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 2026-09-29.

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 11% 4 8% 4 4 4 3 5% 6% 48%	157111
	Night	4 11% 4 8% 4 4 4 3 5% 6% 47%	151673
Transparency	Day	36% 5% 8% 20% 20% 11%	151856
	Night	36% 5% 10% 21% 18% 9%	147931
Seeing	Day	16% 19% 32% 21% 9% 2	16809
	Night	12% 8% 26% 35% 17% 2	26467

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 21% 10% 20% 5% 4 4 3 4 5% 20%	4722
	11th to 20th	Day	2 11% 3 12% 3 2 3 2 3 6% 52%	4740
	21st to end	Day	3 16% 5% 8% 3 4 4 3 4 5% 45%	5187
February	1st to 10th	Day	5% 19% 7% 12% 6% 5% 4 3 4 5% 29%	1844
	11th to 20th	Day	4 18% 9% 18% 6% 4 4 2 3 5% 27%	2033
	21st to end	Day	3 22% 7% 19% 6% 4 4 2 5% 5% 24%	1730
March	1st to 10th	Day	2 14% 7% 16% 5% 4 4 3 6% 7% 31%	4618
	11th to 20th	Day	2 19% 8% 15% 4 3 3 2 2 4 38%	5015
	21st to end	Day	4 26% 10% 14% 6% 5% 5% 3 4 4 18%	5713
April	1st to 10th	Day	5% 22% 8% 12% 6% 4 3 3 4 6% 28%	2520
	11th to 20th	Day	4 18% 5% 11% 6% 4 4 3 4 7% 33%	2599
	21st to end	Day	6% 17% 6% 9% 6% 5% 4 3 5% 7% 31%	2730
May	1st to 10th	Day	3 10% 4 7% 4 5% 4 4 5% 5% 47%	7626
	11th to 20th	Day	4 12% 3 8% 5% 5% 4 4 6% 7% 43%	7702
	21st to end	Day	5% 10% 2 6% 4 4 5% 4 5% 7% 47%	8992
June	1st to 10th	Day	11% 11% 3 7% 5% 5% 5% 4 5% 8% 36%	2940
	11th to 20th	Day	9% 9% 3 7% 4 4 4 4 5% 6% 46%	2940
	21st to end	Day	4 6% 2 5% 3 3 3 3 4 6% 61%	2940
July	1st to 10th	Day	2 2 1 3 3 3 4 3 4 6% 70%	8219
	11th to 20th	Day	2 2 1 4 3 4 4 3 4 5% 69%	8220
	21st to end	Day	3 3 1 4 3 4 5% 4 6% 7% 60%	8922
August	1st to 10th	Day	2 3 1 4 3 3 4 3 5% 7% 64%	7667
	11th to 20th	Day	1 2 1 4 3 4 4 3 5% 6% 67%	7565
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 5% 5% 55%	8035
September	1st to 10th	Day	4 3 1 4 3 4 3 4 4 6% 63%	2494
	11th to 20th	Day	7% 7% 2 5% 4 3 4 3 5% 7% 53%	2310
	21st to end	Day	7% 7% 2 4 3 3 3 3 4 6% 59%	2289
October	1st to 10th	Day	5% 7% 2 5% 3 3 3 2 3 5% 63%	3600
	11th to 20th	Day	6% 10% 2 6% 3 4 3 3 4 6% 53%	3140
	21st to end	Day	4 15% 5% 7% 4 4 4 3 5% 7% 43%	3200
November	1st to 10th	Day	8% 17% 4 10% 4 3 3 4 5% 6% 36%	1854
	11th to 20th	Day	7% 17% 5% 9% 5% 4 4 4 5% 7% 32%	1890
	21st to end	Day	4 16% 5% 9% 5% 4 4 4 5% 8% 38%	1866
December	1st to 10th	Day	3 12% 5% 10% 4 3 2 2 4 5% 50%	2371
	11th to 20th	Day	5% 23% 8% 14% 6% 5% 4 4 6% 6% 19%	2786
	21st to end	Day	5% 21% 4 14% 5% 4 4 3 5% 6% 29%	4092
January	1st to 10th	Night	3 22% 8% 16% 5% 4 4 3 5% 5% 25%	8172
	11th to 20th	Night	3 11% 4 12% 3 2 2 2 4 7% 50%	8220
	21st to end	Night	3 13% 4 8% 3 4 4 3 5% 6% 47%	8994
February	1st to 10th	Night	4 20% 7% 11% 6% 4 5% 4 5% 6% 28%	2774
	11th to 20th	Night	3 17% 8% 13% 6% 4 4 3 4 6% 32%	2474
	21st to end	Night	4 18% 7% 18% 5% 4 4 3 6% 5% 26%	2076
March	1st to 10th	Night	1 15% 8% 13% 5% 4 4 3 5% 7% 36%	5496
