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 2024-03-01.

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	16% 6% 2 5% 5% 3 5% 5% 6% 7% 39%	78851
	Night	17% 6% 2 5% 4 3 4 4 5% 7% 42%	80571
Transparency	Day	43% 7% 11% 18% 16% 5%	78574
	Night	40% 6% 12% 18% 18% 6%	80367
Seeing	Day	21% 12% 23% 22% 15% 8%	14001
	Night	20% 10% 20% 27% 19% 4	24165

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	18% 5% 2 4 3 2 3 4 5% 9% 46%	2836
	11th to 20th	Day	16% 6% 2 5% 4 2 3 3 3 4 51%	2840
	21st to end	Day	26% 4 1 4 3 2 3 4 4 5% 45%	3124
February	1st to 10th	Day	18% 7% 4 5% 4 3 3 4 4 6% 42%	1856
	11th to 20th	Day	20% 6% 2 6% 5% 2 4 4 4 6% 41%	1848
	21st to end	Day	22% 7% 3 5% 4 2 4 4 3 6% 40%	1570
March	1st to 10th	Day	17% 5% 2 4 4 2 4 4 5% 7% 45%	1903
	11th to 20th	Day	22% 7% 3 5% 4 2 5% 4 4 7% 37%	1980
	21st to end	Day	19% 7% 2 6% 4 3 4 5% 5% 6% 38%	2178
April	1st to 10th	Day	13% 4 2 6% 5% 3 5% 5% 6% 9% 42%	1980
	11th to 20th	Day	17% 5% 3 5% 5% 3 5% 5% 5% 6% 41%	2152
	21st to end	Day	15% 4 2 4 4 3 5% 5% 5% 6% 47%	2340
May	1st to 10th	Day	14% 6% 3 5% 6% 3 5% 4 5% 7% 41%	2340
	11th to 20th	Day	19% 6% 3 7% 6% 4 5% 6% 5% 7% 33%	2301
	21st to end	Day	15% 5% 3 5% 6% 4 6% 5% 8% 10% 33%	2531
June	1st to 10th	Day	13% 6% 3 7% 7% 4 8% 6% 7% 10% 29%	2340
	11th to 20th	Day	9% 5% 3 5% 7% 5% 6% 8% 9% 11% 32%	2340
	21st to end	Day	9% 6% 3 6% 8% 5% 8% 6% 7% 9% 33%	2340
July	1st to 10th	Day	10% 7% 4 8% 8% 6% 8% 7% 8% 8% 24%	2340
	11th to 20th	Day	12% 6% 4 8% 9% 5% 9% 8% 8% 9% 23%	2340
	21st to end	Day	10% 6% 3 8% 9% 5% 8% 7% 7% 10% 27%	2547
August	1st to 10th	Day	13% 6% 3 7% 7% 4 8% 8% 8% 9% 25%	2327
	11th to 20th	Day	15% 6% 3 7% 6% 4 8% 6% 8% 10% 27%	2303
	21st to end	Day	15% 7% 3 7% 7% 4 7% 7% 7% 10% 26%	2339
September	1st to 10th	Day	14% 7% 3 5% 6% 4 7% 6% 7% 7% 34%	1980
	11th to 20th	Day	16% 7% 3 6% 6% 4 5% 6% 6% 7% 33%	1980
	21st to end	Day	16% 6% 3 5% 6% 4 5% 5% 5% 6% 38%	1980
October	1st to 10th	Day	16% 6% 2 3 4 3 3 3 4 6% 49%	1980
	11th to 20th	Day	14% 4 1 3 4 2 3 4 5% 6% 54%	1809
	21st to end	Day	15% 4 1 4 4 2 4 4 4 4 55%	1980
November	1st to 10th	Day	13% 5% 1 4 3 2 3 3 4 5% 57%	1740
	11th to 20th	Day	16% 5% 2 3 3 2 3 4 5% 6% 52%	1710
	21st to end	Day	18% 4 2 4 3 2 4 4 5% 7% 47%	1710
December	1st to 10th	Day	23% 6% 2 6% 4 3 5% 4 5% 6% 36%	1710
	11th to 20th	Day	18% 5% 2 4 3 2 3 4 5% 6% 48%	2153
	21st to end	Day	23% 5% 1 3 4 2 3 4 5% 6% 43%	3124
January	1st to 10th	Night	18% 6% 2 4 4 2 3 3 5% 6% 47%	4109
	11th to 20th	Night	14% 6% 3 5% 5% 2 3 4 4 6% 47%	4120
	21st to end	Night	26% 6% 2 4 4 2 3 3 4 6% 41%	4532
February	1st to 10th	Night	16% 6% 3 5% 5% 2 4 3 5% 7% 45%	2322
	11th to 20th	Night	20% 7% 3 4 5% 2 4 4 4 4 43%	2218
	21st to end	Night	22% 6% 2 5% 4 3 4 4 5% 6% 38%	1884
March	1st to 10th	Night	17% 7% 2 4 4 2 4 4 4 6% 46%	2064
