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 2024-04-03.

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	5% 6% 2 5% 5% 3 4 5% 5% 7% 53%	79599
	Night	6% 7% 3 5% 5% 3 4 4 5% 8% 50%	80587
Transparency	Day	29% 6% 12% 19% 20% 14%	79324
	Night	32% 6% 12% 21% 19% 11%	80384
Seeing	Day	8% 9% 22% 32% 22% 7%	14019
	Night	8% 5% 19% 40% 24% 4	23966

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	5% 16% 6% 9% 6% 4 5% 5% 5% 6% 32%	2858
	11th to 20th	Day	7% 14% 5% 10% 8% 4 5% 5% 4 6% 31%	2860
	21st to end	Day	7% 10% 3 7% 6% 4 5% 6% 6% 11% 35%	3146
February	1st to 10th	Day	7% 10% 4 7% 7% 4 4 5% 6% 8% 39%	1710
	11th to 20th	Day	7% 9% 3 6% 5% 4 5% 5% 6% 8% 42%	1802
	21st to end	Day	5% 8% 4 7% 6% 3 4 4 6% 8% 44%	1570
March	1st to 10th	Day	5% 8% 4 6% 6% 3 5% 5% 5% 7% 46%	1985
	11th to 20th	Day	5% 6% 3 5% 6% 4 6% 6% 6% 9% 43%	2090
	21st to end	Day	9% 9% 3 6% 6% 3 5% 6% 6% 7% 40%	2299
April	1st to 10th	Day	6% 9% 2 6% 6% 3 4 4 5% 7% 47%	2026
	11th to 20th	Day	7% 6% 3 7% 5% 3 4 4 4 6% 51%	2271
	21st to end	Day	8% 5% 2 4 4 3 3 4 5% 7% 55%	2340
May	1st to 10th	Day	5% 4 2 4 5% 3 5% 5% 6% 8% 52%	2340
	11th to 20th	Day	5% 4 2 4 5% 3 5% 6% 6% 9% 52%	2301
	21st to end	Day	6% 5% 2 4 6% 3 6% 5% 7% 7% 49%	2530
June	1st to 10th	Day	4 2 1 3 4 3 4 4 6% 9% 60%	2340
	11th to 20th	Day	4 4 1 3 4 2 3 2 4 6% 67%	2358
	21st to end	Day	2 1 1 2 2 2 3 3 4 5% 74%	2520
July	1st to 10th	Day	2 2 1 3 3 3 3 4 5% 8% 65%	2453
	11th to 20th	Day	3 2 2 4 3 2 4 5% 5% 7% 63%	2340
	21st to end	Day	3 2 2 3 4 3 4 5% 7% 9% 58%	2545
August	1st to 10th	Day	3 2 1 3 4 2 4 5% 6% 7% 62%	2328
	11th to 20th	Day	2 2 1 3 3 2 3 4 5% 6% 67%	2299
	21st to end	Day	3 2 1 3 3 3 4 4 5% 6% 66%	2543
September	1st to 10th	Day	2 2 1 1 2 2 2 3 4 6% 74%	2003
	11th to 20th	Day	3 2 1 3 3 2 2 4 4 6% 71%	1980
	21st to end	Day	3 2 1 2 2 1 3 3 3 6% 72%	1980
October	1st to 10th	Day	6% 4 2 3 3 3 4 4 5% 6% 61%	1940
	11th to 20th	Day	5% 4 1 3 3 2 3 4 4 6% 66%	1800
	21st to end	Day	4 5% 1 4 3 2 3 4 5% 7% 61%	1923
November	1st to 10th	Day	6% 5% 2 4 4 2 6% 5% 6% 7% 53%	1674
	11th to 20th	Day	7% 7% 2 5% 4 3 3 4 5% 7% 52%	1710
	21st to end	Day	6% 8% 3 5% 6% 3 5% 5% 5% 6% 49%	1710
December	1st to 10th	Day	8% 8% 3 7% 5% 3 4 5% 5% 7% 46%	1710
	11th to 20th	Day	8% 10% 5% 7% 6% 4 6% 6% 7% 9% 33%	2169
	21st to end	Day	6% 9% 4 8% 7% 4 6% 6% 6% 9% 35%	3146
January	1st to 10th	Night	7% 14% 4 6% 6% 3 4 4 5% 8% 37%	4097
	11th to 20th	Night	10% 13% 6% 9% 7% 4 5% 5% 5% 7% 32%	4110
	21st to end	Night	7% 11% 3 8% 6% 4 5% 5% 6% 13% 33%	4521
February	1st to 10th	Night	6% 9% 3 5% 7% 3 5% 5% 6% 8% 42%	2470
	11th to 20th	Night	7% 10% 4 7% 4 3 5% 4 5% 7% 45%	2263
	21st to end	Night	4 8% 4 6% 5% 3 4 5% 6% 7% 46%	1884
March	1st to 10th	Night	7% 10% 5% 8% 7% 4 5% 4 4 6% 42%	2193
	11th to 20th	Night	4 7% 2 5% 6% 4 6% 6% 6% 8% 46%	2090
