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 2025-02-21.

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% 6% 5% 5% 4 6% 7% 34%	108353
	Night	10% 11% 4 11% 6% 5% 5% 4 6% 6% 30%	110413
Transparency	Day	49% 5% 15% 24% 7%	111788
	Night	52% 4 13% 21% 8%	115138
Seeing	Day	14% 4 20% 34% 22% 6%	16605
	Night	16% 4 25% 36% 17% 3	26731

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	5% 16% 8% 16% 7% 7% 8% 6% 5% 7% 15%	3178
	11th to 20th	Day	4 12% 5% 15% 10% 10% 8% 9% 10% 6% 10%	3152
	21st to end	Day	5% 16% 9% 18% 8% 8% 7% 3 6% 6% 13%	3866
February	1st to 10th	Day	8% 12% 6% 11% 7% 8% 8% 7% 7% 8% 18%	1715
	11th to 20th	Day	7% 10% 5% 10% 6% 7% 7% 5% 7% 10% 26%	1754
	21st to end	Day	6% 12% 5% 12% 7% 6% 7% 5% 8% 6% 27%	1453
March	1st to 10th	Day	8% 12% 4 13% 6% 5% 5% 5% 7% 8% 26%	3221
	11th to 20th	Day	8% 10% 4 12% 8% 6% 6% 5% 6% 6% 30%	3480
	21st to end	Day	8% 11% 4 11% 7% 5% 4 4 5% 5% 36%	3814
April	1st to 10th	Day	11% 9% 3 8% 4 4 5% 4 6% 7% 38%	2323
	11th to 20th	Day	12% 9% 2 6% 5% 5% 5% 4 5% 7% 39%	2442
	21st to end	Day	13% 7% 3 6% 5% 5% 5% 5% 5% 7% 37%	2470
May	1st to 10th	Day	14% 7% 2 6% 4 4 3 3 5% 8% 42%	4326
	11th to 20th	Day	20% 8% 3 7% 4 4 4 4 5% 6% 35%	4629
	21st to end	Day	19% 7% 2 7% 4 4 4 3 4 6% 38%	5173
June	1st to 10th	Day	15% 7% 3 6% 5% 3 5% 4 6% 7% 38%	2850
	11th to 20th	Day	16% 7% 2 6% 4 5% 5% 4 7% 8% 37%	2850
	21st to end	Day	12% 7% 2 7% 5% 5% 5% 4 7% 8% 37%	2850
July	1st to 10th	Day	10% 5% 2 6% 4 4 5% 5% 7% 9% 43%	4750
	11th to 20th	Day	8% 4 2 5% 3 3 4 4 6% 9% 53%	4750
	21st to end	Day	6% 4 2 5% 4 4 5% 4 6% 8% 53%	5151
August	1st to 10th	Day	9% 4 2 5% 4 4 5% 4 7% 10% 45%	4312
	11th to 20th	Day	10% 5% 1 5% 4 5% 5% 4 6% 7% 48%	4041
	21st to end	Day	15% 7% 2 5% 3 4 4 3 5% 9% 43%	4532
September	1st to 10th	Day	11% 6% 2 6% 3 3 5% 3 5% 8% 47%	2370
	11th to 20th	Day	16% 7% 3 7% 5% 5% 4 4 6% 8% 35%	2101
	21st to end	Day	16% 8% 2 6% 4 4 4 3 5% 8% 40%	2090
October	1st to 10th	Day	20% 11% 3 9% 4 4 4 3 4 7% 31%	1914
	11th to 20th	Day	15% 15% 4 7% 4 4 5% 4 6% 5% 29%	1873
	21st to end	Day	16% 18% 4 11% 5% 5% 5% 4 5% 7% 20%	1864
November	1st to 10th	Day	16% 16% 5% 13% 6% 5% 4 3 4 5% 23%	1567
	11th to 20th	Day	11% 17% 6% 16% 10% 6% 4 3 4 4 19%	1600
	21st to end	Day	12% 16% 6% 15% 9% 6% 4 4 4 6% 18%	1465
December	1st to 10th	Day	11% 17% 6% 18% 13% 10% 5% 3 4 4 9%	2290
	11th to 20th	Day	8% 14% 6% 13% 7% 6% 6% 6% 7% 8% 19%	2639
	21st to end	Day	7% 15% 8% 17% 8% 7% 6% 4 6% 7% 16%	3498
January	1st to 10th	Night	5% 13% 7% 16% 7% 9% 9% 6% 6% 5% 17%	6787
	11th to 20th	Night	5% 13% 7% 14% 10% 10% 9% 8% 7% 5% 12%	6753
	21st to end	Night	7% 17% 7% 15% 7% 6% 6% 5% 6% 6% 18%	7094
February	1st to 10th	Night	7% 14% 4 11% 7% 7% 7% 6% 6% 5% 26%	2685
	11th to 20th	Night	7% 10% 4 9% 6% 4 6% 6% 7% 9% 32%	2533
