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 2024-11-02.

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% 12% 4 9% 4 4 4 3 4 6% 45%	100878
	Night	4 12% 4 9% 4 4 4 3 5% 6% 45%	98232
Transparency	Day	40% 5% 9% 19% 18% 11%	100621
	Night	39% 5% 10% 20% 17% 8%	98079
Seeing	Day	15% 19% 32% 22% 9% 2	14376
	Night	11% 8% 25% 36% 18% 2	24062

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 15% 7% 23% 6% 5% 5% 3 5% 6% 22%	2539
	11th to 20th	Day	3 20% 6% 16% 4 4 4 3 4 6% 31%	2540
	21st to end	Day	5% 24% 7% 10% 4 4 4 3 5% 6% 29%	2794
February	1st to 10th	Day	5% 19% 7% 12% 6% 5% 5% 3 4 5% 29%	1668
	11th to 20th	Day	4 18% 9% 18% 6% 4 4 2 3 5% 27%	1834
	21st to end	Day	3 22% 7% 20% 6% 4 4 2 5% 4 21%	1570
March	1st to 10th	Day	2 15% 9% 20% 5% 3 4 3 5% 6% 27%	3138
	11th to 20th	Day	3 17% 5% 9% 4 3 3 2 3 4 47%	3423
	21st to end	Day	4 26% 10% 16% 6% 5% 6% 4 4 4 16%	3903
April	1st to 10th	Day	5% 24% 8% 12% 6% 4 3 3 4 5% 26%	2280
	11th to 20th	Day	5% 19% 5% 11% 6% 4 3 3 4 6% 34%	2348
	21st to end	Day	7% 17% 6% 9% 6% 5% 4 3 5% 7% 31%	2470
May	1st to 10th	Day	5% 18% 7% 10% 5% 5% 5% 4 5% 4 33%	4110
	11th to 20th	Day	5% 7% 3 7% 5% 5% 4 3 5% 7% 49%	4125
	21st to end	Day	7% 16% 3 7% 6% 4 5% 3 4 6% 39%	4813
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	8% 10% 3 6% 4 4 4 3 4 6% 49%	2090
	11th to 20th	Day	7% 12% 3 6% 3 3 3 3 5% 6% 48%	1839
	21st to end	Day	5% 13% 5% 7% 4 4 5% 3 5% 7% 41%	1881
November	1st to 10th	Day	9% 18% 4 10% 4 3 4 3 5% 6% 35%	1683
	11th to 20th	Day	7% 16% 5% 9% 5% 4 4 4 5% 7% 34%	1710
	21st to end	Day	4 16% 5% 9% 6% 4 4 4 5% 8% 37%	1689
December	1st to 10th	Day	5% 18% 6% 11% 5% 3 3 3 5% 6% 35%	1341
	11th to 20th	Day	4 21% 6% 13% 6% 5% 4 3 7% 7% 25%	1726
	21st to end	Day	3 16% 4 14% 6% 4 4 3 6% 7% 34%	2794
January	1st to 10th	Night	3 18% 7% 16% 6% 4 4 3 5% 6% 27%	4406
	11th to 20th	Night	6% 20% 5% 16% 5% 3 3 3 5% 6% 29%	4420
	21st to end	Night	6% 23% 6% 12% 4 4 5% 3 4 5% 30%	4862
February	1st to 10th	Night	4 20% 7% 11% 6% 4 4 4 6% 6% 28%	2510
	11th to 20th	Night	3 17% 8% 13% 6% 4 4 3 4 6% 33%	2233
	21st to end	Night	3 19% 7% 20% 6% 4 4 3 6% 5% 24%	1884
March	1st to 10th	Night	2 18% 8% 14% 5% 4 4 3 5% 7% 31%	3736
	11th to 20th	Night	3 16% 6% 6% 3 3 3 3 4 5% 47%	3557
	21st to end	Night	3 18% 9% 13% 5% 5% 5% 4 6% 7% 24%	3746
April	1st to 10th	Night	5% 17% 5% 10% 6% 4 4 4 5% 5% 35%	1900
	11th to 20th	Night	6% 15% 4 9% 5% 5% 4 4 4 5% 39%	1779
	21st to end	Night	4 12% 4 8% 5% 4 4 4 5% 7% 42%	1710
May	1st to 10th	Night	5% 15% 4 6% 4 3 4 3 5% 8% 42%	2860
	11th to 20th	Night	3 5% 2 6% 4 4 4 4 5% 7% 56%	2774
	21st to end	Night	5% 9% 2 7% 5% 5% 4 4 5% 6% 47%	2762
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	5% 8% 2 5% 4 3 4 3 4 7% 53%	2090
	11th to 20th	Night	9% 8% 3 5% 3 3 3 2 5% 7% 52%	2336
	21st to end	Night	7% 11% 4 7% 4 3 5% 3 6% 7% 43%	2717
November	1st to 10th	Night	9% 15% 5% 8% 3 4 4 4 6% 6% 36%	2418
	11th to 20th	Night	7% 15% 5% 9% 4 4 4 4 6% 7% 36%	2470
	21st to end	Night	5% 11% 5% 9% 6% 5% 5% 3 5% 6% 39%	2470
December	1st to 10th	Night	4 16% 6% 11% 6% 4 5% 4 5% 6% 33%	2831
	11th to 20th	Night	5% 20% 7% 12% 5% 4 5% 4 5% 5% 27%	3474
	21st to end	Night	3 16% 5% 13% 5% 5% 4 3 5% 7% 34%	4862

