Halloran Summit
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Halloran Summit clear sky chart from 2005-11-06 to 2025-02-25.

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	2 2 1 3 2 2 3 2 4 5% 74%	113271
	Night	2 3 1 3 2 2 3 2 4 5% 73%	114340
Transparency	Day	13% 3 4 13% 26% 40%	113035
	Night	13% 3 5% 15% 27% 37%	114166
Seeing	Day	4 8% 23% 36% 24% 5%	15795
	Night	3 4 19% 38% 30% 5%	26141

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	2 3 1 3 3 3 3 2 4 6% 70%	4098
	11th to 20th	Day	2 3 1 4 2 3 3 2 4 5% 71%	4100
	21st to end	Day	6% 7% 2 5% 3 3 3 3 5% 5% 56%	4804
February	1st to 10th	Day	5% 4 1 3 3 4 3 2 4 5% 66%	2000
	11th to 20th	Day	3 3 2 3 3 3 3 2 5% 6% 66%	1950
	21st to end	Day	5% 6% 2 5% 2 3 2 2 3 6% 64%	1611
March	1st to 10th	Day	4 3 1 3 3 3 3 3 4 8% 65%	3138
	11th to 20th	Day	5% 5% 2 5% 3 3 4 3 5% 7% 59%	3428
	21st to end	Day	3 4 1 3 3 3 4 4 6% 8% 62%	3850
April	1st to 10th	Day	3 3 1 4 3 3 2 3 4 6% 67%	2280
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 4 77%	2249
	21st to end	Day	2 2 1 2 1 2 3 2 3 5% 76%	2365
May	1st to 10th	Day	1 2 2 1 1 1 1 3 4 83%	4110
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 4 79%	4071
	21st to end	Day	1 1 1 1 1 1 1 2 3 86%	4477
June	1st to 10th	Day	1 2 1 2 2 2 2 2 3 5% 80%	2470
	11th to 20th	Day	1 1 1 1 1 1 2 2 4 86%	2470
	21st to end	Day	1 2 1 2 2 2 1 2 3 83%	2470
July	1st to 10th	Day	1 1 1 1 1 1 2 1 2 3 86%	4110
	11th to 20th	Day	2 2 1 2 2 3 2 3 4 5% 74%	4110
	21st to end	Day	2 2 1 3 2 2 3 3 4 6% 72%	4454
August	1st to 10th	Day	2 1 1 2 2 3 3 2 4 5% 75%	4001
	11th to 20th	Day	1 1 1 1 1 2 2 3 4 84%	3724
	21st to end	Day	2 1 2 1 2 2 2 3 4 82%	4180
September	1st to 10th	Day	1 2 1 3 2 3 3 3 4 5% 74%	2214
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 86%	2090
	21st to end	Day	1 2 1 2 2 1 2 3 84%	2090
October	1st to 10th	Day	1 1 1 1 1 1 1 1 2 3 85%	3490
	11th to 20th	Day	1 1 1 1 1 1 1 3 4 85%	3441
	21st to end	Day	1 1 2 3 2 2 4 5% 79%	3193
November	1st to 10th	Day	1 3 1 2 1 1 2 3 4 5% 77%	1764
	11th to 20th	Day	2 3 1 3 2 2 3 2 3 5% 75%	1800
	21st to end	Day	3 2 1 3 3 3 3 3 4 7% 69%	1800
December	1st to 10th	Day	2 3 1 3 2 3 3 4 6% 7% 66%	2950
	11th to 20th	Day	3 3 2 4 3 3 4 4 8% 8% 58%	3409
	21st to end	Day	3 3 2 5% 4 4 5% 5% 6% 8% 56%	4510
January	1st to 10th	Night	2 3 1 3 3 3 3 2 4 5% 71%	5867
	11th to 20th	Night	2 3 1 3 3 3 3 3 5% 5% 70%	5880
	21st to end	Night	6% 5% 1 3 3 3 3 3 6% 6% 61%	6186
February	1st to 10th	Night	4 6% 1 5% 2 3 3 3 3 5% 65%	2400
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 64%	2338
	21st to end	Night	4 4 2 3 3 3 3 3 4 6% 65%	1937
March	1st to 10th	Night	3 5% 2 3 2 3 4 3 4 7% 64%	3736
