HiRes 1
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the HiRes 1 clear sky chart from 2005-11-06 to 2025-08-05.

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	4 6% 2 5% 4 4 4 3 5% 7% 57%	130274
	Night	5% 7% 2 6% 4 4 4 3 5% 7% 53%	126757
Transparency	Day	26% 5% 9% 19% 22% 19%	129997
	Night	29% 5% 10% 21% 21% 14%	126549
Seeing	Day	7% 9% 22% 32% 22% 8%	15847
	Night	8% 5% 20% 40% 23% 4	25631

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	4 14% 5% 9% 5% 5% 5% 4 7% 8% 33%	4098
	11th to 20th	Day	5% 11% 4 10% 7% 5% 4 4 5% 7% 38%	4100
	21st to end	Day	5% 10% 3 8% 5% 4 4 4 6% 10% 42%	4510
February	1st to 10th	Day	7% 10% 4 8% 5% 5% 4 4 6% 8% 39%	1800
	11th to 20th	Day	7% 10% 3 7% 4 4 5% 4 6% 8% 42%	1900
	21st to end	Day	4 9% 3 8% 5% 4 4 4 7% 7% 45%	1650
March	1st to 10th	Day	4 11% 3 7% 5% 5% 4 3 6% 7% 44%	4728
	11th to 20th	Day	3 7% 2 7% 5% 5% 5% 4 7% 9% 45%	5000
	21st to end	Day	7% 12% 4 9% 6% 5% 5% 5% 6% 6% 35%	5486
April	1st to 10th	Day	6% 9% 2 6% 5% 5% 4 4 5% 7% 48%	2227
	11th to 20th	Day	6% 7% 3 7% 4 4 3 3 4 6% 51%	2527
	21st to end	Day	8% 5% 2 4 4 4 3 3 6% 7% 54%	2600
May	1st to 10th	Day	5% 5% 2 5% 4 4 4 4 5% 7% 56%	5880
	11th to 20th	Day	5% 5% 2 5% 3 4 4 4 5% 7% 56%	5841
	21st to end	Day	3 4 1 4 3 3 3 3 5% 7% 63%	6423
June	1st to 10th	Day	3 3 1 3 3 4 3 4 6% 9% 60%	2600
	11th to 20th	Day	4 4 1 3 3 3 3 2 4 6% 69%	2621
	21st to end	Day	2 2 1 2 2 3 3 3 4 5% 74%	2800
July	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 76%	6148
	11th to 20th	Day	2 2 1 4 3 3 4 4 6% 8% 64%	5880
	21st to end	Day	1 1 1 2 2 2 3 3 5% 7% 72%	6401
August	1st to 10th	Day	2 1 1 3 3 3 4 4 6% 7% 68%	4145
	11th to 20th	Day	1 1 1 3 3 3 4 4 5% 6% 68%	4028
	21st to end	Day	2 2 1 2 2 2 3 3 4 5% 75%	4464
September	1st to 10th	Day	2 2 1 2 2 2 2 3 4 6% 74%	2114
	11th to 20th	Day	2 2 1 4 3 3 3 3 4 6% 70%	2090
	21st to end	Day	3 2 1 2 2 2 3 2 3 5% 74%	2090
October	1st to 10th	Day	3 2 1 2 2 3 3 2 4 5% 72%	3408
	11th to 20th	Day	5% 5% 1 4 2 2 3 3 4 6% 65%	3150
	21st to end	Day	3 5% 2 4 3 3 2 3 4 6% 65%	3332
November	1st to 10th	Day	5% 5% 2 5% 3 4 5% 4 7% 7% 54%	1764
	11th to 20th	Day	7% 7% 2 6% 3 4 3 4 5% 7% 52%	1800
	21st to end	Day	6% 9% 3 5% 5% 4 5% 4 5% 6% 49%	1800
December	1st to 10th	Day	4 5% 2 4 3 3 3 3 4 7% 62%	2950
	11th to 20th	Day	5% 9% 3 7% 5% 4 5% 4 6% 11% 40%	3409
	21st to end	Day	6% 11% 4 10% 6% 5% 6% 5% 7% 8% 32%	4510
January	1st to 10th	Night	6% 13% 4 8% 5% 4 4 3 5% 8% 40%	5867
	11th to 20th	Night	8% 10% 4 9% 5% 5% 5% 3 4 6% 40%	5880
	21st to end	Night	5% 11% 3 7% 4 3 3 3 5% 11% 44%	6468
February	1st to 10th	Night	7% 10% 3 7% 6% 5% 4 4 6% 8% 42%	2600
	11th to 20th	Night	6% 10% 3 8% 4 4 4 3 5% 7% 44%	2385
	21st to end	Night	4 9% 3 7% 5% 4 4 5% 6% 7% 47%	1980
March	1st to 10th	Night	5% 13% 4 8% 5% 4 4 3 5% 6% 43%	5164
