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 2026-07-31.

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 5% 2 5% 4 4 4 3 5% 7% 57%	147851
	Night	5% 6% 2 6% 4 4 4 4 5% 8% 52%	142693
Transparency	Day	27% 3 8% 20% 23% 19%	142610
	Night	30% 3 9% 22% 22% 14%	138952
Seeing	Day	6% 8% 21% 30% 23% 10%	17178
	Night	8% 5% 19% 41% 24% 4	26897

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 12% 4 9% 6% 6% 5% 4 6% 7% 36%	5318
	11th to 20th	Day	3 9% 3 8% 5% 4 5% 4 6% 7% 46%	5340
	21st to end	Day	4 7% 3 9% 5% 5% 5% 4 6% 9% 43%	5844
February	1st to 10th	Day	5% 10% 4 8% 6% 4 3 5% 7% 8% 40%	1890
	11th to 20th	Day	6% 10% 3 9% 4 5% 4 4 6% 8% 41%	2008
	21st to end	Day	4 9% 3 7% 5% 6% 4 4 6% 7% 47%	1730
March	1st to 10th	Day	4 11% 3 6% 5% 5% 4 4 5% 8% 45%	4815
	11th to 20th	Day	3 8% 3 7% 5% 5% 5% 5% 6% 9% 46%	5110
	21st to end	Day	7% 11% 4 8% 6% 5% 5% 5% 6% 7% 37%	5607
April	1st to 10th	Day	5% 7% 2 6% 5% 5% 4 4 6% 7% 50%	2347
	11th to 20th	Day	5% 7% 3 6% 4 4 4 3 5% 6% 52%	2649
	21st to end	Day	7% 6% 2 5% 3 4 4 4 5% 7% 54%	2730
May	1st to 10th	Day	4 4 1 4 4 4 4 3 5% 6% 59%	7626
	11th to 20th	Day	5% 5% 2 5% 3 4 4 3 5% 7% 59%	7596
	21st to end	Day	3 4 2 5% 3 3 4 3 5% 7% 61%	8370
June	1st to 10th	Day	3 3 1 3 3 4 4 4 6% 8% 62%	2730
	11th to 20th	Day	3 3 1 3 3 3 3 2 4 6% 69%	2773
	21st to end	Day	2 2 1 3 2 2 3 3 4 6% 73%	2940
July	1st to 10th	Day	1 2 1 3 2 2 2 2 4 5% 75%	6329
	11th to 20th	Day	2 2 1 4 3 3 4 4 6% 8% 64%	6010
	21st to end	Day	1 1 1 2 3 2 3 3 5% 7% 71%	6532
August	1st to 10th	Day	1 1 1 2 2 3 3 3 4 6% 74%	5779
	11th to 20th	Day	1 1 1 3 2 2 4 4 5% 5% 72%	5798
	21st to end	Day	2 2 1 4 3 4 4 4 5% 7% 64%	6348
September	1st to 10th	Day	2 2 1 2 2 2 3 3 4 6% 73%	2230
	11th to 20th	Day	2 2 1 3 3 3 2 3 4 6% 71%	2200
	21st to end	Day	3 2 1 3 2 2 2 2 3 4 75%	2200
October	1st to 10th	Day	4 2 1 2 2 2 3 2 4 5% 72%	3537
	11th to 20th	Day	4 5% 2 3 2 2 3 3 4 7% 64%	3250
	21st to end	Day	3 5% 2 4 3 3 3 3 3 6% 66%	3433
November	1st to 10th	Day	5% 5% 2 4 4 4 4 4 6% 8% 55%	1854
	11th to 20th	Day	6% 7% 2 6% 3 4 4 4 5% 7% 52%	1890
	21st to end	Day	6% 8% 3 7% 4 4 4 4 5% 7% 49%	1890
December	1st to 10th	Day	4 6% 2 5% 2 2 3 2 5% 11% 57%	3040
	11th to 20th	Day	5% 9% 3 7% 5% 5% 5% 5% 6% 11% 38%	3499
	21st to end	Day	6% 11% 4 10% 6% 6% 7% 5% 6% 7% 32%	4609
January	1st to 10th	Night	6% 12% 4 8% 5% 5% 4 4 6% 9% 36%	7605
	11th to 20th	Night	6% 8% 4 7% 4 3 4 3 4 11% 46%	7650
	21st to end	Night	4 8% 3 7% 6% 5% 5% 4 6% 10% 43%	8370
February	1st to 10th	Night	6% 9% 3 8% 5% 5% 4 4 6% 8% 41%	2730
	11th to 20th	Night	6% 10% 4 8% 5% 4 6% 4 5% 6% 43%	2496
	21st to end	Night	4 8% 3 6% 5% 5% 5% 4 6% 7% 47%	2076
March	1st to 10th	Night	4 13% 4 8% 4 4 5% 4 5% 6% 42%	5286
	11th to 20th	Night	5% 8% 3 8% 5% 5% 5% 4 6% 7% 44%	5110
