Grandjean
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grandjean 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	4 13% 4 9% 4 4 4 3 4 6% 45%	117060
	Night	4 12% 5% 9% 4 4 4 3 5% 6% 45%	120990
Transparency	Day	40% 5% 9% 19% 18% 10%	116803
	Night	39% 5% 10% 21% 17% 8%	120837
Seeing	Day	15% 19% 32% 22% 9% 2	15032
	Night	12% 8% 25% 35% 18% 2	24854

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	3 16% 9% 21% 5% 5% 5% 3 5% 5% 24%	3639
	11th to 20th	Day	2 14% 4 16% 4 3 3 2 4 6% 41%	3640
	21st to end	Day	4 18% 5% 8% 4 3 3 2 4 5% 44%	4004
February	1st to 10th	Day	5% 20% 7% 13% 6% 5% 4 3 4 5% 28%	1756
	11th to 20th	Day	4 18% 10% 17% 6% 3 4 2 3 5% 27%	1934
	21st to end	Day	3 22% 7% 19% 6% 4 4 2 5% 4 23%	1650
March	1st to 10th	Day	2 14% 7% 16% 5% 4 5% 3 6% 7% 32%	4518
	11th to 20th	Day	2 19% 8% 16% 4 3 3 2 3 4 38%	4907
	21st to end	Day	4 26% 10% 14% 6% 5% 5% 3 4 4 18%	5590
April	1st to 10th	Day	5% 23% 8% 12% 6% 4 3 3 4 5% 27%	2400
	11th to 20th	Day	4 18% 5% 11% 6% 4 4 3 4 6% 35%	2474
	21st to end	Day	7% 17% 6% 9% 6% 5% 4 3 5% 7% 31%	2600
May	1st to 10th	Day	5% 18% 7% 10% 5% 5% 5% 4 5% 4 34%	4240
	11th to 20th	Day	5% 7% 3 7% 5% 5% 4 4 5% 7% 48%	4257
	21st to end	Day	7% 16% 3 7% 6% 4 4 3 4 6% 39%	4886
June	1st to 10th	Day	12% 11% 3 7% 5% 5% 5% 4 5% 8% 34%	2660
	11th to 20th	Day	9% 10% 3 7% 4 4 4 4 5% 5% 44%	2660
	21st to end	Day	3 5% 2 5% 3 3 3 3 4 5% 62%	2660
July	1st to 10th	Day	2 1 1 2 3 2 3 3 3 5% 75%	4420
	11th to 20th	Day	2 2 1 4 2 3 3 3 4 5% 71%	4420
	21st to end	Day	4 3 1 4 3 4 3 3 4 6% 65%	4790
August	1st to 10th	Day	4 6% 2 6% 4 4 5% 4 5% 6% 54%	4158
	11th to 20th	Day	1 2 1 3 3 3 4 3 5% 6% 69%	4028
	21st to end	Day	3 4 1 4 3 2 3 3 3 5% 68%	4338
September	1st to 10th	Day	4 3 1 4 3 4 2 3 4 6% 66%	2256
	11th to 20th	Day	7% 7% 2 6% 4 3 4 3 5% 6% 53%	2090
	21st to end	Day	7% 7% 2 4 3 3 3 3 4 7% 59%	2090
October	1st to 10th	Day	5% 7% 2 5% 3 3 3 2 3 5% 63%	3490
	11th to 20th	Day	6% 9% 2 6% 3 4 3 3 4 6% 53%	3044
	21st to end	Day	4 15% 5% 7% 4 4 4 3 5% 7% 42%	3101
November	1st to 10th	Day	8% 17% 4 10% 4 3 4 4 5% 6% 36%	1764
	11th to 20th	Day	7% 17% 5% 9% 5% 4 4 4 5% 7% 33%	1800
	21st to end	Day	4 16% 5% 9% 5% 4 4 4 5% 8% 37%	1777
December	1st to 10th	Day	3 11% 5% 11% 4 3 2 2 4 5% 51%	2301
	11th to 20th	Day	5% 23% 9% 14% 6% 5% 4 3 6% 6% 19%	2714
	21st to end	Day	5% 21% 4 14% 5% 4 4 3 5% 6% 29%	4004
January	1st to 10th	Night	4 18% 7% 14% 5% 4 4 3 5% 6% 29%	6306
	11th to 20th	Night	4 15% 5% 16% 4 3 3 2 4 6% 39%	6320
	21st to end	Night	4 16% 4 9% 3 3 4 2 4 5% 45%	6952
February	1st to 10th	Night	5% 21% 7% 12% 6% 4 4 4 5% 6% 27%	2642
	11th to 20th	Night	3 18% 8% 13% 6% 4 4 3 4 6% 33%	2353
	21st to end	Night	3 18% 7% 19% 6% 4 4 3 6% 5% 25%	1980
March	1st to 10th	Night	1 15% 8% 13% 5% 4 4 3 5% 7% 36%	5376