	11th to 20th	Night	4 9% 3 7% 6% 6% 6% 4 6% 8% 43%	5000
	21st to end	Night	6% 11% 3 8% 5% 4 4 4 6% 6% 44%	5489
April	1st to 10th	Night	9% 8% 2 5% 4 3 4 2 4 7% 51%	2170
	11th to 20th	Night	6% 7% 2 5% 4 5% 4 4 6% 7% 50%	1819
	21st to end	Night	8% 6% 3 5% 4 4 4 3 5% 7% 50%	1800
May	1st to 10th	Night	7% 6% 2 5% 3 4 5% 4 6% 6% 52%	4100
	11th to 20th	Night	5% 4 1 4 4 4 4 4 6% 7% 57%	4074
	21st to end	Night	4 4 2 5% 4 4 5% 4 6% 7% 56%	4474
June	1st to 10th	Night	4 4 2 4 4 4 5% 4 6% 8% 55%	1800
	11th to 20th	Night	5% 3 1 4 3 3 3 2 4 6% 66%	1768
	21st to end	Night	2 2 1 4 4 3 4 3 6% 8% 63%	1600
July	1st to 10th	Night	1 2 1 3 3 3 3 3 4 5% 71%	3796
	11th to 20th	Night	3 4 2 6% 5% 5% 6% 5% 7% 8% 48%	4100
	21st to end	Night	2 4 2 5% 3 3 4 4 7% 7% 59%	4472
August	1st to 10th	Night	2 3 2 6% 5% 4 6% 4 7% 8% 54%	2883
	11th to 20th	Night	1 2 1 4 3 3 4 3 5% 6% 67%	2811
	21st to end	Night	3 2 1 3 2 2 2 2 3 5% 75%	3202
September	1st to 10th	Night	3 2 1 3 3 3 2 3 4 6% 69%	2066
	11th to 20th	Night	3 3 1 4 2 2 3 2 3 6% 70%	2090
	21st to end	Night	5% 3 1 3 2 2 2 2 3 5% 73%	2090
October	1st to 10th	Night	3 2 1 3 2 3 3 3 4 5% 71%	3567
	11th to 20th	Night	6% 5% 1 3 2 2 3 3 5% 5% 65%	3778
	21st to end	Night	5% 3 1 4 3 3 4 4 6% 8% 60%	4210
November	1st to 10th	Night	6% 7% 2 4 3 3 3 3 6% 8% 55%	2535
	11th to 20th	Night	6% 5% 2 6% 4 3 4 3 5% 7% 55%	2600
	21st to end	Night	6% 8% 3 5% 4 4 4 2 5% 7% 52%	2600
December	1st to 10th	Night	5% 6% 1 4 3 2 3 3 5% 7% 61%	4240
	11th to 20th	Night	6% 9% 3 6% 4 4 4 3 6% 8% 47%	4811
	21st to end	Night	7% 11% 4 10% 7% 4 5% 4 6% 9% 35%	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	44% 9% 23% 16% 8%	4099
	11th to 20th	Day	42% 8% 15% 24% 9% 2	4100
	21st to end	Day	36% 9% 16% 22% 16% 1	4510
February	1st to 10th	Day	42% 7% 17% 21% 11% 2	1800
	11th to 20th	Day	39% 7% 18% 23% 13% 1	1910
	21st to end	Day	35% 6% 15% 27% 14% 3	1650
March	1st to 10th	Day	36% 6% 10% 25% 18% 4	4728
	11th to 20th	Day	31% 7% 8% 31% 19% 3	5000
	21st to end	Day	44% 7% 14% 21% 12% 1	5486
April	1st to 10th	Day	35% 6% 10% 22% 19% 7%	2227
	11th to 20th	Day	34% 4 9% 22% 24% 7%	2530
	21st to end	Day	29% 5% 10% 26% 25% 6%	2600
May	1st to 10th	Day	23% 6% 11% 21% 27% 11%	5880
	11th to 20th	Day	25% 4 7% 19% 34% 10%	5844
	21st to end	Day	19% 5% 6% 20% 38% 12%	6424
June	1st to 10th	Day	20% 4 9% 18% 33% 16%	2600
	11th to 20th	Day	18% 3 5% 14% 29% 30%	2621
	21st to end	Day	13% 3 4 9% 28% 44%	2800
July	1st to 10th	Day	12% 2 3 5% 12% 66%	6083
	11th to 20th	Day	17% 3 3 8% 20% 48%	5811
	21st to end	Day	12% 3 4 8% 21% 52%	6217
August	1st to 10th	Day	15% 4 5% 13% 28% 36%	4131
	11th to 20th	Day	15% 4 5% 9% 31% 35%	4067
	21st to end	Day	12% 3 4 13% 26% 43%	4463
September	1st to 10th	Day	12% 3 5% 13% 30% 36%	2114
	11th to 20th	Day	16% 4 7% 19% 26% 28%	2090
