Sandy
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandy clear sky chart from 2005-11-06 to 2024-01-16.

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	7% 9% 3 6% 5% 3 4 4 5% 7% 46%	74547
	Night	8% 9% 3 6% 5% 3 4 4 5% 7% 47%	75261
Transparency	Day	33% 7% 11% 19% 19% 11%	74272
	Night	34% 7% 12% 20% 18% 10%	75057
Seeing	Day	13% 18% 34% 26% 8% 1	13790
	Night	12% 9% 24% 33% 18% 3	24354

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	14% 16% 3 7% 5% 3 5% 4 5% 7% 32%	1708
	11th to 20th	Day	9% 15% 7% 9% 7% 3 3 3 4 8% 33%	1665
	21st to end	Day	9% 17% 7% 8% 6% 2 4 4 5% 7% 31%	1782
February	1st to 10th	Day	10% 15% 6% 10% 7% 3 5% 3 4 5% 32%	1620
	11th to 20th	Day	9% 16% 6% 9% 7% 3 5% 4 5% 6% 32%	1642
	21st to end	Day	7% 19% 6% 11% 6% 3 4 4 5% 7% 28%	1480
March	1st to 10th	Day	7% 14% 5% 9% 5% 3 5% 5% 6% 7% 34%	1926
	11th to 20th	Day	7% 12% 5% 8% 6% 3 5% 6% 7% 8% 36%	1980
	21st to end	Day	10% 16% 5% 11% 7% 3 4 4 5% 6% 28%	2178
April	1st to 10th	Day	10% 13% 4 7% 6% 3 4 5% 4 7% 37%	1995
	11th to 20th	Day	9% 14% 4 8% 6% 3 5% 4 5% 5% 38%	2268
	21st to end	Day	9% 11% 4 7% 5% 3 3 4 5% 6% 43%	2340
May	1st to 10th	Day	8% 8% 4 5% 6% 3 5% 6% 7% 8% 39%	2340
	11th to 20th	Day	6% 7% 2 7% 5% 3 5% 5% 8% 10% 42%	2301
	21st to end	Day	8% 8% 2 6% 6% 4 6% 6% 7% 8% 39%	2530
June	1st to 10th	Day	4 4 2 5% 4 3 4 5% 6% 8% 52%	2361
	11th to 20th	Day	5% 5% 2 3 4 2 3 4 5% 6% 60%	2483
	21st to end	Day	3 2 1 3 3 3 3 4 4 5% 70%	2340
July	1st to 10th	Day	2 3 1 3 4 3 4 4 6% 8% 63%	2340
	11th to 20th	Day	5% 3 2 4 4 3 5% 6% 5% 7% 56%	2340
	21st to end	Day	4 3 1 4 6% 3 6% 6% 7% 7% 53%	2545
August	1st to 10th	Day	4 3 1 4 5% 3 4 6% 6% 8% 56%	2328
	11th to 20th	Day	3 3 1 3 3 3 4 5% 6% 7% 61%	2299
	21st to end	Day	4 4 2 4 3 2 4 4 6% 7% 59%	2556
September	1st to 10th	Day	3 3 1 3 2 2 4 4 4 5% 69%	2030
	11th to 20th	Day	4 3 1 4 4 3 4 5% 4 6% 62%	1980
	21st to end	Day	7% 3 2 3 3 1 2 3 4 5% 66%	1980
October	1st to 10th	Day	9% 7% 3 4 5% 2 4 4 4 4 53%	1894
	11th to 20th	Day	7% 6% 3 4 4 2 3 3 4 7% 58%	1800
	21st to end	Day	7% 8% 2 5% 3 3 5% 4 5% 5% 54%	1976
November	1st to 10th	Day	10% 9% 3 6% 5% 3 4 3 4 5% 46%	1674
	11th to 20th	Day	13% 9% 3 6% 5% 3 4 4 4 6% 44%	1710
	21st to end	Day	8% 13% 5% 7% 6% 2 4 3 4 5% 42%	1588
December	1st to 10th	Day	11% 12% 5% 7% 4 2 3 4 5% 7% 40%	1520
	11th to 20th	Day	11% 13% 5% 9% 6% 4 4 5% 6% 8% 28%	1926
	21st to end	Day	8% 13% 5% 9% 7% 3 5% 5% 5% 7% 33%	3122
January	1st to 10th	Night	13% 11% 5% 8% 5% 3 3 5% 5% 8% 34%	2457
	11th to 20th	Night	10% 14% 4 8% 5% 3 4 4 5% 8% 38%	2417
	21st to end	Night	8% 16% 5% 7% 6% 2 4 4 5% 7% 36%	2574
February	1st to 10th	Night	9% 13% 4 9% 6% 3 4 5% 4 6% 36%	2340
	11th to 20th	Night	8% 13% 4 8% 6% 3 4 3 4 6% 40%	2204
	21st to end	Night	7% 13% 6% 9% 7% 3 5% 4 5% 7% 35%	1776
March	1st to 10th	Night	8% 14% 7% 9% 5% 4 4 5% 4 6% 34%	2034