	11th to 20th	Night	4 6% 2 5% 3 4 4 4 5% 7% 56%	3538
	21st to end	Night	2 3 1 3 3 3 3 3 5% 7% 68%	3798
April	1st to 10th	Night	2 4 2 4 3 3 2 3 4 7% 68%	1900
	11th to 20th	Night	2 2 1 3 3 2 3 2 4 5% 74%	1879
	21st to end	Night	1 2 2 2 2 2 3 4 6% 77%	1814
May	1st to 10th	Night	1 1 2 1 1 2 2 3 4 83%	2860
	11th to 20th	Night	1 1 1 1 2 1 3 6% 83%	2834
	21st to end	Night	1 1 1 1 1 1 1 3 89%	3110
June	1st to 10th	Night	1 1 1 2 2 2 4 6% 80%	1710
	11th to 20th	Night	1 1 1 2 1 1 1 1 2 3 86%	1710
	21st to end	Night	1 2 1 1 2 2 2 3 86%	1710
July	1st to 10th	Night	1 1 1 1 1 1 1 3 3 88%	2860
	11th to 20th	Night	2 1 1 2 2 2 3 3 4 6% 75%	2860
	21st to end	Night	2 2 1 2 2 2 3 3 5% 6% 72%	3109
August	1st to 10th	Night	1 1 2 1 2 2 2 3 6% 78%	2936
	11th to 20th	Night	1 1 1 1 1 2 2 3 5% 83%	3126
	21st to end	Night	1 1 1 1 1 1 1 2 3 87%	3498
September	1st to 10th	Night	2 1 2 2 2 3 2 3 5% 78%	1966
	11th to 20th	Night	1 1 2 1 1 1 1 3 3 85%	2090
	21st to end	Night	1 1 1 1 1 1 1 1 2 3 86%	2090
October	1st to 10th	Night	2 1 1 1 1 1 2 2 2 87%	3490
	11th to 20th	Night	1 1 1 1 1 1 2 3 4 85%	3494
	21st to end	Night	1 1 1 2 1 2 3 2 3 4 81%	4360
November	1st to 10th	Night	2 4 1 2 2 2 2 2 3 5% 77%	2535
	11th to 20th	Night	1 2 1 3 2 2 3 2 3 6% 75%	2600
	21st to end	Night	3 3 1 3 2 2 3 2 5% 7% 68%	2600
December	1st to 10th	Night	2 3 1 3 2 2 3 3 6% 7% 68%	4240
	11th to 20th	Night	3 3 1 3 3 4 4 3 6% 9% 61%	4811
	21st to end	Night	3 4 2 5% 3 4 4 4 6% 9% 55%	6468

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	16% 3 4 24% 36% 16%	4099
	11th to 20th	Day	15% 3 7% 21% 22% 31%	4100
	21st to end	Day	28% 4 5% 17% 25% 21%	4804
February	1st to 10th	Day	21% 3 6% 15% 29% 25%	2000
	11th to 20th	Day	18% 4 5% 19% 30% 24%	1960
	21st to end	Day	23% 4 6% 17% 29% 20%	1611
March	1st to 10th	Day	16% 7% 5% 21% 30% 21%	3138
	11th to 20th	Day	23% 6% 8% 22% 28% 14%	3428
	21st to end	Day	16% 6% 8% 26% 31% 14%	3850
April	1st to 10th	Day	18% 3 5% 19% 29% 26%	2280
	11th to 20th	Day	12% 3 4 16% 32% 34%	2252
	21st to end	Day	11% 3 4 17% 34% 32%	2365
May	1st to 10th	Day	7% 3 3 12% 33% 42%	4110
	11th to 20th	Day	8% 5% 3 9% 29% 46%	4074
	21st to end	Day	7% 1 3 8% 31% 50%	4477
June	1st to 10th	Day	9% 2 3 11% 25% 50%	2470
	11th to 20th	Day	6% 2 2 5% 19% 66%	2470
	21st to end	Day	9% 2 2 4 14% 69%	2470
July	1st to 10th	Day	7% 1 2 4 8% 77%	4046
	11th to 20th	Day	12% 3 4 6% 20% 55%	4041
	21st to end	Day	13% 4 4 12% 19% 49%	4314
August	1st to 10th	Day	13% 2 3 11% 30% 41%	3987
	11th to 20th	Day	6% 3 3 8% 19% 60%	3760
	21st to end	Day	9% 3 2 6% 16% 65%	4179
September	1st to 10th	Day	13% 4 5% 9% 17% 52%	2214
	11th to 20th	Day	6% 3 3 6% 25% 57%	2090
	21st to end	Day	7% 2 2 9% 27% 52%	2090
