% 6% 3 6% 6% 4 4 4 6% 6% 48%	1470
July	1st to 10th	Night	6% 5% 3 6% 4 4 4 3 5% 5% 55%	4140
	11th to 20th	Night	2 3 1 3 2 2 3 3 4 6% 71%	4414
	21st to end	Night	4 4 2 6% 4 3 4 3 5% 8% 57%	5152
August	1st to 10th	Night	2 2 1 3 2 2 2 3 3 6% 74%	3892
	11th to 20th	Night	7% 5% 2 6% 4 3 4 4 6% 9% 50%	4142
	21st to end	Night	12% 8% 2 6% 4 3 3 3 4 7% 49%	4814
September	1st to 10th	Night	6% 4 1 6% 3 4 4 3 6% 6% 57%	2100
	11th to 20th	Night	12% 8% 3 7% 5% 4 4 3 5% 6% 43%	2100
	21st to end	Night	14% 8% 3 7% 4 4 4 4 5% 7% 40%	2405
October	1st to 10th	Night	12% 8% 3 7% 4 4 4 4 5% 9% 40%	4040
	11th to 20th	Night	22% 8% 2 7% 5% 5% 5% 4 5% 7% 30%	3933
	21st to end	Night	22% 14% 4 8% 5% 4 4 3 5% 5% 25%	4406
November	1st to 10th	Night	28% 11% 3 7% 5% 5% 5% 4 4 5% 23%	2845
	11th to 20th	Night	27% 10% 4 9% 6% 5% 4 3 4 5% 23%	2940
	21st to end	Night	23% 11% 3 9% 7% 5% 4 4 5% 6% 22%	2940
December	1st to 10th	Night	17% 11% 4 9% 7% 6% 6% 4 6% 7% 23%	4700
	11th to 20th	Night	32% 14% 4 12% 7% 5% 4 3 4 4 11%	5338
	21st to end	Night	30% 17% 6% 11% 6% 4 4 3 4 4 11%	7623

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% 9% 9% 3	4125
	11th to 20th	Day	58% 4 12% 16% 9% 1	4638
	21st to end	Day	65% 4 10% 14% 8%	5426
February	1st to 10th	Day	67% 7% 12% 8% 5% 1	1890
	11th to 20th	Day	64% 7% 10% 11% 7% 1	1854
	21st to end	Day	61% 8% 12% 14% 4 1	1607
March	1st to 10th	Day	61% 11% 12% 13% 3	4790
	11th to 20th	Day	57% 6% 11% 22% 4	5110
	21st to end	Day	66% 7% 15% 11% 1	5741
April	1st to 10th	Day	60% 8% 18% 12% 2	2520
	11th to 20th	Day	55% 8% 13% 17% 6% 2	2542
	21st to end	Day	55% 7% 16% 15% 6% 1	2751
May	1st to 10th	Day	41% 6% 17% 26% 9% 1	8195
	11th to 20th	Day	52% 9% 19% 17% 4	8163
	21st to end	Day	48% 8% 24% 17% 3	8994
June	1st to 10th	Day	51% 8% 19% 15% 6% 1	3029
	11th to 20th	Day	57% 9% 15% 15% 4	3150
	21st to end	Day	45% 8% 19% 19% 7% 2	3150
July	1st to 10th	Day	36% 4 16% 19% 19% 6%	8746
	11th to 20th	Day	23% 5% 14% 28% 28% 3	8463
	21st to end	Day	33% 4 20% 32% 10% 1	8723
August	1st to 10th	Day	23% 6% 13% 31% 24% 4	6245
	11th to 20th	Day	39% 5% 25% 24% 6% 1	5962
	21st to end	Day	43% 5% 16% 28% 9% 1	6343
September	1st to 10th	Day	35% 6% 18% 23% 13% 5%	2520
	11th to 20th	Day	43% 7% 15% 18% 15% 2	2520
	21st to end	Day	45% 7% 18% 18% 10% 2	2210
October	1st to 10th	Day	40% 5% 20% 28% 6%	3380
	11th to 20th	Day	56% 7% 14% 14% 8%	3292
	21st to end	Day	63% 6% 12% 12% 7% 1	3399
November	1st to 10th	Day	64% 7% 13% 11% 4 1	1670
	11th to 20th	Day	67% 6% 9% 11% 5% 1	1680
	21st to end	Day	66% 5% 10% 12% 6%	1680
December	1st to 10th	Day	61% 6% 10% 15% 7%	2700
