Sandy Point Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandy Point Observatory clear sky chart from 2005-11-06 to 2025-04-11.

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	13% 6% 2 6% 4 4 4 4 5% 8% 43%	113385
	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	114891
Transparency	Day	37% 7% 12% 22% 18% 4	117024
	Night	37% 6% 10% 22% 19% 5%	119391
Seeing	Day	17% 11% 27% 25% 15% 5%	15690
	Night	17% 10% 29% 30% 13% 1	25439

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% 4 1 4 3 2 3 3 4 4 57%	3898
	11th to 20th	Day	12% 7% 2 5% 3 3 3 2 4 5% 53%	4060
	21st to end	Day	13% 8% 2 7% 4 4 4 4 4 5% 45%	4466
February	1st to 10th	Day	13% 7% 3 5% 4 5% 5% 4 5% 8% 40%	1800
	11th to 20th	Day	21% 7% 2 4 3 4 3 3 4 6% 43%	1861
	21st to end	Day	17% 7% 2 6% 5% 4 4 4 5% 6% 39%	1650
March	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 5% 6% 40%	4877
	11th to 20th	Day	10% 6% 2 5% 3 3 3 3 5% 7% 52%	4980
	21st to end	Day	15% 6% 2 6% 4 4 4 4 5% 6% 45%	5556
April	1st to 10th	Day	12% 6% 2 5% 4 3 4 3 4 7% 50%	2400
	11th to 20th	Day	11% 5% 2 6% 4 4 4 4 4 7% 49%	2254
	21st to end	Day	15% 6% 3 6% 4 4 4 3 5% 7% 44%	2280
May	1st to 10th	Day	15% 7% 3 8% 5% 5% 5% 4 7% 9% 33%	3857
	11th to 20th	Day	21% 6% 2 6% 5% 5% 5% 4 5% 8% 32%	4166
	21st to end	Day	10% 4 2 4 4 4 5% 4 6% 10% 48%	4823
June	1st to 10th	Day	11% 5% 2 5% 4 5% 5% 4 6% 10% 43%	2660
	11th to 20th	Day	11% 6% 1 5% 5% 5% 5% 4 5% 10% 42%	2660
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 6% 8% 11% 41%	2660
July	1st to 10th	Day	6% 4 2 6% 6% 6% 6% 5% 8% 12% 39%	4440
	11th to 20th	Day	12% 6% 2 7% 5% 6% 5% 5% 7% 11% 34%	4128
	21st to end	Day	9% 6% 3 8% 6% 6% 7% 6% 8% 10% 31%	4466
August	1st to 10th	Day	14% 8% 3 8% 5% 5% 6% 5% 8% 10% 26%	4104
	11th to 20th	Day	10% 5% 3 7% 5% 6% 6% 5% 7% 9% 36%	3771
	21st to end	Day	9% 4 2 5% 3 4 5% 4 6% 11% 47%	4180
September	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 6% 9% 42%	2280
	11th to 20th	Day	14% 5% 2 7% 5% 4 4 4 6% 7% 42%	2280
	21st to end	Day	21% 6% 2 7% 6% 4 4 2 5% 6% 36%	2008
October	1st to 10th	Day	16% 7% 2 6% 4 4 3 3 4 6% 46%	1900
	11th to 20th	Day	15% 5% 1 4 3 2 4 3 4 5% 52%	1900
	21st to end	Day	12% 4 2 6% 4 3 3 3 4 6% 53%	2090
November	1st to 10th	Day	13% 6% 2 4 3 3 3 3 4 5% 53%	1903
	11th to 20th	Day	14% 7% 2 5% 3 3 3 2 5% 7% 50%	1800
	21st to end	Day	14% 4 2 5% 3 3 3 3 5% 5% 53%	1677
December	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 5% 6% 54%	2600
	11th to 20th	Day	17% 7% 2 6% 4 2 3 2 4 6% 47%	2990
	21st to end	Day	13% 6% 2 6% 3 4 4 3 5% 7% 48%	3960
January	1st to 10th	Night	13% 6% 3 5% 3 3 3 2 4 5% 53%	6031
	11th to 20th	Night	13% 6% 2 6% 3 3 3 3 3 4 54%	5900
	21st to end	Night	17% 7% 2 5% 3 3 3 3 3 5% 50%	6490
February	1st to 10th	Night	16% 8% 2 5% 4 4 3 3 5% 6% 43%	2600
	11th to 20th	Night	21% 7% 2 5% 4 3 4 3 5% 5% 41%	2422
	21st to end	Night	17% 7% 2 5% 4 3 3 3 4 6% 47%	1980
March	1st to 10th	Night	19% 6% 2 5% 4 3 3 3 4 6% 44%	4972
