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 2025-05-26.

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	24% 9% 3 6% 4 4 4 3 4 5% 34%	118433
	Night	26% 10% 2 6% 4 3 3 2 4 5% 34%	119557
Transparency	Day	52% 7% 10% 15% 12% 4	118199
	Night	54% 6% 10% 16% 11% 3	119437
Seeing	Day	28% 13% 25% 23% 9% 2	15441
	Night	32% 7% 18% 29% 13% 1	24242

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 4 10%	3639
	11th to 20th	Day	30% 10% 3 6% 5% 4 3 3 4 5% 28%	3616
	21st to end	Day	36% 9% 2 5% 3 3 2 2 3 4 31%	4306
February	1st to 10th	Day	37% 13% 3 7% 4 3 3 3 5% 6% 16%	1800
	11th to 20th	Day	38% 13% 4 8% 5% 3 3 2 4 4 18%	1755
	21st to end	Day	36% 17% 3 8% 4 3 3 2 3 4 17%	1565
March	1st to 10th	Day	23% 17% 3 9% 5% 4 4 3 5% 6% 20%	4693
	11th to 20th	Day	31% 16% 3 7% 4 3 3 2 3 3 25%	5000
	21st to end	Day	33% 15% 4 9% 6% 5% 5% 3 4 4 11%	5617
April	1st to 10th	Day	33% 14% 4 9% 5% 3 3 4 4 6% 16%	2400
	11th to 20th	Day	27% 13% 3 7% 5% 4 4 3 4 4 25%	2372
	21st to end	Day	26% 12% 4 7% 5% 5% 4 4 4 6% 23%	2596
May	1st to 10th	Day	22% 10% 2 7% 4 4 4 3 4 5% 34%	4541
	11th to 20th	Day	14% 7% 2 6% 4 5% 5% 5% 7% 8% 36%	4517
	21st to end	Day	26% 9% 3 8% 6% 6% 6% 4 5% 5% 23%	4886
June	1st to 10th	Day	25% 7% 3 7% 5% 4 3 3 5% 6% 30%	2660
	11th to 20th	Day	22% 9% 3 7% 6% 6% 5% 4 6% 8% 25%	2661
	21st to end	Day	12% 5% 3 5% 4 5% 4 3 5% 6% 48%	2850
July	1st to 10th	Day	5% 3 1 4 3 3 3 2 4 6% 67%	4494
	11th to 20th	Day	5% 3 1 3 2 2 2 2 3 5% 71%	4420
	21st to end	Day	8% 3 1 3 2 3 3 2 4 6% 64%	4790
August	1st to 10th	Day	4 4 2 4 3 3 3 3 4 6% 65%	4189
	11th to 20th	Day	7% 5% 2 6% 4 5% 4 4 7% 8% 47%	4026
	21st to end	Day	13% 6% 2 6% 3 3 4 3 4 6% 50%	4306
September	1st to 10th	Day	10% 4 2 4 3 3 3 4 4 6% 58%	2280
	11th to 20th	Day	15% 7% 2 7% 5% 4 4 4 4 8% 42%	2280
	21st to end	Day	20% 6% 2 6% 3 3 3 3 4 6% 45%	2002
October	1st to 10th	Day	19% 6% 2 4 3 3 3 2 4 6% 48%	3180
	11th to 20th	Day	25% 8% 2 7% 4 4 4 4 6% 7% 29%	3161
	21st to end	Day	36% 9% 2 5% 4 4 3 3 4 5% 25%	3333
November	1st to 10th	Day	39% 12% 4 6% 4 3 4 4 5% 4 16%	1649
	11th to 20th	Day	43% 9% 3 5% 4 3 3 3 4 5% 18%	1600
	21st to end	Day	39% 11% 2 6% 3 3 3 2 5% 4 22%	1600
December	1st to 10th	Day	27% 10% 2 5% 3 3 3 3 4 4 35%	2620
	11th to 20th	Day	43% 15% 3 7% 4 3 3 2 3 4 13%	3025
	21st to end	Day	42% 15% 5% 8% 4 4 4 2 3 3 11%	4004
January	1st to 10th	Night	44% 14% 3 7% 4 3 3 2 3 3 13%	6306
	11th to 20th	Night	28% 13% 3 7% 4 3 3 2 4 5% 30%	6282
	21st to end	Night	34% 10% 2 5% 3 3 2 2 3 4 33%	6661
February	1st to 10th	Night	34% 13% 3 8% 5% 4 4 3 4 4 18%	2600
	11th to 20th	Night	34% 13% 2 7% 4 3 3 2 4 3 24%	2532
	21st to end	Night	34% 14% 3 6% 3 3 4 3 4 5% 20%	2065
March	1st to 10th	Night	23% 18% 3 8% 6% 5% 4 3 5% 4 21%	5200
