Hale Hill Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hale Hill Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	14% 9% 3 7% 5% 4 5% 4 6% 7% 36%	131705
	Night	15% 10% 4 8% 4 4 4 3 5% 6% 37%	131645
Transparency	Day	45% 5% 11% 23% 14% 2	135309
	Night	46% 5% 10% 22% 15% 2	136114
Seeing	Day	19% 13% 28% 23% 13% 4	17653
	Night	19% 12% 28% 27% 13% 1	27527

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	14% 14% 5% 11% 4 4 4 4 5% 7% 27%	4691
	11th to 20th	Day	9% 13% 6% 14% 8% 5% 5% 4 4 5% 27%	4710
	21st to end	Day	18% 15% 5% 12% 6% 4 4 3 4 5% 24%	5463
February	1st to 10th	Day	17% 13% 6% 8% 5% 4 4 3 4 6% 30%	2100
	11th to 20th	Day	16% 11% 4 10% 5% 5% 4 3 5% 6% 31%	2048
	21st to end	Day	15% 14% 4 9% 5% 3 4 3 4 6% 33%	1730
March	1st to 10th	Day	19% 15% 4 7% 3 3 3 2 3 3 38%	4580
	11th to 20th	Day	12% 10% 2 5% 4 4 4 3 5% 8% 41%	4844
	21st to end	Day	17% 11% 4 7% 5% 5% 4 4 5% 7% 31%	5953
April	1st to 10th	Day	16% 11% 3 7% 4 4 4 3 5% 7% 35%	2520
	11th to 20th	Day	17% 8% 3 6% 5% 5% 4 3 5% 6% 38%	2494
	21st to end	Day	19% 7% 2 7% 5% 5% 5% 3 4 6% 37%	2646
May	1st to 10th	Day	19% 10% 3 8% 5% 4 5% 4 7% 7% 28%	5992
	11th to 20th	Day	19% 8% 2 7% 5% 4 5% 4 5% 7% 35%	5991
	21st to end	Day	14% 7% 3 8% 5% 5% 5% 5% 7% 8% 32%	6807
June	1st to 10th	Day	12% 6% 2 6% 5% 5% 5% 3 6% 8% 43%	2801
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 4 7% 8% 44%	2800
	21st to end	Day	10% 6% 2 6% 4 4 6% 3 7% 9% 42%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 4 3 6% 11% 42%	6360
	11th to 20th	Day	8% 6% 3 8% 5% 6% 6% 5% 8% 10% 36%	6360
	21st to end	Day	12% 7% 3 7% 5% 6% 6% 5% 7% 9% 33%	6799
August	1st to 10th	Day	8% 4 1 5% 4 3 5% 3 7% 10% 49%	4234
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 6% 8% 11% 38%	3864
	21st to end	Day	6% 4 2 5% 3 4 5% 4 6% 9% 54%	4312
September	1st to 10th	Day	11% 6% 2 5% 4 4 4 5% 5% 8% 47%	2398
	11th to 20th	Day	9% 4 2 6% 4 5% 5% 3 6% 8% 49%	2200
	21st to end	Day	12% 5% 2 5% 4 4 4 3 5% 6% 50%	2200
October	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 4 8% 51%	2200
	11th to 20th	Day	14% 6% 3 6% 4 3 5% 4 5% 6% 45%	2041
	21st to end	Day	19% 9% 3 6% 4 4 3 4 4 6% 38%	1980
November	1st to 10th	Day	9% 7% 2 5% 4 4 4 3 5% 7% 51%	1852
	11th to 20th	Day	17% 10% 3 7% 4 3 3 3 4 5% 40%	1890
	21st to end	Day	19% 11% 4 7% 3 3 4 3 5% 5% 35%	1890
December	1st to 10th	Day	13% 12% 3 8% 4 3 3 2 3 5% 43%	2997
	11th to 20th	Day	16% 14% 6% 9% 5% 5% 5% 4 4 5% 27%	3088
	21st to end	Day	26% 18% 7% 9% 5% 4 4 3 4 5% 16%	4070
January	1st to 10th	Night	16% 14% 7% 12% 5% 4 4 3 5% 5% 26%	8232
	11th to 20th	Night	11% 15% 6% 14% 6% 5% 5% 3 4 4 25%	8280
	21st to end	Night	18% 12% 5% 7% 6% 5% 4 3 5% 6% 30%	8740
February	1st to 10th	Night	18% 12% 5% 10% 5% 4 3 3 5% 5% 28%	2520
	11th to 20th	Night	16% 12% 4 10% 4 5% 4 3 4 6% 32%	2458
