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 2024-11-04.

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	7% 5% 2 4 2 2 3 2 4 5% 65%	104978
	Night	8% 6% 2 4 3 3 3 3 4 6% 59%	102774
Transparency	Day	22% 4 5% 17% 25% 26%	104733
	Night	25% 6% 6% 15% 22% 25%	102607
Seeing	Day	10% 11% 23% 31% 19% 6%	15406
	Night	10% 6% 13% 31% 30% 9%	24770

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% 3 3 3 3 3 5% 40%	2667
	11th to 20th	Day	15% 9% 3 9% 6% 5% 5% 3 5% 6% 35%	2860
	21st to end	Day	21% 10% 3 7% 4 4 5% 4 5% 7% 30%	3146
February	1st to 10th	Day	18% 8% 3 5% 3 2 3 2 5% 5% 44%	1710
	11th to 20th	Day	12% 9% 2 6% 4 4 4 3 4 7% 44%	1789
	21st to end	Day	16% 10% 2 6% 3 4 3 2 4 5% 45%	1570
March	1st to 10th	Day	13% 15% 4 11% 7% 6% 5% 4 5% 5% 26%	3324
	11th to 20th	Day	10% 7% 2 5% 2 3 3 2 5% 6% 53%	3490
	21st to end	Day	12% 12% 4 9% 5% 6% 5% 4 5% 6% 32%	3917
April	1st to 10th	Day	14% 8% 2 7% 4 3 3 3 5% 6% 44%	2280
	11th to 20th	Day	11% 5% 2 4 3 3 3 3 5% 6% 54%	2249
	21st to end	Day	6% 5% 1 3 3 2 3 3 5% 7% 61%	2280
May	1st to 10th	Day	6% 3 1 3 2 2 2 2 3 5% 71%	3825
	11th to 20th	Day	2 2 1 2 1 1 1 1 2 5% 80%	4213
	21st to end	Day	3 2 1 3 3 2 2 2 3 5% 74%	4813
June	1st to 10th	Day	4 4 1 4 2 3 3 3 5% 7% 65%	2660
	11th to 20th	Day	1 1 1 2 2 1 2 4 5% 80%	2660
	21st to end	Day	2 2 1 2 1 1 2 2 2 3 84%	2660
July	1st to 10th	Day	1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 1 2 1 2 3 86%	4184
	21st to end	Day	1 1 1 1 1 2 3 91%	4455
August	1st to 10th	Day	1 1 2 1 1 2 3 4 4 80%	4091
	11th to 20th	Day	1 1 1 1 2 3 92%	3762
	21st to end	Day	1 1 1 1 1 1 2 3 88%	4180
September	1st to 10th	Day	1 1 1 2 1 1 1 1 2 2 88%	2280
	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	9% 6% 2 4 4 4 4 3 5% 8% 51%	1826
	11th to 20th	Day	8% 6% 2 4 4 3 5% 4 5% 7% 53%	1710
	21st to end	Day	13% 6% 2 4 2 3 3 3 5% 7% 54%	1611
December	1st to 10th	Day	12% 8% 3 6% 4 4 4 3 4 7% 45%	1520
	11th to 20th	Day	29% 12% 3 5% 3 3 2 3 4 4 31%	1928
	21st to end	Day	14% 13% 4 7% 4 4 4 4 5% 6% 34%	2794
January	1st to 10th	Night	12% 9% 3 7% 4 4 4 4 5% 6% 42%	4278
	11th to 20th	Night	18% 11% 3 6% 5% 5% 5% 4 5% 6% 31%	4110
	21st to end	Night	19% 12% 4 8% 5% 4 4 4 5% 6% 31%	4521
February	1st to 10th	Night	17% 7% 2 5% 4 3 3 3 4 6% 47%	2470
	11th to 20th	Night	12% 8% 2 6% 4 4 4 3 6% 7% 44%	2279
	21st to end	Night	12% 8% 3 5% 4 2 4 3 5% 7% 47%	1884
March	1st to 10th	Night	17% 16% 4 9% 5% 4 3 3 4 4 31%	3534
	11th to 20th	Night	11% 8% 3 5% 3 3 2 2 3 5% 55%	3490
	21st to end	Night	13% 15% 4 8% 5% 4 4 3 5% 6% 33%	3723