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	60% 8% 10% 15% 6% 1	2539
	11th to 20th	Day	56% 3 11% 14% 11% 4	2540
	21st to end	Day	57% 4 12% 18% 7% 2	2794
February	1st to 10th	Day	57% 6% 10% 18% 6% 2	1668
	11th to 20th	Day	61% 4 10% 14% 8% 2	1844
	21st to end	Day	65% 4 8% 14% 7% 2	1570
March	1st to 10th	Day	57% 4 9% 12% 17% 1	3138
	11th to 20th	Day	41% 5% 9% 31% 13% 1	3423
	21st to end	Day	68% 8% 9% 12% 3	3903
April	1st to 10th	Day	61% 4 13% 14% 7% 1	2280
	11th to 20th	Day	51% 5% 13% 19% 12% 1	2351
	21st to end	Day	52% 6% 12% 21% 9%	2470
May	1st to 10th	Day	51% 7% 9% 20% 12% 1	4110
	11th to 20th	Day	32% 6% 10% 32% 14% 7%	4128
	21st to end	Day	45% 6% 10% 27% 10% 2	4814
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	38% 4 9% 24% 21% 4	2090
	11th to 20th	Day	36% 4 12% 27% 18% 3	1839
	21st to end	Day	41% 4 13% 23% 16% 2	1881
November	1st to 10th	Day	49% 5% 13% 18% 13% 2	1683
	11th to 20th	Day	49% 5% 14% 20% 9% 2	1710
	21st to end	Day	46% 4 13% 18% 12% 6%	1689
December	1st to 10th	Day	50% 4 15% 16% 10% 4	1341
	11th to 20th	Day	59% 5% 15% 11% 9% 1	1725
	21st to end	Day	50% 5% 8% 20% 13% 5%	2794
January	1st to 10th	Night	56% 5% 11% 19% 8% 1	4406
	11th to 20th	Night	58% 3 12% 14% 11% 3	4420
	21st to end	Night	57% 5% 9% 19% 7% 2	4862
February	1st to 10th	Night	55% 5% 13% 17% 8% 2	2510
	11th to 20th	Night	53% 5% 14% 16% 9% 3	2234
	21st to end	Night	60% 5% 8% 15% 8% 3	1884
March	1st to 10th	Night	52% 5% 8% 15% 19% 2	3736
	11th to 20th	Night	39% 5% 13% 29% 14% 1	3557
	21st to end	Night	54% 10% 14% 18% 5%	3746
April	1st to 10th	Night	49% 6% 12% 22% 10% 1	1900
	11th to 20th	Night	46% 5% 13% 21% 14% 1	1779
	21st to end	Night	40% 7% 17% 27% 10%	1710
May	1st to 10th	Night	39% 8% 10% 28% 14% 2	2860
	11th to 20th	Night	25% 7% 12% 36% 17% 2	2775
	21st to end	Night	35% 6% 13% 33% 12%	2762
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	30% 5% 11% 25% 23% 6%	2090
	11th to 20th	Night	32% 4 13% 27% 20% 4	2336
	21st to end	Night	39% 5% 14% 24% 16% 2	2717
November	1st to 10th	Night	46% 5% 12% 20% 15% 3	2418
	11th to 20th	Night	47% 6% 13% 23% 10% 1	2470
	21st to end	Night	44% 6% 10% 18% 15% 7%	2470
December	1st to 10th	Night	50% 5% 16% 15% 9% 4	2831
	11th to 20th	Night	57% 5% 12% 16% 10% 1	3475
	21st to end	Night	50% 5% 9% 18% 13% 6%	4862

