Mount Hamilton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Mount Hamilton clear sky chart from 2005-11-06 to 2025-09-04.

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	6% 5% 1 4 2 2 3 2 3 5% 67%	134498
	Night	7% 6% 2 4 3 3 3 2 4 5% 60%	131654
Transparency	Day	21% 4 5% 17% 26% 26%	129453
	Night	25% 6% 5% 15% 24% 25%	127684
Seeing	Day	10% 11% 23% 31% 19% 6%	16480
	Night	10% 6% 14% 31% 30% 8%	25831

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	13% 9% 3 5% 2 3 3 3 4 7% 49%	3823
	11th to 20th	Day	11% 6% 2 6% 4 4 3 2 3 4 55%	4100
	21st to end	Day	15% 8% 3 6% 4 3 4 3 4 6% 43%	4510
February	1st to 10th	Day	19% 9% 3 5% 3 2 3 2 5% 5% 44%	1800
	11th to 20th	Day	12% 9% 2 6% 4 5% 4 3 5% 7% 43%	1886
	21st to end	Day	15% 10% 3 5% 3 4 3 2 4 6% 45%	1650
March	1st to 10th	Day	11% 14% 4 10% 7% 5% 5% 4 4 5% 31%	4782
	11th to 20th	Day	11% 9% 3 6% 3 4 4 3 5% 6% 46%	5000
	21st to end	Day	11% 11% 3 8% 5% 5% 6% 4 5% 7% 35%	5617
April	1st to 10th	Day	13% 8% 2 7% 4 3 3 2 5% 6% 45%	2400
	11th to 20th	Day	10% 5% 2 4 3 3 3 3 5% 6% 55%	2369
	21st to end	Day	6% 5% 2 4 4 2 3 3 5% 7% 60%	2400
May	1st to 10th	Day	4 3 1 2 2 2 2 2 4 5% 75%	5478
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 5% 78%	6048
	21st to end	Day	2 1 1 2 2 2 2 2 3 5% 80%	6902
June	1st to 10th	Day	3 4 1 4 2 3 3 3 4 6% 67%	2800
	11th to 20th	Day	1 1 1 1 1 1 2 4 5% 81%	2800
	21st to end	Day	1 2 1 1 1 1 2 2 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 2 94%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 90%	5980
	21st to end	Day	1 1 1 1 1 2 3 90%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 3 3 85%	5777
	11th to 20th	Day	1 1 1 2 3 92%	5415
	21st to end	Day	1 1 1 1 1 1 2 4 87%	5981
September	1st to 10th	Day	1 1 1 2 1 1 1 1 2 2 88%	2317
	11th to 20th	Day	1 1 1 2 2 2 3 2 4 6% 76%	2280
	21st to end	Day	3 1 2 2 2 2 2 3 4 80%	2035
October	1st to 10th	Day	2 3 1 3 2 2 3 2 3 6% 74%	3180
	11th to 20th	Day	3 2 1 2 1 2 2 2 4 5% 76%	3161
	21st to end	Day	3 3 1 4 3 3 3 4 5% 6% 64%	3448
November	1st to 10th	Day	8% 6% 2 4 4 4 4 3 5% 8% 52%	1891
	11th to 20th	Day	8% 6% 2 4 4 3 5% 4 5% 8% 52%	1800
	21st to end	Day	13% 6% 2 4 2 3 3 3 5% 7% 53%	1695
December	1st to 10th	Day	7% 6% 2 5% 3 3 4 3 5% 9% 53%	2620
	11th to 20th	Day	21% 11% 3 7% 4 3 3 3 4 6% 35%	3028
	21st to end	Day	15% 15% 4 8% 4 4 5% 4 5% 6% 29%	4004
January	1st to 10th	Night	9% 7% 3 6% 4 3 4 4 6% 8% 47%	6113
	11th to 20th	Night	13% 8% 2 4 4 3 4 3 4 4 51%	5880
	21st to end	Night	14% 9% 3 7% 4 3 4 3 4 5% 45%	6468
February	1st to 10th	Night	18% 8% 2 5% 3 3 3 3 3 6% 46%	2600
	11th to 20th	Night	13% 8% 2 6% 4 4 4 3 6% 7% 44%	2402
	21st to end	Night	12% 8% 3 6% 4 3 4 3 5% 7% 47%	1980
March	1st to 10th	Night	14% 14% 4 11% 6% 4 3 2 4 4 34%	5096
