HiRes 2
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the HiRes 2 clear sky chart from 2005-11-06 to 2025-03-27.

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 4 5% 7% 56%	114120
	Night	5% 7% 2 6% 4 4 4 4 5% 8% 51%	114094
Transparency	Day	28% 3 8% 21% 23% 18%	113884
	Night	32% 3 9% 23% 21% 13%	113920
Seeing	Day	7% 9% 21% 31% 23% 9%	15341
	Night	8% 5% 19% 40% 24% 4	25225

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 13% 4 9% 6% 6% 5% 5% 7% 8% 33%	4098
	11th to 20th	Day	4 11% 4 11% 6% 5% 6% 4 6% 6% 36%	4100
	21st to end	Day	5% 9% 3 10% 5% 5% 5% 4 6% 8% 39%	4510
February	1st to 10th	Day	5% 10% 4 8% 6% 4 3 5% 7% 8% 39%	1800
	11th to 20th	Day	6% 9% 3 8% 4 5% 4 4 5% 8% 43%	1910
	21st to end	Day	4 9% 3 7% 4 6% 3 4 6% 7% 46%	1650
March	1st to 10th	Day	4 9% 3 6% 5% 5% 5% 4 5% 8% 46%	3383
	11th to 20th	Day	4 8% 2 6% 5% 4 5% 4 6% 8% 48%	3600
	21st to end	Day	7% 12% 4 8% 5% 5% 5% 5% 6% 8% 35%	3892
April	1st to 10th	Day	5% 8% 2 6% 5% 5% 4 4 6% 7% 48%	2123
	11th to 20th	Day	6% 7% 3 6% 5% 4 4 3 4 5% 52%	2393
	21st to end	Day	7% 6% 2 4 3 4 4 3 5% 7% 55%	2470
May	1st to 10th	Day	5% 6% 2 5% 4 5% 5% 4 6% 7% 52%	4110
	11th to 20th	Day	4 3 1 3 3 3 4 3 5% 7% 64%	4071
	21st to end	Day	4 4 2 4 3 3 4 4 6% 7% 58%	4477
June	1st to 10th	Day	3 3 1 3 3 4 4 4 6% 8% 61%	2470
	11th to 20th	Day	4 4 1 3 3 3 3 2 4 6% 67%	2509
	21st to end	Day	2 2 1 3 2 2 3 3 4 6% 74%	2660
July	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 79%	4332
	11th to 20th	Day	2 2 1 3 3 3 4 4 7% 8% 63%	4110
	21st to end	Day	1 2 1 2 3 3 3 3 6% 8% 67%	4454
August	1st to 10th	Day	2 1 1 3 3 3 4 3 5% 7% 67%	4092
	11th to 20th	Day	2 2 1 3 3 3 5% 4 5% 5% 68%	4028
	21st to end	Day	2 2 1 2 2 3 3 2 4 5% 75%	4440
September	1st to 10th	Day	2 2 1 2 2 2 3 3 4 6% 74%	2119
	11th to 20th	Day	2 2 1 3 2 3 2 2 4 5% 71%	2090
	21st to end	Day	3 2 1 3 2 2 2 2 3 4 75%	2090
October	1st to 10th	Day	4 2 1 2 2 2 3 2 4 5% 72%	3429
	11th to 20th	Day	4 5% 2 3 2 2 2 3 4 7% 64%	3150
	21st to end	Day	3 5% 2 4 3 2 3 2 3 6% 66%	3327
November	1st to 10th	Day	5% 5% 1 4 4 4 4 4 7% 8% 53%	1764
	11th to 20th	Day	6% 7% 2 6% 3 3 4 4 5% 7% 53%	1800
	21st to end	Day	7% 8% 3 6% 4 4 4 4 5% 7% 49%	1800
December	1st to 10th	Day	4 6% 2 5% 2 2 3 2 5% 11% 57%	2950
	11th to 20th	Day	5% 9% 3 7% 5% 5% 5% 5% 6% 11% 38%	3409
	21st to end	Day	6% 11% 4 10% 6% 6% 7% 5% 6% 7% 32%	4510
January	1st to 10th	Night	5% 13% 4 8% 6% 4 4 4 5% 9% 36%	5867
	11th to 20th	Night	7% 11% 5% 9% 5% 4 5% 4 5% 9% 36%	5880
	21st to end	Night	5% 10% 4 7% 6% 5% 5% 4 6% 8% 39%	6468
February	1st to 10th	Night	7% 10% 3 8% 5% 5% 4 4 6% 8% 40%	2600
	11th to 20th	Night	6% 10% 3 8% 5% 4 6% 3 5% 6% 44%	2374
	21st to end	Night	4 8% 3 6% 5% 5% 5% 4 6% 7% 45%	1980
March	1st to 10th	Night	4 11% 4 8% 5% 5% 6% 4 5% 7% 41%	3710
