Grand Beach
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Grand Beach clear sky chart from 2005-11-06 to 2026-06-16.

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	11% 9% 4 9% 5% 5% 5% 4 6% 7% 35%	138159
	Night	10% 11% 4 10% 6% 5% 6% 5% 6% 7% 31%	134169
Transparency	Day	47% 5% 14% 25% 9% 1	141545
	Night	51% 4 13% 22% 9% 1	138871
Seeing	Day	14% 5% 21% 34% 22% 5%	18505
	Night	15% 4 25% 36% 17% 2	28479

By Month

Percentage Cloud Cover NE (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	6% 15% 8% 16% 8% 8% 8% 5% 5% 6% 16%	4124
	11th to 20th	Day	5% 11% 5% 14% 9% 9% 9% 10% 10% 6% 11%	4112
	21st to end	Day	4 13% 7% 15% 8% 9% 8% 5% 7% 9% 16%	5008
February	1st to 10th	Day	8% 12% 6% 12% 7% 7% 8% 7% 7% 7% 18%	1801
	11th to 20th	Day	7% 10% 5% 10% 6% 6% 7% 5% 7% 9% 26%	1844
	21st to end	Day	6% 11% 5% 12% 7% 7% 6% 5% 7% 6% 27%	1601
March	1st to 10th	Day	8% 10% 4 13% 6% 5% 5% 4 6% 8% 30%	4733
	11th to 20th	Day	9% 9% 4 11% 8% 6% 6% 5% 6% 7% 30%	5090
	21st to end	Day	9% 10% 4 10% 6% 5% 4 4 5% 6% 37%	5585
April	1st to 10th	Day	11% 9% 3 9% 4 4 5% 4 6% 7% 38%	2569
	11th to 20th	Day	12% 9% 3 7% 5% 5% 5% 4 5% 6% 39%	2702
	21st to end	Day	13% 8% 3 6% 5% 5% 5% 5% 5% 7% 38%	2730
May	1st to 10th	Day	9% 7% 3 7% 4 4 4 3 5% 7% 48%	8055
	11th to 20th	Day	18% 8% 3 8% 5% 5% 4 4 5% 7% 33%	8634
	21st to end	Day	13% 6% 2 5% 4 4 4 3 5% 6% 50%	9683
June	1st to 10th	Day	15% 7% 2 6% 5% 4 5% 4 6% 7% 38%	3150
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 7% 8% 37%	3076
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 7% 9% 38%	3000
July	1st to 10th	Day	8% 5% 2 5% 4 4 5% 4 7% 9% 46%	6799
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 4 6% 9% 47%	6800
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 6% 8% 49%	7406
August	1st to 10th	Day	9% 4 2 5% 4 4 5% 4 7% 10% 46%	4448
	11th to 20th	Day	10% 5% 1 6% 4 5% 5% 4 6% 7% 47%	4171
	21st to end	Day	15% 6% 2 5% 4 4 4 3 5% 9% 43%	4675
September	1st to 10th	Day	11% 6% 2 6% 3 3 5% 3 5% 7% 46%	2495
	11th to 20th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 33%	2211
	21st to end	Day	15% 8% 2 6% 4 4 4 3 6% 8% 42%	2200
October	1st to 10th	Day	20% 11% 3 9% 4 4 4 3 4 7% 31%	2015
	11th to 20th	Day	16% 15% 4 8% 4 4 5% 4 6% 5% 28%	1971
	21st to end	Day	17% 17% 4 11% 5% 5% 5% 4 5% 7% 20%	1962
November	1st to 10th	Day	16% 15% 5% 13% 6% 5% 4 3 4 5% 23%	1647
	11th to 20th	Day	11% 17% 6% 15% 10% 5% 4 3 4 5% 21%	1680