	11th to 20th	Night	2 18% 8% 10% 4 4 4 4 4 5% 37%	5093
	21st to end	Night	3 19% 9% 12% 5% 4 4 3 5% 7% 28%	5368
April	1st to 10th	Night	5% 16% 5% 10% 6% 4 4 4 5% 6% 35%	2000
	11th to 20th	Night	5% 14% 4 9% 5% 5% 4 4 4 5% 40%	1873
	21st to end	Night	4 13% 4 8% 5% 4 4 4 5% 7% 42%	1800
May	1st to 10th	Night	5% 15% 4 6% 4 3 4 3 5% 8% 42%	2950
	11th to 20th	Night	3 6% 2 6% 4 4 4 4 5% 7% 55%	2862
	21st to end	Night	5% 9% 2 7% 5% 5% 4 4 5% 6% 48%	2805
June	1st to 10th	Night	10% 7% 2 6% 5% 5% 5% 4 7% 9% 41%	1520
	11th to 20th	Night	8% 9% 3 4 4 3 4 3 5% 4 53%	1520
	21st to end	Night	3 3 1 3 3 3 4 4 6% 6% 64%	1520
July	1st to 10th	Night	1 1 1 2 2 3 2 4 5% 79%	2540
	11th to 20th	Night	1 1 1 2 2 2 3 3 4 6% 75%	2540
	21st to end	Night	1 3 1 3 2 3 3 3 5% 7% 68%	2761
August	1st to 10th	Night	3 4 2 6% 4 4 4 4 5% 6% 58%	2777
	11th to 20th	Night	1 1 1 2 2 2 2 3 4 6% 76%	2811
	21st to end	Night	2 2 1 3 3 3 3 3 4 5% 73%	3332
September	1st to 10th	Night	3 3 1 3 2 2 3 2 4 5% 72%	1924
	11th to 20th	Night	5% 6% 2 4 3 3 4 4 5% 6% 59%	2090
	21st to end	Night	7% 4 1 4 2 2 3 2 4 6% 65%	2090
October	1st to 10th	Night	3 5% 2 4 3 3 3 3 4 6% 65%	3490
	11th to 20th	Night	6% 9% 2 5% 3 4 3 2 4 5% 55%	3880
	21st to end	Night	6% 12% 4 6% 4 4 5% 4 6% 8% 41%	4457
November	1st to 10th	Night	9% 15% 5% 9% 4 4 4 4 6% 6% 37%	2535
	11th to 20th	Night	7% 15% 5% 9% 4 4 4 4 6% 7% 35%	2600
	21st to end	Night	6% 11% 5% 9% 5% 5% 5% 3 5% 7% 40%	2600
December	1st to 10th	Night	3 10% 4 10% 4 3 3 3 4 6% 50%	4871
	11th to 20th	Night	4 22% 8% 12% 5% 4 5% 4 6% 6% 24%	5500
	21st to end	Night	5% 19% 6% 15% 6% 5% 4 3 5% 6% 26%	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	59% 8% 9% 13% 10% 1	3639
	11th to 20th	Day	45% 3 11% 24% 13% 3	3640
	21st to end	Day	44% 3 10% 19% 17% 7%	4004
February	1st to 10th	Day	58% 6% 10% 18% 6% 2	1756
	11th to 20th	Day	61% 4 10% 14% 8% 3	1944
	21st to end	Day	64% 3 8% 14% 8% 2	1650
March	1st to 10th	Day	49% 4 8% 17% 16% 5%	4518
	11th to 20th	Day	51% 5% 7% 26% 11%	4907
	21st to end	Day	65% 8% 8% 15% 3	5590
April	1st to 10th	Day	59% 5% 13% 14% 8% 1	2400
	11th to 20th	Day	50% 5% 12% 20% 12% 1	2477
	21st to end	Day	52% 6% 12% 21% 9%	2600
May	1st to 10th	Day	50% 7% 9% 21% 13% 1	4240
	11th to 20th	Day	33% 6% 10% 31% 13% 7%	4260
	21st to end	Day	44% 6% 10% 27% 10% 2	4887
June	1st to 10th	Day	48% 5% 12% 24% 11%	2660
	11th to 20th	Day	39% 5% 8% 22% 19% 7%	2660
	21st to end	Day	24% 4 5% 22% 35% 10%	2660
July	1st to 10th	Day	12% 3 3 13% 20% 49%	4351
	11th to 20th	Day	15% 3 3 9% 22% 47%	4345
	21st to end	Day	18% 4 4 11% 34% 28%	4639
August	1st to 10th	Day	28% 5% 5% 13% 27% 23%	4142
	11th to 20th	Day	14% 4 5% 17% 41% 19%	4067
	21st to end	Day	20% 3 4 20% 34% 18%	4337
September	1st to 10th	Day	20% 3 5% 20% 32% 20%	2256
	11th to 20th	Day	31% 4 9% 23% 23% 10%	2090