	11th to 20th	Night	19% 5% 2 5% 5% 3 3 4 5% 8% 40%	1980
	21st to end	Night	19% 6% 2 5% 4 3 4 4 5% 6% 41%	2178
April	1st to 10th	Night	14% 5% 2 5% 4 2 3 5% 4 6% 48%	1980
	11th to 20th	Night	16% 6% 2 4 3 2 3 3 5% 6% 49%	1772
	21st to end	Night	13% 5% 2 3 4 2 4 4 5% 7% 50%	1620
May	1st to 10th	Night	13% 5% 2 5% 6% 3 5% 5% 6% 7% 43%	1620
	11th to 20th	Night	17% 7% 3 5% 5% 2 5% 5% 5% 7% 38%	1592
	21st to end	Night	15% 6% 3 5% 5% 3 5% 5% 6% 9% 39%	1744
June	1st to 10th	Night	12% 6% 2 5% 7% 4 6% 6% 8% 11% 33%	1620
	11th to 20th	Night	9% 4 3 5% 5% 4 5% 6% 7% 11% 42%	1620
	21st to end	Night	9% 5% 2 5% 5% 3 5% 6% 8% 11% 41%	1620
July	1st to 10th	Night	11% 6% 4 6% 7% 4 7% 6% 7% 11% 30%	1620
	11th to 20th	Night	10% 6% 2 6% 7% 5% 7% 7% 8% 12% 29%	1620
	21st to end	Night	8% 5% 3 7% 6% 5% 6% 7% 8% 11% 33%	1771
August	1st to 10th	Night	14% 6% 3 6% 6% 3 6% 6% 7% 12% 30%	1611
	11th to 20th	Night	17% 7% 3 5% 5% 4 5% 5% 7% 9% 34%	1598
	21st to end	Night	14% 7% 3 5% 6% 4 5% 6% 6% 10% 34%	1991
September	1st to 10th	Night	14% 6% 2 6% 5% 4 6% 5% 8% 8% 37%	1980
	11th to 20th	Night	18% 8% 3 5% 5% 3 5% 5% 6% 7% 35%	1980
	21st to end	Night	17% 6% 2 4 5% 3 4 4 5% 6% 45%	1980
October	1st to 10th	Night	16% 6% 2 4 3 3 3 3 3 6% 51%	1980
	11th to 20th	Night	16% 6% 2 4 3 2 3 4 4 5% 50%	2150
	21st to end	Night	16% 4 1 3 3 2 4 3 4 5% 55%	2376
November	1st to 10th	Night	15% 5% 2 3 3 2 3 3 4 7% 53%	2335
	11th to 20th	Night	17% 6% 2 3 3 2 4 4 4 5% 50%	2470
	21st to end	Night	17% 5% 2 4 4 3 3 4 6% 7% 45%	2470
December	1st to 10th	Night	22% 8% 3 6% 4 2 3 4 4 6% 39%	2470
	11th to 20th	Night	16% 6% 2 4 4 2 3 3 4 6% 49%	3042
	21st to end	Night	23% 8% 2 4 4 3 4 5% 5% 8% 35%	4532

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	35% 3 9% 21% 22% 10%	2837
	11th to 20th	Day	38% 4 6% 15% 25% 11%	2840
	21st to end	Day	42% 2 6% 21% 20% 9%	3124
February	1st to 10th	Day	43% 5% 8% 19% 19% 6%	1856
	11th to 20th	Day	45% 4 8% 17% 19% 7%	1858
	21st to end	Day	46% 5% 8% 15% 17% 9%	1570
March	1st to 10th	Day	37% 5% 9% 19% 21% 9%	1903
	11th to 20th	Day	47% 6% 10% 14% 16% 7%	1980
	21st to end	Day	46% 6% 11% 19% 15% 3	2178
April	1st to 10th	Day	38% 6% 12% 19% 19% 5%	1980
	11th to 20th	Day	42% 5% 9% 17% 20% 7%	2152
	21st to end	Day	37% 6% 13% 25% 17% 3	2340
May	1st to 10th	Day	41% 7% 13% 19% 15% 4	2340
	11th to 20th	Day	50% 10% 13% 16% 11% 1	2305
	21st to end	Day	43% 11% 14% 21% 11% 1	2531
June	1st to 10th	Day	48% 11% 15% 19% 6% 1	2340
	11th to 20th	Day	42% 13% 17% 17% 9% 2	2340
	21st to end	Day	44% 11% 16% 17% 10% 2	2340
July	1st to 10th	Day	50% 13% 15% 15% 6% 1	2275
	11th to 20th	Day	52% 13% 17% 10% 6% 2	2271
	21st to end	Day	48% 13% 17% 15% 6% 1	2405
August	1st to 10th	Day	51% 13% 16% 16% 4	2314
	11th to 20th	Day	49% 12% 15% 16% 6% 1	2303
	21st to end	Day	49% 12% 14% 15% 8% 1	2339