	11th to 20th	Night	7% 11% 4 5% 4 3 6% 5% 6% 8% 41%	1980
	21st to end	Night	8% 15% 4 7% 6% 3 4 5% 6% 7% 36%	2178
April	1st to 10th	Night	13% 12% 3 5% 4 3 3 4 4 7% 42%	1963
	11th to 20th	Night	7% 10% 3 6% 5% 3 5% 5% 6% 7% 43%	1636
	21st to end	Night	10% 9% 3 6% 5% 4 3 5% 4 5% 46%	1620
May	1st to 10th	Night	8% 8% 3 5% 5% 3 6% 6% 6% 7% 42%	1620
	11th to 20th	Night	7% 6% 2 5% 5% 3 4 4 6% 7% 50%	1594
	21st to end	Night	10% 6% 2 5% 5% 3 4 5% 7% 8% 45%	1746
June	1st to 10th	Night	7% 4 2 4 4 3 4 4 5% 8% 54%	1584
	11th to 20th	Night	5% 5% 2 2 3 2 3 4 4 6% 64%	1460
	21st to end	Night	3 4 1 2 3 2 2 4 6% 7% 65%	1620
July	1st to 10th	Night	4 3 2 5% 6% 3 4 5% 5% 7% 56%	1620
	11th to 20th	Night	4 4 2 6% 6% 3 5% 5% 6% 7% 53%	1620
	21st to end	Night	3 4 2 5% 4 3 5% 5% 7% 8% 52%	1773
August	1st to 10th	Night	4 3 1 5% 4 3 5% 5% 6% 8% 56%	1611
	11th to 20th	Night	3 3 2 5% 5% 3 6% 5% 5% 7% 58%	1602
	21st to end	Night	3 4 1 3 4 3 4 5% 6% 8% 59%	1800
September	1st to 10th	Night	3 3 1 4 4 2 4 3 3 5% 67%	1926
	11th to 20th	Night	5% 3 1 3 3 3 3 3 5% 7% 64%	1980
	21st to end	Night	6% 3 2 4 3 1 3 3 3 4 69%	1980
October	1st to 10th	Night	7% 5% 2 4 4 2 5% 3 4 6% 57%	2065
	11th to 20th	Night	7% 7% 3 4 4 2 2 3 3 5% 60%	2160
	21st to end	Night	7% 7% 2 4 4 2 4 3 5% 5% 57%	2376
November	1st to 10th	Night	10% 9% 3 4 3 2 3 4 6% 7% 48%	2405
	11th to 20th	Night	11% 7% 3 5% 4 2 3 4 5% 8% 48%	2470
	21st to end	Night	9% 10% 4 6% 5% 2 4 3 5% 7% 46%	2568
December	1st to 10th	Night	11% 11% 4 6% 4 2 3 3 5% 7% 43%	2660
	11th to 20th	Night	13% 12% 4 7% 6% 3 4 4 5% 8% 34%	3271
	21st to end	Night	9% 12% 5% 10% 7% 3 4 5% 4 7% 33%	4571

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	50% 10% 17% 18% 4 1	1709
	11th to 20th	Day	49% 8% 16% 17% 8% 2	1665
	21st to end	Day	52% 11% 16% 14% 5% 2	1782
February	1st to 10th	Day	49% 9% 16% 15% 9% 1	1620
	11th to 20th	Day	48% 9% 12% 19% 11% 1	1651
	21st to end	Day	50% 9% 13% 18% 9% 1	1480
March	1st to 10th	Day	41% 12% 14% 22% 9% 2	1926
	11th to 20th	Day	40% 10% 13% 23% 12% 2	1980
	21st to end	Day	50% 9% 15% 16% 8% 2	2178
April	1st to 10th	Day	42% 8% 12% 17% 15% 5%	1995
	11th to 20th	Day	41% 8% 12% 21% 16% 2	2271
	21st to end	Day	35% 8% 11% 24% 20% 2	2340
May	1st to 10th	Day	33% 9% 11% 22% 20% 4	2340
	11th to 20th	Day	30% 9% 9% 25% 22% 6%	2304
	21st to end	Day	34% 9% 11% 23% 18% 4	2530
June	1st to 10th	Day	24% 7% 8% 22% 29% 11%	2361
	11th to 20th	Day	21% 5% 7% 18% 28% 20%	2483
	21st to end	Day	15% 3 4 11% 30% 36%	2340
July	1st to 10th	Day	17% 5% 6% 12% 26% 34%	2276
	11th to 20th	Day	21% 6% 7% 12% 24% 30%	2271
	21st to end	Day	23% 7% 9% 13% 22% 27%	2404
August	1st to 10th	Day	22% 6% 7% 17% 22% 26%	2313
	11th to 20th	Day	19% 5% 7% 11% 30% 28%	2299
	21st to end	Day	21% 4 6% 16% 32% 20%	2556
