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 2024-03-28.

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	8% 5% 2 4 3 2 3 3 4 6% 60%	78931
	Night	10% 6% 2 4 4 2 3 4 4 6% 55%	81100
Transparency	Day	26% 4 6% 18% 25% 20%	78649
	Night	29% 6% 6% 16% 22% 20%	80902
Seeing	Day	10% 11% 23% 31% 19% 6%	14522
	Night	11% 6% 13% 31% 30% 9%	24036

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	17% 11% 3 6% 4 2 4 4 3 5% 40%	2667
	11th to 20th	Day	15% 8% 3 8% 7% 3 5% 4 4 6% 35%	2860
	21st to end	Day	21% 10% 3 6% 5% 3 4 5% 5% 7% 30%	3146
February	1st to 10th	Day	18% 8% 3 5% 3 1 3 3 5% 6% 44%	1710
	11th to 20th	Day	13% 9% 2 6% 4 3 5% 4 5% 7% 44%	1789
	21st to end	Day	17% 9% 2 5% 4 2 4 3 4 5% 45%	1570
March	1st to 10th	Day	16% 12% 3 7% 6% 3 5% 4 5% 7% 31%	2014
	11th to 20th	Day	16% 9% 3 6% 4 4 4 4 5% 7% 36%	2090
	21st to end	Day	13% 8% 4 5% 5% 4 4 4 4 6% 43%	2310
April	1st to 10th	Day	15% 7% 3 6% 5% 2 4 3 5% 7% 43%	2160
	11th to 20th	Day	11% 5% 2 3 3 2 3 4 5% 6% 55%	2129
	21st to end	Day	6% 5% 2 3 4 2 3 3 4 7% 61%	2160
May	1st to 10th	Day	5% 4 2 3 3 2 3 4 5% 7% 61%	2172
	11th to 20th	Day	4 4 1 3 3 1 3 3 3 7% 66%	2378
	21st to end	Day	5% 3 1 3 3 2 3 4 5% 7% 62%	2723
June	1st to 10th	Day	4 4 1 3 3 2 4 3 4 7% 65%	2520
	11th to 20th	Day	1 1 1 1 2 1 2 2 4 6% 80%	2520
	21st to end	Day	2 2 1 1 1 1 1 2 2 3 84%	2520
July	1st to 10th	Day	1 1 1 1 1 2 2 3 88%	2520
	11th to 20th	Day	1 1 1 1 1 1 2 2 2 3 85%	2388
	21st to end	Day	1 1 1 1 1 2 2 3 4 85%	2546
August	1st to 10th	Day	1 1 1 2 2 1 2 2 2 3 83%	2328
	11th to 20th	Day	1 1 1 1 1 1 2 4 88%	2148
	21st to end	Day	1 1 1 1 1 2 2 3 89%	2376
September	1st to 10th	Day	1 1 1 1 1 1 1 1 2 2 87%	2160
	11th to 20th	Day	1 1 1 1 2 1 3 3 4 6% 76%	2160
	21st to end	Day	3 1 1 1 2 1 2 2 3 4 79%	1929
October	1st to 10th	Day	3 3 1 3 2 2 2 4 3 6% 70%	1800
	11th to 20th	Day	5% 3 1 2 2 2 3 3 5% 6% 68%	1800
	21st to end	Day	5% 4 2 3 4 2 3 3 4 5% 65%	1980
November	1st to 10th	Day	9% 5% 3 4 4 3 5% 4 5% 8% 51%	1795
	11th to 20th	Day	8% 5% 2 4 4 3 5% 4 4 7% 53%	1710
	21st to end	Day	13% 5% 2 4 3 2 3 3 5% 6% 54%	1611
December	1st to 10th	Day	12% 8% 3 6% 5% 4 3 4 4 7% 45%	1520
	11th to 20th	Day	29% 11% 3 5% 4 2 3 3 4 4 31%	1928
	21st to end	Day	14% 12% 4 6% 5% 3 4 5% 5% 6% 34%	2794
January	1st to 10th	Night	12% 8% 3 6% 5% 2 5% 5% 5% 6% 42%	4278
	11th to 20th	Night	18% 11% 3 5% 6% 3 5% 5% 6% 6% 31%	4110
	21st to end	Night	19% 11% 4 7% 5% 3 4 5% 5% 6% 31%	4521
February	1st to 10th	Night	17% 7% 2 5% 4 2 3 3 3 6% 47%	2470
	11th to 20th	Night	12% 7% 3 5% 4 3 4 4 6% 7% 44%	2279
	21st to end	Night	13% 8% 3 5% 4 2 3 4 5% 7% 47%	1884
March	1st to 10th	Night	17% 12% 3 6% 5% 2 4 4 4 5% 37%	2161
	11th to 20th	Night	15% 9% 3 5% 5% 2 4 3 4 9% 42%	2090