	11th to 20th	Night	32% 13% 3 7% 4 3 3 2 3 3 28%	4999
	21st to end	Night	38% 14% 3 7% 4 4 4 3 4 5% 14%	5345
April	1st to 10th	Night	29% 15% 4 7% 5% 4 4 3 3 5% 22%	2000
	11th to 20th	Night	24% 12% 3 6% 4 4 4 3 5% 5% 30%	1979
	21st to end	Night	25% 10% 3 6% 4 4 4 3 5% 7% 30%	1800
May	1st to 10th	Night	24% 8% 3 6% 4 3 3 3 3 4 38%	2637
	11th to 20th	Night	12% 5% 2 5% 4 3 3 3 5% 7% 51%	2596
	21st to end	Night	15% 9% 2 8% 6% 4 5% 4 5% 8% 34%	2805
June	1st to 10th	Night	20% 10% 3 8% 5% 3 3 2 3 6% 37%	1520
	11th to 20th	Night	17% 8% 3 6% 3 3 3 3 5% 8% 41%	1508
	21st to end	Night	9% 5% 2 4 3 4 3 2 4 5% 61%	1330
July	1st to 10th	Night	3 2 1 1 2 2 2 1 3 4 79%	2444
	11th to 20th	Night	4 2 1 3 2 2 2 2 3 4 76%	2540
	21st to end	Night	5% 1 1 1 1 1 1 2 3 5% 78%	2762
August	1st to 10th	Night	2 2 1 2 2 2 2 2 4 5% 76%	2745
	11th to 20th	Night	7% 4 2 4 3 3 4 3 5% 6% 61%	2812
	21st to end	Night	12% 4 1 3 3 3 3 2 3 5% 59%	3367
September	1st to 10th	Night	8% 3 1 3 3 3 3 3 4 5% 65%	1900
	11th to 20th	Night	13% 6% 3 5% 4 4 4 3 4 5% 48%	1900
	21st to end	Night	14% 6% 3 5% 3 4 4 3 4 6% 49%	2174
October	1st to 10th	Night	15% 4 1 4 3 3 3 2 4 7% 53%	3800
	11th to 20th	Night	23% 7% 2 6% 4 4 4 3 6% 7% 34%	3777
	21st to end	Night	34% 9% 2 7% 3 3 3 3 4 5% 27%	4232
November	1st to 10th	Night	38% 11% 3 6% 4 4 3 3 3 4 22%	2647
	11th to 20th	Night	43% 10% 2 5% 3 4 3 2 3 4 22%	2800
	21st to end	Night	34% 10% 3 7% 3 4 3 3 3 4 26%	2800
December	1st to 10th	Night	25% 9% 2 5% 3 3 3 3 4 6% 36%	4560
	11th to 20th	Night	44% 14% 3 7% 4 3 3 2 3 4 13%	5180
	21st to end	Night	43% 17% 4 7% 4 4 3 2 2 2 12%	6952

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	80% 4 5% 6% 4	3639
	11th to 20th	Day	59% 5% 7% 19% 9% 1	3640
	21st to end	Day	59% 4 5% 13% 15% 4	4306
February	1st to 10th	Day	71% 7% 9% 9% 5%	1800
	11th to 20th	Day	73% 5% 7% 9% 4 2	1764
	21st to end	Day	72% 6% 7% 7% 7% 1	1565
March	1st to 10th	Day	63% 7% 15% 10% 6%	4693
	11th to 20th	Day	66% 4 6% 19% 5%	5000
	21st to end	Day	75% 7% 10% 6% 2	5617
April	1st to 10th	Day	71% 7% 10% 10% 2	2400
	11th to 20th	Day	62% 7% 10% 11% 8% 2	2372
	21st to end	Day	64% 9% 8% 12% 6% 1	2596
May	1st to 10th	Day	51% 9% 12% 15% 12%	4541
	11th to 20th	Day	41% 16% 20% 16% 7% 1	4520
	21st to end	Day	62% 12% 14% 10% 1	4887
June	1st to 10th	Day	56% 8% 15% 14% 7%	2660
	11th to 20th	Day	58% 10% 16% 11% 5%	2661
	21st to end	Day	37% 8% 16% 25% 10% 4	2850
July	1st to 10th	Day	20% 5% 8% 17% 20% 29%	4425
	11th to 20th	Day	17% 4 6% 19% 38% 15%	4345
	21st to end	Day	21% 4 12% 27% 32% 4	4639
August	1st to 10th	Day	21% 4 10% 25% 29% 11%	4172
	11th to 20th	Day	33% 6% 18% 27% 11% 5%	4065
	21st to end	Day	36% 5% 11% 22% 17% 8%	4305
September	1st to 10th	Day	28% 6% 13% 23% 19% 11%	2280
	11th to 20th	Day	43% 8% 12% 15% 14% 7%	2280
