Sand Island Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sand Island Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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% 6% 2 6% 5% 5% 5% 4 6% 8% 43%	100948
	Night	11% 6% 2 6% 4 4 4 4 6% 8% 44%	98615
Transparency	Day	37% 6% 12% 20% 19% 7%	104802
	Night	36% 6% 11% 18% 20% 9%	102887
Seeing	Day	13% 9% 20% 24% 22% 11%	14556
	Night	13% 7% 21% 31% 23% 5%	24520

By Month

Percentage Cloud Cover SE (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	15% 10% 2 5% 3 3 4 4 6% 6% 41%	2845
	11th to 20th	Day	11% 5% 2 5% 4 4 3 3 5% 6% 51%	3160
	21st to end	Day	20% 11% 3 5% 2 2 3 3 5% 5% 41%	3476
February	1st to 10th	Day	16% 8% 3 6% 5% 3 4 4 6% 6% 38%	1900
	11th to 20th	Day	19% 8% 2 6% 4 4 3 2 5% 4 43%	1848
	21st to end	Day	17% 8% 3 6% 4 5% 5% 2 5% 6% 40%	1570
March	1st to 10th	Day	20% 10% 3 7% 4 4 4 4 6% 6% 32%	3120
	11th to 20th	Day	14% 7% 2 6% 5% 4 5% 4 6% 7% 41%	3281
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 6% 6% 42%	4101
April	1st to 10th	Day	12% 6% 2 5% 5% 4 4 5% 5% 7% 45%	2280
	11th to 20th	Day	11% 7% 2 6% 5% 5% 4 3 5% 6% 45%	2254
	21st to end	Day	7% 4 2 5% 4 5% 5% 2 5% 8% 54%	2280
May	1st to 10th	Day	9% 5% 2 6% 4 4 5% 4 7% 9% 46%	3850
	11th to 20th	Day	14% 6% 2 7% 4 4 4 4 6% 8% 40%	4081
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 7% 10% 46%	4476
June	1st to 10th	Day	8% 6% 3 8% 6% 6% 6% 5% 8% 10% 35%	2470
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 5% 8% 12% 40%	2470
	21st to end	Day	6% 5% 2 6% 7% 6% 6% 5% 9% 12% 36%	2470
July	1st to 10th	Day	6% 5% 2 8% 7% 6% 8% 7% 10% 13% 27%	4120
	11th to 20th	Day	7% 5% 2 6% 5% 7% 8% 6% 9% 11% 34%	4098
	21st to end	Day	7% 6% 3 8% 7% 7% 8% 7% 10% 13% 24%	4118
August	1st to 10th	Day	3 2 1 5% 4 4 6% 5% 7% 10% 53%	3785
	11th to 20th	Day	6% 4 2 5% 4 5% 5% 4 6% 10% 50%	3729
	21st to end	Day	6% 4 2 5% 5% 5% 6% 4 7% 10% 47%	4180
September	1st to 10th	Day	11% 5% 3 8% 6% 6% 5% 4 6% 8% 39%	2257
	11th to 20th	Day	13% 6% 2 7% 5% 5% 5% 4 5% 6% 42%	2090
	21st to end	Day	9% 5% 2 5% 4 4 4 3 5% 7% 51%	2090
October	1st to 10th	Day	8% 5% 2 5% 4 4 4 3 5% 6% 54%	2090
	11th to 20th	Day	9% 5% 2 4 3 3 4 3 5% 6% 55%	2087
	21st to end	Day	11% 5% 2 5% 3 3 3 3 4 6% 55%	2026
November	1st to 10th	Day	11% 5% 2 5% 3 3 3 3 5% 5% 55%	1762
	11th to 20th	Day	12% 7% 2 4 4 3 3 3 4 6% 52%	1800
	21st to end	Day	13% 6% 2 5% 4 4 3 3 5% 7% 48%	1797
December	1st to 10th	Day	14% 8% 3 6% 4 5% 4 3 5% 5% 44%	1710
	11th to 20th	Day	15% 10% 3 7% 4 3 4 3 4 4 42%	2153
	21st to end	Day	15% 10% 3 6% 4 5% 4 3 4 6% 40%	3124
January	1st to 10th	Night	17% 8% 3 5% 4 4 4 3 5% 6% 42%	4097
	11th to 20th	Night	13% 7% 2 6% 4 3 3 2 3 6% 49%	3800
	21st to end	Night	21% 12% 3 6% 3 3 3 2 4 5% 40%	4180
February	1st to 10th	Night	14% 8% 3 6% 5% 4 4 3 5% 6% 41%	2280
	11th to 20th	Night	20% 9% 2 4 3 3 3 2 4 4 46%	2218