	21st to end	Night	6% 10% 3 7% 5% 4 4 4 6% 6% 44%	5610
April	1st to 10th	Night	8% 8% 2 6% 4 4 4 3 5% 7% 51%	2268
	11th to 20th	Night	5% 8% 2 6% 4 4 4 4 5% 6% 51%	1918
	21st to end	Night	8% 5% 3 5% 4 3 4 3 5% 7% 52%	1890
May	1st to 10th	Night	6% 5% 2 5% 4 4 4 4 5% 6% 55%	5321
	11th to 20th	Night	5% 4 1 4 3 4 4 3 5% 7% 60%	5296
	21st to end	Night	4 4 2 6% 4 4 5% 4 6% 7% 54%	5837
June	1st to 10th	Night	4 4 2 4 4 4 4 4 6% 8% 56%	1890
	11th to 20th	Night	4 3 2 4 2 2 3 3 4 6% 67%	1836
	21st to end	Night	2 2 1 4 3 4 4 4 6% 7% 63%	1680
July	1st to 10th	Night	2 3 1 3 3 3 3 2 4 5% 71%	3832
	11th to 20th	Night	3 4 2 6% 5% 6% 6% 6% 6% 8% 48%	4190
	21st to end	Night	2 4 2 6% 4 4 4 3 6% 7% 58%	4562
August	1st to 10th	Night	1 2 1 5% 4 4 4 4 6% 8% 62%	4033
	11th to 20th	Night	2 2 1 4 3 3 4 3 5% 6% 68%	4051
	21st to end	Night	3 3 2 6% 4 4 4 3 4 5% 62%	4618
September	1st to 10th	Night	2 2 1 4 3 3 3 3 4 8% 67%	2170
	11th to 20th	Night	3 2 1 4 3 3 3 3 4 6% 69%	2200
	21st to end	Night	5% 3 1 3 2 2 2 2 4 4 72%	2200
October	1st to 10th	Night	3 3 1 3 2 3 4 3 4 5% 70%	3659
	11th to 20th	Night	6% 5% 1 3 3 3 2 3 5% 5% 64%	3898
	21st to end	Night	5% 3 1 4 3 3 4 3 6% 7% 61%	4364
November	1st to 10th	Night	6% 7% 2 4 3 3 4 4 5% 8% 56%	2665
	11th to 20th	Night	5% 6% 2 5% 4 4 4 4 5% 7% 55%	2730
	21st to end	Night	6% 8% 2 6% 4 3 3 3 5% 7% 52%	2730
December	1st to 10th	Night	5% 6% 2 5% 3 3 3 3 5% 17% 50%	4370
	11th to 20th	Night	6% 9% 3 5% 4 4 4 4 6% 10% 45%	4941
	21st to end	Night	7% 10% 4 10% 6% 5% 5% 4 6% 9% 33%	6611

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% 23% 19% 7%	5321
	11th to 20th	Day	36% 4 14% 33% 11% 2	5340
	21st to end	Day	36% 3 18% 27% 14% 1	5844
February	1st to 10th	Day	40% 5% 15% 25% 12% 3	1890
	11th to 20th	Day	41% 4 13% 27% 14% 1	2018
	21st to end	Day	36% 3 12% 30% 17% 3	1730
March	1st to 10th	Day	38% 3 7% 28% 21% 4	4815
	11th to 20th	Day	34% 3 6% 32% 20% 4	5110
	21st to end	Day	47% 3 10% 23% 15% 2	5607
April	1st to 10th	Day	34% 3 8% 26% 21% 8%	2347
	11th to 20th	Day	34% 3 6% 21% 27% 8%	2652
	21st to end	Day	30% 3 6% 25% 29% 7%	2730
May	1st to 10th	Day	25% 4 6% 19% 30% 17%	7626
	11th to 20th	Day	25% 4 3 15% 35% 17%	7599
	21st to end	Day	22% 3 3 18% 39% 15%	8371
June	1st to 10th	Day	21% 2 5% 18% 35% 20%	2730
	11th to 20th	Day	18% 2 3 13% 29% 34%	2773
	21st to end	Day	14% 2 1 8% 27% 47%	2940
July	1st to 10th	Day	12% 2 2 5% 12% 66%	6263
	11th to 20th	Day	17% 4 3 9% 20% 47%	5941
	21st to end	Day	12% 3 3 8% 22% 52%	6348
August	1st to 10th	Day	15% 3 3 13% 29% 36%	4208
	11th to 20th	Day	15% 4 4 9% 30% 38%	4197
	21st to end	Day	15% 2 2 12% 26% 43%	4579
September	1st to 10th	Day	14% 2 3 14% 30% 37%	2230
	11th to 20th	Day	17% 3 4 18% 28% 30%	2200
	21st to end	Day	15% 2 4 18% 32% 28%	2200