	21st to end	Night	12% 8% 3 5% 6% 2 4 4 5% 7% 44%	2201
April	1st to 10th	Night	12% 6% 2 5% 4 2 5% 5% 5% 7% 45%	1800
	11th to 20th	Night	8% 6% 3 5% 4 2 4 3 4 7% 53%	1779
	21st to end	Night	5% 5% 2 3 4 3 4 3 5% 10% 57%	1800
May	1st to 10th	Night	4 4 2 4 4 2 4 3 5% 8% 60%	1784
	11th to 20th	Night	4 3 2 3 4 2 3 3 5% 10% 60%	1510
	21st to end	Night	3 2 1 3 3 3 4 5% 6% 10% 60%	1552
June	1st to 10th	Night	3 3 1 4 3 2 3 3 3 8% 68%	1440
	11th to 20th	Night	1 1 1 2 3 1 3 3 4 6% 76%	1440
	21st to end	Night	1 1 1 1 2 1 1 1 3 4 84%	1440
July	1st to 10th	Night	1 1 1 2 2 2 5% 86%	1440
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 4 85%	1568
	21st to end	Night	1 1 1 1 1 1 1 3 4 6% 78%	1773
August	1st to 10th	Night	1 1 1 2 2 1 2 3 3 6% 78%	1611
	11th to 20th	Night	1 1 1 1 2 3 3 6% 82%	1753
	21st to end	Night	1 1 1 1 1 1 2 3 4 85%	1980
September	1st to 10th	Night	1 1 1 1 1 1 2 3 4 85%	1800
	11th to 20th	Night	2 3 1 2 2 1 2 3 3 5% 76%	1800
	21st to end	Night	2 2 1 2 2 1 2 2 3 5% 79%	2026
October	1st to 10th	Night	4 3 2 2 2 1 3 3 3 6% 71%	2160
	11th to 20th	Night	4 3 2 3 3 2 3 3 4 6% 67%	2160
	21st to end	Night	6% 2 1 3 3 2 3 3 5% 6% 67%	2376
November	1st to 10th	Night	7% 6% 2 4 4 2 3 4 4 7% 57%	2281
	11th to 20th	Night	8% 6% 2 4 3 2 4 4 6% 9% 52%	2470
	21st to end	Night	9% 6% 2 4 5% 2 4 4 4 7% 55%	2565
December	1st to 10th	Night	14% 7% 2 5% 4 3 3 3 4 6% 48%	2660
	11th to 20th	Night	27% 11% 3 5% 4 2 3 4 5% 6% 31%	3276
	21st to end	Night	17% 9% 4 8% 7% 4 5% 5% 5% 6% 32%	4862

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	45% 5% 8% 28% 11% 3	2667
	11th to 20th	Day	50% 5% 9% 17% 12% 8%	2860
	21st to end	Day	50% 8% 6% 17% 13% 5%	3146
February	1st to 10th	Day	41% 4 5% 19% 20% 10%	1710
	11th to 20th	Day	40% 4 8% 18% 23% 8%	1798
	21st to end	Day	41% 4 6% 21% 20% 7%	1570
March	1st to 10th	Day	50% 7% 7% 17% 16% 4	2014
	11th to 20th	Day	45% 6% 6% 19% 18% 6%	2090
	21st to end	Day	40% 7% 12% 19% 17% 5%	2310
April	1st to 10th	Day	40% 5% 10% 19% 20% 6%	2160
	11th to 20th	Day	29% 5% 12% 22% 24% 7%	2132
	21st to end	Day	23% 5% 9% 30% 26% 6%	2160
May	1st to 10th	Day	20% 6% 8% 26% 32% 8%	2172
	11th to 20th	Day	20% 3 6% 28% 33% 10%	2381
	21st to end	Day	21% 5% 10% 30% 28% 6%	2724
June	1st to 10th	Day	20% 4 8% 24% 31% 13%	2520
	11th to 20th	Day	8% 3 7% 21% 36% 25%	2520
	21st to end	Day	8% 2 2 12% 31% 44%	2520
July	1st to 10th	Day	5% 2 1 8% 27% 57%	2451
	11th to 20th	Day	8% 2 3 10% 26% 52%	2316
	21st to end	Day	5% 2 2 7% 30% 54%	2405
August	1st to 10th	Day	9% 3 2 10% 29% 47%	2312
	11th to 20th	Day	4 2 1 11% 34% 48%	2148
	21st to end	Day	5% 1 11% 36% 46%	2376
September	1st to 10th	Day	7% 2 1 9% 30% 52%	2160
	11th to 20th	Day	10% 5% 3 20% 38% 24%	2160
	21st to end	Day	11% 2 2 20% 40% 25%	1929
