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 2024-03-08.

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	6% 12% 5% 8% 6% 3 4 4 5% 6% 41%	78102
	Night	5% 12% 5% 8% 5% 3 4 4 5% 6% 42%	81448
Transparency	Day	43% 5% 10% 19% 16% 7%	77805
	Night	42% 5% 11% 20% 15% 6%	81267
Seeing	Day	15% 19% 32% 22% 10% 2	13442
	Night	12% 8% 24% 36% 18% 2	23317

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 14% 8% 20% 8% 4 5% 4 5% 6% 23%	2539
	11th to 20th	Day	3 18% 8% 13% 7% 3 3 4 4 6% 31%	2540
	21st to end	Day	5% 23% 8% 8% 6% 3 4 4 5% 6% 29%	2794
February	1st to 10th	Day	5% 18% 7% 11% 7% 4 5% 4 4 5% 30%	1668
	11th to 20th	Day	4 17% 10% 17% 7% 3 4 3 3 5% 27%	1834
	21st to end	Day	4 19% 9% 19% 8% 3 4 3 5% 5% 21%	1570
March	1st to 10th	Day	3 19% 8% 15% 8% 3 6% 4 5% 6% 23%	1871
	11th to 20th	Day	5% 19% 8% 10% 6% 3 5% 5% 4 7% 29%	1939
	21st to end	Day	6% 23% 8% 12% 6% 3 5% 4 4 5% 23%	2218
April	1st to 10th	Day	6% 22% 9% 11% 7% 3 3 4 4 6% 25%	2160
	11th to 20th	Day	5% 18% 7% 10% 6% 3 4 4 4 6% 33%	2222
	21st to end	Day	7% 16% 6% 8% 7% 4 4 4 5% 7% 32%	2340
May	1st to 10th	Day	5% 14% 4 9% 7% 4 5% 5% 5% 6% 36%	2340
	11th to 20th	Day	9% 10% 4 8% 8% 4 5% 6% 6% 9% 32%	2329
	21st to end	Day	10% 14% 5% 9% 8% 4 5% 5% 5% 6% 28%	2723
June	1st to 10th	Day	12% 11% 4 7% 6% 4 5% 5% 5% 8% 34%	2520
	11th to 20th	Day	10% 9% 4 7% 5% 3 5% 5% 5% 6% 43%	2520
	21st to end	Day	3 6% 2 4 4 2 4 4 4 6% 61%	2520
July	1st to 10th	Day	3 2 1 3 4 2 4 5% 4 7% 64%	2520
	11th to 20th	Day	3 2 1 3 3 2 3 3 3 5% 73%	2520
	21st to end	Day	4 3 2 4 4 3 4 3 4 5% 64%	2741
August	1st to 10th	Day	4 5% 2 4 5% 3 5% 5% 5% 6% 57%	2369
	11th to 20th	Day	2 2 1 3 4 2 3 4 5% 6% 68%	2299
	21st to end	Day	4 4 1 4 4 2 3 4 4 6% 64%	2469
September	1st to 10th	Day	5% 3 1 3 3 3 3 4 3 5% 67%	2138
	11th to 20th	Day	7% 7% 2 4 4 2 4 4 5% 7% 54%	1980
	21st to end	Day	7% 7% 2 3 4 2 3 4 5% 7% 57%	1980
October	1st to 10th	Day	9% 10% 3 5% 5% 2 4 4 4 6% 48%	1980
	11th to 20th	Day	8% 12% 3 5% 4 2 3 4 4 6% 48%	1743
	21st to end	Day	5% 13% 5% 7% 5% 3 4 3 5% 8% 42%	1782
November	1st to 10th	Day	10% 16% 5% 9% 5% 2 4 4 4 6% 36%	1674
	11th to 20th	Day	8% 14% 6% 8% 6% 2 5% 5% 5% 7% 34%	1710
	21st to end	Day	4 14% 6% 8% 6% 3 4 5% 5% 8% 37%	1689
December	1st to 10th	Day	5% 17% 6% 11% 5% 2 3 4 5% 6% 35%	1341
	11th to 20th	Day	4 20% 7% 13% 7% 3 5% 4 7% 6% 25%	1726