October	1st to 10th	Day	15% 2 4 11% 39% 27%	3537
	11th to 20th	Day	21% 2 5% 26% 35% 12%	3250
	21st to end	Day	22% 2 6% 28% 36% 6%	3433
November	1st to 10th	Day	28% 3 12% 27% 25% 5%	1854
	11th to 20th	Day	33% 2 10% 34% 19% 2	1890
	21st to end	Day	35% 3 13% 31% 17% 1	1890
December	1st to 10th	Day	24% 2 16% 44% 12% 1	3040
	11th to 20th	Day	38% 3 17% 33% 9%	3498
	21st to end	Day	49% 4 14% 22% 11% 1	4609
January	1st to 10th	Night	44% 4 21% 24% 8%	7605
	11th to 20th	Night	35% 2 14% 33% 15% 1	7650
	21st to end	Night	34% 2 17% 29% 17% 1	8370
February	1st to 10th	Night	40% 4 14% 30% 11% 2	2730
	11th to 20th	Night	41% 3 11% 24% 19% 2	2497
	21st to end	Night	35% 4 8% 32% 18% 3	2076
March	1st to 10th	Night	42% 2 6% 26% 20% 4	5286
	11th to 20th	Night	40% 2 7% 27% 20% 3	5110
	21st to end	Night	41% 3 8% 25% 21% 3	5610
April	1st to 10th	Night	33% 3 7% 25% 23% 9%	2268
	11th to 20th	Night	33% 3 7% 24% 27% 7%	1918
	21st to end	Night	32% 3 7% 25% 28% 5%	1890
May	1st to 10th	Night	29% 4 7% 14% 32% 15%	5319
	11th to 20th	Night	24% 3 3 18% 32% 19%	5297
	21st to end	Night	26% 4 5% 14% 42% 9%	5838
June	1st to 10th	Night	25% 3 5% 18% 29% 20%	1890
	11th to 20th	Night	20% 2 4 12% 30% 32%	1836
	21st to end	Night	19% 3 2 9% 28% 39%	1680
July	1st to 10th	Night	16% 3 2 5% 10% 64%	3790
	11th to 20th	Night	29% 5% 4 8% 15% 38%	4136
	21st to end	Night	21% 4 4 7% 18% 45%	4434
August	1st to 10th	Night	24% 4 5% 13% 22% 32%	2928
	11th to 20th	Night	18% 4 4 11% 27% 37%	2928
	21st to end	Night	16% 2 3 10% 21% 48%	3328
September	1st to 10th	Night	17% 3 4 13% 25% 38%	2170
	11th to 20th	Night	18% 3 4 15% 27% 33%	2200
	21st to end	Night	18% 2 3 15% 29% 34%	2200
October	1st to 10th	Night	17% 2 4 11% 32% 33%	3659
	11th to 20th	Night	23% 3 3 20% 35% 16%	3898
	21st to end	Night	22% 3 5% 29% 34% 7%	4364
November	1st to 10th	Night	28% 3 8% 28% 26% 8%	2665
	11th to 20th	Night	29% 3 9% 33% 22% 4	2730
	21st to end	Night	32% 3 9% 33% 21% 2	2730
December	1st to 10th	Night	25% 3 15% 41% 15% 2	4370
	11th to 20th	Night	36% 3 17% 33% 11%	4941
	21st to end	Night	47% 3 15% 23% 11%	6611

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% 10% 22% 28% 22% 6%	330
	11th to 20th	Day	6% 5% 24% 37% 18% 10%	330
	21st to end	Day	7% 5% 17% 38% 24% 9%	363
February	1st to 10th	Day	11% 11% 26% 26% 16% 9%	330
	11th to 20th	Day	12% 10% 27% 34% 13% 4	493
	21st to end	Day	4 13% 32% 34% 13% 4	446
March	1st to 10th	Day	11% 11% 23% 33% 18% 3	540
	11th to 20th	Day	9% 11% 30% 27% 16% 6%	540
	21st to end	Day	12% 13% 31% 25% 14% 4	594
April	1st to 10th	Day	10% 13% 26% 26% 18% 8%	540
	11th to 20th	Day	11% 18% 27% 25% 16% 3	532
	21st to end	Day	11% 16% 25% 24% 17% 6%	540
May	1st to 10th	Day	5% 13% 31% 29% 14% 8%	543
	11th to 20th	Day	7% 11% 25% 27% 22% 8%	568