April	1st to 10th	Night	11% 8% 2 6% 4 3 5% 4 5% 6% 46%	1900
	11th to 20th	Night	8% 8% 2 5% 3 4 3 3 4 7% 53%	1879
	21st to end	Night	5% 6% 2 3 3 4 4 3 6% 10% 56%	1900
May	1st to 10th	Night	2 4 2 4 3 2 3 2 5% 8% 64%	3150
	11th to 20th	Night	2 2 1 2 2 1 2 2 5% 7% 74%	2680
	21st to end	Night	2 1 1 2 2 2 3 3 4 6% 73%	2762
June	1st to 10th	Night	3 3 1 4 3 2 2 3 3 8% 67%	1520
	11th to 20th	Night	1 1 1 2 2 2 3 2 3 6% 77%	1520
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 85%	1520
July	1st to 10th	Night	1 1 1 1 1 3 92%	2540
	11th to 20th	Night	1 1 2 1 1 1 1 2 3 86%	2780
	21st to end	Night	1 1 1 1 1 1 2 3 4 85%	3109
August	1st to 10th	Night	1 1 1 2 1 1 2 2 4 6% 79%	2848
	11th to 20th	Night	1 1 1 1 1 2 4 89%	3086
	21st to end	Night	1 1 1 1 1 1 1 2 3 86%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 2 4 85%	1900
	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% 7% 2 4 3 3 3 3 5% 7% 56%	2322
	11th to 20th	Night	8% 6% 2 4 3 4 3 4 6% 8% 51%	2470
	21st to end	Night	9% 7% 2 4 4 4 4 3 4 7% 54%	2565
December	1st to 10th	Night	14% 7% 2 5% 4 4 3 2 4 6% 48%	2660
	11th to 20th	Night	26% 12% 3 5% 4 3 3 3 5% 6% 31%	3276
	21st to end	Night	16% 10% 3 9% 6% 5% 5% 4 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	54% 10% 7% 14% 12% 3	3324
	11th to 20th	Day	31% 5% 8% 27% 17% 12%	3490
	21st to end	Day	44% 13% 12% 18% 11% 3	3917
April	1st to 10th	Day	39% 5% 10% 20% 20% 6%	2280
	11th to 20th	Day	29% 5% 13% 23% 23% 7%	2252
	21st to end	Day	23% 6% 10% 30% 26% 6%	2280
May	1st to 10th	Day	16% 5% 8% 29% 29% 13%	3825
	11th to 20th	Day	11% 3 3 23% 45% 15%	4216
	21st to end	Day	12% 6% 6% 30% 39% 6%	4814
June	1st to 10th	Day	20% 4 8% 23% 32% 13%	2660
	11th to 20th	Day	8% 3 6% 20% 37% 26%	2660
	21st to end	Day	8% 2 2 11% 30% 46%	2660
July	1st to 10th	Day	3 1 1 4 15% 76%	4351
	11th to 20th	Day	8% 2 2 6% 15% 68%	4112
	21st to end	Day	3 1 1 5% 26% 64%	4315
August	1st to 10th	Day	8% 3 2 7% 23% 57%	4076
	11th to 20th	Day	3 1 1 6% 31% 58%	3798
	21st to end	Day	5% 2 2 9% 34% 49%	4179
September	1st to 10th	Day	7% 2 1 8% 30% 52%	2280
	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% 6% 6% 20% 24% 15%	1826
	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	48% 12% 8% 13% 13% 5%	3534
	11th to 20th	Night	31% 6% 5% 23% 23% 12%	3490
	21st to end	Night	42% 15% 8% 19% 13% 5%	3723
April	1st to 10th	Night	35% 7% 12% 18% 21% 7%	1900
	11th to 20th	Night	30% 6% 8% 24% 23% 10%	1879
	21st to end	Night	22% 8% 7% 27% 27% 8%	1900