October	1st to 10th	Day	8% 2 2 6% 17% 64%	3490
	11th to 20th	Day	6% 2 2 9% 32% 50%	3441
	21st to end	Day	10% 1 2 8% 33% 46%	3193
November	1st to 10th	Day	10% 2 4 15% 38% 30%	1764
	11th to 20th	Day	13% 2 5% 14% 33% 32%	1800
	21st to end	Day	16% 3 8% 15% 31% 27%	1800
December	1st to 10th	Day	18% 2 5% 14% 31% 30%	2950
	11th to 20th	Day	21% 3 8% 22% 31% 15%	3408
	21st to end	Day	24% 4 11% 26% 27% 8%	4510
January	1st to 10th	Night	15% 5% 4 21% 37% 20%	5867
	11th to 20th	Night	14% 5% 6% 19% 24% 31%	5880
	21st to end	Night	23% 3 5% 17% 26% 25%	6186
February	1st to 10th	Night	23% 3 7% 16% 27% 25%	2400
	11th to 20th	Night	20% 3 5% 19% 26% 26%	2339
	21st to end	Night	20% 5% 6% 17% 29% 23%	1937
March	1st to 10th	Night	17% 6% 8% 22% 29% 18%	3736
	11th to 20th	Night	23% 7% 7% 25% 20% 18%	3538
	21st to end	Night	14% 7% 8% 24% 35% 12%	3798
April	1st to 10th	Night	18% 3 6% 19% 29% 25%	1900
	11th to 20th	Night	14% 3 4 17% 32% 30%	1879
	21st to end	Night	10% 3 4 17% 36% 31%	1814
May	1st to 10th	Night	7% 3 7% 10% 36% 38%	2860
	11th to 20th	Night	5% 3 3 10% 31% 48%	2835
	21st to end	Night	5% 1 3 11% 38% 43%	3110
June	1st to 10th	Night	7% 2 4 11% 27% 50%	1710
	11th to 20th	Night	8% 1 2 7% 18% 64%	1710
	21st to end	Night	6% 2 1 5% 16% 70%	1710
July	1st to 10th	Night	4 1 2 5% 8% 79%	2815
	11th to 20th	Night	11% 3 3 7% 23% 54%	2806
	21st to end	Night	13% 3 4 15% 18% 47%	3011
August	1st to 10th	Night	9% 3 3 13% 30% 43%	2927
	11th to 20th	Night	6% 2 3 11% 21% 57%	3156
	21st to end	Night	6% 2 2 5% 17% 68%	3498
September	1st to 10th	Night	11% 3 5% 7% 21% 53%	1966
	11th to 20th	Night	7% 2 2 8% 25% 56%	2090
	21st to end	Night	7% 2 2 8% 24% 58%	2090
October	1st to 10th	Night	7% 1 2 7% 17% 66%	3490
	11th to 20th	Night	6% 1 2 7% 29% 56%	3494
	21st to end	Night	10% 2 2 7% 29% 50%	4360
November	1st to 10th	Night	13% 2 3 14% 35% 33%	2535
	11th to 20th	Night	13% 2 4 14% 32% 35%	2600
	21st to end	Night	17% 3 5% 14% 28% 33%	2600
December	1st to 10th	Night	15% 2 4 11% 34% 34%	4240
	11th to 20th	Night	20% 4 6% 21% 31% 19%	4811
	21st to end	Night	22% 5% 10% 28% 26% 9%	6468

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	11% 8% 31% 33% 15% 2	300
	11th to 20th	Day	8% 10% 25% 35% 20% 2	300
	21st to end	Day	4 12% 28% 31% 21% 3	455
February	1st to 10th	Day	10% 11% 27% 34% 15% 3	500
	11th to 20th	Day	9% 13% 31% 30% 14% 3	495
	21st to end	Day	8% 15% 34% 28% 14% 1	399
March	1st to 10th	Day	9% 17% 30% 30% 14%	460
	11th to 20th	Day	7% 15% 27% 34% 15% 2	460
	21st to end	Day	6% 16% 35% 33% 9% 2	506
April	1st to 10th	Day	7% 12% 31% 31% 16% 3	460
	11th to 20th	Day	5% 18% 32% 30% 12% 2	452
	21st to end	Day	3 17% 28% 28% 20% 4	460
May	1st to 10th	Day	5% 13% 29% 33% 16% 3	463