	21st to end	Night	5% 11% 4 13% 6% 6% 6% 4 5% 7% 32%	2018
March	1st to 10th	Night	10% 12% 4 11% 6% 5% 6% 5% 7% 6% 29%	3613
	11th to 20th	Night	5% 8% 3 8% 5% 5% 5% 5% 6% 8% 42%	3479
	21st to end	Night	9% 14% 4 8% 6% 5% 4 3 5% 6% 36%	3817
April	1st to 10th	Night	11% 8% 2 7% 4 4 5% 4 6% 8% 41%	1853
	11th to 20th	Night	13% 10% 3 7% 6% 4 4 4 6% 6% 36%	1689
	21st to end	Night	8% 9% 3 9% 6% 6% 6% 4 6% 6% 37%	1710
May	1st to 10th	Night	18% 9% 3 6% 4 4 4 4 5% 6% 37%	2624
	11th to 20th	Night	19% 6% 3 5% 4 4 3 3 5% 7% 40%	2274
	21st to end	Night	18% 10% 2 6% 4 3 4 3 6% 6% 38%	2414
June	1st to 10th	Night	13% 8% 3 7% 5% 5% 5% 4 7% 9% 36%	1330
	11th to 20th	Night	16% 7% 2 5% 4 5% 5% 3 7% 7% 41%	1330
	21st to end	Night	13% 4 2 5% 4 5% 5% 3 7% 12% 39%	1330
July	1st to 10th	Night	10% 4 2 5% 3 3 4 4 7% 9% 50%	2220
	11th to 20th	Night	8% 4 1 3 4 3 4 4 7% 9% 53%	2220
	21st to end	Night	5% 3 1 4 4 4 4 4 7% 9% 53%	2414
August	1st to 10th	Night	9% 5% 1 6% 4 5% 6% 4 6% 9% 46%	2625
	11th to 20th	Night	9% 3 1 4 3 3 4 4 7% 9% 51%	2798
	21st to end	Night	10% 6% 2 6% 4 4 5% 4 6% 9% 44%	3135
September	1st to 10th	Night	11% 6% 2 7% 3 4 4 4 6% 7% 47%	1810
	11th to 20th	Night	15% 6% 3 7% 4 4 4 4 6% 7% 42%	2076
	21st to end	Night	17% 6% 3 6% 4 4 4 4 5% 7% 40%	2090
October	1st to 10th	Night	18% 10% 4 6% 4 4 4 3 5% 7% 35%	2247
	11th to 20th	Night	18% 12% 5% 7% 4 4 5% 3 4 6% 31%	2304
	21st to end	Night	16% 15% 5% 10% 5% 4 5% 3 5% 6% 26%	2732
November	1st to 10th	Night	15% 17% 5% 10% 7% 5% 4 3 5% 5% 25%	2731
	11th to 20th	Night	12% 19% 7% 15% 8% 4 3 3 4 5% 19%	2800
	21st to end	Night	13% 17% 7% 15% 10% 6% 4 3 4 4 17%	2930
December	1st to 10th	Night	10% 19% 8% 20% 13% 7% 4 2 4 3 11%	4900
	11th to 20th	Night	8% 13% 6% 14% 7% 6% 6% 7% 6% 6% 21%	5568
	21st to end	Night	8% 12% 6% 19% 9% 7% 6% 5% 5% 5% 20%	7480

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	68% 4 9% 13% 6%	3179
	11th to 20th	Day	67% 3 9% 17% 4	3180
	21st to end	Day	73% 4 8% 12% 4	3866
February	1st to 10th	Day	60% 6% 12% 14% 7% 1	1715
	11th to 20th	Day	53% 5% 13% 21% 8%	1763
	21st to end	Day	55% 5% 12% 20% 8%	1453
March	1st to 10th	Day	56% 5% 13% 21% 5%	3223
	11th to 20th	Day	55% 5% 11% 21% 9%	3480
	21st to end	Day	51% 4 11% 26% 7% 1	3814
April	1st to 10th	Day	46% 4 13% 25% 12% 1	2323
	11th to 20th	Day	45% 4 10% 24% 15% 2	2442
	21st to end	Day	44% 5% 14% 26% 9% 1	2470
May	1st to 10th	Day	42% 4 11% 30% 12%	4337
	11th to 20th	Day	50% 4 10% 23% 12% 2	4633
	21st to end	Day	49% 4 9% 25% 11% 1	5173
June	1st to 10th	Day	45% 5% 13% 25% 12% 1	2850
	11th to 20th	Day	45% 7% 14% 21% 12% 1	2850
	21st to end	Day	43% 7% 17% 26% 7%	2850
July	1st to 10th	Day	36% 7% 21% 28% 8%	4676
	11th to 20th	Day	28% 6% 23% 38% 5%	4669
	21st to end	Day	30% 5% 21% 35% 8%	4988
August	1st to 10th	Day	34% 6% 20% 32% 8%	4295