	21st to end	Night	8% 10% 3 6% 5% 3 5% 5% 6% 8% 43%	2299
April	1st to 10th	Night	9% 8% 2 5% 5% 3 4 3 4 7% 50%	1976
	11th to 20th	Night	6% 7% 2 5% 5% 4 4 5% 5% 7% 50%	1636
	21st to end	Night	9% 5% 3 4 4 2 4 4 5% 8% 53%	1620
May	1st to 10th	Night	6% 5% 2 5% 4 4 6% 6% 8% 7% 47%	1620
	11th to 20th	Night	6% 5% 1 4 5% 3 5% 5% 6% 8% 53%	1594
	21st to end	Night	7% 5% 3 6% 5% 3 5% 5% 6% 8% 46%	1746
June	1st to 10th	Night	5% 4 2 4 4 2 5% 4 6% 9% 55%	1620
	11th to 20th	Night	6% 2 1 3 3 2 3 3 4 6% 66%	1592
	21st to end	Night	2 2 1 3 4 2 4 4 6% 8% 65%	1440
July	1st to 10th	Night	2 2 2 3 4 3 5% 6% 7% 8% 59%	1498
	11th to 20th	Night	5% 4 2 5% 4 4 5% 6% 6% 8% 52%	1620
	21st to end	Night	3 4 1 4 4 3 4 6% 8% 9% 52%	1773
August	1st to 10th	Night	2 3 2 5% 5% 3 5% 5% 7% 8% 56%	1611
	11th to 20th	Night	2 2 2 4 4 2 4 4 5% 7% 63%	1602
	21st to end	Night	3 3 1 4 3 3 4 4 5% 7% 62%	1810
September	1st to 10th	Night	3 2 1 3 3 2 2 3 4 7% 70%	1957
	11th to 20th	Night	3 3 1 3 3 2 3 3 3 6% 70%	1980
	21st to end	Night	5% 3 1 2 3 2 2 3 3 5% 72%	1980
October	1st to 10th	Night	5% 3 2 4 4 2 3 4 4 6% 63%	2018
	11th to 20th	Night	5% 3 1 3 3 2 3 3 4 6% 65%	2160
	21st to end	Night	5% 3 1 3 4 2 3 4 6% 7% 63%	2430
November	1st to 10th	Night	7% 6% 2 4 4 2 4 3 6% 8% 55%	2405
	11th to 20th	Night	6% 4 2 5% 5% 2 4 4 5% 6% 56%	2470
	21st to end	Night	7% 7% 3 5% 4 3 3 3 5% 7% 53%	2470
December	1st to 10th	Night	8% 8% 2 5% 5% 2 4 4 6% 8% 47%	2470
	11th to 20th	Night	9% 9% 3 6% 5% 4 5% 5% 6% 8% 41%	3041
	21st to end	Night	7% 11% 5% 8% 8% 3 4 5% 5% 8% 35%	4521

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	49% 10% 19% 17% 5%	2859
	11th to 20th	Day	51% 9% 19% 17% 4	2860
	21st to end	Day	41% 12% 23% 15% 7% 1	3146
February	1st to 10th	Day	42% 7% 17% 21% 10% 1	1710
	11th to 20th	Day	38% 7% 18% 23% 13% 1	1812
	21st to end	Day	36% 6% 16% 27% 13% 1	1570
March	1st to 10th	Day	36% 7% 15% 24% 17% 2	1985
	11th to 20th	Day	35% 8% 12% 28% 14% 3	2090
	21st to end	Day	42% 7% 14% 23% 11% 3	2299
April	1st to 10th	Day	37% 6% 11% 22% 17% 8%	2026
	11th to 20th	Day	35% 4 10% 21% 23% 6%	2274
	21st to end	Day	29% 4 10% 25% 25% 6%	2340
May	1st to 10th	Day	28% 5% 11% 21% 26% 9%	2340
	11th to 20th	Day	27% 6% 11% 20% 26% 9%	2304
	21st to end	Day	30% 8% 10% 22% 22% 8%	2530
June	1st to 10th	Day	20% 4 10% 17% 32% 17%	2340
	11th to 20th	Day	20% 3 6% 15% 29% 27%	2358
	21st to end	Day	13% 3 4 9% 27% 44%	2520
July	1st to 10th	Day	16% 4 6% 11% 20% 43%	2387
	11th to 20th	Day	19% 3 7% 12% 22% 37%	2271
	21st to end	Day	20% 6% 10% 14% 21% 29%	2404
August	1st to 10th	Day	19% 5% 7% 15% 21% 32%	2313
	11th to 20th	Day	17% 4 6% 10% 28% 36%	2299
	21st to end	Day	18% 4 6% 15% 28% 29%	2543
September	1st to 10th	Day	12% 3 6% 13% 29% 36%	2003
	11th to 20th	Day	16% 4 7% 19% 25% 29%	1980