	21st to end	Day	43% 6% 13% 19% 14% 5%	2002
October	1st to 10th	Day	39% 4 7% 25% 24% 2	3180
	11th to 20th	Day	54% 6% 13% 15% 12% 1	3161
	21st to end	Day	63% 4 8% 15% 10% 1	3333
November	1st to 10th	Day	71% 8% 7% 10% 4	1649
	11th to 20th	Day	69% 7% 9% 8% 5% 2	1600
	21st to end	Day	67% 5% 6% 9% 13% 1	1600
December	1st to 10th	Day	53% 4 7% 17% 18% 1	2620
	11th to 20th	Day	76% 5% 9% 8% 2	3028
	21st to end	Day	78% 6% 8% 5% 2	4004
January	1st to 10th	Night	78% 4 6% 7% 5% 1	6306
	11th to 20th	Night	57% 5% 8% 19% 9% 1	6320
	21st to end	Night	58% 3 5% 14% 14% 6%	6661
February	1st to 10th	Night	69% 8% 7% 10% 4 1	2600
	11th to 20th	Night	66% 6% 8% 10% 6% 2	2534
	21st to end	Night	67% 6% 9% 9% 7% 1	2065
March	1st to 10th	Night	64% 7% 11% 12% 6%	5200
	11th to 20th	Night	63% 6% 9% 16% 6% 1	5000
	21st to end	Night	74% 6% 10% 8% 1	5345
April	1st to 10th	Night	68% 6% 10% 11% 4	2000
	11th to 20th	Night	56% 8% 11% 13% 9% 2	1979
	21st to end	Night	57% 10% 13% 14% 5% 2	1800
May	1st to 10th	Night	48% 10% 11% 19% 11% 1	2637
	11th to 20th	Night	33% 12% 21% 25% 8% 2	2596
	21st to end	Night	48% 16% 21% 11% 3	2805
June	1st to 10th	Night	50% 6% 16% 20% 6% 1	1520
	11th to 20th	Night	45% 9% 19% 20% 7%	1508
	21st to end	Night	30% 5% 19% 32% 10% 5%	1330
July	1st to 10th	Night	12% 5% 9% 23% 21% 30%	2404
	11th to 20th	Night	15% 3 7% 20% 35% 19%	2492
	21st to end	Night	12% 4 13% 29% 35% 7%	2674
August	1st to 10th	Night	12% 4 11% 32% 32% 8%	2737
	11th to 20th	Night	26% 5% 26% 27% 11% 5%	2839
	21st to end	Night	29% 5% 15% 24% 19% 8%	3367
September	1st to 10th	Night	23% 6% 14% 27% 20% 11%	1900
	11th to 20th	Night	39% 7% 13% 21% 11% 10%	1900
	21st to end	Night	37% 8% 16% 18% 16% 7%	2174
October	1st to 10th	Night	33% 4 10% 29% 22% 2	3800
	11th to 20th	Night	48% 5% 14% 18% 13% 1	3777
	21st to end	Night	61% 5% 7% 16% 9% 2	4232
November	1st to 10th	Night	68% 6% 8% 12% 6% 1	2647
	11th to 20th	Night	69% 5% 8% 8% 7% 2	2800
	21st to end	Night	64% 5% 8% 8% 13% 2	2800
December	1st to 10th	Night	50% 5% 8% 16% 21% 1	4560
	11th to 20th	Night	78% 5% 7% 7% 4	5176
	21st to end	Night	80% 7% 6% 6% 2	6952

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% 11% 5%	200
	11th to 20th	Day	48% 13% 21% 12% 7%	200
	21st to end	Day	45% 10% 18% 18% 8% 1	353
February	1st to 10th	Day	47% 14% 19% 15% 6% 1	400
	11th to 20th	Day	47% 14% 22% 14% 2 1	395
	21st to end	Day	47% 15% 19% 17% 2	371
March	1st to 10th	Day	40% 16% 26% 14% 4	500
	11th to 20th	Day	38% 14% 24% 16% 6% 2	500
	21st to end	Day	44% 18% 18% 16% 4 1	550
April	1st to 10th	Day	42% 15% 23% 13% 6% 1	500
	11th to 20th	Day	32% 18% 27% 16% 5% 1	492
	21st to end	Day	30% 19% 27% 17% 6% 1	500
May	1st to 10th	Day	21% 19% 28% 22% 8% 2	503
	11th to 20th	Day	31% 17% 21% 21% 7% 3	528