September	1st to 10th	Day	15% 5% 5% 15% 28% 30%	2030
	11th to 20th	Day	19% 7% 8% 19% 25% 22%	1980
	21st to end	Day	19% 4 7% 20% 33% 18%	1980
October	1st to 10th	Day	30% 7% 7% 20% 28% 8%	1894
	11th to 20th	Day	26% 5% 8% 30% 25% 6%	1800
	21st to end	Day	28% 7% 15% 27% 20% 3	1976
November	1st to 10th	Day	37% 7% 13% 21% 18% 4	1674
	11th to 20th	Day	37% 7% 14% 27% 15% 1	1710
	21st to end	Day	42% 7% 15% 22% 12% 2	1588
December	1st to 10th	Day	42% 9% 17% 21% 8% 2	1520
	11th to 20th	Day	51% 8% 15% 18% 8%	1924
	21st to end	Day	47% 9% 18% 14% 11%	3122
January	1st to 10th	Night	46% 9% 19% 19% 6% 1	2457
	11th to 20th	Night	45% 9% 17% 19% 8% 2	2417
	21st to end	Night	47% 8% 18% 18% 7% 2	2574
February	1st to 10th	Night	46% 8% 18% 20% 8% 1	2340
	11th to 20th	Night	42% 7% 13% 20% 16% 1	2205
	21st to end	Night	44% 7% 14% 22% 11% 2	1776
March	1st to 10th	Night	46% 8% 11% 22% 12%	2034
	11th to 20th	Night	35% 9% 12% 25% 17% 3	1980
	21st to end	Night	42% 9% 14% 23% 10% 3	2178
April	1st to 10th	Night	39% 6% 12% 20% 16% 7%	1963
	11th to 20th	Night	35% 6% 12% 22% 21% 3	1636
	21st to end	Night	36% 7% 12% 21% 23% 2	1620
May	1st to 10th	Night	34% 8% 11% 23% 21% 3	1620
	11th to 20th	Night	30% 5% 9% 26% 21% 8%	1595
	21st to end	Night	30% 7% 14% 27% 18% 4	1746
June	1st to 10th	Night	25% 6% 9% 20% 29% 11%	1584
	11th to 20th	Night	19% 5% 7% 19% 29% 22%	1460
	21st to end	Night	16% 4 4 10% 27% 38%	1620
July	1st to 10th	Night	24% 4 6% 14% 17% 35%	1575
	11th to 20th	Night	26% 5% 6% 14% 19% 29%	1566
	21st to end	Night	26% 5% 10% 15% 19% 25%	1674
August	1st to 10th	Night	22% 7% 9% 16% 21% 25%	1602
	11th to 20th	Night	22% 6% 7% 11% 26% 28%	1602
	21st to end	Night	20% 5% 7% 14% 29% 24%	1800
September	1st to 10th	Night	19% 5% 5% 14% 25% 33%	1926
	11th to 20th	Night	19% 4 7% 17% 27% 25%	1980
	21st to end	Night	20% 4 4 17% 33% 23%	1980
October	1st to 10th	Night	26% 6% 8% 21% 29% 10%	2065
	11th to 20th	Night	26% 4 6% 25% 29% 8%	2160
	21st to end	Night	25% 6% 11% 27% 26% 5%	2376
November	1st to 10th	Night	32% 6% 11% 25% 21% 6%	2405
	11th to 20th	Night	31% 8% 14% 27% 17% 3	2470
	21st to end	Night	36% 7% 14% 25% 15% 4	2568
December	1st to 10th	Night	39% 7% 16% 25% 10% 3	2660
	11th to 20th	Night	47% 7% 17% 20% 10%	3272
	21st to end	Night	48% 9% 18% 14% 10% 1	4571

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	28% 16% 23% 22% 11%	270
	11th to 20th	Day	17% 24% 29% 19% 9% 2	255
	21st to end	Day	18% 20% 30% 21% 9% 1	264
February	1st to 10th	Day	21% 24% 33% 15% 6%	240
	11th to 20th	Day	16% 23% 36% 17% 6% 1	307
	21st to end	Day	16% 28% 36% 16% 4	346
March	1st to 10th	Day	18% 22% 38% 16% 5% 1	420
	11th to 20th	Day	17% 20% 36% 22% 5%	420
	21st to end	Day	23% 25% 34% 17% 2	462
April	1st to 10th	Day	18% 25% 32% 21% 4 1	420
	11th to 20th	Day	23% 28% 28% 16% 5%	412
	21st to end	Day	19% 26% 25% 22% 7% 1	420