	21st to end	Day	4 15% 5% 13% 7% 3 4 4 5% 7% 34%	2794
January	1st to 10th	Night	3 16% 7% 15% 7% 3 5% 4 5% 7% 27%	4406
	11th to 20th	Night	6% 19% 6% 14% 7% 2 3 3 4 6% 29%	4420
	21st to end	Night	6% 22% 7% 11% 6% 3 5% 4 4 5% 30%	4862
February	1st to 10th	Night	5% 18% 8% 10% 7% 3 5% 5% 6% 6% 28%	2510
	11th to 20th	Night	3 16% 8% 12% 7% 3 4 4 4 6% 33%	2233
	21st to end	Night	4 16% 9% 18% 8% 3 4 4 6% 5% 24%	1884
March	1st to 10th	Night	3 17% 8% 12% 6% 3 5% 4 6% 7% 29%	2249
	11th to 20th	Night	5% 17% 6% 6% 5% 4 5% 6% 5% 8% 33%	2021
	21st to end	Night	5% 15% 6% 11% 6% 4 5% 5% 5% 8% 30%	2134
April	1st to 10th	Night	6% 15% 6% 10% 6% 3 4 5% 6% 6% 34%	1800
	11th to 20th	Night	6% 14% 5% 9% 6% 3 5% 5% 4 5% 38%	1685
	21st to end	Night	5% 12% 5% 7% 5% 3 4 5% 5% 7% 43%	1620
May	1st to 10th	Night	4 10% 3 5% 6% 3 4 4 6% 9% 46%	1620
	11th to 20th	Night	5% 6% 3 6% 7% 3 7% 7% 7% 9% 40%	1562
	21st to end	Night	7% 11% 3 8% 6% 4 5% 6% 6% 9% 35%	1552
June	1st to 10th	Night	10% 7% 2 5% 6% 4 5% 5% 6% 9% 41%	1440
	11th to 20th	Night	8% 9% 3 4 4 2 4 4 6% 4 52%	1440
	21st to end	Night	3 3 1 3 4 2 4 5% 5% 6% 64%	1440
July	1st to 10th	Night	2 1 1 2 3 2 4 5% 5% 6% 69%	1440
	11th to 20th	Night	1 2 1 2 3 1 2 3 5% 6% 74%	1440
	21st to end	Night	2 2 2 3 2 2 3 4 5% 5% 72%	1576
August	1st to 10th	Night	3 4 2 4 4 3 4 4 5% 6% 62%	1568
	11th to 20th	Night	1 1 1 2 3 2 2 4 4 6% 73%	1602
	21st to end	Night	2 3 1 3 3 2 3 4 4 5% 69%	1884
September	1st to 10th	Night	3 3 1 2 3 1 4 3 3 5% 72%	1822
	11th to 20th	Night	5% 5% 2 3 3 2 3 5% 5% 6% 61%	1980
	21st to end	Night	7% 4 1 4 3 2 3 3 4 6% 64%	1980
October	1st to 10th	Night	6% 8% 2 4 4 3 4 5% 4 7% 52%	1980
	11th to 20th	Night	9% 8% 3 5% 4 2 3 3 5% 7% 51%	2212
	21st to end	Night	7% 11% 3 5% 5% 3 4 4 5% 8% 44%	2574
November	1st to 10th	Night	10% 14% 5% 7% 4 3 4 5% 6% 6% 37%	2405
	11th to 20th	Night	8% 14% 5% 8% 5% 2 5% 4 6% 7% 36%	2470
	21st to end	Night	6% 9% 6% 8% 7% 3 6% 4 5% 7% 39%	2470
December	1st to 10th	Night	4 15% 8% 10% 6% 3 5% 5% 5% 7% 33%	2831
	11th to 20th	Night	5% 19% 7% 11% 7% 3 5% 5% 5% 6% 28%	3474
	21st to end	Night	4 14% 6% 12% 7% 3 4 4 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	61% 4 12% 14% 8%	1871
	11th to 20th	Day	55% 6% 12% 17% 9% 1	1939
	21st to end	Day	64% 5% 11% 15% 4 1	2218