	21st to end	Day	26% 3 9% 27% 27% 7%	2090
October	1st to 10th	Day	27% 2 6% 21% 35% 8%	3490
	11th to 20th	Day	32% 4 14% 22% 24% 3	3044
	21st to end	Day	39% 4 11% 24% 20% 2	3101
November	1st to 10th	Day	48% 5% 13% 19% 13% 2	1764
	11th to 20th	Day	51% 5% 14% 20% 9% 2	1800
	21st to end	Day	45% 4 13% 20% 12% 6%	1777
December	1st to 10th	Day	36% 3 16% 25% 16% 2	2301
	11th to 20th	Day	64% 6% 12% 12% 6% 1	2713
	21st to end	Day	56% 5% 8% 18% 10% 3	4004
January	1st to 10th	Night	54% 5% 9% 19% 11% 1	6306
	11th to 20th	Night	48% 3 13% 20% 12% 4	6320
	21st to end	Night	42% 4 7% 20% 17% 9%	6952
February	1st to 10th	Night	57% 5% 12% 17% 8% 1	2642
	11th to 20th	Night	53% 5% 13% 16% 10% 3	2354
	21st to end	Night	59% 5% 8% 15% 9% 3	1980
March	1st to 10th	Night	46% 6% 8% 19% 15% 7%	5376
	11th to 20th	Night	47% 5% 10% 24% 12% 1	5093
	21st to end	Night	52% 9% 15% 19% 5%	5368
April	1st to 10th	Night	48% 6% 13% 22% 11% 1	2000
	11th to 20th	Night	45% 5% 13% 23% 14% 1	1873
	21st to end	Night	40% 7% 17% 27% 10%	1800
May	1st to 10th	Night	38% 8% 10% 28% 14% 2	2950
	11th to 20th	Night	26% 7% 12% 35% 17% 2	2863
	21st to end	Night	35% 6% 13% 33% 13%	2805
June	1st to 10th	Night	38% 6% 18% 28% 10%	1520
	11th to 20th	Night	32% 4 11% 25% 20% 7%	1520
	21st to end	Night	18% 4 8% 25% 36% 9%	1520
July	1st to 10th	Night	9% 3 4 11% 26% 47%	2500
	11th to 20th	Night	10% 2 3 11% 22% 50%	2492
	21st to end	Night	16% 3 4 13% 32% 32%	2674
August	1st to 10th	Night	25% 4 5% 14% 31% 21%	2769
	11th to 20th	Night	10% 3 5% 17% 44% 22%	2838
	21st to end	Night	14% 4 5% 22% 34% 20%	3332
September	1st to 10th	Night	16% 2 5% 21% 35% 21%	1924
	11th to 20th	Night	24% 5% 10% 24% 24% 14%	2090
	21st to end	Night	22% 3 9% 31% 27% 9%	2090
October	1st to 10th	Night	22% 3 7% 21% 36% 10%	3490
	11th to 20th	Night	30% 4 14% 23% 24% 5%	3880
	21st to end	Night	39% 6% 12% 23% 19% 1	4457
November	1st to 10th	Night	46% 5% 11% 20% 15% 3	2535
	11th to 20th	Night	48% 6% 13% 22% 10% 1	2600
	21st to end	Night	44% 5% 10% 19% 15% 6%	2600
December	1st to 10th	Night	35% 4 19% 25% 14% 2	4871
	11th to 20th	Night	59% 5% 12% 16% 6% 1	5501
	21st to end	Night	58% 5% 8% 15% 10% 4	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	20% 22% 28% 21% 7% 3	200
	11th to 20th	Day	16% 21% 30% 19% 11% 4	200
	21st to end	Day	16% 20% 31% 18% 12% 3	220
February	1st to 10th	Day	29% 21% 33% 11% 5% 1	278
	11th to 20th	Day	19% 26% 33% 15% 6% 1	479
	21st to end	Day	22% 20% 36% 19% 3	414
March	1st to 10th	Day	21% 26% 37% 13% 3	500
	11th to 20th	Day	18% 22% 35% 16% 8% 1	500
	21st to end	Day	20% 26% 31% 17% 5%	550
April	1st to 10th	Day	24% 20% 30% 18% 6% 1	500
	11th to 20th	Day	22% 25% 29% 17% 5% 1	492
	21st to end	Day	19% 22% 30% 22% 5% 2	500
May	1st to 10th	Day	15% 19% 34% 22% 8% 2	503
	11th to 20th	Day	20% 23% 25% 21% 9% 2	528