	21st to end	Night	16% 8% 3 7% 4 4 3 3 5% 6% 41%	1884
March	1st to 10th	Night	22% 12% 3 7% 4 3 3 3 4 5% 35%	3733
	11th to 20th	Night	12% 7% 2 6% 5% 4 5% 4 6% 8% 43%	3677
	21st to end	Night	14% 7% 2 7% 4 4 4 3 5% 6% 42%	3544
April	1st to 10th	Night	10% 6% 2 5% 5% 3 4 4 5% 9% 46%	1900
	11th to 20th	Night	12% 5% 2 5% 4 4 4 4 7% 8% 47%	1875
	21st to end	Night	7% 3 1 5% 4 3 4 3 6% 8% 56%	1900
May	1st to 10th	Night	8% 6% 2 6% 5% 5% 4 4 6% 8% 47%	3044
	11th to 20th	Night	14% 7% 2 6% 4 5% 4 4 6% 7% 41%	2812
	21st to end	Night	5% 4 2 5% 4 5% 5% 5% 8% 12% 45%	3079
June	1st to 10th	Night	6% 5% 2 6% 6% 6% 6% 6% 9% 12% 35%	1710
	11th to 20th	Night	5% 3 2 4 5% 5% 5% 6% 10% 12% 44%	1710
	21st to end	Night	5% 5% 3 8% 6% 4 5% 5% 8% 10% 38%	1710
July	1st to 10th	Night	7% 7% 3 8% 6% 6% 7% 6% 8% 13% 29%	2840
	11th to 20th	Night	6% 4 2 7% 5% 5% 6% 6% 10% 15% 34%	2858
	21st to end	Night	6% 8% 3 10% 8% 6% 7% 6% 9% 10% 25%	3435
August	1st to 10th	Night	2 2 2 5% 4 5% 5% 5% 8% 13% 49%	3144
	11th to 20th	Night	5% 4 2 5% 4 6% 6% 5% 8% 11% 45%	3100
	21st to end	Night	5% 4 2 6% 5% 5% 6% 5% 8% 9% 45%	3476
September	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 5% 7% 9% 39%	1902
	11th to 20th	Night	12% 5% 2 5% 3 5% 5% 4 7% 8% 43%	2090
	21st to end	Night	7% 5% 2 7% 4 4 4 3 6% 8% 50%	2090
October	1st to 10th	Night	7% 4 2 5% 4 3 4 3 6% 7% 54%	2090
	11th to 20th	Night	9% 4 1 4 3 2 3 3 4 6% 60%	2090
	21st to end	Night	11% 5% 2 5% 3 3 3 2 4 5% 59%	2568
November	1st to 10th	Night	9% 5% 2 5% 3 3 4 3 4 5% 57%	2536
	11th to 20th	Night	13% 5% 2 4 3 3 3 3 4 6% 55%	2600
	21st to end	Night	12% 7% 2 6% 3 4 4 3 4 7% 49%	2599
December	1st to 10th	Night	15% 7% 2 6% 3 3 4 3 5% 6% 47%	2470
	11th to 20th	Night	16% 7% 3 6% 4 3 3 3 4 5% 47%	3042
	21st to end	Night	15% 8% 2 6% 4 3 4 4 5% 7% 40%	4532

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	42% 3 6% 19% 23% 6%	2846
	11th to 20th	Day	33% 4 7% 22% 26% 8%	3160
	21st to end	Day	46% 4 6% 17% 19% 9%	3476
February	1st to 10th	Day	45% 4 7% 16% 20% 8%	1900
	11th to 20th	Day	44% 3 5% 16% 23% 9%	1858
	21st to end	Day	45% 5% 7% 15% 17% 11%	1570
March	1st to 10th	Day	50% 5% 9% 14% 16% 6%	3120
	11th to 20th	Day	41% 5% 7% 20% 19% 8%	3281
	21st to end	Day	42% 5% 10% 16% 19% 8%	4101
April	1st to 10th	Day	38% 5% 9% 19% 19% 9%	2280
	11th to 20th	Day	38% 5% 9% 18% 21% 9%	2254
	21st to end	Day	29% 6% 10% 21% 28% 5%	2280
May	1st to 10th	Day	31% 7% 12% 27% 18% 4	3885
	11th to 20th	Day	41% 7% 13% 24% 15% 1	4085
	21st to end	Day	30% 9% 13% 30% 16% 2	4489
June	1st to 10th	Day	41% 10% 12% 23% 11% 2	2470
	11th to 20th	Day	34% 10% 16% 23% 14% 3	2470
	21st to end	Day	38% 10% 18% 21% 12% 2	2470
July	1st to 10th	Day	43% 10% 28% 13% 5%	4055
	11th to 20th	Day	39% 11% 25% 21% 4	4029
	21st to end	Day	47% 14% 18% 18% 3	3987
August	1st to 10th	Day	24% 8% 14% 31% 23% 1	3773
