Ariel
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ariel clear sky chart from 2005-11-06 to 2026-10-14.

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	21% 8% 2 6% 4 4 4 3 5% 6% 38%	154282
	Night	24% 9% 2 6% 3 3 3 3 4 5% 38%	146208
Transparency	Day	47% 6% 12% 19% 13% 4	154007
	Night	50% 5% 11% 18% 12% 3	146065
Seeing	Day	28% 12% 25% 24% 9% 2	17289
	Night	31% 8% 19% 29% 13% 1	25884

By Month

Percentage Cloud Cover NW (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	46% 17% 3 7% 4 3 3 2 3 3 8%	4723
	11th to 20th	Day	24% 8% 2 5% 4 3 3 3 4 5% 38%	4716
	21st to end	Day	35% 8% 3 6% 3 3 3 2 4 4 30%	5582
February	1st to 10th	Day	37% 13% 3 7% 4 3 3 3 5% 6% 16%	1890
	11th to 20th	Day	37% 14% 4 7% 5% 3 3 2 4 4 18%	1845
	21st to end	Day	36% 17% 3 8% 3 3 3 2 3 4 17%	1641
March	1st to 10th	Day	24% 17% 3 9% 5% 4 4 3 5% 6% 19%	4797
	11th to 20th	Day	32% 16% 3 7% 4 3 3 2 3 3 25%	5110
	21st to end	Day	33% 15% 4 9% 5% 5% 5% 3 4 4 12%	5741
April	1st to 10th	Day	32% 13% 3 9% 5% 3 3 3 4 6% 17%	2520
	11th to 20th	Day	27% 13% 3 8% 5% 4 4 3 4 5% 24%	2492
	21st to end	Day	25% 12% 4 8% 6% 5% 4 4 4 5% 24%	2726
May	1st to 10th	Day	14% 7% 2 6% 4 4 4 4 5% 6% 44%	8151
	11th to 20th	Day	17% 9% 3 8% 5% 5% 6% 5% 7% 8% 28%	8160
	21st to end	Day	18% 7% 2 6% 5% 4 5% 4 7% 8% 35%	8993
June	1st to 10th	Day	24% 7% 3 7% 5% 4 3 3 5% 6% 32%	2940
	11th to 20th	Day	20% 8% 3 7% 6% 5% 5% 4 6% 8% 28%	2941
	21st to end	Day	12% 6% 3 5% 4 5% 4 3 5% 6% 47%	3150
July	1st to 10th	Day	4 3 1 4 3 3 3 3 5% 6% 66%	8364
	11th to 20th	Day	4 3 1 2 2 2 2 2 3 4 74%	8220
	21st to end	Day	5% 3 1 3 2 3 3 3 5% 7% 66%	8916
August	1st to 10th	Day	4 3 1 3 2 2 3 2 3 5% 70%	6131
	11th to 20th	Day	5% 3 2 5% 4 5% 5% 4 6% 9% 52%	5923
	21st to end	Day	11% 6% 2 5% 3 3 4 3 5% 8% 51%	6256
September	1st to 10th	Day	10% 4 2 5% 3 3 4 3 5% 7% 54%	2520
	11th to 20th	Day	14% 7% 2 6% 4 4 4 4 4 7% 43%	2520
	21st to end	Day	19% 6% 2 5% 3 4 3 3 4 6% 45%	2213
October	1st to 10th	Day	19% 6% 2 4 3 3 3 3 4 6% 49%	3380
	11th to 20th	Day	25% 8% 2 7% 4 4 4 4 6% 7% 29%	3292
	21st to end	Day	36% 9% 2 6% 4 4 3 3 4 5% 24%	3439
November	1st to 10th	Day	39% 12% 3 6% 4 3 4 4 5% 4 15%	1733
	11th to 20th	Day	42% 9% 3 5% 4 3 3 3 4 5% 18%	1680