April	1st to 10th	Day	61% 4 13% 13% 8% 1	2160
	11th to 20th	Day	52% 5% 12% 18% 11% 1	2225
	21st to end	Day	52% 6% 12% 21% 9%	2340
May	1st to 10th	Day	46% 6% 14% 21% 12% 1	2340
	11th to 20th	Day	48% 6% 15% 24% 7% 1	2332
	21st to end	Day	56% 6% 12% 18% 6% 1	2724
June	1st to 10th	Day	48% 5% 13% 24% 10%	2520
	11th to 20th	Day	41% 5% 8% 22% 18% 6%	2520
	21st to end	Day	25% 4 5% 22% 34% 9%	2520
July	1st to 10th	Day	20% 4 4 21% 30% 21%	2451
	11th to 20th	Day	16% 3 3 13% 31% 34%	2445
	21st to end	Day	21% 3 4 10% 31% 30%	2589
August	1st to 10th	Day	28% 5% 6% 18% 26% 17%	2352
	11th to 20th	Day	17% 4 4 23% 36% 16%	2299
	21st to end	Day	21% 4 5% 21% 33% 16%	2469
September	1st to 10th	Day	21% 3 5% 19% 30% 21%	2138
	11th to 20th	Day	30% 4 9% 23% 24% 11%	1980
	21st to end	Day	28% 3 9% 27% 26% 7%	1980
October	1st to 10th	Day	39% 4 10% 25% 19% 4	1980
	11th to 20th	Day	37% 4 12% 28% 17% 3	1743
	21st to end	Day	41% 4 14% 23% 15% 2	1782
November	1st to 10th	Day	49% 5% 13% 18% 13% 2	1674
	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	54% 6% 11% 17% 11% 1	2249
	11th to 20th	Night	47% 6% 14% 19% 12% 1	2021
	21st to end	Night	52% 8% 16% 18% 6%	2134
April	1st to 10th	Night	49% 6% 12% 21% 10% 1	1800
	11th to 20th	Night	48% 5% 13% 21% 13% 1	1685
	21st to end	Night	39% 7% 17% 26% 10%	1620
May	1st to 10th	Night	33% 7% 14% 31% 14% 1	1620
	11th to 20th	Night	37% 8% 18% 29% 9%	1563
	21st to end	Night	44% 8% 16% 25% 7%	1552
June	1st to 10th	Night	39% 7% 18% 28% 9%	1440
	11th to 20th	Night	33% 5% 11% 25% 19% 7%	1440
	21st to end	Night	19% 4 8% 25% 35% 9%	1440
July	1st to 10th	Night	14% 4 7% 19% 36% 20%	1400
	11th to 20th	Night	12% 2 4 17% 29% 35%	1392
	21st to end	Night	15% 3 5% 13% 32% 32%	1488
August	1st to 10th	Night	23% 4 7% 20% 27% 20%	1560
	11th to 20th	Night	12% 3 6% 22% 37% 20%	1602
	21st to end	Night	16% 4 7% 21% 33% 19%	1884
September	1st to 10th	Night	17% 2 5% 21% 33% 22%	1822
	11th to 20th	Night	22% 4 10% 24% 25% 14%	1980
	21st to end	Night	23% 3 9% 30% 28% 8%	1980
October	1st to 10th	Night	31% 5% 12% 26% 21% 5%	1980
	11th to 20th	Night	33% 4 12% 28% 20% 3	2212
	21st to end	Night	39% 5% 14% 24% 16% 2	2574
November	1st to 10th	Night	46% 5% 12% 20% 15% 3	2405
	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% 38% 12% 3	449
	11th to 20th	Day	16% 22% 37% 18% 7%	420