	21st to end	Night	19% 12% 4 9% 4 3 3 2 4 5% 34%	2076
March	1st to 10th	Night	21% 15% 3 6% 3 3 3 2 3 4 36%	5493
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 4 7% 46%	5336
	21st to end	Night	17% 10% 3 7% 5% 5% 5% 3 5% 6% 33%	5260
April	1st to 10th	Night	14% 10% 2 7% 4 4 4 3 5% 7% 39%	2100
	11th to 20th	Night	17% 8% 3 6% 4 3 3 3 5% 7% 41%	2075
	21st to end	Night	15% 9% 3 7% 4 4 4 4 6% 7% 38%	1973
May	1st to 10th	Night	20% 10% 4 8% 5% 3 5% 3 5% 7% 29%	4123
	11th to 20th	Night	16% 7% 3 6% 4 4 4 3 7% 8% 38%	4122
	21st to end	Night	14% 7% 2 7% 5% 5% 5% 4 6% 8% 36%	4282
June	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	1600
	11th to 20th	Night	9% 5% 2 6% 5% 3 4 4 5% 9% 48%	1600
	21st to end	Night	6% 5% 1 5% 4 4 5% 5% 7% 10% 47%	1600
July	1st to 10th	Night	9% 6% 2 6% 4 5% 5% 4 7% 8% 46%	3618
	11th to 20th	Night	7% 6% 2 6% 4 5% 5% 5% 7% 10% 44%	3617
	21st to end	Night	9% 6% 3 7% 5% 5% 5% 5% 8% 10% 38%	4064
August	1st to 10th	Night	8% 5% 2 6% 4 4 4 3 5% 8% 52%	2915
	11th to 20th	Night	7% 5% 2 6% 4 5% 4 4 6% 9% 49%	3169
	21st to end	Night	5% 3 1 5% 4 4 4 3 6% 9% 57%	3586
September	1st to 10th	Night	10% 6% 1 6% 4 3 3 4 5% 6% 51%	2000
	11th to 20th	Night	7% 4 2 5% 3 3 3 4 6% 7% 56%	2180
	21st to end	Night	13% 6% 2 5% 4 4 3 3 5% 7% 48%	2200
October	1st to 10th	Night	9% 4 2 5% 3 4 4 3 5% 6% 57%	2200
	11th to 20th	Night	16% 7% 2 4 3 3 3 3 4 6% 49%	2353
	21st to end	Night	18% 9% 2 6% 3 3 4 3 5% 6% 40%	2860
November	1st to 10th	Night	11% 8% 3 5% 3 3 4 3 5% 6% 50%	2666
	11th to 20th	Night	14% 10% 3 7% 4 4 3 3 5% 7% 42%	2730
	21st to end	Night	20% 13% 4 7% 4 3 3 3 4 5% 34%	2730
December	1st to 10th	Night	16% 11% 4 9% 4 3 3 3 3 5% 39%	4398
	11th to 20th	Night	19% 13% 4 9% 5% 4 3 4 5% 5% 29%	5339
	21st to end	Night	29% 18% 6% 9% 4 3 4 3 3 4 18%	7150

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	56% 4 11% 21% 7% 1	4692
	11th to 20th	Day	58% 4 11% 14% 12% 2	4710
	21st to end	Day	64% 2 7% 17% 6% 4	5463
February	1st to 10th	Day	55% 5% 10% 20% 9% 2	2100
	11th to 20th	Day	53% 5% 10% 21% 10% 1	2058
	21st to end	Day	53% 4 8% 23% 10% 2	1730
March	1st to 10th	Day	53% 3 5% 19% 16% 4	4580
	11th to 20th	Day	41% 4 10% 31% 13% 2	4844
	21st to end	Day	51% 6% 12% 20% 9% 1	5955
April	1st to 10th	Day	47% 6% 11% 21% 14% 1	2520
	11th to 20th	Day	45% 5% 11% 24% 12% 3	2494
	21st to end	Day	48% 6% 9% 25% 11% 3	2646
May	1st to 10th	Day	52% 7% 12% 19% 10%	6027
	11th to 20th	Day	48% 7% 9% 25% 11%	5995
	21st to end	Day	46% 7% 13% 19% 13% 2	6821
June	1st to 10th	Day	37% 7% 11% 25% 17% 3	2801
	11th to 20th	Day	35% 8% 12% 23% 19% 3	2800
	21st to end	Day	37% 8% 15% 19% 17% 4	2800
July	1st to 10th	Day	37% 5% 11% 28% 18% 2	6291
	11th to 20th	Day	40% 8% 14% 25% 13%	6285
	21st to end	Day	44% 8% 13% 22% 11% 1	6604
August	1st to 10th	Day	29% 7% 14% 33% 16% 1	4221
	11th to 20th	Day	36% 7% 12% 28% 15% 1	3900