	11th to 20th	Night	2 19% 7% 10% 4 4 4 3 4 5% 38%	5205
	21st to end	Night	3 19% 9% 12% 5% 4 4 3 5% 7% 28%	5487
April	1st to 10th	Night	5% 16% 5% 10% 5% 4 4 4 6% 6% 36%	2100
	11th to 20th	Night	5% 14% 5% 9% 5% 4 4 4 5% 5% 40%	1968
	21st to end	Night	4 13% 4 9% 5% 4 4 4 5% 7% 42%	1890
May	1st to 10th	Night	3 9% 3 5% 4 3 3 3 5% 8% 55%	5321
	11th to 20th	Night	4 8% 2 6% 4 5% 5% 4 5% 7% 50%	5180
	21st to end	Night	3 6% 2 6% 4 4 4 3 5% 7% 56%	5181
June	1st to 10th	Night	9% 6% 2 5% 5% 5% 4 4 6% 9% 44%	1680
	11th to 20th	Night	7% 8% 3 4 4 3 4 3 5% 4 55%	1680
	21st to end	Night	3 4 1 4 3 3 4 4 6% 6% 63%	1680
July	1st to 10th	Night	1 1 1 2 2 2 3 3 5% 6% 74%	4740
	11th to 20th	Night	1 2 1 3 2 2 3 2 4 6% 74%	4740
	21st to end	Night	1 3 1 3 2 3 3 3 5% 8% 66%	5153
August	1st to 10th	Night	2 2 1 3 3 3 3 3 4 6% 70%	5147
	11th to 20th	Night	1 1 1 3 3 3 4 4 5% 7% 69%	5291
	21st to end	Night	5% 4 2 4 3 4 4 4 6% 6% 58%	6196
September	1st to 10th	Night	3 3 1 3 2 3 3 3 4 6% 70%	2126
	11th to 20th	Night	5% 6% 2 4 3 3 4 4 5% 7% 59%	2310
	21st to end	Night	7% 4 1 4 2 2 3 3 4 6% 64%	2288
October	1st to 10th	Night	3 5% 2 4 3 3 3 3 4 6% 64%	3600
	11th to 20th	Night	6% 9% 3 5% 3 4 3 2 4 6% 55%	4004
	21st to end	Night	6% 12% 4 6% 4 4 5% 4 6% 8% 41%	4600
November	1st to 10th	Night	9% 14% 5% 9% 4 4 4 4 5% 6% 37%	2665
	11th to 20th	Night	7% 15% 5% 9% 5% 4 4 4 6% 7% 34%	2730
	21st to end	Night	6% 11% 5% 9% 5% 5% 5% 3 5% 7% 39%	2730
December	1st to 10th	Night	3 11% 4 10% 4 3 3 3 4 6% 49%	5020
	11th to 20th	Night	4 22% 7% 12% 5% 4 5% 4 6% 6% 24%	5649
	21st to end	Night	5% 19% 6% 15% 6% 5% 4 3 5% 6% 26%	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	65% 7% 10% 11% 8%	4723
	11th to 20th	Day	35% 3 10% 29% 17% 7%	4740
	21st to end	Day	42% 4 9% 17% 21% 7%	5187
February	1st to 10th	Day	57% 6% 10% 19% 6% 2	1844
	11th to 20th	Day	61% 5% 10% 14% 8% 3	2043
	21st to end	Day	63% 3 7% 15% 9% 2	1730
March	1st to 10th	Day	50% 4 9% 17% 15% 5%	4618
	11th to 20th	Day	50% 5% 7% 26% 11% 1	5015
	21st to end	Day	65% 8% 8% 15% 3	5713
April	1st to 10th	Day	58% 5% 12% 17% 8% 1	2520
	11th to 20th	Day	51% 5% 12% 19% 11% 1	2602
	21st to end	Day	51% 6% 12% 21% 9%	2730
May	1st to 10th	Day	32% 8% 9% 29% 19% 2	7626
	11th to 20th	Day	35% 7% 9% 25% 19% 5%	7705
	21st to end	Day	32% 7% 9% 30% 21% 2	8994
June	1st to 10th	Day	45% 5% 12% 24% 13% 1	2940
	11th to 20th	Day	37% 5% 8% 22% 21% 7%	2940
	21st to end	Day	25% 4 5% 22% 35% 9%	2940
July	1st to 10th	Day	13% 4 4 13% 29% 37%	8151
	11th to 20th	Day	16% 3 4 11% 27% 38%	8145
	21st to end	Day	18% 6% 6% 15% 31% 24%	8723
August	1st to 10th	Day	20% 3 4 10% 26% 37%	6070
	11th to 20th	Day	18% 6% 7% 21% 35% 14%	5964
	21st to end	Day	24% 5% 6% 21% 30% 14%	6302
September	1st to 10th	Day	21% 4 5% 20% 31% 18%	2494
	11th to 20th	Day	30% 5% 8% 23% 24% 9%	2310
