Mountain High observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mountain High observatory clear sky chart from 2005-11-06 to 2025-05-11.

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	15% 6% 2 6% 4 4 5% 4 6% 7% 40%	114221
	Night	16% 6% 2 5% 4 4 4 3 5% 7% 43%	114744
Transparency	Day	23% 44% 7% 11% 11% 5%	118002
	Night	22% 42% 7% 11% 11% 6%	119102
Seeing	Day	21% 13% 24% 22% 14% 7%	15620
	Night	20% 10% 21% 27% 18% 4	25741

By Month

Percentage Cloud Cover SE (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	16% 5% 1 4 2 2 2 2 4 6% 54%	4056
	11th to 20th	Day	12% 7% 2 4 3 3 2 1 3 3 60%	4060
	21st to end	Day	21% 5% 2 6% 3 2 3 3 4 6% 45%	4466
February	1st to 10th	Day	17% 8% 3 6% 4 4 3 3 5% 5% 43%	1954
	11th to 20th	Day	21% 6% 2 6% 4 3 3 3 4 6% 40%	1948
	21st to end	Day	21% 7% 2 6% 4 3 4 3 3 6% 41%	1650
March	1st to 10th	Day	21% 7% 3 6% 4 4 4 3 5% 5% 40%	4751
	11th to 20th	Day	13% 6% 2 5% 3 3 3 3 4 6% 52%	4980
	21st to end	Day	20% 6% 2 5% 4 3 4 3 4 5% 44%	5465
April	1st to 10th	Day	14% 5% 2 7% 4 4 5% 4 7% 8% 41%	2200
	11th to 20th	Day	16% 5% 2 5% 4 4 5% 4 5% 6% 42%	2396
	21st to end	Day	16% 5% 2 5% 4 4 4 4 5% 6% 45%	2600
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 7% 35%	4212
	11th to 20th	Day	21% 7% 3 7% 5% 5% 5% 4 6% 7% 31%	4081
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 5% 8% 10% 38%	4476
June	1st to 10th	Day	13% 7% 3 8% 6% 6% 7% 5% 7% 9% 28%	2470
	11th to 20th	Day	8% 6% 2 6% 6% 6% 7% 6% 9% 11% 33%	2470
	21st to end	Day	9% 6% 2 7% 7% 6% 7% 6% 7% 9% 34%	2470
July	1st to 10th	Day	8% 6% 3 9% 7% 7% 7% 6% 10% 9% 28%	4120
	11th to 20th	Day	11% 6% 3 9% 7% 7% 8% 5% 7% 10% 26%	4120
	21st to end	Day	15% 10% 4 12% 8% 8% 8% 6% 6% 7% 16%	4466
August	1st to 10th	Day	11% 7% 3 8% 6% 7% 7% 6% 8% 10% 27%	4104
	11th to 20th	Day	11% 6% 3 7% 5% 7% 7% 5% 8% 11% 32%	4044
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 9% 41%	4101
September	1st to 10th	Day	14% 8% 3 6% 6% 6% 7% 4 7% 7% 34%	2090
	11th to 20th	Day	17% 8% 2 7% 5% 6% 5% 5% 6% 7% 32%	2090
	21st to end	Day	17% 7% 3 6% 6% 5% 4 4 5% 7% 37%	2090
October	1st to 10th	Day	16% 6% 2 4 3 3 3 3 4 6% 50%	2090
	11th to 20th	Day	13% 4 1 4 3 3 3 3 5% 6% 55%	1909
	21st to end	Day	15% 4 1 4 4 2 4 3 4 4 55%	2090
November	1st to 10th	Day	14% 6% 1 4 3 3 3 2 4 5% 55%	1833
	11th to 20th	Day	16% 5% 1 3 3 3 3 3 5% 6% 52%	1800