	21st to end	Day	14% 4 6% 17% 31% 27%	2090
October	1st to 10th	Day	16% 3 5% 11% 37% 28%	3408
	11th to 20th	Day	21% 3 5% 25% 33% 12%	3150
	21st to end	Day	22% 3 7% 26% 35% 7%	3332
November	1st to 10th	Day	28% 7% 12% 26% 23% 5%	1764
	11th to 20th	Day	30% 6% 10% 33% 19% 2	1800
	21st to end	Day	34% 5% 14% 29% 16% 2	1800
December	1st to 10th	Day	22% 5% 14% 45% 12% 1	2950
	11th to 20th	Day	37% 7% 17% 30% 9%	3408
	21st to end	Day	46% 8% 13% 22% 11% 1	4510
January	1st to 10th	Night	39% 8% 17% 24% 11%	5867
	11th to 20th	Night	41% 8% 14% 24% 11% 2	5880
	21st to end	Night	33% 7% 16% 26% 17% 1	6468
February	1st to 10th	Night	40% 7% 16% 26% 10% 2	2600
	11th to 20th	Night	38% 6% 12% 24% 17% 2	2386
	21st to end	Night	34% 6% 12% 28% 17% 4	1980
March	1st to 10th	Night	39% 5% 9% 23% 18% 6%	5164
	11th to 20th	Night	36% 6% 9% 26% 20% 3	5000
	21st to end	Night	36% 7% 11% 24% 19% 3	5489
April	1st to 10th	Night	33% 5% 9% 25% 21% 8%	2170
	11th to 20th	Night	31% 6% 10% 23% 25% 5%	1819
	21st to end	Night	31% 5% 11% 25% 24% 4	1800
May	1st to 10th	Night	28% 7% 12% 16% 27% 11%	4097
	11th to 20th	Night	23% 6% 6% 22% 34% 9%	4075
	21st to end	Night	23% 6% 8% 16% 40% 8%	4474
June	1st to 10th	Night	24% 6% 7% 18% 27% 17%	1800
	11th to 20th	Night	20% 3 6% 15% 29% 28%	1768
	21st to end	Night	17% 3 5% 11% 26% 38%	1600
July	1st to 10th	Night	16% 2 3 5% 8% 65%	3753
	11th to 20th	Night	29% 5% 4 7% 17% 39%	4046
	21st to end	Night	20% 4 5% 8% 16% 47%	4344
August	1st to 10th	Night	23% 5% 6% 13% 22% 31%	2874
	11th to 20th	Night	17% 4 6% 12% 26% 36%	2838
	21st to end	Night	14% 2 4 11% 20% 48%	3202
September	1st to 10th	Night	16% 3 6% 13% 25% 37%	2066
	11th to 20th	Night	17% 4 7% 16% 26% 31%	2090
	21st to end	Night	16% 3 4 13% 31% 32%	2090
October	1st to 10th	Night	17% 3 5% 11% 32% 32%	3567
	11th to 20th	Night	23% 3 4 21% 33% 16%	3778
	21st to end	Night	22% 4 5% 27% 35% 7%	4210
November	1st to 10th	Night	28% 4 10% 27% 24% 7%	2535
	11th to 20th	Night	28% 5% 11% 31% 22% 4	2600
	21st to end	Night	33% 4 11% 30% 21% 2	2600
December	1st to 10th	Night	23% 5% 13% 41% 16% 2	4240
	11th to 20th	Night	35% 5% 17% 32% 11%	4811
	21st to end	Night	45% 6% 14% 24% 11%	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	13% 8% 26% 26% 21% 5%	300
	11th to 20th	Day	7% 6% 26% 36% 16% 8%	300
	21st to end	Day	9% 6% 17% 38% 23% 8%	330
February	1st to 10th	Day	15% 10% 27% 28% 13% 6%	300
	11th to 20th	Day	11% 8% 30% 33% 13% 4	445
	21st to end	Day	6% 11% 31% 36% 13% 3	414
March	1st to 10th	Day	12% 13% 24% 31% 17% 3	500
	11th to 20th	Day	9% 15% 29% 26% 16% 4	500
	21st to end	Day	15% 15% 30% 27% 11% 2	550
April	1st to 10th	Day	12% 15% 26% 27% 16% 5%	500
	11th to 20th	Day	13% 19% 26% 27% 15% 1	492
	21st to end	Day	12% 17% 28% 24% 15% 5%	500
May	1st to 10th	Day	6% 14% 31% 31% 13% 5%	503