	21st to end	Day	11% 15% 6% 16% 10% 6% 4 4 4 6% 18%	1538
December	1st to 10th	Day	11% 17% 6% 18% 13% 10% 5% 3 4 4 9%	2360
	11th to 20th	Day	8% 14% 6% 13% 7% 6% 6% 6% 7% 8% 19%	2709
	21st to end	Day	7% 15% 8% 17% 8% 7% 6% 4 6% 6% 16%	3575
January	1st to 10th	Night	5% 14% 7% 16% 7% 8% 8% 6% 6% 5% 17%	8800
	11th to 20th	Night	6% 14% 7% 13% 10% 9% 9% 8% 6% 5% 13%	8803
	21st to end	Night	6% 13% 5% 12% 7% 7% 8% 5% 8% 7% 21%	9188
February	1st to 10th	Night	7% 14% 5% 11% 7% 6% 7% 6% 6% 5% 26%	2819
	11th to 20th	Night	7% 11% 3 9% 6% 4 6% 6% 7% 9% 33%	2663
	21st to end	Night	6% 12% 5% 13% 6% 6% 6% 4 5% 7% 31%	2223
March	1st to 10th	Night	8% 12% 4 11% 5% 5% 6% 4 7% 6% 30%	5328
	11th to 20th	Night	7% 9% 3 8% 5% 5% 6% 5% 7% 9% 38%	5089
	21st to end	Night	11% 12% 4 8% 6% 5% 4 3 5% 6% 37%	5588
April	1st to 10th	Night	11% 8% 3 8% 5% 4 5% 4 6% 7% 41%	2047
	11th to 20th	Night	13% 10% 3 7% 6% 4 4 4 6% 6% 37%	1869
	21st to end	Night	9% 9% 3 9% 6% 6% 6% 4 6% 6% 37%	1890
May	1st to 10th	Night	11% 7% 2 6% 4 4 5% 4 6% 7% 44%	4872
	11th to 20th	Night	18% 7% 3 6% 4 4 4 3 5% 7% 39%	4243
	21st to end	Night	12% 8% 2 6% 3 3 3 3 5% 6% 49%	4526
June	1st to 10th	Night	13% 8% 3 8% 5% 4 5% 4 6% 9% 36%	1470
	11th to 20th	Night	15% 7% 2 5% 4 5% 5% 3 7% 7% 40%	1435
	21st to end	Night	13% 4 2 6% 4 5% 5% 3 7% 11% 40%	1400
July	1st to 10th	Night	9% 4 2 6% 3 4 4 4 6% 9% 50%	3180
	11th to 20th	Night	9% 5% 2 5% 4 4 5% 5% 7% 9% 46%	3180
	21st to end	Night	6% 4 2 5% 4 4 4 4 7% 10% 51%	3469
August	1st to 10th	Night	9% 5% 1 6% 4 5% 6% 4 6% 9% 46%	2708
	11th to 20th	Night	9% 3 1 4 3 3 4 4 8% 9% 51%	2888
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 45%	3234
September	1st to 10th	Night	11% 6% 2 7% 3 4 4 4 6% 7% 46%	1905
	11th to 20th	Night	16% 6% 3 7% 4 4 4 4 5% 7% 41%	2185
	21st to end	Night	16% 6% 3 6% 4 4 4 4 5% 8% 41%	2200
October	1st to 10th	Night	17% 10% 4 6% 4 4 4 3 5% 7% 35%	2365
	11th to 20th	Night	19% 12% 5% 8% 4 4 5% 3 4 6% 30%	2425
	21st to end	Night	17% 15% 5% 10% 5% 4 4 3 5% 6% 25%	2876
November	1st to 10th	Night	14% 17% 5% 10% 7% 5% 4 3 5% 4 25%	2871
	11th to 20th	Night	12% 19% 7% 14% 8% 4 3 3 4 5% 21%	2940
	21st to end	Night	13% 17% 7% 15% 10% 6% 4 3 4 4 17%	3077
December	1st to 10th	Night	9% 19% 7% 19% 13% 7% 4 2 4 4 12%	5050
	11th to 20th	Night	8% 13% 6% 13% 7% 6% 6% 7% 6% 6% 21%	5718
	21st to end	Night	8% 12% 6% 18% 9% 7% 6% 5% 5% 5% 19%	7645