	21st to end	Day	17% 4 1 5% 3 3 4 3 5% 7% 48%	1800
December	1st to 10th	Day	18% 5% 1 4 3 3 4 3 4 5% 49%	2930
	11th to 20th	Day	17% 8% 2 5% 3 5% 5% 3 5% 5% 42%	3373
	21st to end	Day	23% 5% 1 3 3 2 3 3 5% 6% 45%	4466
January	1st to 10th	Night	16% 4 1 4 3 3 3 3 4 5% 55%	5889
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 4 5% 54%	5900
	21st to end	Night	18% 6% 2 4 3 3 4 2 4 7% 46%	6490
February	1st to 10th	Night	15% 6% 3 5% 4 3 4 3 6% 7% 44%	2444
	11th to 20th	Night	21% 7% 2 4 5% 4 4 3 4 4 42%	2338
	21st to end	Night	21% 6% 2 6% 4 4 4 3 5% 6% 39%	1980
March	1st to 10th	Night	21% 8% 2 5% 3 4 5% 5% 5% 7% 36%	5102
	11th to 20th	Night	12% 4 1 4 3 3 4 4 5% 8% 51%	4980
	21st to end	Night	18% 6% 2 5% 4 4 5% 3 4 6% 43%	5467
April	1st to 10th	Night	15% 6% 2 5% 4 3 3 4 4 6% 47%	2200
	11th to 20th	Night	15% 6% 2 4 3 3 4 2 5% 6% 49%	1970
	21st to end	Night	14% 6% 2 5% 3 3 4 3 5% 7% 48%	1800
May	1st to 10th	Night	15% 8% 2 7% 4 4 5% 4 7% 9% 36%	2903
	11th to 20th	Night	21% 8% 2 5% 4 3 4 4 6% 8% 35%	2818
	21st to end	Night	10% 5% 2 5% 4 4 5% 4 8% 11% 41%	3079
June	1st to 10th	Night	13% 7% 2 6% 6% 5% 5% 5% 8% 10% 32%	1710
	11th to 20th	Night	9% 6% 2 6% 4 5% 5% 4 7% 11% 41%	1710
	21st to end	Night	9% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	1710
July	1st to 10th	Night	12% 8% 3 8% 7% 6% 6% 5% 7% 10% 29%	2840
	11th to 20th	Night	10% 6% 3 6% 6% 6% 6% 5% 9% 11% 32%	2840
	21st to end	Night	16% 10% 4 10% 6% 6% 5% 5% 7% 8% 22%	3087
August	1st to 10th	Night	12% 7% 3 8% 5% 5% 6% 5% 8% 12% 30%	2825
	11th to 20th	Night	16% 9% 3 7% 5% 5% 5% 4 7% 8% 32%	2785
	21st to end	Night	9% 5% 2 5% 5% 5% 5% 4 7% 10% 45%	3515
September	1st to 10th	Night	14% 6% 2 7% 5% 5% 5% 4 8% 7% 37%	2090
	11th to 20th	Night	19% 9% 2 6% 5% 4 5% 4 6% 6% 34%	2090
	21st to end	Night	19% 6% 2 5% 4 4 4 3 5% 6% 43%	2090
October	1st to 10th	Night	16% 5% 2 5% 3 3 3 3 4 6% 51%	2090
	11th to 20th	Night	15% 6% 2 5% 3 3 3 3 4 5% 52%	2270
	21st to end	Night	15% 5% 1 3 3 3 4 2 4 5% 55%	2508
November	1st to 10th	Night	15% 6% 2 4 3 3 3 2 4 7% 51%	2462
	11th to 20th	Night	16% 6% 2 4 3 2 4 3 4 6% 50%	2600
	21st to end	Night	16% 5% 2 5% 3 3 4 3 6% 6% 46%	2600
December	1st to 10th	Night	17% 6% 2 5% 3 3 3 3 4 5% 50%	4250
	11th to 20th	Night	20% 7% 2 6% 3 3 4 3 4 6% 42%	4822
	21st to end	Night	22% 7% 2 4 3 3 4 3 4 6% 41%	6490