	21st to end	Day	26% 4 9% 26% 27% 8%	2289
October	1st to 10th	Day	27% 3 6% 22% 35% 8%	3600
	11th to 20th	Day	32% 4 14% 22% 24% 3	3140
	21st to end	Day	39% 4 11% 25% 20% 2	3200
November	1st to 10th	Day	48% 5% 14% 19% 12% 2	1854
	11th to 20th	Day	51% 6% 15% 19% 8% 2	1890
	21st to end	Day	44% 4 14% 20% 12% 6%	1866
December	1st to 10th	Day	37% 4 16% 25% 16% 2	2371
	11th to 20th	Day	63% 6% 12% 12% 6% 1	2785
	21st to end	Day	56% 5% 8% 19% 10% 3	4092
January	1st to 10th	Night	59% 5% 9% 16% 9% 1	8172
	11th to 20th	Night	37% 3 12% 27% 16% 5%	8220
	21st to end	Night	37% 6% 8% 20% 18% 10%	8994
February	1st to 10th	Night	56% 6% 13% 17% 7% 1	2774
	11th to 20th	Night	53% 5% 14% 15% 10% 3	2475
	21st to end	Night	58% 5% 8% 15% 10% 3	2076
March	1st to 10th	Night	45% 6% 8% 19% 15% 7%	5496
	11th to 20th	Night	47% 5% 10% 24% 13% 1	5205
	21st to end	Night	51% 9% 15% 19% 6%	5487
April	1st to 10th	Night	47% 6% 12% 24% 10% 1	2100
	11th to 20th	Night	45% 5% 13% 23% 14% 1	1968
	21st to end	Night	40% 7% 16% 26% 10%	1890
May	1st to 10th	Night	25% 8% 10% 34% 20% 2	5319
	11th to 20th	Night	29% 7% 10% 29% 22% 2	5181
	21st to end	Night	27% 6% 9% 36% 22% 1	5182
June	1st to 10th	Night	36% 6% 16% 28% 13%	1680
	11th to 20th	Night	31% 4 10% 25% 23% 7%	1680
	21st to end	Night	20% 4 8% 24% 35% 9%	1680
July	1st to 10th	Night	11% 3 4 15% 28% 39%	4700
	11th to 20th	Night	13% 2 4 15% 26% 39%	4692
	21st to end	Night	15% 4 6% 17% 28% 29%	5040
August	1st to 10th	Night	18% 3 4 11% 28% 37%	4069
	11th to 20th	Night	15% 5% 8% 17% 38% 18%	4168
	21st to end	Night	21% 5% 8% 22% 28% 16%	4854
September	1st to 10th	Night	17% 3 6% 22% 34% 19%	2126
	11th to 20th	Night	23% 5% 10% 25% 24% 12%	2310
	21st to end	Night	22% 3 8% 30% 28% 8%	2288
October	1st to 10th	Night	22% 3 7% 22% 36% 10%	3600
	11th to 20th	Night	30% 4 14% 23% 24% 5%	4004
	21st to end	Night	38% 6% 12% 24% 18% 1	4600
November	1st to 10th	Night	46% 5% 12% 20% 15% 2	2665
	11th to 20th	Night	48% 6% 14% 22% 9% 1	2730
	21st to end	Night	44% 5% 11% 19% 14% 6%	2730
December	1st to 10th	Night	36% 4 19% 25% 14% 2	5020
	11th to 20th	Night	58% 5% 13% 16% 7% 1	5650
	21st to end	Night	58% 5% 8% 15% 9% 4	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	22% 22% 26% 19% 7% 3	220
	11th to 20th	Day	15% 19% 31% 20% 11% 4	220
	21st to end	Day	15% 19% 32% 19% 12% 2	242
February	1st to 10th	Day	28% 20% 32% 12% 7% 1	306
	11th to 20th	Day	19% 25% 33% 16% 6% 1	518
	21st to end	Day	24% 20% 35% 18% 3	446
March	1st to 10th	Day	21% 26% 36% 14% 3	540
	11th to 20th	Day	20% 23% 34% 16% 7% 1	540
	21st to end	Day	21% 26% 31% 16% 5%	594
April	1st to 10th	Day	23% 20% 30% 18% 7% 1	540
	11th to 20th	Day	23% 25% 30% 16% 5% 1	532
	21st to end	Day	19% 22% 29% 22% 6% 2	540
May	1st to 10th	Day	14% 19% 35% 22% 9% 1	543
	11th to 20th	Day	21% 23% 26% 20% 9% 2	568