	21st to end	Day	39% 11% 2 6% 3 3 3 2 5% 4 22%	1680
December	1st to 10th	Day	28% 10% 2 5% 3 3 3 3 4 4 34%	2700
	11th to 20th	Day	44% 15% 3 7% 4 3 3 2 3 4 13%	3105
	21st to end	Day	42% 15% 5% 8% 4 4 4 2 3 3 11%	4092
January	1st to 10th	Night	44% 15% 3 7% 4 4 3 2 3 3 11%	8172
	11th to 20th	Night	22% 10% 2 6% 3 3 3 2 4 5% 39%	8182
	21st to end	Night	32% 10% 2 5% 3 3 2 2 4 5% 32%	8616
February	1st to 10th	Night	34% 13% 3 7% 4 4 4 3 4 4 19%	2730
	11th to 20th	Night	34% 13% 2 7% 4 3 3 2 3 4 24%	2662
	21st to end	Night	34% 14% 3 6% 3 3 4 3 4 5% 21%	2165
March	1st to 10th	Night	23% 17% 4 8% 6% 5% 4 3 5% 4 21%	5316
	11th to 20th	Night	32% 13% 3 7% 4 3 3 2 3 3 27%	5109
	21st to end	Night	38% 14% 3 7% 4 4 4 3 4 5% 14%	5463
April	1st to 10th	Night	28% 15% 3 7% 5% 4 4 2 4 5% 23%	2100
	11th to 20th	Night	24% 12% 3 6% 4 4 4 4 4 5% 29%	2079
	21st to end	Night	25% 10% 3 6% 4 4 4 3 5% 7% 31%	1890
May	1st to 10th	Night	16% 6% 2 5% 3 3 4 3 5% 7% 48%	4768
	11th to 20th	Night	15% 7% 3 7% 5% 4 4 4 5% 7% 38%	4700
	21st to end	Night	11% 8% 2 6% 4 4 4 4 6% 9% 41%	5178
June	1st to 10th	Night	19% 9% 2 8% 5% 3 3 2 3 6% 39%	1680
	11th to 20th	Night	16% 8% 3 6% 3 3 3 3 5% 8% 44%	1666
	21st to end	Night	9% 5% 2 4 3 4 3 3 4 5% 59%	1470
July	1st to 10th	Night	2 1 1 1 2 2 2 2 4 6% 78%	4560
	11th to 20th	Night	2 2 1 3 2 2 2 2 3 4 77%	4740
	21st to end	Night	4 2 1 2 2 2 2 3 4 6% 74%	5152
August	1st to 10th	Night	1 1 1 2 1 1 1 2 4 5% 81%	4030
	11th to 20th	Night	5% 3 1 3 2 3 4 3 5% 7% 65%	4142
	21st to end	Night	10% 4 2 4 3 3 3 2 3 5% 61%	4905
September	1st to 10th	Night	8% 4 2 4 3 3 4 3 4 5% 62%	2100
	11th to 20th	Night	13% 6% 3 5% 4 4 4 3 4 5% 50%	2100
	21st to end	Night	14% 6% 2 5% 3 4 4 3 4 6% 49%	2403
October	1st to 10th	Night	14% 4 2 4 3 3 4 2 4 7% 53%	4040
	11th to 20th	Night	23% 7% 2 6% 3 4 4 3 6% 7% 35%	3933
	21st to end	Night	33% 9% 2 7% 3 3 3 3 4 5% 27%	4368
November	1st to 10th	Night	38% 11% 3 6% 4 4 4 3 3 4 21%	2783
	11th to 20th	Night	42% 10% 3 5% 3 4 3 2 3 4 21%	2940
	21st to end	Night	34% 10% 3 7% 3 4 3 3 3 4 26%	2940
December	1st to 10th	Night	26% 9% 2 5% 3 3 3 3 4 6% 35%	4700
	11th to 20th	Night	44% 14% 3 7% 4 3 3 2 3 4 13%	5320
	21st to end	Night	42% 17% 4 7% 4 4 3 2 2 2 12%	7106