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	21% 26% 4 14% 25% 10%	4057
	11th to 20th	Day	19% 27% 3 9% 27% 14%	4060
	21st to end	Day	28% 30% 4 12% 15% 11%	4466
February	1st to 10th	Day	16% 62% 3 9% 8% 2	1954
	11th to 20th	Day	19% 62% 3 6% 7% 3	1958
	21st to end	Day	19% 62% 2 5% 8% 4	1650
March	1st to 10th	Day	33% 31% 5% 9% 12% 10%	4751
	11th to 20th	Day	19% 30% 3 9% 20% 19%	4980
	21st to end	Day	29% 29% 5% 19% 13% 5%	5465
April	1st to 10th	Day	17% 63% 6% 6% 7% 2	2200
	11th to 20th	Day	18% 63% 4 8% 6% 2	2396
	21st to end	Day	15% 63% 6% 9% 7% 1	2600
May	1st to 10th	Day	28% 43% 8% 13% 7% 1	4250
	11th to 20th	Day	31% 43% 7% 13% 6%	4085
	21st to end	Day	21% 44% 10% 11% 13% 1	4489
June	1st to 10th	Day	19% 67% 4 7% 2 1	2470
	11th to 20th	Day	15% 68% 8% 6% 3	2470
	21st to end	Day	16% 65% 8% 7% 4	2470
July	1st to 10th	Day	26% 45% 13% 14% 2	4055
	11th to 20th	Day	29% 44% 17% 9% 1	4051
	21st to end	Day	47% 40% 6% 5% 2	4324
August	1st to 10th	Day	29% 43% 12% 13% 2	4091
	11th to 20th	Day	19% 46% 17% 7% 8% 3	4083
	21st to end	Day	16% 42% 11% 23% 7% 1	4101
September	1st to 10th	Day	18% 62% 5% 8% 7% 1	2090
	11th to 20th	Day	20% 62% 5% 7% 5% 1	2090
	21st to end	Day	19% 62% 6% 8% 5%	2090
October	1st to 10th	Day	15% 38% 6% 10% 23% 8%	3480
	11th to 20th	Day	9% 37% 3 9% 17% 25%	3149
	21st to end	Day	15% 39% 5% 15% 14% 13%	3428
November	1st to 10th	Day	18% 56% 3 9% 10% 4	1833
	11th to 20th	Day	15% 56% 2 10% 12% 5%	1800
	21st to end	Day	14% 57% 4 10% 13% 3	1800
December	1st to 10th	Day	21% 37% 5% 10% 17% 11%	2930
	11th to 20th	Day	28% 33% 5% 12% 14% 8%	3370
	21st to end	Day	27% 27% 7% 22% 15% 2	4466
January	1st to 10th	Night	22% 26% 7% 11% 21% 12%	5889
	11th to 20th	Night	21% 27% 5% 9% 23% 15%	5900
	21st to end	Night	26% 29% 6% 9% 18% 12%	6490
February	1st to 10th	Night	14% 62% 5% 10% 7% 2	2444
	11th to 20th	Night	20% 61% 4 6% 7% 2	2339
	21st to end	Night	17% 61% 3 7% 8% 3	1980
March	1st to 10th	Night	31% 32% 7% 12% 13% 4	5102
	11th to 20th	Night	18% 30% 6% 12% 17% 17%	4980
	21st to end	Night	29% 30% 8% 16% 11% 7%	5467
April	1st to 10th	Night	16% 62% 5% 7% 8% 2	2200
	11th to 20th	Night	16% 63% 4 9% 6% 2	1970
	21st to end	Night	14% 62% 5% 11% 7% 1	1800
May	1st to 10th	Night	26% 44% 8% 14% 6% 1	2930
	11th to 20th	Night	27% 44% 9% 12% 8%	2819
	21st to end	Night	15% 46% 13% 18% 8%	3087
June	1st to 10th	Night	17% 66% 6% 6% 3 1	1710
	11th to 20th	Night	12% 67% 9% 7% 4	1710
	21st to end	Night	12% 66% 9% 9% 5%	1710
July	1st to 10th	Night	28% 45% 15% 11% 1	2795
	11th to 20th	Night	22% 46% 19% 11% 1	2786
	21st to end	Night	39% 43% 10% 5% 2	2988
August	1st to 10th	Night	27% 45% 16% 10% 1	2814
	11th to 20th	Night	26% 45% 13% 9% 6% 1	2812
	21st to end	Night	13% 42% 13% 24% 7% 1	3515
September	1st to 10th	Night	16% 62% 7% 7% 7% 1	2090
	11th to 20th	Night	19% 63% 6% 7% 6%	2090
	21st to end	Night	18% 61% 6% 9% 5% 1	2090
October	1st to 10th	Night	14% 42% 9% 11% 20% 5%	3480
	11th to 20th	Night	8% 37% 4 10% 18% 23%	3769
	21st to end	Night	18% 38% 3 13% 15% 13%	4120
November	1st to 10th	Night	19% 56% 5% 9% 7% 4	2462
	11th to 20th	Night	16% 57% 4 6% 11% 6%	2600
	21st to end	Night	15% 57% 3 10% 11% 3	2600
December	1st to 10th	Night	21% 37% 6% 12% 17% 9%	4250
	11th to 20th	Night	28% 34% 5% 14% 14% 5%	4824
	21st to end	Night	31% 27% 9% 17% 14% 2	6490