	21st to end	Day	15% 4 7% 17% 31% 26%	1980
October	1st to 10th	Day	25% 4 9% 17% 33% 12%	1940
	11th to 20th	Day	20% 4 7% 26% 33% 9%	1800
	21st to end	Day	22% 5% 12% 30% 27% 4	1923
November	1st to 10th	Day	28% 7% 13% 26% 23% 4	1674
	11th to 20th	Day	31% 6% 11% 33% 18% 1	1710
	21st to end	Day	35% 5% 15% 28% 15% 2	1710
December	1st to 10th	Day	36% 8% 18% 25% 11% 2	1710
	11th to 20th	Day	44% 10% 17% 20% 9%	2168
	21st to end	Day	43% 9% 17% 17% 14% 1	3146
January	1st to 10th	Night	41% 10% 16% 25% 8%	4097
	11th to 20th	Night	49% 10% 18% 16% 7% 1	4110
	21st to end	Night	40% 10% 22% 18% 8% 1	4521
February	1st to 10th	Night	39% 7% 16% 27% 9% 1	2470
	11th to 20th	Night	38% 6% 13% 24% 17% 2	2264
	21st to end	Night	35% 6% 12% 27% 17% 3	1884
March	1st to 10th	Night	43% 5% 11% 23% 16% 1	2193
	11th to 20th	Night	33% 6% 12% 28% 18% 4	2090
	21st to end	Night	38% 7% 13% 24% 14% 4	2299
April	1st to 10th	Night	34% 5% 10% 24% 19% 8%	1976
	11th to 20th	Night	32% 6% 10% 23% 24% 5%	1636
	21st to end	Night	30% 5% 11% 25% 25% 4	1620
May	1st to 10th	Night	32% 7% 10% 20% 24% 7%	1620
	11th to 20th	Night	26% 7% 9% 24% 24% 10%	1595
	21st to end	Night	32% 8% 13% 21% 20% 6%	1746
June	1st to 10th	Night	25% 6% 7% 19% 27% 16%	1620
	11th to 20th	Night	20% 3 7% 16% 28% 25%	1592
	21st to end	Night	16% 3 6% 12% 26% 38%	1440
July	1st to 10th	Night	21% 5% 8% 11% 15% 41%	1455
	11th to 20th	Night	27% 5% 9% 10% 17% 32%	1566
	21st to end	Night	24% 7% 12% 13% 17% 27%	1674
August	1st to 10th	Night	22% 6% 9% 15% 19% 29%	1602
	11th to 20th	Night	20% 3 8% 12% 23% 34%	1602
	21st to end	Night	20% 4 7% 14% 25% 31%	1810
September	1st to 10th	Night	16% 3 6% 13% 25% 37%	1957
	11th to 20th	Night	17% 4 7% 16% 25% 32%	1980
	21st to end	Night	17% 3 5% 14% 30% 31%	1980
October	1st to 10th	Night	23% 4 9% 17% 33% 15%	2018
	11th to 20th	Night	21% 4 7% 21% 37% 11%	2160
	21st to end	Night	20% 5% 8% 31% 28% 7%	2430
November	1st to 10th	Night	28% 4 10% 26% 24% 8%	2405
	11th to 20th	Night	28% 5% 11% 32% 21% 4	2470
	21st to end	Night	32% 4 11% 30% 20% 2	2470
December	1st to 10th	Night	35% 7% 14% 28% 14% 3	2470
	11th to 20th	Night	40% 7% 15% 27% 11%	3041
	21st to end	Night	45% 8% 14% 19% 13%	4521

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	14% 7% 27% 25% 22% 5%	270
	11th to 20th	Day	8% 6% 27% 36% 17% 7%	270
	21st to end	Day	9% 6% 16% 38% 23% 8%	297
February	1st to 10th	Day	14% 9% 28% 29% 14% 6%	270
	11th to 20th	Day	11% 8% 29% 33% 14% 5%	407
	21st to end	Day	6% 11% 32% 36% 12% 2	382
March	1st to 10th	Day	12% 12% 24% 32% 17% 2	460
	11th to 20th	Day	8% 14% 30% 27% 17% 5%	460
	21st to end	Day	15% 16% 31% 27% 11% 1	506
April	1st to 10th	Day	13% 15% 26% 27% 16% 3	429
	11th to 20th	Day	13% 20% 27% 25% 13% 1	412
	21st to end	Day	12% 18% 28% 23% 14% 5%	420