	11th to 20th	Night	13% 5% 3 5% 3 4 3 3 5% 6% 50%	4980
	21st to end	Night	14% 6% 2 5% 4 4 3 3 5% 6% 48%	5376
April	1st to 10th	Night	12% 7% 3 6% 4 4 4 3 5% 7% 43%	2000
	11th to 20th	Night	13% 6% 2 5% 3 4 3 3 4 8% 49%	1882
	21st to end	Night	15% 6% 1 5% 4 4 5% 3 6% 7% 45%	1900
May	1st to 10th	Night	16% 9% 3 8% 5% 5% 4 4 6% 7% 35%	3052
	11th to 20th	Night	21% 7% 2 6% 4 4 4 3 5% 6% 37%	2727
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 10% 44%	2732
June	1st to 10th	Night	12% 5% 1 5% 4 5% 5% 6% 8% 10% 40%	1520
	11th to 20th	Night	9% 6% 2 6% 5% 5% 4 3 7% 9% 42%	1520
	21st to end	Night	8% 6% 2 6% 6% 4 6% 5% 7% 10% 41%	1520
July	1st to 10th	Night	8% 4 2 6% 5% 6% 6% 6% 9% 11% 38%	2520
	11th to 20th	Night	11% 6% 2 6% 5% 5% 5% 5% 8% 11% 36%	2828
	21st to end	Night	14% 8% 4 8% 7% 6% 5% 5% 7% 8% 30%	3087
August	1st to 10th	Night	17% 8% 3 7% 5% 5% 5% 5% 7% 10% 28%	2825
	11th to 20th	Night	9% 6% 3 7% 5% 5% 6% 4 7% 8% 41%	3024
	21st to end	Night	9% 5% 2 5% 4 4 4 4 7% 9% 48%	3476
September	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 6% 7% 44%	1900
	11th to 20th	Night	13% 5% 3 6% 4 4 4 3 5% 8% 45%	1900
	21st to end	Night	22% 7% 2 7% 4 3 4 4 5% 6% 37%	2166
October	1st to 10th	Night	15% 5% 2 6% 3 3 4 4 5% 6% 47%	2280
	11th to 20th	Night	16% 5% 2 5% 4 3 3 3 4 6% 49%	2280
	21st to end	Night	15% 4 2 4 3 4 3 3 4 6% 53%	2508
November	1st to 10th	Night	14% 6% 2 5% 4 3 3 3 5% 5% 51%	2392
	11th to 20th	Night	16% 5% 2 5% 4 3 4 3 4 6% 48%	2600
	21st to end	Night	12% 5% 2 5% 4 4 4 3 5% 6% 50%	2720
December	1st to 10th	Night	11% 4 2 5% 3 4 4 3 5% 5% 55%	4580
	11th to 20th	Night	18% 7% 3 7% 4 4 4 3 5% 5% 39%	5205
	21st to end	Night	13% 6% 2 6% 4 4 3 3 4 7% 48%	6996

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	32% 3 5% 26% 24% 11%	3899
	11th to 20th	Day	34% 4 6% 19% 29% 8%	4060
	21st to end	Day	42% 5% 8% 26% 17% 3	4466
February	1st to 10th	Day	42% 5% 11% 24% 15% 3	1800
	11th to 20th	Day	42% 5% 7% 21% 20% 5%	1870
	21st to end	Day	44% 6% 11% 16% 17% 5%	1650
March	1st to 10th	Day	44% 5% 7% 18% 19% 7%	4877
	11th to 20th	Day	31% 5% 9% 23% 23% 9%	4980
	21st to end	Day	40% 7% 11% 17% 19% 6%	5556
April	1st to 10th	Day	35% 5% 11% 22% 20% 7%	2400
	11th to 20th	Day	35% 6% 11% 22% 21% 6%	2254
	21st to end	Day	39% 6% 12% 21% 16% 6%	2280
May	1st to 10th	Day	45% 11% 14% 19% 9% 2	3892
	11th to 20th	Day	46% 10% 13% 21% 8% 1	4170
	21st to end	Day	29% 10% 15% 25% 18% 2	4837
June	1st to 10th	Day	35% 9% 16% 24% 15% 2	2660
	11th to 20th	Day	36% 9% 15% 24% 15% 1	2660
	21st to end	Day	35% 10% 17% 22% 15% 2	2660
July	1st to 10th	Day	34% 11% 23% 18% 11% 4	4370
	11th to 20th	Day	42% 10% 23% 19% 6% 1	4058
	21st to end	Day	43% 11% 15% 16% 15% 1	4324
August	1st to 10th	Day	49% 9% 17% 20% 5%	4091
	11th to 20th	Day	40% 9% 14% 24% 12%	3808
	21st to end	Day	30% 8% 14% 28% 18% 1	4180
September	1st to 10th	Day	38% 9% 14% 16% 19% 4	2280
	11th to 20th	Day	40% 6% 11% 21% 18% 3	2280
	21st to end	Day	50% 6% 8% 19% 15% 3	2008