	11th to 20th	Night	4 6% 2 7% 4 5% 5% 3 7% 7% 50%	3600
	21st to end	Night	7% 13% 4 8% 5% 4 4 3 6% 6% 40%	3899
April	1st to 10th	Night	8% 8% 2 6% 4 4 4 3 5% 7% 51%	2052
	11th to 20th	Night	5% 8% 2 6% 4 4 4 4 5% 6% 51%	1734
	21st to end	Night	8% 5% 2 5% 4 3 4 4 5% 7% 52%	1710
May	1st to 10th	Night	9% 7% 2 6% 4 4 5% 5% 6% 7% 45%	2860
	11th to 20th	Night	4 4 1 4 3 4 4 3 5% 8% 60%	2834
	21st to end	Night	5% 5% 2 5% 4 3 5% 4 6% 7% 54%	3110
June	1st to 10th	Night	4 3 3 4 4 4 5% 4 6% 8% 55%	1710
	11th to 20th	Night	4 4 2 4 3 3 3 3 3 6% 66%	1662
	21st to end	Night	2 2 1 4 3 3 4 4 6% 7% 64%	1520
July	1st to 10th	Night	1 1 1 3 2 3 3 3 4 5% 75%	2622
	11th to 20th	Night	4 4 2 7% 5% 5% 6% 6% 7% 9% 45%	2860
	21st to end	Night	3 4 2 6% 4 4 3 3 7% 7% 56%	3109
August	1st to 10th	Night	2 3 1 6% 5% 5% 5% 5% 6% 8% 52%	2847
	11th to 20th	Night	1 2 1 4 3 3 4 3 5% 6% 67%	2811
	21st to end	Night	3 2 1 2 2 2 2 2 3 4 75%	3227
September	1st to 10th	Night	2 2 1 3 3 3 3 3 4 7% 68%	2061
	11th to 20th	Night	3 2 1 4 2 3 3 3 4 7% 69%	2090
	21st to end	Night	5% 3 1 3 2 2 2 2 4 4 73%	2090
October	1st to 10th	Night	3 3 1 3 2 3 4 3 4 5% 70%	3547
	11th to 20th	Night	6% 5% 1 3 2 3 2 3 5% 5% 65%	3778
	21st to end	Night	5% 3 1 4 3 3 4 3 6% 7% 61%	4228
November	1st to 10th	Night	6% 7% 2 4 3 3 3 4 5% 8% 55%	2535
	11th to 20th	Night	5% 6% 2 5% 4 4 3 3 5% 7% 56%	2600
	21st to end	Night	6% 8% 2 6% 4 4 3 3 5% 7% 52%	2600
December	1st to 10th	Night	5% 6% 2 5% 3 3 3 3 5% 17% 50%	4240
	11th to 20th	Night	6% 10% 3 6% 4 4 4 4 6% 10% 45%	4811
	21st to end	Night	7% 10% 4 10% 6% 5% 5% 4 6% 9% 33%	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	46% 5% 22% 18% 8%	4099
	11th to 20th	Day	45% 4 16% 25% 8% 2	4100
	21st to end	Day	41% 3 18% 26% 12% 1	4510
February	1st to 10th	Day	41% 5% 16% 24% 12% 3	1800
	11th to 20th	Day	40% 4 13% 27% 14% 1	1920
	21st to end	Day	36% 4 12% 30% 17% 3	1650
March	1st to 10th	Day	36% 3 8% 31% 20% 2	3383
	11th to 20th	Day	36% 3 7% 34% 18% 3	3600
	21st to end	Day	49% 3 9% 22% 14% 2	3892
April	1st to 10th	Day	36% 4 8% 25% 19% 9%	2123
	11th to 20th	Day	36% 3 7% 21% 26% 8%	2396
	21st to end	Day	30% 3 6% 25% 29% 7%	2470
May	1st to 10th	Day	30% 5% 4 22% 28% 11%	4110
	11th to 20th	Day	21% 4 3 20% 38% 13%	4074
	21st to end	Day	26% 3 4 21% 33% 13%	4477
June	1st to 10th	Day	22% 2 5% 19% 34% 19%	2470
	11th to 20th	Day	20% 2 4 14% 30% 30%	2509
	21st to end	Day	14% 2 1 8% 26% 49%	2660
July	1st to 10th	Day	10% 2 2 6% 11% 69%	4265
	11th to 20th	Day	17% 4 3 10% 23% 44%	4041
	21st to end	Day	15% 3 4 11% 27% 40%	4314
August	1st to 10th	Day	16% 3 3 13% 29% 36%	4078
	11th to 20th	Day	15% 4 4 9% 30% 37%	4067
	21st to end	Day	14% 2 2 11% 26% 45%	4439
September	1st to 10th	Day	13% 2 3 13% 30% 38%	2119
	11th to 20th	Day	17% 3 4 18% 28% 30%	2090