	21st to end	Day	26% 6% 16% 31% 17% 4	4311
September	1st to 10th	Day	35% 7% 11% 25% 19% 4	2398
	11th to 20th	Day	32% 5% 12% 27% 22% 3	2200
	21st to end	Day	35% 5% 10% 23% 26% 2	2200
October	1st to 10th	Day	22% 4 7% 29% 36% 2	3590
	11th to 20th	Day	30% 4 9% 26% 30% 2	3309
	21st to end	Day	40% 3 9% 23% 25% 1	3180
November	1st to 10th	Day	33% 5% 12% 29% 17% 5%	1852
	11th to 20th	Day	46% 4 9% 22% 16% 4	1890
	21st to end	Day	50% 5% 8% 22% 14% 1	1890
December	1st to 10th	Day	46% 3 10% 27% 14%	2997
	11th to 20th	Day	59% 4 8% 21% 7% 1	3085
	21st to end	Day	71% 5% 10% 10% 5%	4070
January	1st to 10th	Night	61% 4 10% 18% 6%	8233
	11th to 20th	Night	62% 3 9% 11% 11% 3	8280
	21st to end	Night	56% 3 11% 21% 9% 1	8740
February	1st to 10th	Night	58% 5% 11% 19% 7% 1	2520
	11th to 20th	Night	53% 4 10% 20% 12% 1	2459
	21st to end	Night	53% 5% 9% 20% 10% 2	2076
March	1st to 10th	Night	53% 3 6% 23% 11% 4	5493
	11th to 20th	Night	38% 5% 10% 26% 20% 1	5336
	21st to end	Night	51% 5% 10% 26% 7% 1	5260
April	1st to 10th	Night	45% 5% 10% 25% 14% 2	2100
	11th to 20th	Night	43% 4 11% 20% 17% 5%	2075
	21st to end	Night	44% 7% 9% 25% 12% 3	1973
May	1st to 10th	Night	52% 8% 12% 18% 9% 1	4150
	11th to 20th	Night	44% 6% 13% 25% 12%	4123
	21st to end	Night	44% 7% 14% 20% 12% 3	4290
June	1st to 10th	Night	35% 6% 10% 25% 22% 3	1600
	11th to 20th	Night	34% 6% 11% 23% 21% 6%	1600
	21st to end	Night	28% 9% 16% 22% 21% 5%	1600
July	1st to 10th	Night	35% 6% 10% 26% 20% 2	3580
	11th to 20th	Night	33% 8% 15% 27% 15% 1	3572
	21st to end	Night	38% 7% 16% 25% 12% 2	3947
August	1st to 10th	Night	30% 6% 13% 37% 12% 2	2904
	11th to 20th	Night	31% 6% 15% 28% 18% 1	3198
	21st to end	Night	24% 6% 14% 30% 22% 4	3586
September	1st to 10th	Night	33% 6% 11% 24% 20% 6%	2000
	11th to 20th	Night	26% 5% 9% 27% 28% 5%	2180
	21st to end	Night	35% 6% 10% 21% 24% 5%	2200
October	1st to 10th	Night	19% 5% 7% 21% 45% 3	3590
	11th to 20th	Night	26% 2 7% 26% 35% 3	3824
	21st to end	Night	36% 3 8% 23% 26% 5%	4610
November	1st to 10th	Night	36% 5% 9% 24% 22% 4	2666
	11th to 20th	Night	43% 3 9% 23% 18% 3	2730
	21st to end	Night	52% 4 9% 22% 13% 1	2730
December	1st to 10th	Night	49% 3 11% 23% 13%	4398
	11th to 20th	Night	57% 4 11% 18% 10%	5341
	21st to end	Night	72% 4 7% 12% 5%	7150

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	28% 19% 34% 15% 3	330
	11th to 20th	Day	35% 22% 32% 10% 1	330
	21st to end	Day	31% 24% 33% 10% 2	363
February	1st to 10th	Day	26% 27% 32% 13% 1	330
	11th to 20th	Day	28% 30% 29% 10% 3	330
	21st to end	Day	31% 21% 39% 8% 1	273
March	1st to 10th	Day	27% 28% 34% 10% 1	330
	11th to 20th	Day	30% 22% 33% 11% 4	424
	21st to end	Day	31% 20% 31% 15% 3	594
April	1st to 10th	Day	25% 20% 31% 17% 6% 1	540
	11th to 20th	Day	27% 19% 26% 20% 7% 1	532
	21st to end	Day	24% 18% 35% 15% 7% 1	540