May	1st to 10th	Night	14% 13% 6% 20% 30% 17%	3150
	11th to 20th	Night	11% 6% 4 13% 49% 17%	2680
	21st to end	Night	10% 8% 5% 22% 41% 14%	2762
June	1st to 10th	Night	17% 6% 8% 20% 32% 17%	1520
	11th to 20th	Night	10% 5% 5% 13% 31% 36%	1520
	21st to end	Night	7% 3 2 11% 29% 47%	1520
July	1st to 10th	Night	3 2 1 2 16% 76%	2500
	11th to 20th	Night	6% 2 2 4 16% 70%	2729
	21st to end	Night	5% 2 1 5% 22% 65%	3011
August	1st to 10th	Night	9% 4 1 7% 19% 60%	2839
	11th to 20th	Night	3 2 1 5% 24% 65%	3116
	21st to end	Night	6% 3 2 7% 30% 51%	3498
September	1st to 10th	Night	7% 2 1 5% 23% 61%	1900
	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% 22% 20%	2322
	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% 31% 21% 8%	506
April	1st to 10th	Day	19% 16% 29% 22% 13% 1	460
	11th to 20th	Day	15% 19% 29% 23% 13% 2	452
	21st to end	Day	8% 18% 29% 27% 12% 6%	460
May	1st to 10th	Day	11% 16% 32% 28% 10% 3	463
	11th to 20th	Day	7% 16% 29% 27% 16% 6%	488
	21st to end	Day	8% 11% 24% 32% 19% 5%	529
June	1st to 10th	Day	6% 12% 27% 28% 18% 8%	490
	11th to 20th	Day	3 13% 28% 35% 16% 6%	490
	21st to end	Day	2 4 24% 39% 21% 9%	490
July	1st to 10th	Day	1 2 17% 41% 31% 7%	486
	11th to 20th	Day	1 13% 41% 34% 11%	484
	21st to end	Day	1 1 12% 45% 27% 16%	531
August	1st to 10th	Day	2 3 17% 40% 27% 11%	487
	11th to 20th	Day	1 3 19% 39% 26% 12%	477
	21st to end	Day	1 3 12% 41% 32% 12%	539
September	1st to 10th	Day	2 3 16% 42% 30% 7%	490
	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% 11% 22% 28% 25% 3	404
	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	836
April	1st to 10th	Night	15% 10% 20% 31% 21% 4	760
	11th to 20th	Night	12% 12% 18% 32% 21% 5%	751
	21st to end	Night	7% 11% 25% 27% 22% 8%	760
May	1st to 10th	Night	6% 8% 25% 30% 25% 7%	743
	11th to 20th	Night	5% 7% 12% 33% 36% 7%	473
	21st to end	Night	4 6% 15% 34% 35% 6%	411
June	1st to 10th	Night	3 6% 16% 34% 33% 8%	380
	11th to 20th	Night	1 6% 14% 38% 36% 5%	380
	21st to end	Night	2 1 7% 37% 45% 8%	380
July	1st to 10th	Night	1 8% 33% 50% 8%	370
	11th to 20th	Night	1 2 26% 55% 16%	501
	21st to end	Night	1 1 2 24% 54% 17%	624
August	1st to 10th	Night	2 1 6% 29% 50% 13%	567
	11th to 20th	Night	1 7% 35% 44% 13%	724
	21st to end	Night	1 1 4 28% 53% 12%	836
September	1st to 10th	Night	1 2 7% 32% 46% 12%	760
	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% 29% 10%	754
	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

Page generated at 2024-11-01 19:47:45. Version ac07

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