	21st to end	Day	16% 2 4 17% 33% 29%	2090
October	1st to 10th	Day	15% 2 4 11% 39% 28%	3429
	11th to 20th	Day	21% 2 5% 25% 34% 12%	3150
	21st to end	Day	22% 2 6% 28% 36% 6%	3327
November	1st to 10th	Day	29% 3 12% 26% 25% 5%	1764
	11th to 20th	Day	32% 2 10% 35% 20% 2	1800
	21st to end	Day	35% 3 12% 31% 18% 1	1800
December	1st to 10th	Day	24% 2 16% 44% 12% 1	2950
	11th to 20th	Day	39% 3 17% 33% 9%	3408
	21st to end	Day	49% 4 14% 21% 11% 1	4510
January	1st to 10th	Night	44% 4 18% 25% 9%	5867
	11th to 20th	Night	45% 3 14% 26% 10% 1	5880
	21st to end	Night	38% 3 16% 28% 14% 1	6468
February	1st to 10th	Night	41% 4 14% 28% 11% 2	2600
	11th to 20th	Night	41% 3 11% 24% 19% 2	2375
	21st to end	Night	36% 5% 8% 30% 18% 3	1980
March	1st to 10th	Night	44% 2 7% 26% 19% 2	3710
	11th to 20th	Night	35% 2 7% 34% 19% 3	3600
	21st to end	Night	47% 3 6% 22% 19% 3	3899
April	1st to 10th	Night	34% 3 8% 27% 20% 9%	2052
	11th to 20th	Night	34% 3 7% 23% 26% 6%	1734
	21st to end	Night	31% 3 8% 26% 27% 5%	1710
May	1st to 10th	Night	37% 3 4 20% 27% 9%	2860
	11th to 20th	Night	25% 3 3 24% 33% 12%	2835
	21st to end	Night	28% 3 6% 18% 37% 7%	3110
June	1st to 10th	Night	27% 3 5% 19% 28% 18%	1710
	11th to 20th	Night	22% 2 4 13% 31% 28%	1662
	21st to end	Night	18% 3 2 10% 27% 41%	1520
July	1st to 10th	Night	13% 3 2 6% 10% 66%	2580
	11th to 20th	Night	30% 5% 5% 9% 18% 33%	2806
	21st to end	Night	26% 4 5% 9% 21% 36%	3011
August	1st to 10th	Night	24% 4 5% 13% 22% 31%	2838
	11th to 20th	Night	18% 4 4 11% 27% 36%	2838
	21st to end	Night	15% 2 3 10% 21% 49%	3227
September	1st to 10th	Night	17% 3 4 12% 25% 40%	2061
	11th to 20th	Night	18% 3 4 16% 27% 32%	2090
	21st to end	Night	17% 2 3 14% 30% 34%	2090
October	1st to 10th	Night	17% 2 4 11% 32% 33%	3547
	11th to 20th	Night	23% 3 3 20% 36% 16%	3778
	21st to end	Night	22% 3 5% 29% 34% 7%	4228
November	1st to 10th	Night	29% 3 8% 28% 26% 8%	2535
	11th to 20th	Night	28% 3 9% 34% 22% 4	2600
	21st to end	Night	33% 3 8% 32% 22% 2	2600
December	1st to 10th	Night	25% 3 15% 41% 15% 2	4240
	11th to 20th	Night	36% 3 17% 33% 10%	4811
	21st to end	Night	47% 3 15% 23% 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	12% 9% 22% 28% 22% 6%	300
	11th to 20th	Day	6% 6% 26% 38% 16% 8%	300
	21st to end	Day	8% 5% 18% 38% 22% 9%	330
February	1st to 10th	Day	12% 12% 29% 26% 15% 7%	300
	11th to 20th	Day	11% 9% 26% 35% 14% 5%	455
	21st to end	Day	4 13% 32% 35% 11% 5%	414
March	1st to 10th	Day	11% 12% 23% 34% 18% 3	500
	11th to 20th	Day	9% 12% 30% 28% 15% 6%	500
	21st to end	Day	14% 14% 32% 25% 12% 4	531
April	1st to 10th	Day	11% 14% 27% 26% 17% 5%	460
	11th to 20th	Day	11% 20% 27% 25% 15% 2	452
	21st to end	Day	12% 17% 28% 23% 15% 6%	460
May	1st to 10th	Day	5% 15% 32% 31% 12% 5%	463