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	83% 4 4 5% 4	4723
	11th to 20th	Day	48% 4 8% 16% 16% 8%	4740
	21st to end	Day	59% 4 5% 13% 17% 3	5583
February	1st to 10th	Day	70% 7% 9% 9% 5%	1890
	11th to 20th	Day	73% 5% 7% 9% 5% 2	1854
	21st to end	Day	72% 5% 6% 8% 7% 1	1641
March	1st to 10th	Day	63% 7% 15% 9% 6%	4797
	11th to 20th	Day	66% 4 6% 19% 5%	5110
	21st to end	Day	74% 7% 10% 6% 2	5741
April	1st to 10th	Day	69% 7% 11% 11% 2	2520
	11th to 20th	Day	63% 6% 11% 11% 8% 1	2492
	21st to end	Day	63% 8% 8% 13% 7% 1	2726
May	1st to 10th	Day	40% 6% 13% 26% 15% 1	8151
	11th to 20th	Day	50% 11% 18% 15% 6%	8163
	21st to end	Day	47% 8% 20% 20% 5%	8994
June	1st to 10th	Day	54% 8% 15% 15% 8% 1	2940
	11th to 20th	Day	55% 9% 15% 14% 7%	2941
	21st to end	Day	39% 8% 16% 25% 10% 4	3150
July	1st to 10th	Day	20% 5% 13% 28% 18% 16%	8295
	11th to 20th	Day	16% 3 7% 20% 38% 16%	8145
	21st to end	Day	19% 3 17% 34% 25% 3	8723
August	1st to 10th	Day	18% 3 9% 25% 27% 18%	6114
	11th to 20th	Day	29% 6% 18% 34% 10% 3	5962
	21st to end	Day	32% 4 14% 28% 15% 6%	6255
September	1st to 10th	Day	31% 6% 13% 22% 18% 10%	2520
	11th to 20th	Day	42% 7% 12% 17% 15% 7%	2520
	21st to end	Day	42% 6% 13% 19% 14% 5%	2213
October	1st to 10th	Day	38% 4 7% 26% 23% 2	3380
	11th to 20th	Day	53% 6% 13% 15% 12% 1	3292
	21st to end	Day	63% 4 8% 15% 9% 1	3439
November	1st to 10th	Day	71% 7% 7% 10% 4	1733
	11th to 20th	Day	69% 7% 9% 9% 5% 2	1680
	21st to end	Day	67% 5% 6% 9% 12% 1	1680
December	1st to 10th	Day	54% 4 7% 16% 18% 1	2700
	11th to 20th	Day	77% 5% 9% 8% 2	3108
	21st to end	Day	78% 6% 8% 6% 2	4092
January	1st to 10th	Night	80% 4 6% 7% 4 1	8172
	11th to 20th	Night	48% 5% 8% 16% 17% 5%	8220
	21st to end	Night	58% 3 4 12% 18% 5%	8616
February	1st to 10th	Night	69% 7% 8% 11% 4 1	2730
	11th to 20th	Night	67% 6% 8% 11% 6% 2	2664
	21st to end	Night	67% 6% 9% 10% 7% 1	2165
March	1st to 10th	Night	64% 7% 11% 12% 6%	5316
	11th to 20th	Night	63% 6% 9% 15% 6%	5110
	21st to end	Night	74% 6% 10% 8% 2	5463
April	1st to 10th	Night	66% 6% 11% 13% 4	2100
	11th to 20th	Night	57% 8% 12% 13% 9% 2	2079
	21st to end	Night	56% 9% 12% 15% 6% 2	1890
May	1st to 10th	Night	35% 6% 12% 30% 16% 1	4765
	11th to 20th	Night	45% 9% 19% 20% 6% 1	4700
	21st to end	Night	39% 10% 24% 23% 4	5182
June	1st to 10th	Night	49% 6% 16% 21% 7% 1	1680
	11th to 20th	Night	42% 9% 18% 23% 9%	1666
	21st to end	Night	31% 5% 19% 30% 10% 4	1470
July	1st to 10th	Night	11% 3 16% 34% 17% 17%	4520
	11th to 20th	Night	15% 2 8% 19% 42% 13%	4692
	21st to end	Night	15% 3 19% 34% 25% 4	5040
August	1st to 10th	Night	9% 3 8% 32% 33% 15%	4022
	11th to 20th	Night	22% 4 25% 35% 10% 4	4169
	21st to end	Night	26% 4 19% 30% 15% 6%	4905
September	1st to 10th	Night	25% 6% 14% 26% 19% 10%	2100
	11th to 20th	Night	38% 6% 12% 21% 13% 9%	2100
	21st to end	Night	37% 7% 15% 19% 16% 6%	2403
October	1st to 10th	Night	33% 4 10% 29% 22% 2	4040
	11th to 20th	Night	48% 5% 14% 18% 14% 1	3933
	21st to end	Night	61% 5% 7% 16% 9% 2	4368
November	1st to 10th	Night	69% 6% 8% 12% 6% 1	2783
	11th to 20th	Night	69% 5% 8% 9% 6% 2	2940
	21st to end	Night	64% 5% 8% 9% 13% 2	2940
December	1st to 10th	Night	51% 5% 8% 15% 20% 1	4700
	11th to 20th	Night	78% 5% 6% 7% 4	5316
	21st to end	Night	79% 7% 6% 6% 2	7106