	21st to end	Day	26% 13% 25% 24% 10% 2	550
June	1st to 10th	Day	27% 13% 23% 24% 10% 1	490
	11th to 20th	Day	25% 17% 30% 20% 6% 1	491
	21st to end	Day	12% 13% 33% 30% 11% 2	680
July	1st to 10th	Day	11% 10% 32% 35% 11% 1	534
	11th to 20th	Day	8% 9% 32% 34% 14% 3	484
	21st to end	Day	6% 7% 31% 35% 17% 4	531
August	1st to 10th	Day	9% 6% 25% 39% 16% 5%	487
	11th to 20th	Day	5% 5% 26% 39% 20% 4	477
	21st to end	Day	12% 11% 29% 33% 13% 2	539
September	1st to 10th	Day	10% 8% 31% 39% 10% 2	490
	11th to 20th	Day	19% 9% 24% 32% 14% 1	490
	21st to end	Day	22% 11% 27% 28% 11% 1	490
October	1st to 10th	Day	26% 9% 26% 24% 14% 1	490
	11th to 20th	Day	30% 11% 22% 22% 14% 1	490
	21st to end	Day	38% 14% 19% 19% 8% 2	473
November	1st to 10th	Day	42% 16% 20% 15% 6%	243
	11th to 20th	Day	48% 11% 20% 14% 6% 2	200
	21st to end	Day	43% 11% 18% 24% 5% 1	200
December	1st to 10th	Day	45% 10% 21% 17% 8% 1	200
	11th to 20th	Day	53% 12% 22% 12% 3	200
	21st to end	Day	53% 11% 20% 13% 3	220
January	1st to 10th	Night	50% 11% 16% 17% 6%	800
	11th to 20th	Night	43% 9% 19% 21% 8%	800
	21st to end	Night	40% 6% 23% 22% 8% 1	880
February	1st to 10th	Night	40% 10% 20% 23% 6% 1	800
	11th to 20th	Night	42% 10% 18% 22% 8%	784
	21st to end	Night	43% 10% 19% 22% 6%	660
March	1st to 10th	Night	38% 12% 16% 24% 9% 1	800
	11th to 20th	Night	40% 9% 20% 24% 8%	800
	21st to end	Night	44% 11% 16% 21% 8%	880
April	1st to 10th	Night	36% 10% 20% 22% 11% 1	800
	11th to 20th	Night	29% 10% 24% 26% 10% 1	791
	21st to end	Night	32% 7% 17% 32% 12%	600
May	1st to 10th	Night	21% 9% 24% 32% 14% 1	417
	11th to 20th	Night	24% 5% 18% 35% 18% 1	394
	21st to end	Night	18% 8% 19% 35% 19% 1	422
June	1st to 10th	Night	24% 5% 18% 36% 16%	380
	11th to 20th	Night	22% 9% 19% 36% 14%	380
	21st to end	Night	12% 5% 21% 41% 21%	380
July	1st to 10th	Night	7% 2 17% 50% 24%	370
	11th to 20th	Night	7% 3 17% 49% 24% 1	368
	21st to end	Night	4 3 16% 51% 25% 2	416
August	1st to 10th	Night	5% 2 12% 42% 35% 4	503
	11th to 20th	Night	7% 2 8% 43% 38% 3	564
	21st to end	Night	10% 5% 17% 43% 23% 1	757
September	1st to 10th	Night	11% 4 16% 44% 23% 2	760
	11th to 20th	Night	17% 4 14% 34% 29% 2	760
	21st to end	Night	18% 5% 19% 38% 20% 1	760
October	1st to 10th	Night	22% 6% 18% 36% 16% 2	760
	11th to 20th	Night	28% 6% 14% 32% 17% 2	760
	21st to end	Night	37% 7% 19% 25% 11% 1	836
November	1st to 10th	Night	44% 8% 18% 24% 6%	780
	11th to 20th	Night	49% 8% 18% 19% 5% 1	800
	21st to end	Night	39% 8% 21% 24% 8%	800
December	1st to 10th	Night	38% 8% 24% 23% 7% 1	800
	11th to 20th	Night	50% 8% 18% 20% 4	800
	21st to end	Night	56% 10% 17% 13% 5%	880

Page generated at 2025-05-23 06:21:00. Version ac07

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