	11th to 20th	Day	30% 8% 15% 31% 15% 2	3765
	21st to end	Day	32% 7% 20% 22% 16% 2	4179
September	1st to 10th	Day	41% 9% 12% 16% 16% 6%	2257
	11th to 20th	Day	41% 7% 11% 17% 19% 5%	2090
	21st to end	Day	32% 6% 10% 21% 23% 8%	2090
October	1st to 10th	Day	26% 3 6% 12% 29% 24%	3480
	11th to 20th	Day	18% 3 4 16% 27% 31%	3443
	21st to end	Day	22% 2 5% 16% 42% 13%	3316
November	1st to 10th	Day	32% 4 6% 18% 27% 12%	1762
	11th to 20th	Day	33% 4 7% 17% 27% 13%	1800
	21st to end	Day	36% 4 9% 17% 25% 9%	1797
December	1st to 10th	Day	42% 5% 8% 19% 20% 6%	1710
	11th to 20th	Day	45% 4 6% 14% 26% 5%	2150
	21st to end	Day	45% 4 7% 20% 20% 4	3124
January	1st to 10th	Night	42% 3 7% 14% 27% 7%	4097
	11th to 20th	Night	38% 3 5% 20% 24% 10%	3800
	21st to end	Night	49% 2 5% 14% 24% 6%	4180
February	1st to 10th	Night	43% 5% 7% 16% 19% 10%	2280
	11th to 20th	Night	43% 4 5% 15% 21% 12%	2219
	21st to end	Night	44% 6% 7% 16% 16% 12%	1884
March	1st to 10th	Night	52% 4 8% 14% 14% 8%	3733
	11th to 20th	Night	38% 6% 9% 17% 21% 9%	3677
	21st to end	Night	41% 5% 10% 17% 20% 8%	3544
April	1st to 10th	Night	35% 6% 12% 16% 19% 12%	1900
	11th to 20th	Night	35% 5% 9% 18% 22% 11%	1875
	21st to end	Night	26% 6% 10% 23% 26% 8%	1900
May	1st to 10th	Night	32% 8% 14% 20% 21% 5%	3074
	11th to 20th	Night	41% 7% 13% 21% 16% 2	2813
	21st to end	Night	28% 10% 15% 25% 19% 3	3087
June	1st to 10th	Night	36% 12% 17% 18% 14% 2	1710
	11th to 20th	Night	27% 10% 19% 25% 15% 3	1710
	21st to end	Night	36% 9% 20% 17% 13% 4	1710
July	1st to 10th	Night	43% 11% 28% 12% 5% 1	2795
	11th to 20th	Night	36% 11% 28% 21% 4	2804
	21st to end	Night	49% 14% 17% 16% 4	3325
August	1st to 10th	Night	23% 10% 14% 32% 19% 1	3132
	11th to 20th	Night	30% 9% 18% 28% 12% 3	3130
	21st to end	Night	32% 9% 17% 19% 20% 3	3476
September	1st to 10th	Night	39% 11% 13% 15% 15% 7%	1902
	11th to 20th	Night	36% 10% 11% 17% 19% 7%	2090
	21st to end	Night	32% 6% 10% 22% 21% 9%	2090
October	1st to 10th	Night	30% 4 6% 11% 21% 27%	3480
	11th to 20th	Night	16% 3 4 16% 28% 34%	3461
	21st to end	Night	21% 1 4 17% 37% 21%	4228
November	1st to 10th	Night	30% 5% 8% 16% 26% 16%	2536
	11th to 20th	Night	31% 4 5% 18% 28% 14%	2600
	21st to end	Night	36% 4 7% 19% 21% 13%	2599
December	1st to 10th	Night	37% 5% 10% 20% 23% 5%	2470
	11th to 20th	Night	41% 3 6% 13% 30% 7%	3044
	21st to end	Night	42% 4 9% 17% 24% 3	4532

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	21% 26% 34% 16% 3 1	270
	11th to 20th	Day	20% 20% 33% 16% 10% 2	270
	21st to end	Day	19% 27% 33% 18% 2 1	297
February	1st to 10th	Day	22% 23% 32% 17% 5%	270
	11th to 20th	Day	30% 20% 33% 14% 3	270
	21st to end	Day	21% 24% 40% 13% 3	225
March	1st to 10th	Day	20% 22% 36% 15% 6%	270
	11th to 20th	Day	18% 18% 33% 22% 8% 1	339
	21st to end	Day	20% 17% 31% 23% 10%	506
April	1st to 10th	Day	18% 15% 29% 24% 12% 2	460