	11th to 20th	Day	35% 5% 22% 32% 4	4080
	21st to end	Day	40% 5% 23% 28% 4	4531
September	1st to 10th	Day	36% 6% 18% 32% 7% 1	2370
	11th to 20th	Day	47% 6% 19% 26% 3	2101
	21st to end	Day	43% 4 16% 29% 7%	2090
October	1st to 10th	Day	46% 4 16% 28% 5%	3174
	11th to 20th	Day	45% 4 15% 25% 11%	3091
	21st to end	Day	55% 4 20% 19% 2	3062
November	1st to 10th	Day	63% 3 15% 15% 3 1	1567
	11th to 20th	Day	66% 6% 12% 13% 3	1600
	21st to end	Day	69% 3 16% 10% 2	1465
December	1st to 10th	Day	78% 5% 8% 7% 2	2290
	11th to 20th	Day	60% 4 15% 15% 6%	2640
	21st to end	Day	68% 5% 8% 15% 3	3498
January	1st to 10th	Night	66% 4 9% 12% 8%	6787
	11th to 20th	Night	69% 3 7% 17% 4	6800
	21st to end	Night	66% 4 11% 14% 6%	7094
February	1st to 10th	Night	56% 4 13% 18% 9% 1	2685
	11th to 20th	Night	44% 5% 18% 23% 9%	2534
	21st to end	Night	52% 4 11% 21% 11%	2018
March	1st to 10th	Night	54% 5% 10% 22% 9%	3624
	11th to 20th	Night	38% 6% 14% 28% 13% 1	3480
	21st to end	Night	50% 3 9% 26% 11% 1	3817
April	1st to 10th	Night	42% 4 10% 25% 17% 3	1853
	11th to 20th	Night	46% 4 9% 22% 15% 3	1689
	21st to end	Night	45% 5% 10% 24% 14% 1	1710
May	1st to 10th	Night	48% 4 8% 26% 14%	2632
	11th to 20th	Night	43% 4 12% 29% 10% 2	2275
	21st to end	Night	47% 4 11% 25% 13% 1	2414
June	1st to 10th	Night	43% 6% 15% 25% 11% 1	1330
	11th to 20th	Night	42% 5% 14% 23% 15% 2	1330
	21st to end	Night	36% 7% 21% 27% 9% 1	1330
July	1st to 10th	Night	29% 8% 18% 35% 9% 1	2185
	11th to 20th	Night	25% 5% 22% 39% 7% 1	2178
	21st to end	Night	27% 5% 22% 37% 9%	2337
August	1st to 10th	Night	33% 5% 15% 35% 12%	2618
	11th to 20th	Night	28% 5% 28% 32% 7%	2825
	21st to end	Night	37% 5% 21% 30% 7%	3135
September	1st to 10th	Night	35% 5% 19% 30% 11%	1810
	11th to 20th	Night	41% 5% 20% 24% 9% 1	2076
	21st to end	Night	43% 5% 15% 28% 9%	2090
October	1st to 10th	Night	41% 3 15% 30% 11%	3754
	11th to 20th	Night	47% 4 14% 26% 9%	3827
	21st to end	Night	52% 4 16% 24% 3	4494
November	1st to 10th	Night	62% 5% 15% 14% 4	2731
	11th to 20th	Night	68% 4 13% 11% 4	2800
	21st to end	Night	71% 4 13% 10% 3	2930
December	1st to 10th	Night	77% 4 11% 6% 2	4900
	11th to 20th	Night	61% 3 14% 12% 10%	5566
	21st to end	Night	66% 5% 8% 16% 5%	7480

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	10% 2 31% 35% 19% 4	300
	11th to 20th	Day	11% 4 21% 43% 19% 2	300
	21st to end	Day	15% 5% 23% 36% 17% 4	330
February	1st to 10th	Day	13% 5% 27% 36% 15% 3	300
	11th to 20th	Day	10% 5% 25% 35% 20% 6%	300
	21st to end	Day	7% 4 22% 42% 24% 1	225
March	1st to 10th	Day	12% 4 22% 32% 25% 4	270
	11th to 20th	Day	11% 3 24% 36% 21% 4	270
	21st to end	Day	13% 3 21% 34% 24% 5%	297
April	1st to 10th	Day	12% 4 17% 41% 21% 5%	503
	11th to 20th	Day	15% 5% 21% 34% 21% 4	640
	21st to end	Day	14% 4 19% 35% 24% 4	650