	11th to 20th	Day	2 9% 26% 37% 20% 5%	488
	21st to end	Day	3 8% 27% 37% 21% 4	529
June	1st to 10th	Day	2 6% 21% 38% 27% 6%	490
	11th to 20th	Day	1 6% 22% 40% 24% 7%	490
	21st to end	Day	1 3 24% 39% 26% 7%	490
July	1st to 10th	Day	2 1 12% 43% 32% 10%	486
	11th to 20th	Day	3 1 5% 39% 41% 11%	484
	21st to end	Day	2 1 6% 41% 37% 12%	531
August	1st to 10th	Day	3 1 12% 37% 37% 11%	487
	11th to 20th	Day	2 1 10% 41% 35% 10%	477
	21st to end	Day	3 1 11% 38% 38% 9%	539
September	1st to 10th	Day	2 2 14% 41% 31% 10%	490
	11th to 20th	Day	1 3 21% 46% 24% 5%	490
	21st to end	Day	1 6% 20% 36% 30% 7%	490
October	1st to 10th	Day	2 6% 20% 41% 24% 7%	490
	11th to 20th	Day	2 7% 18% 44% 26% 3	474
	21st to end	Day	2 7% 23% 31% 32% 6%	330
November	1st to 10th	Day	3 11% 22% 35% 23% 6%	300
	11th to 20th	Day	1 8% 24% 42% 20% 6%	300
	21st to end	Day	3 10% 24% 34% 26% 3	300
December	1st to 10th	Day	5% 4 25% 43% 20% 2	300
	11th to 20th	Day	4 7% 22% 38% 25% 4	300
	21st to end	Day	8% 15% 27% 33% 15% 2	330
January	1st to 10th	Night	5% 5% 22% 37% 23% 7%	800
	11th to 20th	Night	5% 5% 20% 43% 22% 5%	800
	21st to end	Night	8% 6% 24% 38% 19% 4	880
February	1st to 10th	Night	6% 6% 25% 39% 20% 5%	800
	11th to 20th	Night	5% 8% 26% 38% 19% 5%	784
	21st to end	Night	8% 7% 28% 37% 15% 4	646
March	1st to 10th	Night	4 8% 29% 38% 17% 3	760
	11th to 20th	Night	5% 8% 25% 34% 21% 7%	760
	21st to end	Night	4 8% 31% 39% 16% 3	836
April	1st to 10th	Night	4 10% 26% 37% 21% 3	760
	11th to 20th	Night	2 9% 31% 35% 20% 3	751
	21st to end	Night	1 8% 26% 37% 24% 4	674
May	1st to 10th	Night	2 6% 32% 38% 20% 2	570
	11th to 20th	Night	1 7% 24% 34% 32% 2	561
	21st to end	Night	1 7% 23% 38% 27% 2	617
June	1st to 10th	Night	1 4 19% 43% 31% 1	570
	11th to 20th	Night	2 4 22% 41% 29% 2	570
	21st to end	Night	2 14% 46% 36% 1	570
July	1st to 10th	Night	1 5% 43% 46% 5%	555
	11th to 20th	Night	4 5% 38% 51% 3	552
	21st to end	Night	3 1 5% 36% 52% 2	624
August	1st to 10th	Night	2 1 4 41% 48% 4	617
	11th to 20th	Night	1 5% 30% 56% 7%	752
	21st to end	Night	2 6% 31% 50% 12%	836
September	1st to 10th	Night	1 2 6% 29% 52% 10%	760
	11th to 20th	Night	1 11% 42% 37% 9%	760
	21st to end	Night	1 2 15% 33% 36% 13%	760
October	1st to 10th	Night	3 3 15% 41% 35% 3	760
	11th to 20th	Night	3 2 15% 39% 31% 10%	760
	21st to end	Night	2 4 21% 32% 32% 10%	836
November	1st to 10th	Night	3 4 17% 37% 33% 6%	780
	11th to 20th	Night	2 3 15% 42% 32% 6%	800
	21st to end	Night	5% 4 20% 38% 27% 5%	800
December	1st to 10th	Night	6% 3 22% 39% 26% 5%	800
	11th to 20th	Night	8% 4 20% 38% 26% 4	800
	21st to end	Night	6% 7% 25% 39% 19% 4	880

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