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	35% 23% 30% 9% 2	300
	11th to 20th	Day	38% 22% 27% 11% 2	300
	21st to end	Day	34% 27% 28% 8% 2	330
February	1st to 10th	Day	27% 26% 37% 9% 1	300
	11th to 20th	Day	47% 23% 26% 4	300
	21st to end	Day	41% 27% 27% 4 2	249
March	1st to 10th	Day	35% 16% 29% 15% 6%	415
	11th to 20th	Day	31% 21% 30% 14% 4 1	500
	21st to end	Day	29% 23% 27% 15% 4 1	550
April	1st to 10th	Day	28% 16% 29% 18% 8% 1	500
	11th to 20th	Day	20% 22% 27% 20% 10% 1	492
	21st to end	Day	21% 16% 26% 21% 12% 4	500
May	1st to 10th	Day	19% 22% 26% 19% 9% 5%	503
	11th to 20th	Day	22% 10% 23% 25% 13% 7%	487
	21st to end	Day	19% 9% 19% 27% 14% 12%	529
June	1st to 10th	Day	16% 7% 20% 28% 17% 11%	490
	11th to 20th	Day	11% 6% 21% 23% 23% 15%	490
	21st to end	Day	11% 7% 26% 29% 15% 13%	490
July	1st to 10th	Day	9% 4 13% 30% 26% 17%	485
	11th to 20th	Day	10% 3 15% 27% 25% 20%	484
	21st to end	Day	10% 3 14% 29% 25% 19%	532
August	1st to 10th	Day	17% 5% 21% 23% 19% 16%	486
	11th to 20th	Day	11% 5% 15% 29% 25% 15%	479
	21st to end	Day	16% 3 13% 25% 29% 14%	539
September	1st to 10th	Day	12% 5% 18% 26% 29% 10%	490
	11th to 20th	Day	18% 6% 16% 24% 26% 10%	490
	21st to end	Day	21% 6% 17% 28% 22% 5%	490
October	1st to 10th	Day	19% 7% 21% 27% 18% 8%	490
	11th to 20th	Day	15% 12% 26% 28% 14% 4	490
	21st to end	Day	20% 11% 25% 30% 11% 3	539
November	1st to 10th	Day	21% 12% 29% 25% 11% 2	371
	11th to 20th	Day	22% 22% 31% 21% 4	300
	21st to end	Day	21% 17% 37% 19% 6% 1	300
December	1st to 10th	Day	34% 22% 29% 12% 3 1	300
	11th to 20th	Day	28% 17% 32% 18% 4	300
	21st to end	Day	33% 18% 33% 12% 4 1	330
January	1st to 10th	Night	29% 14% 29% 21% 6%	800
	11th to 20th	Night	25% 19% 27% 24% 6%	800
	21st to end	Night	24% 17% 34% 21% 3	880
February	1st to 10th	Night	21% 15% 28% 32% 4	800
	11th to 20th	Night	29% 18% 32% 20% 2	783
	21st to end	Night	28% 19% 29% 20% 3	660
March	1st to 10th	Night	25% 15% 28% 27% 5%	800
	11th to 20th	Night	24% 14% 31% 23% 8% 1	800
	21st to end	Night	24% 15% 30% 23% 8%	880
April	1st to 10th	Night	23% 17% 26% 25% 9% 1	800
	11th to 20th	Night	23% 11% 25% 26% 13% 2	641
	21st to end	Night	15% 11% 22% 33% 18% 2	600
May	1st to 10th	Night	16% 12% 23% 29% 20% 1	600
	11th to 20th	Night	20% 7% 18% 29% 23% 4	563
	21st to end	Night	16% 3 13% 28% 30% 9%	617
June	1st to 10th	Night	14% 3 14% 31% 31% 7%	570
	11th to 20th	Night	9% 3 15% 28% 32% 13%	570
	21st to end	Night	8% 2 14% 28% 40% 7%	570
July	1st to 10th	Night	11% 3 8% 23% 45% 9%	555
	11th to 20th	Night	12% 1 10% 22% 41% 13%	552
	21st to end	Night	9% 3 9% 26% 42% 12%	624
August	1st to 10th	Night	17% 3 10% 24% 35% 11%	567
	11th to 20th	Night	16% 5% 11% 25% 34% 10%	564
	21st to end	Night	13% 3 9% 26% 38% 11%	649
September	1st to 10th	Night	13% 3 12% 28% 34% 10%	760
	11th to 20th	Night	22% 3 12% 25% 32% 5%	760
	21st to end	Night	21% 5% 10% 34% 26% 4	760
October	1st to 10th	Night	17% 7% 17% 25% 26% 8%	760
	11th to 20th	Night	19% 11% 17% 31% 19% 3	760
	21st to end	Night	19% 9% 18% 35% 16% 3	836
November	1st to 10th	Night	18% 8% 21% 31% 19% 3	780
	11th to 20th	Night	20% 14% 22% 31% 11% 3	800
	21st to end	Night	20% 13% 20% 33% 13% 1	800
December	1st to 10th	Night	28% 15% 22% 28% 7% 1	800
	11th to 20th	Night	22% 14% 25% 31% 8% 1	800
	21st to end	Night	30% 13% 23% 29% 5%	880

Page generated at 2025-05-08 09:33:56. Version ac07

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