May	1st to 10th	Day	6% 15% 32% 30% 13% 4	423
	11th to 20th	Day	9% 9% 26% 29% 22% 5%	448
	21st to end	Day	8% 11% 20% 32% 22% 7%	485
June	1st to 10th	Day	5% 9% 24% 31% 22% 9%	450
	11th to 20th	Day	6% 14% 24% 28% 22% 6%	450
	21st to end	Day	2 6% 26% 36% 25% 6%	450
July	1st to 10th	Day	2 3 18% 39% 29% 8%	446
	11th to 20th	Day	4 3 18% 39% 28% 8%	444
	21st to end	Day	3 1 12% 39% 34% 11%	487
August	1st to 10th	Day	4 3 19% 39% 27% 9%	447
	11th to 20th	Day	4 3 14% 33% 30% 15%	437
	21st to end	Day	4 3 15% 32% 34% 11%	495
September	1st to 10th	Day	4 5% 16% 31% 28% 16%	450
	11th to 20th	Day	5% 6% 17% 32% 29% 11%	450
	21st to end	Day	6% 8% 18% 27% 32% 10%	450
October	1st to 10th	Day	6% 10% 22% 26% 23% 13%	410
	11th to 20th	Day	4 9% 17% 29% 24% 16%	270
	21st to end	Day	6% 8% 20% 32% 28% 5%	297
November	1st to 10th	Day	13% 7% 19% 30% 20% 11%	270
	11th to 20th	Day	7% 7% 22% 32% 23% 10%	270
	21st to end	Day	13% 8% 15% 28% 25% 11%	270
December	1st to 10th	Day	9% 8% 17% 38% 20% 9%	270
	11th to 20th	Day	11% 6% 20% 36% 22% 6%	270
	21st to end	Day	11% 10% 22% 36% 16% 4	297
January	1st to 10th	Night	13% 4 19% 36% 23% 6%	760
	11th to 20th	Night	9% 6% 17% 39% 24% 5%	760
	21st to end	Night	10% 5% 16% 41% 22% 6%	836
February	1st to 10th	Night	10% 6% 22% 41% 17% 4	760
	11th to 20th	Night	11% 4 21% 42% 20% 2	744
	21st to end	Night	8% 6% 25% 44% 16% 1	628
March	1st to 10th	Night	11% 8% 22% 43% 14% 3	760
	11th to 20th	Night	6% 7% 23% 40% 21% 2	760
	21st to end	Night	9% 9% 30% 37% 13% 2	836
April	1st to 10th	Night	13% 10% 23% 38% 15% 2	728
	11th to 20th	Night	8% 12% 28% 36% 14% 2	568
	21st to end	Night	11% 8% 25% 37% 17% 2	540
May	1st to 10th	Night	7% 8% 26% 42% 15% 2	540
	11th to 20th	Night	8% 5% 21% 37% 24% 5%	531
	21st to end	Night	10% 8% 20% 33% 26% 3	584
June	1st to 10th	Night	6% 9% 17% 41% 26% 1	540
	11th to 20th	Night	8% 10% 19% 40% 23% 1	512
	21st to end	Night	2 2 20% 44% 30% 3	360
July	1st to 10th	Night	3 2 15% 43% 34% 3	405
	11th to 20th	Night	5% 4 16% 45% 28% 2	522
	21st to end	Night	5% 1 10% 42% 39% 2	591
August	1st to 10th	Night	4 2 18% 45% 29% 2	537
	11th to 20th	Night	3 1 17% 45% 32% 3	534
	21st to end	Night	4 2 15% 40% 34% 4	622
September	1st to 10th	Night	3 3 12% 39% 35% 7%	720
	11th to 20th	Night	4 3 16% 39% 32% 7%	720
	21st to end	Night	5% 4 16% 35% 31% 10%	720
October	1st to 10th	Night	7% 4 18% 39% 27% 5%	720
	11th to 20th	Night	7% 3 15% 37% 31% 6%	720
	21st to end	Night	6% 3 20% 37% 27% 6%	792
November	1st to 10th	Night	10% 4 17% 38% 25% 6%	740
	11th to 20th	Night	8% 4 17% 46% 21% 5%	760
	21st to end	Night	9% 4 17% 39% 24% 7%	760
December	1st to 10th	Night	10% 5% 16% 44% 21% 4	760
	11th to 20th	Night	15% 4 18% 38% 22% 2	760
	21st to end	Night	16% 6% 22% 38% 15% 3	836

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