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	69% 3 8% 13% 6%	4125
	11th to 20th	Day	68% 3 9% 16% 4	4140
	21st to end	Day	67% 3 7% 18% 5%	5008
February	1st to 10th	Day	61% 5% 12% 14% 7% 1	1801
	11th to 20th	Day	53% 5% 13% 21% 8%	1853
	21st to end	Day	55% 5% 12% 20% 8% 1	1601
March	1st to 10th	Day	52% 5% 14% 16% 10% 2	4735
	11th to 20th	Day	55% 4 13% 19% 10%	5090
	21st to end	Day	49% 3 10% 26% 11% 1	5585
April	1st to 10th	Day	47% 3 13% 24% 12% 1	2569
	11th to 20th	Day	46% 4 10% 24% 14% 1	2702
	21st to end	Day	44% 5% 14% 26% 9% 1	2730
May	1st to 10th	Day	38% 3 11% 31% 17%	8066
	11th to 20th	Day	51% 3 9% 26% 10% 1	8638
	21st to end	Day	37% 3 8% 27% 21% 3	9683
June	1st to 10th	Day	44% 5% 13% 26% 11% 1	3150
	11th to 20th	Day	45% 7% 14% 22% 11% 1	3076
	21st to end	Day	42% 7% 18% 26% 7%	3000
July	1st to 10th	Day	34% 7% 22% 29% 8%	6726
	11th to 20th	Day	33% 5% 21% 35% 4	6719
	21st to end	Day	32% 5% 22% 34% 6%	7193
August	1st to 10th	Day	33% 6% 20% 33% 8%	4431
	11th to 20th	Day	36% 5% 23% 32% 4	4210
	21st to end	Day	39% 5% 23% 28% 4	4674
September	1st to 10th	Day	37% 6% 18% 32% 7% 1	2495
	11th to 20th	Day	48% 6% 18% 25% 3	2211
	21st to end	Day	42% 4 17% 30% 7%	2200
October	1st to 10th	Day	46% 4 16% 28% 5%	3275
	11th to 20th	Day	46% 4 15% 24% 11%	3189
	21st to end	Day	55% 4 20% 19% 2	3160
November	1st to 10th	Day	62% 3 15% 15% 4 1	1647
	11th to 20th	Day	64% 6% 12% 13% 4	1680
	21st to end	Day	68% 3 15% 11% 2	1538
December	1st to 10th	Day	78% 5% 8% 7% 2	2360
	11th to 20th	Day	61% 4 15% 14% 6%	2710
	21st to end	Day	69% 5% 8% 15% 3	3575
January	1st to 10th	Night	67% 4 8% 14% 7%	8800
	11th to 20th	Night	69% 3 8% 15% 5%	8850
	21st to end	Night	59% 4 10% 20% 8%	9188
February	1st to 10th	Night	56% 4 13% 18% 8% 1	2819
	11th to 20th	Night	44% 5% 18% 24% 9%	2664
	21st to end	Night	54% 4 11% 20% 11%	2223
March	1st to 10th	Night	53% 4 12% 16% 10% 5%	5339
	11th to 20th	Night	41% 5% 13% 25% 14% 1	5090
	21st to end	Night	50% 3 8% 25% 13% 1	5588
April	1st to 10th	Night	42% 4 9% 26% 16% 2	2047
	11th to 20th	Night	46% 4 10% 23% 14% 3	1869
	21st to end	Night	45% 5% 10% 25% 14% 1	1890
May	1st to 10th	Night	38% 4 8% 32% 17%	4881
	11th to 20th	Night	45% 4 11% 29% 11% 1	4244
	21st to end	Night	37% 3 8% 20% 28% 4	4526
June	1st to 10th	Night	43% 6% 15% 24% 11% 1	1470
	11th to 20th	Night	41% 5% 14% 24% 14% 2	1435
	21st to end	Night	36% 7% 20% 28% 9% 1	1400
July	1st to 10th	Night	31% 7% 17% 35% 8% 1	3145
	11th to 20th	Night	33% 5% 19% 37% 5% 1	3138
	21st to end	Night	29% 5% 23% 34% 8%	3368
August	1st to 10th	Night	33% 5% 15% 35% 12%	2701
	11th to 20th	Night	29% 5% 28% 31% 6%	2915
	21st to end	Night	36% 5% 21% 30% 7%	3234
September	1st to 10th	Night	36% 5% 19% 30% 11%	1905
	11th to 20th	Night	42% 5% 20% 24% 8% 1	2185
	21st to end	Night	42% 5% 15% 29% 9% 1	2200
October	1st to 10th	Night	41% 3 15% 30% 11%	3872
	11th to 20th	Night	47% 4 14% 25% 9%	3948
	21st to end	Night	53% 4 16% 24% 3	4638
November	1st to 10th	Night	62% 5% 15% 14% 5% 1	2871
	11th to 20th	Night	66% 4 13% 12% 5%	2940
	21st to end	Night	71% 4 12% 10% 3	3077
December	1st to 10th	Night	77% 4 11% 6% 2	5050
	11th to 20th	Night	61% 3 14% 12% 10%	5716
	21st to end	Night	66% 5% 8% 16% 5%	7645