	21st to end	Day	17% 15% 29% 21% 13% 4	550
June	1st to 10th	Day	17% 20% 29% 24% 8% 2	490
	11th to 20th	Day	21% 23% 32% 17% 5% 1	490
	21st to end	Day	9% 19% 35% 26% 9% 3	490
July	1st to 10th	Day	5% 19% 38% 24% 10% 3	486
	11th to 20th	Day	5% 15% 43% 24% 11% 1	484
	21st to end	Day	5% 18% 37% 25% 13% 3	531
August	1st to 10th	Day	9% 13% 38% 27% 12% 1	487
	11th to 20th	Day	4 16% 34% 29% 14% 4	477
	21st to end	Day	8% 17% 32% 28% 11% 4	539
September	1st to 10th	Day	8% 14% 33% 26% 15% 3	490
	11th to 20th	Day	14% 15% 30% 27% 12% 3	490
	21st to end	Day	10% 16% 32% 25% 14% 2	490
October	1st to 10th	Day	17% 15% 29% 25% 12% 2	490
	11th to 20th	Day	15% 17% 28% 21% 15% 5%	329
	21st to end	Day	18% 21% 28% 24% 8% 2	330
November	1st to 10th	Day	22% 18% 26% 22% 11% 1	300
	11th to 20th	Day	18% 20% 27% 23% 8% 3	300
	21st to end	Day	16% 13% 33% 25% 11% 2	300
December	1st to 10th	Day	20% 11% 29% 17% 15% 7%	205
	11th to 20th	Day	19% 25% 33% 15% 7% 2	200
	21st to end	Day	20% 20% 30% 19% 9% 2	220
January	1st to 10th	Night	15% 13% 30% 31% 11% 1	800
	11th to 20th	Night	15% 8% 30% 30% 14% 2	800
	21st to end	Night	10% 9% 34% 31% 14% 2	880
February	1st to 10th	Night	17% 13% 36% 25% 8% 2	800
	11th to 20th	Night	10% 15% 34% 30% 11% 1	784
	21st to end	Night	14% 12% 34% 29% 9% 2	660
March	1st to 10th	Night	14% 14% 31% 32% 10% 1	800
	11th to 20th	Night	15% 10% 28% 33% 13% 1	800
	21st to end	Night	13% 13% 29% 34% 10% 1	880
April	1st to 10th	Night	14% 11% 25% 34% 13% 2	800
	11th to 20th	Night	14% 12% 31% 31% 10% 1	685
	21st to end	Night	11% 9% 26% 40% 13% 2	600
May	1st to 10th	Night	11% 7% 23% 37% 20% 2	600
	11th to 20th	Night	10% 6% 20% 35% 25% 3	556
	21st to end	Night	10% 6% 21% 32% 28% 3	422
June	1st to 10th	Night	13% 5% 16% 40% 24% 2	380
	11th to 20th	Night	12% 6% 20% 42% 20% 2	380
	21st to end	Night	4 1 14% 45% 34% 2	380
July	1st to 10th	Night	2 1 11% 48% 35% 4	370
	11th to 20th	Night	2 1 11% 59% 27% 1	368
	21st to end	Night	3 2 7% 51% 33% 3	416
August	1st to 10th	Night	4 3 14% 46% 29% 3	522
	11th to 20th	Night	2 2 13% 45% 38% 1	564
	21st to end	Night	3 3 14% 46% 30% 3	735
September	1st to 10th	Night	5% 3 12% 45% 32% 3	760
	11th to 20th	Night	8% 3 12% 39% 33% 5%	760
	21st to end	Night	9% 4 18% 37% 27% 6%	760
October	1st to 10th	Night	11% 4 21% 38% 23% 2	760
	11th to 20th	Night	14% 8% 18% 39% 18% 3	760
	21st to end	Night	13% 8% 26% 37% 13% 3	836
November	1st to 10th	Night	17% 8% 28% 32% 14% 1	780
	11th to 20th	Night	17% 10% 33% 32% 8% 1	800
	21st to end	Night	14% 10% 31% 29% 14% 4	800
December	1st to 10th	Night	14% 11% 30% 35% 9% 1	996
	11th to 20th	Night	15% 13% 31% 30% 9% 2	980
	21st to end	Night	18% 12% 34% 25% 9% 1	880

Page generated at 2025-05-23 07:21:03. Version ac07

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