	11th to 20th	Day	15% 14% 31% 25% 13% 2	452
	21st to end	Day	10% 12% 27% 28% 20% 3	460
May	1st to 10th	Day	9% 11% 24% 29% 22% 5%	463
	11th to 20th	Day	8% 4 22% 31% 22% 13%	487
	21st to end	Day	12% 3 19% 29% 23% 15%	529
June	1st to 10th	Day	10% 4 14% 33% 26% 13%	490
	11th to 20th	Day	8% 3 13% 25% 32% 19%	490
	21st to end	Day	8% 2 12% 27% 30% 21%	490
July	1st to 10th	Day	7% 1 6% 24% 37% 25%	485
	11th to 20th	Day	6% 1 5% 17% 40% 31%	484
	21st to end	Day	5% 1 7% 21% 40% 26%	532
August	1st to 10th	Day	6% 1 8% 20% 34% 31%	486
	11th to 20th	Day	8% 1 5% 21% 35% 29%	479
	21st to end	Day	12% 2 8% 21% 32% 25%	539
September	1st to 10th	Day	10% 2 10% 25% 33% 19%	490
	11th to 20th	Day	15% 2 9% 23% 37% 13%	490
	21st to end	Day	10% 3 13% 30% 30% 15%	490
October	1st to 10th	Day	11% 4 20% 35% 22% 8%	490
	11th to 20th	Day	10% 6% 19% 32% 29% 4	487
	21st to end	Day	18% 12% 18% 29% 16% 6%	330
November	1st to 10th	Day	22% 6% 24% 33% 12% 4	300
	11th to 20th	Day	17% 13% 31% 26% 10% 3	300
	21st to end	Day	12% 16% 33% 26% 11% 2	299
December	1st to 10th	Day	23% 14% 34% 19% 8% 1	270
	11th to 20th	Day	26% 13% 34% 20% 7%	270
	21st to end	Day	23% 17% 31% 21% 7% 1	297
January	1st to 10th	Night	19% 16% 32% 29% 5%	760
	11th to 20th	Night	18% 16% 26% 32% 8%	760
	21st to end	Night	21% 13% 33% 28% 4	836
February	1st to 10th	Night	19% 12% 32% 32% 6%	760
	11th to 20th	Night	24% 14% 28% 28% 5%	743
	21st to end	Night	19% 17% 36% 25% 3	628
March	1st to 10th	Night	16% 15% 32% 29% 8%	760
	11th to 20th	Night	14% 13% 34% 28% 10%	760
	21st to end	Night	15% 11% 32% 30% 11%	679
April	1st to 10th	Night	13% 11% 32% 31% 12%	570
	11th to 20th	Night	14% 9% 30% 30% 16% 1	563
	21st to end	Night	8% 7% 26% 40% 18% 1	570
May	1st to 10th	Night	9% 5% 26% 35% 21% 4	570
	11th to 20th	Night	9% 2 20% 37% 26% 4	561
	21st to end	Night	8% 1 13% 37% 35% 6%	617
June	1st to 10th	Night	9% 1 8% 38% 39% 5%	570
	11th to 20th	Night	5% 1 7% 30% 45% 12%	570
	21st to end	Night	7% 2 9% 28% 43% 12%	570
July	1st to 10th	Night	6% 1 4 24% 46% 19%	555
	11th to 20th	Night	6% 5% 19% 49% 21%	552
	21st to end	Night	4 6% 24% 52% 13%	624
August	1st to 10th	Night	5% 3 22% 50% 19%	567
	11th to 20th	Night	8% 1 5% 24% 47% 15%	564
	21st to end	Night	10% 2 7% 25% 40% 17%	627
September	1st to 10th	Night	12% 2 9% 23% 41% 14%	572
	11th to 20th	Night	14% 1 10% 32% 37% 5%	760
	21st to end	Night	9% 2 13% 31% 39% 6%	760
October	1st to 10th	Night	10% 1 23% 32% 29% 5%	760
	11th to 20th	Night	11% 4 21% 34% 27% 3	760
	21st to end	Night	13% 7% 18% 35% 24% 2	836
November	1st to 10th	Night	10% 7% 22% 38% 21% 3	780
	11th to 20th	Night	15% 8% 23% 38% 15% 1	800
	21st to end	Night	14% 9% 27% 36% 13% 1	800
December	1st to 10th	Night	18% 11% 29% 29% 13% 1	760
	11th to 20th	Night	20% 9% 31% 30% 10%	760
	21st to end	Night	21% 12% 29% 30% 8%	836

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