October	1st to 10th	Day	16% 4 5% 18% 29% 28%	1800
	11th to 20th	Day	17% 4 7% 20% 24% 28%	1800
	21st to end	Day	22% 4 6% 18% 26% 25%	1980
November	1st to 10th	Day	30% 6% 6% 19% 24% 15%	1795
	11th to 20th	Day	29% 6% 8% 20% 25% 11%	1710
	21st to end	Day	30% 4 8% 17% 29% 12%	1611
December	1st to 10th	Day	38% 4 8% 18% 19% 13%	1520
	11th to 20th	Day	56% 3 6% 13% 15% 6%	1928
	21st to end	Day	47% 7% 9% 20% 13% 4	2794
January	1st to 10th	Night	37% 10% 13% 22% 13% 5%	4278
	11th to 20th	Night	49% 7% 10% 15% 10% 9%	4110
	21st to end	Night	48% 10% 7% 14% 12% 8%	4521
February	1st to 10th	Night	38% 4 6% 18% 20% 14%	2470
	11th to 20th	Night	37% 5% 7% 17% 23% 11%	2280
	21st to end	Night	34% 6% 7% 26% 19% 7%	1884
March	1st to 10th	Night	46% 8% 8% 16% 15% 7%	2161
	11th to 20th	Night	40% 5% 6% 19% 20% 9%	2090
	21st to end	Night	37% 9% 10% 18% 19% 8%	2201
April	1st to 10th	Night	36% 7% 12% 17% 21% 7%	1800
	11th to 20th	Night	31% 5% 7% 24% 22% 10%	1779
	21st to end	Night	23% 7% 7% 28% 27% 8%	1800
May	1st to 10th	Night	21% 8% 6% 22% 30% 14%	1784
	11th to 20th	Night	19% 6% 6% 21% 37% 11%	1510
	21st to end	Night	18% 9% 9% 25% 31% 9%	1552
June	1st to 10th	Night	18% 6% 9% 20% 30% 18%	1440
	11th to 20th	Night	11% 5% 5% 14% 31% 35%	1440
	21st to end	Night	7% 4 2 12% 31% 45%	1440
July	1st to 10th	Night	5% 4 1 4 29% 58%	1400
	11th to 20th	Night	6% 3 3 6% 28% 54%	1517
	21st to end	Night	8% 4 2 7% 28% 51%	1674
August	1st to 10th	Night	11% 6% 1 11% 26% 46%	1602
	11th to 20th	Night	6% 4 1 8% 32% 49%	1753
	21st to end	Night	6% 3 1 8% 37% 45%	1980
September	1st to 10th	Night	7% 2 1 5% 24% 60%	1800
	11th to 20th	Night	11% 6% 2 17% 33% 31%	1800
	21st to end	Night	11% 3 1 18% 36% 31%	2026
October	1st to 10th	Night	16% 4 5% 17% 24% 33%	2160
	11th to 20th	Night	19% 5% 4 17% 26% 30%	2160
	21st to end	Night	19% 4 4 16% 30% 27%	2376
November	1st to 10th	Night	26% 6% 7% 19% 23% 20%	2281
	11th to 20th	Night	27% 6% 8% 20% 27% 12%	2470
	21st to end	Night	30% 4 7% 18% 24% 18%	2565
December	1st to 10th	Night	36% 5% 7% 17% 19% 16%	2660
	11th to 20th	Night	53% 4 7% 13% 16% 7%	3276
	21st to end	Night	47% 10% 10% 15% 12% 6%	4862

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% 13% 23% 18% 11% 1	256
	11th to 20th	Day	18% 15% 24% 27% 14% 3	370
	21st to end	Day	19% 12% 24% 24% 19% 3	407
February	1st to 10th	Day	24% 16% 22% 20% 14% 5%	370
	11th to 20th	Day	20% 20% 26% 24% 10%	425
	21st to end	Day	21% 15% 27% 27% 9% 2	382
March	1st to 10th	Day	23% 18% 28% 22% 7% 2	460
	11th to 20th	Day	25% 18% 20% 20% 14% 3	460
	21st to end	Day	21% 18% 30% 21% 9%	491
April	1st to 10th	Day	19% 16% 29% 23% 13%	420
	11th to 20th	Day	16% 20% 30% 21% 13% 1	412
	21st to end	Day	8% 19% 28% 27% 12% 5%	420
May	1st to 10th	Day	11% 16% 30% 29% 10% 4	423