	11th to 20th	Night	13% 9% 3 7% 4 3 3 2 3 6% 46%	5000
	21st to end	Night	13% 14% 4 7% 5% 3 3 3 5% 6% 36%	5345
April	1st to 10th	Night	10% 7% 2 6% 4 3 5% 4 6% 6% 46%	2000
	11th to 20th	Night	7% 8% 2 5% 3 4 3 3 5% 8% 52%	1979
	21st to end	Night	4 6% 2 3 4 4 4 3 5% 9% 55%	2000
May	1st to 10th	Night	2 4 1 4 2 2 2 2 5% 6% 70%	4516
	11th to 20th	Night	2 2 1 4 3 3 2 2 5% 7% 71%	3850
	21st to end	Night	1 1 1 2 2 2 2 2 4 5% 78%	3972
June	1st to 10th	Night	3 3 1 4 3 2 2 3 3 8% 69%	1600
	11th to 20th	Night	1 1 1 2 2 2 3 2 3 5% 78%	1600
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 85%	1600
July	1st to 10th	Night	1 1 1 2 94%	3640
	11th to 20th	Night	1 1 1 1 1 1 2 2 90%	3978
	21st to end	Night	1 1 1 1 1 1 2 3 4 85%	4472
August	1st to 10th	Night	1 1 1 1 1 1 1 2 3 4 85%	4035
	11th to 20th	Night	1 1 1 1 2 4 91%	4452
	21st to end	Night	1 1 1 1 1 2 1 3 3 85%	5015
September	1st to 10th	Night	1 1 1 1 1 1 2 2 4 85%	1930
	11th to 20th	Night	2 3 1 3 2 2 2 2 4 5% 76%	1900
	21st to end	Night	2 2 1 2 2 1 1 2 3 5% 80%	2140
October	1st to 10th	Night	2 2 1 3 2 2 2 2 3 5% 77%	3800
	11th to 20th	Night	3 2 1 3 2 2 3 2 4 5% 73%	3777
	21st to end	Night	4 3 1 4 4 3 4 3 5% 5% 64%	4121
November	1st to 10th	Night	7% 6% 2 4 3 3 3 3 4 7% 57%	2405
	11th to 20th	Night	8% 6% 2 5% 3 3 3 4 6% 8% 51%	2600
	21st to end	Night	9% 7% 2 5% 4 4 4 3 4 7% 53%	2700
December	1st to 10th	Night	8% 5% 1 4 3 4 3 3 5% 6% 58%	4560
	11th to 20th	Night	19% 12% 3 6% 4 4 3 4 7% 9% 29%	5176
	21st to end	Night	19% 11% 4 10% 6% 5% 5% 4 5% 5% 27%	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	35% 5% 13% 25% 13% 9%	3823
	11th to 20th	Day	35% 3 6% 12% 22% 21%	4100
	21st to end	Day	40% 7% 4 13% 17% 18%	4510
February	1st to 10th	Day	42% 4 5% 19% 20% 11%	1800
	11th to 20th	Day	40% 4 9% 18% 22% 8%	1895
	21st to end	Day	41% 4 6% 21% 20% 8%	1650
March	1st to 10th	Day	48% 11% 6% 14% 18% 4	4782
	11th to 20th	Day	36% 6% 11% 27% 13% 8%	5000
	21st to end	Day	41% 12% 12% 21% 11% 2	5617
April	1st to 10th	Day	38% 5% 10% 21% 20% 5%	2400
	11th to 20th	Day	29% 5% 12% 23% 23% 8%	2372
	21st to end	Day	24% 6% 10% 30% 25% 6%	2400
May	1st to 10th	Day	12% 4 6% 25% 37% 15%	5478
	11th to 20th	Day	10% 4 4 28% 42% 12%	6051
	21st to end	Day	9% 5% 5% 25% 46% 11%	6904
June	1st to 10th	Day	19% 4 8% 23% 35% 12%	2800
	11th to 20th	Day	7% 3 6% 19% 36% 28%	2800
	21st to end	Day	8% 2 2 11% 31% 47%	2800
July	1st to 10th	Day	2 1 1 3 19% 74%	6251
	11th to 20th	Day	5% 1 1 4 13% 74%	5908
	21st to end	Day	3 1 1 12% 31% 51%	6217
August	1st to 10th	Day	8% 3 2 7% 24% 56%	4206
	11th to 20th	Day	3 1 1 6% 31% 58%	3919
	21st to end	Day	5% 2 2 9% 34% 49%	4311
September	1st to 10th	Day	7% 2 1 8% 31% 51%	2317
	11th to 20th	Day	11% 5% 3 20% 38% 23%	2280
	21st to end	Day	11% 2 2 19% 40% 27%	2035