	21st to end	Day	16% 17% 30% 21% 13% 4	617
June	1st to 10th	Day	17% 21% 29% 23% 9% 2	570
	11th to 20th	Day	20% 24% 34% 16% 5% 1	570
	21st to end	Day	10% 18% 36% 25% 8% 2	570
July	1st to 10th	Day	5% 19% 40% 23% 10% 3	566
	11th to 20th	Day	5% 16% 43% 24% 11% 1	564
	21st to end	Day	5% 18% 36% 26% 12% 3	619
August	1st to 10th	Day	9% 14% 39% 25% 12% 1	567
	11th to 20th	Day	5% 16% 35% 27% 14% 3	557
	21st to end	Day	9% 17% 32% 28% 11% 3	627
September	1st to 10th	Day	8% 16% 33% 25% 15% 3	570
	11th to 20th	Day	14% 15% 29% 26% 13% 3	570
	21st to end	Day	11% 16% 32% 25% 14% 2	563
October	1st to 10th	Day	17% 16% 28% 25% 12% 2	530
	11th to 20th	Day	16% 19% 28% 19% 14% 4	360
	21st to end	Day	18% 20% 28% 24% 8% 2	363
November	1st to 10th	Day	23% 19% 25% 21% 10% 1	330
	11th to 20th	Day	18% 21% 28% 23% 8% 3	330
	21st to end	Day	16% 14% 34% 24% 11% 2	330
December	1st to 10th	Day	24% 12% 28% 16% 14% 7%	225
	11th to 20th	Day	21% 25% 31% 15% 6% 1	220
	21st to end	Day	24% 18% 28% 19% 9% 2	242
January	1st to 10th	Night	16% 13% 30% 30% 10% 1	840
	11th to 20th	Night	15% 8% 32% 30% 13% 2	840
	21st to end	Night	9% 9% 35% 31% 14% 2	924
February	1st to 10th	Night	17% 13% 36% 25% 8% 2	840
	11th to 20th	Night	10% 15% 34% 29% 11% 1	824
	21st to end	Night	14% 12% 35% 28% 9% 2	692
March	1st to 10th	Night	13% 14% 32% 30% 10% 1	840
	11th to 20th	Night	16% 10% 29% 31% 12% 1	840
	21st to end	Night	14% 14% 30% 33% 10% 1	924
April	1st to 10th	Night	14% 11% 25% 36% 13% 2	840
	11th to 20th	Night	14% 12% 32% 31% 10% 1	720
	21st to end	Night	11% 9% 27% 39% 13% 2	630
May	1st to 10th	Night	11% 7% 24% 37% 19% 2	630
	11th to 20th	Night	10% 7% 21% 35% 24% 3	584
	21st to end	Night	10% 6% 21% 34% 27% 3	455
June	1st to 10th	Night	12% 5% 17% 41% 23% 2	420
	11th to 20th	Night	11% 5% 22% 42% 18% 2	420
	21st to end	Night	5% 1 15% 46% 31% 1	420
July	1st to 10th	Night	2 1 11% 50% 33% 3	410
	11th to 20th	Night	2 13% 58% 26%	408
	21st to end	Night	2 2 10% 51% 32% 3	460
August	1st to 10th	Night	4 3 15% 48% 28% 3	578
	11th to 20th	Night	2 2 14% 47% 34% 1	624
	21st to end	Night	4 4 15% 46% 28% 3	813
September	1st to 10th	Night	5% 3 13% 46% 31% 3	840
	11th to 20th	Night	7% 3 13% 39% 32% 5%	840
	21st to end	Night	9% 4 19% 37% 26% 6%	832
October	1st to 10th	Night	12% 5% 22% 37% 23% 2	800
	11th to 20th	Night	14% 8% 19% 38% 18% 3	800
	21st to end	Night	13% 8% 26% 37% 12% 3	880
November	1st to 10th	Night	17% 8% 28% 32% 14% 1	820
	11th to 20th	Night	17% 10% 33% 31% 8% 1	840
	21st to end	Night	13% 10% 31% 29% 14% 3	840
December	1st to 10th	Night	16% 11% 30% 33% 8% 1	1046
	11th to 20th	Night	16% 14% 31% 29% 9% 1	1029
	21st to end	Night	19% 12% 33% 25% 9% 1	924

Page generated at 2026-09-26 02:18:43. Version ac07

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