	11th to 20th	Day	77% 7% 7% 8% 1	3084
	21st to end	Day	74% 7% 9% 8% 2	3575
January	1st to 10th	Night	75% 5% 9% 9% 2	8770
	11th to 20th	Night	52% 4 12% 18% 12% 1	8305
	21st to end	Night	62% 4 10% 17% 7%	8753
February	1st to 10th	Night	63% 8% 12% 10% 7%	2730
	11th to 20th	Night	59% 8% 12% 12% 7% 1	2664
	21st to end	Night	59% 8% 13% 15% 5% 1	2198
March	1st to 10th	Night	58% 9% 14% 13% 6%	5321
	11th to 20th	Night	53% 7% 13% 23% 4 1	5110
	21st to end	Night	67% 8% 14% 10% 2	5463
April	1st to 10th	Night	56% 9% 17% 15% 3	2100
	11th to 20th	Night	48% 10% 16% 15% 9% 1	2026
	21st to end	Night	50% 7% 17% 17% 7% 1	1869
May	1st to 10th	Night	37% 7% 12% 31% 12% 1	4721
	11th to 20th	Night	45% 8% 20% 21% 5% 1	4700
	21st to end	Night	42% 12% 25% 19% 1	5182
June	1st to 10th	Night	43% 8% 21% 22% 5% 1	1586
	11th to 20th	Night	44% 10% 21% 20% 4	1470
	21st to end	Night	35% 7% 26% 21% 8% 3	1470
July	1st to 10th	Night	30% 4 22% 20% 14% 9%	4105
	11th to 20th	Night	16% 3 14% 31% 33% 3	4372
	21st to end	Night	26% 4 24% 32% 12% 3	5040
August	1st to 10th	Night	14% 3 15% 37% 27% 5%	3884
	11th to 20th	Night	31% 4 31% 25% 7% 2	4169
	21st to end	Night	37% 4 20% 26% 11% 1	4814
September	1st to 10th	Night	27% 5% 21% 27% 15% 5%	2100
	11th to 20th	Night	41% 7% 14% 18% 16% 5%	2100
	21st to end	Night	43% 7% 14% 22% 12% 2	2405
October	1st to 10th	Night	40% 5% 17% 32% 6%	4040
	11th to 20th	Night	52% 5% 14% 21% 7%	3933
	21st to end	Night	59% 6% 11% 14% 9% 1	4406
November	1st to 10th	Night	62% 7% 13% 14% 4 1	2845
	11th to 20th	Night	64% 7% 11% 10% 6% 2	2940
	21st to end	Night	61% 7% 13% 10% 6% 1	2940
December	1st to 10th	Night	59% 6% 12% 16% 7%	4700
	11th to 20th	Night	77% 6% 9% 6% 2	5334
	21st to end	Night	74% 8% 8% 8% 2	7623

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	38% 16% 19% 14% 10% 4	220
	11th to 20th	Day	34% 11% 16% 17% 18% 3	220
	21st to end	Day	33% 15% 18% 19% 13% 2	326
February	1st to 10th	Day	35% 15% 19% 20% 9% 2	430
	11th to 20th	Day	35% 18% 21% 19% 6% 1	425
	21st to end	Day	30% 20% 24% 17% 8% 1	383
March	1st to 10th	Day	29% 18% 25% 18% 8% 2	540
	11th to 20th	Day	31% 17% 21% 15% 10% 5%	540
	21st to end	Day	29% 18% 25% 18% 8% 2	594
April	1st to 10th	Day	31% 14% 23% 20% 10% 4	540
	11th to 20th	Day	23% 17% 27% 19% 10% 5%	532
	21st to end	Day	21% 15% 26% 26% 9% 3	540
May	1st to 10th	Day	16% 12% 24% 30% 15% 3	543
	11th to 20th	Day	25% 10% 21% 21% 17% 7%	568
	21st to end	Day	21% 7% 21% 27% 17% 6%	617
June	1st to 10th	Day	20% 12% 24% 24% 16% 4	659
	11th to 20th	Day	21% 11% 23% 29% 12% 3	780
	21st to end	Day	16% 8% 23% 28% 20% 6%	780
July	1st to 10th	Day	12% 6% 19% 31% 25% 7%	771