	21st to end	Day	5% 12% 19% 29% 24% 11%	617
June	1st to 10th	Day	4 9% 20% 31% 26% 12%	570
	11th to 20th	Day	4 11% 23% 29% 24% 9%	570
	21st to end	Day	3 5% 22% 34% 28% 9%	570
July	1st to 10th	Day	2 2 17% 35% 31% 13%	566
	11th to 20th	Day	3 3 13% 36% 30% 14%	564
	21st to end	Day	2 1 10% 36% 34% 17%	616
August	1st to 10th	Day	3 3 16% 36% 29% 13%	527
	11th to 20th	Day	3 3 14% 33% 32% 15%	517
	21st to end	Day	4 4 14% 26% 36% 15%	583
September	1st to 10th	Day	3 4 14% 29% 31% 20%	530
	11th to 20th	Day	4 6% 18% 31% 29% 13%	530
	21st to end	Day	4 7% 17% 27% 32% 13%	530
October	1st to 10th	Day	6% 8% 21% 25% 23% 17%	493
	11th to 20th	Day	5% 9% 16% 28% 26% 16%	330
	21st to end	Day	6% 7% 18% 32% 28% 9%	363
November	1st to 10th	Day	10% 8% 19% 30% 20% 14%	330
	11th to 20th	Day	6% 5% 24% 35% 20% 11%	330
	21st to end	Day	9% 8% 16% 28% 27% 12%	330
December	1st to 10th	Day	7% 6% 20% 31% 23% 13%	330
	11th to 20th	Day	9% 5% 19% 35% 22% 8%	330
	21st to end	Day	12% 12% 22% 33% 17% 4	363
January	1st to 10th	Night	10% 6% 18% 38% 22% 6%	840
	11th to 20th	Night	10% 5% 17% 39% 25% 5%	840
	21st to end	Night	10% 5% 15% 43% 23% 5%	924
February	1st to 10th	Night	11% 6% 21% 40% 17% 4	840
	11th to 20th	Night	11% 5% 21% 44% 19% 2	824
	21st to end	Night	7% 6% 25% 46% 15% 1	692
March	1st to 10th	Night	10% 10% 23% 40% 15% 3	840
	11th to 20th	Night	8% 7% 23% 40% 20% 2	840
	21st to end	Night	10% 8% 28% 39% 14% 2	924
April	1st to 10th	Night	12% 8% 23% 38% 17% 2	840
	11th to 20th	Night	7% 11% 29% 38% 14% 1	670
	21st to end	Night	10% 8% 25% 39% 16% 2	630
May	1st to 10th	Night	8% 8% 26% 40% 17% 2	630
	11th to 20th	Night	9% 4 23% 38% 23% 3	621
	21st to end	Night	9% 8% 20% 35% 26% 2	683
June	1st to 10th	Night	6% 8% 16% 43% 26% 1	630
	11th to 20th	Night	6% 8% 21% 40% 24% 1	576
	21st to end	Night	4 3 19% 43% 30% 1	420
July	1st to 10th	Night	2 2 16% 45% 34% 2	455
	11th to 20th	Night	6% 3 16% 45% 28% 2	612
	21st to end	Night	6% 1 12% 44% 35% 2	687
August	1st to 10th	Night	3 2 21% 45% 28% 1	597
	11th to 20th	Night	4 1 18% 46% 30% 2	594
	21st to end	Night	5% 3 14% 41% 33% 4	704
September	1st to 10th	Night	3 3 13% 40% 37% 6%	800
	11th to 20th	Night	3 3 17% 40% 30% 7%	800
	21st to end	Night	6% 3 15% 35% 33% 8%	800
October	1st to 10th	Night	7% 3 17% 39% 30% 4	800
	11th to 20th	Night	6% 4 16% 39% 29% 6%	800
	21st to end	Night	6% 3 19% 38% 29% 5%	880
November	1st to 10th	Night	9% 5% 15% 40% 27% 5%	820
	11th to 20th	Night	6% 4 18% 45% 22% 5%	840
	21st to end	Night	8% 4 18% 39% 25% 6%	840
December	1st to 10th	Night	10% 4 15% 44% 22% 4	840
	11th to 20th	Night	13% 4 16% 41% 24% 2	840
	21st to end	Night	16% 5% 21% 39% 16% 3	924

Page generated at 2026-07-28 01:25:25. Version ac07

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