	11th to 20th	Day	9% 11% 27% 27% 22% 5%	528
	21st to end	Day	7% 11% 20% 31% 23% 8%	573
June	1st to 10th	Day	4 8% 23% 31% 23% 9%	530
	11th to 20th	Day	6% 13% 26% 29% 21% 5%	530
	21st to end	Day	2 6% 26% 35% 25% 6%	530
July	1st to 10th	Day	2 4 19% 39% 29% 7%	526
	11th to 20th	Day	3 3 18% 38% 28% 9%	524
	21st to end	Day	3 2 14% 38% 32% 11%	575
August	1st to 10th	Day	4 3 18% 39% 27% 9%	504
	11th to 20th	Day	4 3 17% 33% 29% 14%	477
	21st to end	Day	4 5% 16% 31% 34% 11%	539
September	1st to 10th	Day	4 5% 16% 30% 28% 17%	490
	11th to 20th	Day	5% 7% 18% 31% 28% 10%	490
	21st to end	Day	5% 8% 17% 28% 32% 10%	490
October	1st to 10th	Day	6% 9% 21% 25% 24% 15%	447
	11th to 20th	Day	5% 8% 17% 30% 24% 16%	300
	21st to end	Day	6% 8% 20% 32% 28% 5%	330
November	1st to 10th	Day	12% 7% 20% 30% 20% 10%	300
	11th to 20th	Day	6% 7% 24% 33% 21% 9%	300
	21st to end	Day	11% 8% 15% 31% 25% 10%	300
December	1st to 10th	Day	8% 7% 15% 38% 21% 11%	300
	11th to 20th	Day	10% 6% 20% 37% 22% 5%	300
	21st to end	Day	11% 10% 24% 36% 15% 4	330
January	1st to 10th	Night	13% 4 19% 36% 22% 5%	800
	11th to 20th	Night	9% 6% 18% 39% 23% 5%	800
	21st to end	Night	10% 5% 17% 42% 21% 5%	880
February	1st to 10th	Night	10% 7% 23% 40% 16% 4	800
	11th to 20th	Night	11% 4 22% 42% 19% 2	784
	21st to end	Night	8% 6% 26% 44% 16% 1	660
March	1st to 10th	Night	12% 8% 22% 42% 14% 3	800
	11th to 20th	Night	7% 8% 24% 40% 20% 2	800
	21st to end	Night	9% 9% 30% 37% 13% 1	880
April	1st to 10th	Night	12% 10% 23% 39% 15% 1	800
	11th to 20th	Night	8% 11% 29% 37% 14% 1	631
	21st to end	Night	11% 8% 25% 39% 16% 2	600
May	1st to 10th	Night	7% 8% 26% 42% 16% 2	600
	11th to 20th	Night	7% 6% 22% 39% 22% 4	591
	21st to end	Night	9% 8% 20% 34% 26% 3	650
June	1st to 10th	Night	5% 9% 17% 42% 26% 1	600
	11th to 20th	Night	7% 10% 19% 41% 22% 1	568
	21st to end	Night	2 2 20% 44% 30% 3	400
July	1st to 10th	Night	3 2 17% 44% 32% 3	452
	11th to 20th	Night	5% 4 16% 46% 27% 3	582
	21st to end	Night	4 2 12% 43% 37% 2	657
August	1st to 10th	Night	3 2 19% 45% 28% 2	579
	11th to 20th	Night	3 2 18% 44% 30% 3	564
	21st to end	Night	4 3 16% 40% 33% 4	657
September	1st to 10th	Night	3 3 12% 40% 35% 7%	760
	11th to 20th	Night	4 3 18% 39% 30% 7%	760
	21st to end	Night	5% 4 16% 36% 31% 9%	760
October	1st to 10th	Night	6% 4 17% 39% 28% 5%	760
	11th to 20th	Night	7% 3 15% 39% 30% 6%	760
	21st to end	Night	6% 3 21% 37% 27% 6%	836
November	1st to 10th	Night	9% 4 18% 38% 24% 6%	780
	11th to 20th	Night	8% 4 19% 45% 20% 5%	800
	21st to end	Night	9% 5% 18% 39% 23% 7%	800
December	1st to 10th	Night	10% 5% 16% 44% 21% 4	800
	11th to 20th	Night	15% 5% 18% 39% 21% 2	800
	21st to end	Night	15% 6% 23% 38% 15% 3	880

Page generated at 2025-08-01 23:32:26. Version ac07

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