October	1st to 10th	Day	13% 2 4 11% 26% 43%	3180
	11th to 20th	Day	12% 3 4 22% 35% 23%	3161
	21st to end	Day	18% 5% 5% 19% 32% 21%	3448
November	1st to 10th	Day	29% 5% 6% 20% 25% 14%	1891
	11th to 20th	Day	29% 6% 8% 21% 25% 11%	1800
	21st to end	Day	31% 4 8% 17% 29% 12%	1695
December	1st to 10th	Day	28% 4 5% 17% 33% 12%	2620
	11th to 20th	Day	49% 5% 6% 14% 17% 8%	3028
	21st to end	Day	51% 8% 9% 18% 11% 4	4004
January	1st to 10th	Night	29% 10% 14% 21% 18% 7%	6113
	11th to 20th	Night	34% 5% 7% 11% 24% 19%	5880
	21st to end	Night	37% 8% 5% 11% 14% 24%	6468
February	1st to 10th	Night	39% 4 6% 17% 19% 15%	2600
	11th to 20th	Night	38% 5% 7% 17% 22% 11%	2403
	21st to end	Night	34% 6% 7% 25% 19% 9%	1980
March	1st to 10th	Night	45% 13% 6% 12% 18% 6%	5096
	11th to 20th	Night	38% 7% 5% 22% 19% 9%	5000
	21st to end	Night	40% 13% 9% 20% 14% 4	5345
April	1st to 10th	Night	34% 8% 12% 19% 22% 7%	2000
	11th to 20th	Night	30% 6% 8% 24% 23% 10%	1979
	21st to end	Night	23% 9% 7% 27% 27% 8%	2000
May	1st to 10th	Night	13% 10% 5% 20% 35% 19%	4513
	11th to 20th	Night	10% 8% 4 18% 45% 14%	3850
	21st to end	Night	8% 6% 4 18% 42% 22%	3972
June	1st to 10th	Night	16% 6% 8% 19% 34% 17%	1600
	11th to 20th	Night	10% 4 5% 13% 30% 38%	1600
	21st to end	Night	7% 3 2 11% 30% 48%	1600
July	1st to 10th	Night	2 1 1 16% 78%	3600
	11th to 20th	Night	4 1 1 3 13% 78%	3927
	21st to end	Night	6% 2 1 7% 31% 52%	4345
August	1st to 10th	Night	9% 4 1 7% 19% 60%	2929
	11th to 20th	Night	3 2 1 5% 24% 64%	3215
	21st to end	Night	6% 3 2 7% 30% 52%	3608
September	1st to 10th	Night	7% 2 1 5% 24% 61%	1930
	11th to 20th	Night	11% 6% 2 17% 33% 30%	1900
	21st to end	Night	10% 3 1 17% 35% 34%	2140
October	1st to 10th	Night	12% 3 3 10% 19% 53%	3800
	11th to 20th	Night	13% 4 4 19% 33% 26%	3777
	21st to end	Night	19% 7% 3 16% 29% 26%	4121
November	1st to 10th	Night	26% 6% 7% 19% 24% 19%	2405
	11th to 20th	Night	27% 6% 8% 21% 27% 12%	2600
	21st to end	Night	31% 4 6% 18% 23% 17%	2700
December	1st to 10th	Night	27% 4 5% 17% 28% 20%	4560
	11th to 20th	Night	48% 6% 7% 11% 19% 8%	5176
	21st to end	Night	52% 10% 9% 14% 10% 5%	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	34% 12% 24% 20% 10% 1	280
	11th to 20th	Day	18% 14% 24% 27% 14% 3	400
	21st to end	Day	18% 12% 24% 26% 18% 3	440
February	1st to 10th	Day	26% 15% 22% 19% 13% 5%	400
	11th to 20th	Day	20% 20% 27% 24% 10%	462
	21st to end	Day	19% 15% 28% 28% 9% 1	414
March	1st to 10th	Day	23% 19% 28% 22% 7% 1	500
	11th to 20th	Day	26% 17% 21% 20% 13% 3	500
	21st to end	Day	22% 18% 30% 21% 9%	550
April	1st to 10th	Day	18% 16% 30% 22% 13% 1	500
	11th to 20th	Day	14% 19% 29% 23% 13% 2	492
	21st to end	Day	8% 18% 29% 28% 12% 6%	500
May	1st to 10th	Day	11% 16% 32% 27% 11% 4	503
	11th to 20th	Day	9% 16% 29% 26% 16% 5%	528