	11th to 20th	Day	8% 17% 30% 26% 14% 5%	448
	21st to end	Day	8% 12% 25% 32% 18% 5%	485
June	1st to 10th	Day	7% 12% 26% 28% 18% 8%	450
	11th to 20th	Day	3 12% 28% 36% 16% 5%	450
	21st to end	Day	2 5% 25% 38% 22% 9%	450
July	1st to 10th	Day	1 3 17% 42% 31% 7%	446
	11th to 20th	Day	1 12% 42% 33% 11%	444
	21st to end	Day	12% 44% 28% 16%	487
August	1st to 10th	Day	2 3 18% 39% 27% 10%	447
	11th to 20th	Day	2 1 18% 40% 27% 13%	437
	21st to end	Day	1 1 12% 42% 32% 12%	495
September	1st to 10th	Day	2 3 16% 41% 30% 7%	450
	11th to 20th	Day	2 7% 22% 40% 23% 6%	450
	21st to end	Day	4 8% 18% 42% 26% 3	450
October	1st to 10th	Day	4 12% 24% 34% 21% 5%	450
	11th to 20th	Day	7% 8% 22% 32% 25% 6%	450
	21st to end	Day	8% 7% 27% 33% 20% 5%	495
November	1st to 10th	Day	11% 10% 21% 28% 26% 4	391
	11th to 20th	Day	7% 10% 21% 30% 22% 9%	270
	21st to end	Day	12% 10% 22% 32% 19% 5%	223
December	1st to 10th	Day	17% 14% 16% 29% 16% 7%	180
	11th to 20th	Day	20% 17% 16% 27% 13% 7%	180
	21st to end	Day	22% 17% 22% 19% 14% 6%	198
January	1st to 10th	Night	19% 8% 15% 29% 20% 9%	760
	11th to 20th	Night	16% 7% 14% 33% 22% 9%	760
	21st to end	Night	20% 6% 12% 32% 23% 7%	836
February	1st to 10th	Night	21% 8% 12% 31% 21% 6%	760
	11th to 20th	Night	18% 11% 15% 31% 19% 7%	744
	21st to end	Night	16% 11% 18% 34% 18% 3	628
March	1st to 10th	Night	22% 10% 15% 30% 19% 4	760
	11th to 20th	Night	18% 10% 16% 27% 24% 5%	760
	21st to end	Night	16% 11% 20% 30% 20% 3	820
April	1st to 10th	Night	15% 9% 19% 31% 21% 4	720
	11th to 20th	Night	12% 12% 18% 32% 20% 6%	711
	21st to end	Night	7% 11% 24% 27% 23% 8%	720
May	1st to 10th	Night	6% 8% 23% 30% 26% 7%	704
	11th to 20th	Night	5% 7% 13% 33% 35% 7%	448
	21st to end	Night	4 6% 15% 33% 35% 6%	389
June	1st to 10th	Night	4 6% 16% 32% 35% 8%	360
	11th to 20th	Night	1 5% 13% 38% 37% 5%	360
	21st to end	Night	2 1 7% 38% 44% 8%	360
July	1st to 10th	Night	1 8% 33% 49% 9%	350
	11th to 20th	Night	1 2 26% 54% 16%	473
	21st to end	Night	2 1 2 24% 54% 17%	591
August	1st to 10th	Night	2 1 6% 29% 49% 13%	537
	11th to 20th	Night	5% 35% 46% 14%	685
	21st to end	Night	1 3 29% 54% 12%	792
September	1st to 10th	Night	1 2 7% 31% 46% 12%	720
	11th to 20th	Night	3 3 9% 32% 43% 10%	720
	21st to end	Night	3 3 10% 30% 39% 14%	720
October	1st to 10th	Night	5% 3 12% 34% 33% 12%	720
	11th to 20th	Night	8% 5% 10% 33% 34% 11%	720
	21st to end	Night	8% 4 13% 30% 27% 18%	792
November	1st to 10th	Night	9% 8% 10% 32% 30% 11%	740
	11th to 20th	Night	11% 4 15% 35% 27% 9%	760
	21st to end	Night	11% 6% 16% 34% 25% 8%	760
December	1st to 10th	Night	18% 8% 14% 31% 23% 7%	760
	11th to 20th	Night	23% 9% 14% 29% 20% 6%	760
	21st to end	Night	19% 13% 18% 28% 19% 4	836

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