May	1st to 10th	Day	12% 4 21% 36% 23% 4	653
	11th to 20th	Day	17% 4 16% 33% 23% 6%	675
	21st to end	Day	18% 5% 15% 28% 29% 5%	734
June	1st to 10th	Day	16% 4 18% 33% 22% 7%	680
	11th to 20th	Day	13% 5% 22% 33% 23% 4	680
	21st to end	Day	12% 4 15% 34% 27% 9%	680
July	1st to 10th	Day	9% 3 17% 32% 27% 12%	671
	11th to 20th	Day	11% 3 17% 32% 27% 9%	668
	21st to end	Day	8% 3 16% 34% 31% 9%	739
August	1st to 10th	Day	9% 3 19% 33% 26% 10%	676
	11th to 20th	Day	11% 3 13% 32% 28% 12%	665
	21st to end	Day	13% 5% 17% 31% 26% 7%	748
September	1st to 10th	Day	11% 4 25% 34% 23% 3	580
	11th to 20th	Day	16% 9% 19% 38% 15% 3	311
	21st to end	Day	19% 6% 22% 34% 18% 1	300
October	1st to 10th	Day	21% 7% 22% 30% 17% 3	300
	11th to 20th	Day	17% 9% 23% 39% 12% 1	300
	21st to end	Day	28% 5% 26% 28% 11% 3	330
November	1st to 10th	Day	21% 5% 26% 31% 16% 1	300
	11th to 20th	Day	19% 7% 34% 29% 10% 2	300
	21st to end	Day	19% 9% 30% 34% 7%	300
December	1st to 10th	Day	14% 5% 27% 37% 15% 3	300
	11th to 20th	Day	15% 6% 34% 33% 10% 2	300
	21st to end	Day	10% 5% 23% 37% 21% 4	330
January	1st to 10th	Night	13% 4 29% 35% 18% 2	1000
	11th to 20th	Night	14% 4 27% 39% 15% 2	1000
	21st to end	Night	14% 5% 26% 38% 15% 1	1100
February	1st to 10th	Night	12% 4 24% 40% 17% 2	1000
	11th to 20th	Night	10% 3 24% 39% 20% 4	979
	21st to end	Night	10% 2 29% 36% 21% 2	762
March	1st to 10th	Night	12% 4 24% 37% 20% 2	702
	11th to 20th	Night	12% 2 30% 40% 13% 2	570
	21st to end	Night	16% 4 25% 34% 18% 4	627
April	1st to 10th	Night	16% 2 24% 41% 15% 3	570
	11th to 20th	Night	15% 3 23% 37% 20% 2	563
	21st to end	Night	11% 4 22% 44% 17% 2	570
May	1st to 10th	Night	19% 4 23% 39% 14% 2	570
	11th to 20th	Night	16% 3 20% 37% 21% 4	561
	21st to end	Night	19% 3 19% 38% 19% 2	617
June	1st to 10th	Night	18% 2 22% 33% 24% 3	570
	11th to 20th	Night	17% 5% 20% 33% 22% 2	570
	21st to end	Night	13% 4 12% 38% 28% 4	570
July	1st to 10th	Night	8% 2 17% 39% 28% 6%	555
	11th to 20th	Night	10% 2 19% 37% 26% 5%	552
	21st to end	Night	10% 2 16% 37% 30% 6%	624
August	1st to 10th	Night	11% 2 17% 40% 26% 4	567
	11th to 20th	Night	9% 2 16% 37% 29% 7%	564
	21st to end	Night	13% 4 19% 36% 24% 3	627
September	1st to 10th	Night	16% 3 24% 36% 17% 4	570
	11th to 20th	Night	17% 5% 24% 39% 14% 2	570
	21st to end	Night	17% 5% 28% 33% 15% 2	570
October	1st to 10th	Night	24% 7% 28% 29% 11% 1	727
	11th to 20th	Night	24% 8% 26% 30% 10% 1	784
	21st to end	Night	26% 9% 25% 30% 9% 1	1045
November	1st to 10th	Night	22% 7% 31% 26% 12% 1	975
	11th to 20th	Night	21% 7% 31% 30% 8% 3	1000
	21st to end	Night	20% 5% 35% 31% 8% 1	1000
December	1st to 10th	Night	13% 5% 28% 36% 15% 3	1000
	11th to 20th	Night	15% 5% 33% 35% 11% 1	1000
	21st to end	Night	16% 4 29% 32% 17% 2	1100

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