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	11% 2 32% 34% 18% 4	330
	11th to 20th	Day	11% 4 22% 44% 17% 2	330
	21st to end	Day	14% 5% 24% 37% 16% 3	363
February	1st to 10th	Day	14% 7% 27% 35% 14% 3	330
	11th to 20th	Day	11% 4 25% 34% 20% 6%	330
	21st to end	Day	7% 5% 22% 43% 22% 1	273
March	1st to 10th	Day	12% 5% 25% 32% 22% 4	330
	11th to 20th	Day	11% 5% 23% 38% 19% 3	330
	21st to end	Day	13% 4 23% 34% 22% 4	363
April	1st to 10th	Day	12% 5% 17% 41% 20% 5%	589
	11th to 20th	Day	15% 5% 21% 34% 20% 4	740
	21st to end	Day	14% 4 18% 34% 25% 4	750
May	1st to 10th	Day	11% 4 21% 37% 23% 4	753
	11th to 20th	Day	18% 5% 16% 34% 22% 5%	775
	21st to end	Day	16% 5% 14% 29% 30% 6%	844
June	1st to 10th	Day	16% 5% 18% 34% 22% 6%	780
	11th to 20th	Day	13% 5% 21% 34% 23% 4	756
	21st to end	Day	12% 4 16% 35% 26% 8%	730
July	1st to 10th	Day	9% 3 16% 33% 27% 11%	721
	11th to 20th	Day	11% 4 17% 33% 27% 8%	718
	21st to end	Day	8% 3 15% 35% 31% 8%	794
August	1st to 10th	Day	9% 2 18% 33% 27% 10%	726
	11th to 20th	Day	11% 3 13% 33% 28% 11%	715
	21st to end	Day	13% 5% 17% 31% 27% 7%	803
September	1st to 10th	Day	12% 5% 24% 33% 22% 4	625
	11th to 20th	Day	17% 9% 18% 37% 16% 3	341
	21st to end	Day	17% 5% 22% 34% 19% 2	330
October	1st to 10th	Day	21% 7% 22% 32% 16% 3	330
	11th to 20th	Day	20% 8% 22% 39% 11% 1	330
	21st to end	Day	28% 5% 25% 28% 12% 3	363
November	1st to 10th	Day	20% 8% 27% 29% 14% 1	330
	11th to 20th	Day	18% 7% 33% 28% 11% 2	330
	21st to end	Day	18% 9% 31% 35% 8%	330
December	1st to 10th	Day	15% 5% 27% 35% 14% 3	330
	11th to 20th	Day	15% 7% 35% 32% 9% 2	330
	21st to end	Day	10% 5% 25% 37% 19% 3	363
January	1st to 10th	Night	13% 4 30% 35% 17% 2	1050
	11th to 20th	Night	14% 4 27% 39% 14% 2	1050
	21st to end	Night	14% 5% 27% 39% 15% 1	1155
February	1st to 10th	Night	13% 5% 25% 39% 17% 2	1050
	11th to 20th	Night	10% 3 24% 38% 20% 4	1029
	21st to end	Night	10% 4 29% 36% 19% 2	839
March	1st to 10th	Night	13% 5% 26% 36% 19% 2	776
	11th to 20th	Night	11% 3 30% 41% 13% 2	630
	21st to end	Night	16% 4 25% 33% 17% 4	693
April	1st to 10th	Night	15% 2 23% 41% 16% 3	630
	11th to 20th	Night	16% 4 23% 36% 19% 2	623
	21st to end	Night	12% 4 22% 43% 17% 2	630
May	1st to 10th	Night	18% 4 23% 39% 14% 2	630
	11th to 20th	Night	17% 3 20% 38% 20% 3	621
	21st to end	Night	19% 2 18% 39% 19% 2	683
June	1st to 10th	Night	17% 2 22% 33% 23% 3	630
	11th to 20th	Night	17% 5% 21% 35% 21% 2	615
	21st to end	Night	13% 4 13% 38% 28% 4	600
July	1st to 10th	Night	8% 2 17% 39% 29% 5%	585
	11th to 20th	Night	11% 2 19% 37% 26% 5%	582
	21st to end	Night	9% 2 16% 38% 29% 6%	657
August	1st to 10th	Night	11% 2 17% 40% 27% 3	597
	11th to 20th	Night	9% 3 16% 37% 28% 7%	594
	21st to end	Night	13% 4 19% 36% 25% 3	660
September	1st to 10th	Night	16% 3 24% 37% 17% 4	600
	11th to 20th	Night	17% 5% 23% 39% 14% 2	600
	21st to end	Night	17% 5% 28% 33% 16% 2	600
October	1st to 10th	Night	23% 7% 28% 30% 10% 1	765
	11th to 20th	Night	25% 8% 26% 30% 10% 1	825
	21st to end	Night	26% 8% 25% 30% 9% 1	1100
November	1st to 10th	Night	21% 8% 31% 26% 12% 1	1025
	11th to 20th	Night	21% 7% 32% 30% 8% 2	1050
	21st to end	Night	19% 5% 36% 31% 7% 1	1050
December	1st to 10th	Night	13% 4 29% 36% 15% 3	1050
	11th to 20th	Night	16% 5% 33% 35% 10% 1	1050
	21st to end	Night	16% 4 29% 32% 16% 2	1155

Page generated at 2026-06-13 01:31:41. Version ac07

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