	21st to end	Day	7% 11% 26% 32% 19% 5%	573
June	1st to 10th	Day	6% 11% 27% 29% 19% 8%	530
	11th to 20th	Day	3 13% 29% 35% 16% 5%	530
	21st to end	Day	2 5% 24% 38% 21% 10%	530
July	1st to 10th	Day	1 2 18% 41% 30% 7%	526
	11th to 20th	Day	1 12% 41% 33% 12%	524
	21st to end	Day	1 1 12% 45% 27% 15%	575
August	1st to 10th	Day	2 3 19% 38% 27% 11%	527
	11th to 20th	Day	1 3 20% 39% 26% 12%	517
	21st to end	Day	1 2 12% 42% 31% 12%	583
September	1st to 10th	Day	2 3 17% 42% 30% 7%	503
	11th to 20th	Day	2 9% 23% 39% 22% 6%	490
	21st to end	Day	3 7% 18% 41% 26% 4	490
October	1st to 10th	Day	4 11% 23% 33% 23% 6%	490
	11th to 20th	Day	7% 9% 22% 32% 25% 6%	490
	21st to end	Day	9% 8% 27% 32% 20% 5%	539
November	1st to 10th	Day	11% 12% 21% 27% 26% 3	427
	11th to 20th	Day	8% 14% 21% 29% 20% 8%	300
	21st to end	Day	13% 11% 23% 31% 17% 4	247
December	1st to 10th	Day	15% 13% 18% 30% 17% 8%	200
	11th to 20th	Day	19% 18% 17% 26% 15% 7%	200
	21st to end	Day	22% 19% 23% 19% 13% 5%	220
January	1st to 10th	Night	19% 9% 16% 30% 19% 9%	800
	11th to 20th	Night	15% 7% 15% 33% 22% 9%	800
	21st to end	Night	19% 6% 13% 32% 23% 6%	880
February	1st to 10th	Night	22% 9% 13% 30% 20% 6%	800
	11th to 20th	Night	18% 11% 16% 30% 18% 7%	784
	21st to end	Night	15% 10% 19% 35% 17% 3	660
March	1st to 10th	Night	22% 10% 15% 30% 18% 4	800
	11th to 20th	Night	19% 11% 16% 26% 23% 5%	800
	21st to end	Night	16% 11% 20% 29% 20% 3	880
April	1st to 10th	Night	14% 10% 20% 31% 22% 4	800
	11th to 20th	Night	11% 11% 18% 33% 21% 5%	791
	21st to end	Night	7% 11% 24% 27% 23% 8%	800
May	1st to 10th	Night	6% 7% 25% 30% 25% 7%	782
	11th to 20th	Night	5% 7% 13% 34% 34% 7%	498
	21st to end	Night	4 6% 16% 35% 34% 6%	433
June	1st to 10th	Night	3 5% 16% 34% 34% 8%	400
	11th to 20th	Night	1 6% 13% 41% 35% 5%	400
	21st to end	Night	2 1 8% 36% 45% 9%	400
July	1st to 10th	Night	1 7% 34% 50% 8%	390
	11th to 20th	Night	1 2 24% 55% 17%	528
	21st to end	Night	1 1 2 24% 55% 17%	657
August	1st to 10th	Night	2 1 6% 28% 50% 13%	597
	11th to 20th	Night	1 7% 35% 43% 13%	763
	21st to end	Night	1 1 4 29% 53% 12%	880
September	1st to 10th	Night	1 2 7% 33% 46% 11%	772
	11th to 20th	Night	3 3 10% 33% 42% 10%	760
	21st to end	Night	3 3 10% 32% 38% 14%	760
October	1st to 10th	Night	5% 3 11% 35% 34% 12%	760
	11th to 20th	Night	8% 5% 10% 33% 34% 11%	760
	21st to end	Night	7% 4 13% 31% 27% 17%	836
November	1st to 10th	Night	9% 8% 11% 32% 30% 10%	780
	11th to 20th	Night	11% 5% 17% 34% 25% 8%	800
	21st to end	Night	12% 6% 16% 34% 24% 8%	800
December	1st to 10th	Night	17% 8% 15% 32% 23% 7%	800
	11th to 20th	Night	23% 9% 13% 30% 20% 5%	800
	21st to end	Night	19% 13% 19% 27% 18% 4	880

Page generated at 2025-08-31 22:51:03. Version ac07

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