May	1st to 10th	Day	14% 25% 33% 20% 7% 1	423
	11th to 20th	Day	13% 23% 33% 22% 8% 2	448
	21st to end	Day	11% 20% 32% 25% 9% 2	485
June	1st to 10th	Day	6% 18% 38% 30% 6% 2	471
	11th to 20th	Day	9% 18% 31% 28% 13% 1	593
	21st to end	Day	5% 17% 38% 33% 5% 1	450
July	1st to 10th	Day	2 11% 44% 34% 8% 1	446
	11th to 20th	Day	5% 13% 42% 31% 7% 2	444
	21st to end	Day	3 5% 44% 35% 11% 2	487
August	1st to 10th	Day	7% 9% 41% 33% 8% 1	447
	11th to 20th	Day	5% 9% 35% 40% 10% 3	437
	21st to end	Day	5% 11% 35% 38% 8% 3	495
September	1st to 10th	Day	6% 10% 34% 36% 12% 2	450
	11th to 20th	Day	7% 16% 32% 34% 9% 1	450
	21st to end	Day	10% 15% 29% 35% 10% 1	450
October	1st to 10th	Day	12% 21% 30% 26% 11%	364
	11th to 20th	Day	10% 17% 30% 27% 14% 1	270
	21st to end	Day	11% 22% 36% 25% 7%	297
November	1st to 10th	Day	20% 21% 26% 26% 6% 1	270
	11th to 20th	Day	17% 19% 34% 18% 10% 3	270
	21st to end	Day	17% 14% 26% 30% 11% 2	270
December	1st to 10th	Day	21% 13% 31% 24% 9% 2	270
	11th to 20th	Day	20% 13% 36% 23% 7% 1	270
	21st to end	Day	24% 18% 28% 26% 5%	297
January	1st to 10th	Night	19% 9% 25% 27% 17% 4	760
	11th to 20th	Night	16% 10% 26% 29% 14% 5%	744
	21st to end	Night	17% 9% 22% 31% 16% 4	792
February	1st to 10th	Night	15% 11% 30% 30% 11% 3	720
	11th to 20th	Night	14% 11% 26% 32% 14% 2	704
	21st to end	Night	16% 12% 30% 28% 13% 1	592
March	1st to 10th	Night	16% 14% 29% 28% 10% 4	720
	11th to 20th	Night	12% 12% 25% 32% 15% 4	720
	21st to end	Night	14% 17% 26% 30% 10% 2	792
April	1st to 10th	Night	19% 13% 27% 28% 11% 2	703
	11th to 20th	Night	13% 13% 28% 33% 12% 2	533
	21st to end	Night	14% 11% 23% 34% 15% 3	540
May	1st to 10th	Night	11% 10% 30% 34% 14% 2	540
	11th to 20th	Night	10% 9% 22% 34% 21% 5%	531
	21st to end	Night	11% 10% 24% 31% 20% 4	584
June	1st to 10th	Night	7% 12% 21% 36% 21% 2	540
	11th to 20th	Night	6% 11% 25% 39% 16% 2	540
	21st to end	Night	5% 5% 24% 39% 22% 5%	540
July	1st to 10th	Night	3 9% 20% 34% 31% 3	525
	11th to 20th	Night	5% 4 23% 42% 22% 4	522
	21st to end	Night	4 4 15% 40% 35% 2	591
August	1st to 10th	Night	5% 4 22% 40% 26% 2	537
	11th to 20th	Night	4 3 19% 42% 28% 4	534
	21st to end	Night	4 5% 24% 42% 23% 3	594
September	1st to 10th	Night	5% 6% 17% 43% 25% 5%	666
	11th to 20th	Night	6% 6% 20% 36% 26% 5%	720
	21st to end	Night	8% 7% 20% 30% 26% 9%	720
October	1st to 10th	Night	9% 8% 26% 32% 20% 5%	720
	11th to 20th	Night	10% 6% 22% 35% 23% 5%	720
	21st to end	Night	12% 9% 26% 30% 19% 5%	792
November	1st to 10th	Night	15% 11% 21% 30% 18% 4	740
	11th to 20th	Night	14% 9% 28% 28% 16% 5%	760
	21st to end	Night	14% 10% 25% 31% 16% 4	858
December	1st to 10th	Night	17% 11% 25% 31% 15% 1	950
	11th to 20th	Night	21% 10% 27% 30% 11% 2	950
	21st to end	Night	23% 12% 24% 31% 8% 1	860

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