September	1st to 10th	Day	45% 10% 11% 18% 13% 3	1980
	11th to 20th	Day	48% 8% 11% 17% 14% 2	1980
	21st to end	Day	45% 6% 10% 17% 16% 6%	1980
October	1st to 10th	Day	36% 5% 9% 17% 25% 7%	1980
	11th to 20th	Day	31% 6% 11% 22% 20% 10%	1809
	21st to end	Day	33% 4 7% 22% 24% 11%	1980
November	1st to 10th	Day	31% 4 8% 18% 29% 11%	1740
	11th to 20th	Day	33% 3 6% 22% 26% 9%	1710
	21st to end	Day	37% 4 7% 20% 26% 7%	1710
December	1st to 10th	Day	48% 5% 7% 18% 16% 7%	1710
	11th to 20th	Day	37% 3 9% 16% 23% 12%	2150
	21st to end	Day	39% 4 9% 23% 21% 5%	3124
January	1st to 10th	Night	38% 3 9% 16% 24% 10%	4109
	11th to 20th	Night	38% 4 9% 11% 27% 11%	4120
	21st to end	Night	45% 2 8% 15% 20% 9%	4532
February	1st to 10th	Night	38% 4 12% 20% 20% 6%	2322
	11th to 20th	Night	45% 4 8% 18% 19% 6%	2219
	21st to end	Night	46% 4 9% 18% 16% 7%	1884
March	1st to 10th	Night	38% 6% 10% 16% 20% 10%	2064
	11th to 20th	Night	42% 6% 12% 19% 16% 6%	1980
	21st to end	Night	43% 6% 12% 20% 17% 2	2178
April	1st to 10th	Night	35% 4 12% 22% 23% 4	1980
	11th to 20th	Night	36% 3 10% 23% 21% 7%	1772
	21st to end	Night	34% 6% 14% 25% 19% 2	1620
May	1st to 10th	Night	39% 7% 15% 21% 16% 2	1620
	11th to 20th	Night	43% 9% 17% 18% 12% 1	1593
	21st to end	Night	40% 10% 15% 25% 9% 1	1745
June	1st to 10th	Night	41% 13% 17% 20% 8% 1	1620
	11th to 20th	Night	34% 10% 24% 22% 9% 1	1620
	21st to end	Night	33% 14% 20% 19% 11% 3	1620
July	1st to 10th	Night	44% 14% 19% 14% 8% 1	1575
	11th to 20th	Night	42% 16% 22% 14% 5% 2	1566
	21st to end	Night	40% 14% 21% 17% 6% 1	1672
August	1st to 10th	Night	45% 14% 18% 16% 7%	1600
	11th to 20th	Night	45% 13% 15% 15% 10% 1	1598
	21st to end	Night	43% 11% 18% 19% 6% 2	1991
September	1st to 10th	Night	41% 11% 15% 19% 11% 3	1980
	11th to 20th	Night	46% 9% 12% 17% 14% 1	1980
	21st to end	Night	40% 6% 10% 22% 17% 5%	1980
October	1st to 10th	Night	36% 6% 11% 18% 25% 5%	1980
	11th to 20th	Night	36% 5% 10% 21% 19% 9%	2150
	21st to end	Night	32% 3 7% 22% 26% 10%	2376
November	1st to 10th	Night	33% 4 9% 19% 23% 11%	2335
	11th to 20th	Night	35% 4 7% 17% 25% 12%	2470
	21st to end	Night	39% 4 6% 21% 23% 7%	2470
December	1st to 10th	Night	47% 4 9% 17% 17% 7%	2470
	11th to 20th	Night	36% 4 9% 19% 24% 8%	3044
	21st to end	Night	46% 4 10% 19% 17% 4	4532

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	32% 21% 33% 10% 3	270
	11th to 20th	Day	36% 21% 28% 13% 2	270
	21st to end	Day	32% 27% 29% 9% 2	297
February	1st to 10th	Day	28% 24% 38% 9% 1	270
	11th to 20th	Day	44% 23% 27% 5%	270
	21st to end	Day	42% 26% 26% 4 2	225
March	1st to 10th	Day	31% 16% 31% 16% 5%	343
	11th to 20th	Day	31% 20% 28% 16% 4 1	420
	21st to end	Day	29% 21% 27% 17% 5% 2	462
April	1st to 10th	Day	26% 15% 29% 21% 9% 1	420
	11th to 20th	Day	19% 22% 26% 21% 11% 2	412
	21st to end	Day	22% 15% 24% 22% 13% 4	420