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% 19% 26% 23% 8% 3	180
	11th to 20th	Day	18% 20% 28% 19% 11% 4	180
	21st to end	Day	17% 20% 31% 18% 12% 3	198
February	1st to 10th	Day	26% 22% 35% 11% 6% 1	250
	11th to 20th	Day	20% 26% 31% 16% 6% 1	439
	21st to end	Day	22% 20% 36% 19% 3	382
March	1st to 10th	Day	22% 25% 37% 13% 2	460
	11th to 20th	Day	15% 22% 36% 17% 8% 1	460
	21st to end	Day	19% 27% 32% 18% 4	506
April	1st to 10th	Day	24% 21% 30% 18% 6% 1	460
	11th to 20th	Day	22% 25% 30% 16% 5% 2	452
	21st to end	Day	20% 22% 29% 23% 5% 2	460
May	1st to 10th	Day	15% 19% 33% 23% 8% 2	463
	11th to 20th	Day	18% 23% 26% 22% 10% 2	488
	21st to end	Day	17% 15% 30% 21% 13% 4	529
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% 20% 30% 23% 8% 2	330
November	1st to 10th	Day	22% 18% 26% 22% 11% 1	273
	11th to 20th	Day	16% 19% 27% 26% 9% 3	270
	21st to end	Day	14% 13% 34% 25% 12% 1	270
December	1st to 10th	Day	22% 11% 31% 16% 14% 6%	185
	11th to 20th	Day	17% 24% 34% 15% 7% 2	180
	21st to end	Day	18% 18% 32% 21% 10% 2	198
January	1st to 10th	Night	14% 13% 28% 32% 11% 1	760
	11th to 20th	Night	16% 8% 29% 30% 14% 3	760
	21st to end	Night	10% 9% 32% 32% 15% 2	836
February	1st to 10th	Night	15% 12% 38% 26% 8% 2	760
	11th to 20th	Night	11% 13% 34% 30% 11% 1	744
	21st to end	Night	13% 11% 35% 29% 10% 3	628
March	1st to 10th	Night	14% 13% 31% 30% 10% 1	760
	11th to 20th	Night	14% 10% 27% 34% 13% 1	760
	21st to end	Night	13% 13% 29% 34% 11% 1	836
April	1st to 10th	Night	14% 11% 25% 34% 13% 2	760
	11th to 20th	Night	15% 12% 31% 31% 11% 1	651
	21st to end	Night	11% 8% 26% 39% 14% 2	570
May	1st to 10th	Night	11% 7% 23% 36% 21% 2	570
	11th to 20th	Night	9% 6% 20% 37% 26% 3	528
	21st to end	Night	11% 6% 20% 32% 28% 3	411
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% 15% 1	744
	11th to 20th	Night	17% 9% 32% 34% 8% 1	760
	21st to end	Night	13% 9% 30% 30% 14% 4	760
December	1st to 10th	Night	14% 11% 30% 34% 8% 1	946
	11th to 20th	Night	15% 13% 31% 31% 10% 2	931
	21st to end	Night	17% 12% 34% 26% 10% 1	836

Page generated at 2024-10-29 23:23:00. Version ac07

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