May	1st to 10th	Day	22% 19% 31% 20% 6% 2	543
	11th to 20th	Day	23% 10% 32% 19% 11% 5%	567
	21st to end	Day	15% 9% 29% 26% 14% 7%	718
June	1st to 10th	Day	13% 7% 29% 30% 17% 4	731
	11th to 20th	Day	12% 7% 26% 33% 16% 5%	730
	21st to end	Day	12% 7% 30% 28% 18% 6%	730
July	1st to 10th	Day	12% 4 18% 27% 24% 13%	721
	11th to 20th	Day	7% 5% 25% 32% 22% 9%	718
	21st to end	Day	12% 3 19% 34% 25% 7%	744
August	1st to 10th	Day	10% 5% 17% 29% 30% 10%	526
	11th to 20th	Day	13% 4 21% 31% 23% 7%	519
	21st to end	Day	8% 3 17% 34% 28% 11%	583
September	1st to 10th	Day	10% 6% 19% 32% 25% 7%	530
	11th to 20th	Day	8% 3 17% 30% 29% 13%	530
	21st to end	Day	15% 10% 23% 23% 21% 8%	530
October	1st to 10th	Day	10% 8% 25% 34% 18% 5%	530
	11th to 20th	Day	15% 16% 33% 24% 11% 2	411
	21st to end	Day	29% 17% 36% 16% 2	363
November	1st to 10th	Day	15% 25% 29% 21% 8% 2	330
	11th to 20th	Day	26% 22% 30% 15% 5% 1	330
	21st to end	Day	29% 15% 37% 16% 2 1	330
December	1st to 10th	Day	19% 28% 34% 14% 3 1	330
	11th to 20th	Day	27% 21% 37% 11% 4 1	330
	21st to end	Day	34% 20% 34% 10% 2 1	363
January	1st to 10th	Night	23% 19% 34% 20% 4	1050
	11th to 20th	Night	26% 22% 32% 17% 3	1050
	21st to end	Night	24% 20% 34% 21% 2	1113
February	1st to 10th	Night	24% 16% 39% 20% 2	840
	11th to 20th	Night	24% 19% 35% 19% 2	823
	21st to end	Night	26% 18% 37% 18% 2	692
March	1st to 10th	Night	28% 17% 32% 22% 2	840
	11th to 20th	Night	24% 15% 35% 20% 5%	840
	21st to end	Night	27% 11% 33% 25% 4	777
April	1st to 10th	Night	21% 14% 33% 26% 6%	630
	11th to 20th	Night	22% 14% 29% 25% 9% 1	623
	21st to end	Night	20% 10% 37% 25% 8%	630
May	1st to 10th	Night	19% 10% 33% 28% 10%	630
	11th to 20th	Night	15% 9% 29% 29% 14% 3	621
	21st to end	Night	11% 5% 25% 36% 19% 3	683
June	1st to 10th	Night	12% 4 24% 38% 22% 1	600
	11th to 20th	Night	11% 5% 21% 44% 19% 1	600
	21st to end	Night	9% 3 23% 38% 26% 2	600
July	1st to 10th	Night	9% 2 15% 36% 31% 7%	585
	11th to 20th	Night	7% 4 16% 36% 33% 4	582
	21st to end	Night	8% 4 13% 42% 29% 4	657
August	1st to 10th	Night	8% 2 12% 40% 33% 5%	597
	11th to 20th	Night	7% 2 15% 37% 35% 4	594
	21st to end	Night	6% 3 15% 33% 37% 5%	660
September	1st to 10th	Night	13% 3 15% 38% 27% 4	600
	11th to 20th	Night	8% 3 16% 36% 33% 4	780
	21st to end	Night	15% 7% 19% 31% 25% 3	800
October	1st to 10th	Night	11% 6% 24% 33% 23% 3	800
	11th to 20th	Night	19% 12% 27% 28% 14% 1	800
	21st to end	Night	22% 15% 32% 23% 6%	880
November	1st to 10th	Night	16% 11% 34% 29% 10% 1	820
	11th to 20th	Night	19% 18% 29% 26% 7%	840
	21st to end	Night	26% 18% 25% 25% 6%	840
December	1st to 10th	Night	25% 15% 37% 18% 4	845
	11th to 20th	Night	26% 14% 33% 22% 5% 1	1050
	21st to end	Night	33% 16% 31% 17% 3	1155

Page generated at 2026-05-29 01:53:30. Version ac07

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