May	1st to 10th	Day	18% 21% 26% 19% 10% 5%	423
	11th to 20th	Day	21% 9% 22% 25% 14% 8%	447
	21st to end	Day	20% 9% 16% 27% 15% 13%	485
June	1st to 10th	Day	17% 6% 19% 28% 18% 12%	450
	11th to 20th	Day	12% 6% 20% 23% 24% 16%	450
	21st to end	Day	11% 7% 25% 28% 16% 14%	450
July	1st to 10th	Day	10% 4 13% 29% 26% 18%	445
	11th to 20th	Day	10% 2 15% 27% 25% 21%	444
	21st to end	Day	10% 2 12% 29% 26% 21%	488
August	1st to 10th	Day	17% 4 18% 23% 20% 17%	446
	11th to 20th	Day	11% 4 12% 30% 27% 16%	439
	21st to end	Day	17% 2 12% 25% 30% 14%	495
September	1st to 10th	Day	12% 5% 16% 25% 31% 11%	450
	11th to 20th	Day	17% 4 15% 25% 28% 11%	450
	21st to end	Day	20% 5% 17% 29% 24% 6%	450
October	1st to 10th	Day	20% 7% 19% 27% 18% 8%	450
	11th to 20th	Day	16% 12% 26% 28% 15% 4	450
	21st to end	Day	21% 11% 24% 30% 11% 3	495
November	1st to 10th	Day	19% 11% 30% 25% 12% 2	338
	11th to 20th	Day	22% 20% 33% 21% 4	270
	21st to end	Day	18% 17% 37% 20% 6% 1	270
December	1st to 10th	Day	34% 20% 30% 13% 3 1	270
	11th to 20th	Day	27% 16% 33% 20% 4	270
	21st to end	Day	33% 18% 33% 11% 4 1	297
January	1st to 10th	Night	28% 15% 29% 23% 6%	760
	11th to 20th	Night	23% 19% 27% 25% 6%	760
	21st to end	Night	24% 16% 35% 22% 3	836
February	1st to 10th	Night	21% 14% 27% 33% 4	760
	11th to 20th	Night	27% 17% 32% 21% 2	743
	21st to end	Night	29% 19% 28% 20% 4	628
March	1st to 10th	Night	24% 15% 28% 28% 4	723
	11th to 20th	Night	24% 13% 30% 23% 8% 1	720
	21st to end	Night	23% 14% 30% 24% 9%	792
April	1st to 10th	Night	20% 17% 26% 26% 9% 1	720
	11th to 20th	Night	23% 10% 25% 27% 14% 2	577
	21st to end	Night	15% 10% 20% 34% 19% 2	540
May	1st to 10th	Night	16% 11% 21% 29% 21% 1	540
	11th to 20th	Night	19% 7% 17% 29% 24% 5%	531
	21st to end	Night	17% 3 12% 27% 32% 10%	584
June	1st to 10th	Night	15% 2 14% 30% 32% 7%	540
	11th to 20th	Night	8% 2 15% 28% 32% 14%	540
	21st to end	Night	8% 2 14% 29% 40% 7%	540
July	1st to 10th	Night	11% 3 7% 24% 45% 10%	525
	11th to 20th	Night	12% 1 10% 22% 42% 13%	522
	21st to end	Night	9% 2 8% 25% 44% 12%	591
August	1st to 10th	Night	18% 2 8% 23% 37% 11%	537
	11th to 20th	Night	16% 4 9% 25% 36% 10%	534
	21st to end	Night	14% 2 9% 26% 37% 11%	614
September	1st to 10th	Night	13% 3 11% 27% 35% 10%	720
	11th to 20th	Night	20% 3 12% 25% 33% 6%	720
	21st to end	Night	20% 5% 10% 35% 27% 4	720
October	1st to 10th	Night	18% 8% 17% 24% 26% 8%	720
	11th to 20th	Night	19% 10% 18% 30% 19% 3	720
	21st to end	Night	19% 9% 19% 35% 15% 3	792
November	1st to 10th	Night	18% 8% 20% 31% 20% 3	740
	11th to 20th	Night	20% 13% 21% 32% 12% 3	760
	21st to end	Night	19% 13% 21% 33% 14% 1	760
December	1st to 10th	Night	27% 14% 22% 29% 7% 1	760
	11th to 20th	Night	21% 13% 25% 32% 9% 1	760
	21st to end	Night	30% 13% 24% 29% 5%	836

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