	21st to end	Day	20% 27% 32% 18% 4	462
April	1st to 10th	Day	24% 19% 31% 18% 6% 1	420
	11th to 20th	Day	24% 25% 28% 16% 5% 2	412
	21st to end	Day	21% 22% 28% 23% 5% 1	420
May	1st to 10th	Day	14% 18% 34% 23% 9% 2	423
	11th to 20th	Day	19% 21% 25% 23% 9% 2	448
	21st to end	Day	17% 14% 29% 22% 13% 5%	485
June	1st to 10th	Day	17% 20% 27% 24% 9% 2	450
	11th to 20th	Day	20% 23% 33% 17% 5% 1	450
	21st to end	Day	8% 17% 35% 27% 9% 3	450
July	1st to 10th	Day	5% 19% 38% 25% 10% 3	446
	11th to 20th	Day	5% 16% 44% 23% 11% 1	444
	21st to end	Day	4 17% 37% 26% 13% 3	487
August	1st to 10th	Day	8% 13% 38% 27% 12% 2	447
	11th to 20th	Day	4 15% 32% 30% 15% 4	437
	21st to end	Day	7% 16% 33% 28% 12% 3	495
September	1st to 10th	Day	8% 15% 33% 26% 15% 3	450
	11th to 20th	Day	13% 14% 29% 29% 13% 3	450
	21st to end	Day	11% 16% 32% 25% 14% 2	450
October	1st to 10th	Day	18% 15% 27% 25% 13% 2	450
	11th to 20th	Day	15% 18% 28% 20% 14% 5%	298
	21st to end	Day	17% 20% 29% 24% 8% 2	297
November	1st to 10th	Day	21% 18% 27% 22% 11% 1	270
	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	750
	11th to 20th	Night	15% 9% 27% 35% 13% 1	720
	21st to end	Night	13% 13% 29% 33% 11% 1	792
April	1st to 10th	Night	15% 11% 24% 34% 14% 2	720
	11th to 20th	Night	16% 12% 30% 30% 11% 1	617
	21st to end	Night	11% 9% 25% 40% 14% 2	540
May	1st to 10th	Night	11% 7% 21% 38% 21% 2	540
	11th to 20th	Night	8% 6% 19% 37% 27% 3	500
	21st to end	Night	11% 6% 19% 31% 30% 3	389
June	1st to 10th	Night	13% 4 15% 40% 25% 3	360
	11th to 20th	Night	12% 6% 20% 40% 21% 2	360
	21st to end	Night	4 1 13% 44% 36% 2	360
July	1st to 10th	Night	2 1 10% 47% 37% 4	350
	11th to 20th	Night	2 1 11% 58% 27% 1	348
	21st to end	Night	2 2 8% 50% 35% 4	394
August	1st to 10th	Night	4 3 13% 46% 31% 3	494
	11th to 20th	Night	2 1 10% 45% 40% 1	534
	21st to end	Night	3 3 13% 47% 31% 3	696
September	1st to 10th	Night	5% 3 12% 44% 33% 3	720
	11th to 20th	Night	8% 2 12% 39% 34% 6%	720
	21st to end	Night	9% 3 17% 36% 27% 7%	720
October	1st to 10th	Night	12% 4 21% 36% 24% 2	720
	11th to 20th	Night	14% 7% 18% 39% 19% 4	720
	21st to end	Night	13% 8% 25% 37% 13% 4	792
November	1st to 10th	Night	17% 8% 28% 32% 15% 1	740
	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 2024-03-05 05:24:01. Version ac07

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