	11th to 20th	Day	6% 11% 24% 28% 23% 8%	488
	21st to end	Day	5% 12% 19% 31% 24% 9%	529
June	1st to 10th	Day	4 9% 19% 32% 24% 11%	490
	11th to 20th	Day	4 12% 24% 28% 24% 8%	490
	21st to end	Day	2 5% 23% 35% 28% 8%	490
July	1st to 10th	Day	2 2 18% 36% 32% 11%	486
	11th to 20th	Day	4 4 14% 38% 29% 12%	484
	21st to end	Day	2 1 10% 35% 36% 15%	531
August	1st to 10th	Day	3 3 15% 37% 29% 13%	487
	11th to 20th	Day	4 3 15% 32% 32% 14%	477
	21st to end	Day	4 4 15% 27% 35% 15%	539
September	1st to 10th	Day	2 4 14% 30% 30% 20%	490
	11th to 20th	Day	3 6% 18% 30% 29% 13%	490
	21st to end	Day	4 7% 18% 27% 32% 12%	490
October	1st to 10th	Day	6% 7% 21% 25% 22% 18%	455
	11th to 20th	Day	5% 8% 15% 29% 25% 18%	300
	21st to end	Day	7% 7% 19% 33% 28% 6%	330
November	1st to 10th	Day	11% 8% 20% 30% 19% 12%	300
	11th to 20th	Day	6% 5% 24% 36% 19% 10%	300
	21st to end	Day	10% 9% 17% 27% 26% 12%	300
December	1st to 10th	Day	7% 7% 19% 31% 23% 14%	300
	11th to 20th	Day	10% 6% 19% 36% 23% 6%	300
	21st to end	Day	13% 9% 23% 35% 16% 3	330
January	1st to 10th	Night	10% 6% 17% 38% 22% 6%	800
	11th to 20th	Night	11% 5% 17% 39% 24% 5%	800
	21st to end	Night	10% 5% 16% 41% 23% 5%	880
February	1st to 10th	Night	12% 6% 22% 40% 17% 4	800
	11th to 20th	Night	10% 5% 21% 43% 19% 2	784
	21st to end	Night	7% 6% 25% 45% 15% 2	660
March	1st to 10th	Night	10% 10% 23% 40% 16% 2	800
	11th to 20th	Night	8% 7% 23% 40% 20% 2	800
	21st to end	Night	10% 8% 29% 38% 13% 2	862
April	1st to 10th	Night	12% 8% 24% 38% 15% 2	760
	11th to 20th	Night	7% 11% 29% 37% 14% 2	606
	21st to end	Night	10% 8% 26% 37% 17% 2	570
May	1st to 10th	Night	8% 8% 26% 40% 15% 2	570
	11th to 20th	Night	8% 4 22% 39% 24% 3	561
	21st to end	Night	9% 9% 19% 35% 25% 2	617
June	1st to 10th	Night	6% 9% 15% 42% 26% 1	570
	11th to 20th	Night	6% 8% 22% 40% 24% 1	522
	21st to end	Night	2 3 19% 44% 31% 1	380
July	1st to 10th	Night	2 2 16% 43% 36% 3	411
	11th to 20th	Night	6% 3 16% 45% 28% 2	552
	21st to end	Night	6% 1 12% 44% 36% 2	624
August	1st to 10th	Night	3 2 20% 44% 29% 1	567
	11th to 20th	Night	3 1 18% 45% 31% 2	564
	21st to end	Night	4 3 15% 41% 33% 4	669
September	1st to 10th	Night	2 3 13% 39% 37% 6%	760
	11th to 20th	Night	3 3 17% 39% 31% 7%	760
	21st to end	Night	6% 4 15% 34% 33% 8%	760
October	1st to 10th	Night	7% 3 17% 39% 30% 4	760
	11th to 20th	Night	6% 3 15% 39% 30% 7%	760
	21st to end	Night	6% 3 20% 37% 28% 5%	836
November	1st to 10th	Night	9% 5% 15% 40% 26% 5%	780
	11th to 20th	Night	7% 4 18% 45% 22% 5%	800
	21st to end	Night	8% 4 19% 38% 25% 6%	800
December	1st to 10th	Night	11% 4 15% 44% 22% 4	800
	11th to 20th	Night	13% 4 16% 42% 23% 2	800
	21st to end	Night	17% 4 21% 40% 16% 3	880

Page generated at 2025-03-24 07:04:27. Version ac07

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