	11th to 20th	Day	5% 4 21% 35% 26% 9%	674
	21st to end	Day	9% 4 13% 27% 34% 13%	619
August	1st to 10th	Day	9% 7% 14% 28% 31% 11%	567
	11th to 20th	Day	10% 4 13% 31% 32% 11%	557
	21st to end	Day	14% 9% 17% 30% 22% 7%	627
September	1st to 10th	Day	14% 7% 22% 33% 19% 6%	570
	11th to 20th	Day	17% 9% 19% 30% 18% 7%	570
	21st to end	Day	18% 12% 23% 29% 14% 3	570
October	1st to 10th	Day	20% 10% 24% 22% 19% 4	570
	11th to 20th	Day	27% 12% 17% 24% 15% 5%	543
	21st to end	Day	33% 16% 19% 19% 11% 3	486
November	1st to 10th	Day	32% 18% 20% 18% 10% 2	231
	11th to 20th	Day	35% 16% 20% 15% 10% 4	220
	21st to end	Day	31% 14% 19% 16% 14% 5%	220
December	1st to 10th	Day	39% 14% 9% 19% 14% 5%	220
	11th to 20th	Day	41% 18% 18% 13% 9% 1	220
	21st to end	Day	38% 14% 18% 18% 10% 2	242
January	1st to 10th	Night	40% 13% 14% 19% 12% 2	950
	11th to 20th	Night	31% 11% 14% 26% 16% 2	856
	21st to end	Night	28% 11% 15% 27% 17% 3	924
February	1st to 10th	Night	30% 15% 16% 25% 11% 2	840
	11th to 20th	Night	27% 13% 18% 25% 14% 2	824
	21st to end	Night	27% 14% 21% 25% 11% 2	692
March	1st to 10th	Night	26% 13% 21% 26% 13% 1	840
	11th to 20th	Night	28% 15% 19% 24% 11% 2	840
	21st to end	Night	27% 16% 20% 25% 11% 2	924
April	1st to 10th	Night	23% 13% 20% 26% 16% 3	840
	11th to 20th	Night	17% 12% 22% 31% 15% 2	778
	21st to end	Night	19% 10% 18% 34% 16% 2	609
May	1st to 10th	Night	15% 10% 15% 39% 19% 2	420
	11th to 20th	Night	15% 7% 15% 32% 27% 3	414
	21st to end	Night	13% 7% 13% 37% 26% 5%	455
June	1st to 10th	Night	16% 10% 14% 32% 25% 3	420
	11th to 20th	Night	13% 9% 21% 34% 21% 2	420
	21st to end	Night	10% 6% 15% 37% 28% 3	420
July	1st to 10th	Night	8% 2 11% 36% 39% 3	410
	11th to 20th	Night	7% 3 11% 37% 38% 3	408
	21st to end	Night	5% 3 10% 32% 43% 7%	460
August	1st to 10th	Night	5% 3 7% 30% 42% 12%	497
	11th to 20th	Night	7% 3 6% 26% 46% 11%	624
	21st to end	Night	11% 4 11% 36% 34% 4	798
September	1st to 10th	Night	8% 5% 15% 37% 31% 5%	840
	11th to 20th	Night	15% 5% 14% 28% 32% 6%	840
	21st to end	Night	18% 8% 17% 33% 22% 3	840
October	1st to 10th	Night	17% 8% 16% 31% 24% 4	840
	11th to 20th	Night	24% 9% 15% 27% 21% 3	812
	21st to end	Night	28% 10% 16% 28% 16% 2	880
November	1st to 10th	Night	34% 11% 15% 28% 12% 1	820
	11th to 20th	Night	34% 13% 18% 22% 13% 1	840
	21st to end	Night	29% 11% 18% 25% 15% 2	840
December	1st to 10th	Night	29% 9% 13% 30% 17% 2	840
	11th to 20th	Night	38% 13% 16% 21% 11% 2	843
	21st to end	Night	38% 13% 17% 23% 9% 1	1045

Page generated at 2026-10-11 02:06:29. Version ac07

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