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	55% 14% 17% 10% 4	220
	11th to 20th	Day	45% 12% 24% 11% 7%	220
	21st to end	Day	45% 10% 20% 18% 7% 1	382
February	1st to 10th	Day	46% 14% 19% 15% 5%	430
	11th to 20th	Day	46% 15% 22% 14% 2 1	425
	21st to end	Day	46% 15% 20% 17% 2	399
March	1st to 10th	Day	41% 16% 26% 14% 4	540
	11th to 20th	Day	40% 14% 23% 15% 5% 2	540
	21st to end	Day	42% 18% 20% 16% 4 1	594
April	1st to 10th	Day	39% 14% 23% 14% 7% 2	540
	11th to 20th	Day	33% 18% 26% 17% 5% 1	532
	21st to end	Day	30% 18% 28% 18% 6% 1	540
May	1st to 10th	Day	20% 19% 28% 23% 8% 2	543
	11th to 20th	Day	32% 17% 20% 21% 8% 2	568
	21st to end	Day	25% 12% 27% 23% 11% 2	617
June	1st to 10th	Day	26% 14% 25% 24% 10% 2	570
	11th to 20th	Day	22% 18% 31% 21% 6% 2	571
	21st to end	Day	12% 13% 33% 29% 11% 3	780
July	1st to 10th	Day	11% 9% 34% 34% 11% 2	619
	11th to 20th	Day	8% 10% 33% 33% 13% 4	564
	21st to end	Day	5% 7% 31% 36% 16% 5%	619
August	1st to 10th	Day	10% 5% 26% 38% 16% 5%	567
	11th to 20th	Day	5% 5% 27% 39% 19% 4	557
	21st to end	Day	13% 11% 28% 33% 15% 2	627
September	1st to 10th	Day	11% 8% 30% 37% 11% 2	570
	11th to 20th	Day	18% 9% 25% 32% 14% 2	570
	21st to end	Day	22% 10% 27% 28% 11% 1	570
October	1st to 10th	Day	25% 8% 26% 26% 14% 1	570
	11th to 20th	Day	29% 11% 22% 22% 14% 1	543
	21st to end	Day	39% 13% 19% 19% 8% 2	513
November	1st to 10th	Day	44% 15% 21% 14% 5%	267
	11th to 20th	Day	46% 11% 22% 13% 5% 2	220
	21st to end	Day	44% 11% 17% 23% 5%	220
December	1st to 10th	Day	46% 9% 20% 17% 7% 1	220
	11th to 20th	Day	55% 11% 20% 10% 3	220
	21st to end	Day	53% 10% 22% 12% 3	242
January	1st to 10th	Night	50% 12% 16% 17% 5%	840
	11th to 20th	Night	41% 9% 20% 22% 8%	840
	21st to end	Night	40% 7% 24% 21% 8% 1	924
February	1st to 10th	Night	40% 10% 21% 23% 6% 1	840
	11th to 20th	Night	41% 10% 18% 22% 8%	824
	21st to end	Night	43% 10% 20% 22% 6%	692
March	1st to 10th	Night	38% 12% 16% 24% 9% 1	840
	11th to 20th	Night	41% 9% 20% 23% 7%	840
	21st to end	Night	43% 12% 17% 21% 8%	924
April	1st to 10th	Night	35% 10% 20% 23% 11% 1	840
	11th to 20th	Night	29% 10% 24% 25% 10% 1	831
	21st to end	Night	31% 7% 18% 32% 11%	630
May	1st to 10th	Night	20% 8% 24% 33% 14% 1	438
	11th to 20th	Night	23% 5% 20% 34% 17% 1	414
	21st to end	Night	17% 7% 20% 36% 19% 1	455
June	1st to 10th	Night	23% 6% 20% 35% 16%	420
	11th to 20th	Night	20% 8% 22% 36% 13%	420
	21st to end	Night	12% 5% 21% 42% 20%	420
July	1st to 10th	Night	6% 2 19% 49% 23%	410
	11th to 20th	Night	6% 3 21% 48% 21%	408
	21st to end	Night	4 3 16% 51% 24% 2	460
August	1st to 10th	Night	5% 2 13% 43% 33% 3	557
	11th to 20th	Night	7% 2 9% 43% 36% 3	624
	21st to end	Night	10% 5% 17% 43% 24% 1	837
September	1st to 10th	Night	12% 4 16% 43% 24% 2	840
	11th to 20th	Night	16% 5% 16% 34% 28% 2	840
	21st to end	Night	18% 5% 19% 38% 20% 1	840
October	1st to 10th	Night	21% 5% 18% 36% 17% 2	840
	11th to 20th	Night	28% 7% 16% 31% 17% 2	812
	21st to end	Night	38% 7% 19% 24% 10% 1	880
November	1st to 10th	Night	45% 8% 18% 24% 5%	820
	11th to 20th	Night	50% 8% 18% 19% 5% 1	840
	21st to end	Night	39% 8% 21% 24% 7%	840
December	1st to 10th	Night	39% 8% 23% 23% 7% 1	840
	11th to 20th	Night	50% 8% 18% 20% 4	840
	21st to end	Night	55% 10% 18% 13% 4	924

Page generated at 2026-10-11 02:06:28. Version ac07

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