October	1st to 10th	Day	36% 5% 8% 20% 26% 5%	3160
	11th to 20th	Day	24% 4 6% 20% 37% 10%	3140
	21st to end	Day	22% 3 8% 28% 27% 12%	3428
November	1st to 10th	Day	34% 5% 9% 25% 24% 4	1903
	11th to 20th	Day	36% 4 10% 20% 23% 6%	1800
	21st to end	Day	33% 5% 8% 26% 23% 5%	1677
December	1st to 10th	Day	32% 4 8% 28% 21% 6%	2600
	11th to 20th	Day	41% 4 12% 22% 17% 5%	2986
	21st to end	Day	36% 5% 12% 29% 17% 2	3960
January	1st to 10th	Night	34% 4 6% 20% 26% 10%	6031
	11th to 20th	Night	35% 4 5% 22% 28% 6%	5900
	21st to end	Night	38% 4 6% 26% 21% 5%	6490
February	1st to 10th	Night	42% 5% 11% 22% 16% 4	2600
	11th to 20th	Night	45% 4 7% 20% 18% 6%	2424
	21st to end	Night	40% 4 9% 21% 19% 8%	1980
March	1st to 10th	Night	41% 5% 7% 18% 21% 7%	4972
	11th to 20th	Night	34% 5% 8% 19% 23% 10%	4980
	21st to end	Night	38% 4 9% 24% 18% 7%	5376
April	1st to 10th	Night	39% 7% 10% 18% 21% 6%	2000
	11th to 20th	Night	35% 5% 11% 23% 21% 6%	1882
	21st to end	Night	37% 7% 12% 23% 17% 4	1900
May	1st to 10th	Night	46% 10% 14% 18% 11% 1	3082
	11th to 20th	Night	45% 10% 11% 25% 9% 1	2728
	21st to end	Night	33% 11% 16% 23% 16% 1	2739
June	1st to 10th	Night	35% 12% 15% 22% 15% 1	1520
	11th to 20th	Night	37% 10% 16% 22% 14% 1	1520
	21st to end	Night	36% 10% 13% 22% 16% 4	1520
July	1st to 10th	Night	37% 14% 16% 20% 10% 3	2480
	11th to 20th	Night	40% 11% 22% 22% 5% 1	2775
	21st to end	Night	50% 10% 11% 17% 12%	2988
August	1st to 10th	Night	48% 9% 18% 20% 5%	2814
	11th to 20th	Night	38% 9% 17% 23% 12%	3053
	21st to end	Night	31% 8% 11% 25% 23% 1	3476
September	1st to 10th	Night	37% 9% 15% 17% 17% 5%	1900
	11th to 20th	Night	37% 8% 12% 20% 21% 2	1900
	21st to end	Night	47% 6% 10% 17% 16% 3	2166
October	1st to 10th	Night	37% 6% 11% 20% 22% 4	3800
	11th to 20th	Night	26% 3 7% 21% 30% 14%	3779
	21st to end	Night	21% 3 6% 31% 27% 12%	4120
November	1st to 10th	Night	36% 4 10% 23% 23% 4	2392
	11th to 20th	Night	39% 4 8% 19% 21% 10%	2600
	21st to end	Night	34% 5% 8% 26% 23% 5%	2720
December	1st to 10th	Night	31% 4 8% 27% 25% 5%	4580
	11th to 20th	Night	47% 5% 10% 17% 15% 6%	5208
	21st to end	Night	36% 5% 11% 29% 16% 1	6996

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	24% 24% 33% 15% 4	267
	11th to 20th	Day	21% 29% 35% 14% 1	300
	21st to end	Day	18% 22% 43% 15% 2	330
February	1st to 10th	Day	19% 26% 38% 13% 4	300
	11th to 20th	Day	32% 22% 33% 11% 2	409
	21st to end	Day	25% 20% 35% 15% 5%	414
March	1st to 10th	Day	23% 20% 36% 16% 5%	500
	11th to 20th	Day	23% 21% 32% 18% 5% 1	500
	21st to end	Day	21% 19% 37% 17% 5% 1	550
April	1st to 10th	Day	21% 19% 35% 16% 7% 1	500
	11th to 20th	Day	14% 16% 33% 25% 10% 3	452
	21st to end	Day	18% 14% 35% 25% 6% 2	460
May	1st to 10th	Day	16% 15% 35% 22% 10% 2	463
	11th to 20th	Day	21% 8% 28% 27% 13% 3	487
	21st to end	Day	19% 4 24% 28% 20% 6%	529
June	1st to 10th	Day	12% 6% 25% 31% 21% 5%	490
	11th to 20th	Day	12% 7% 20% 33% 22% 7%	490
	21st to end	Day	11% 4 21% 37% 22% 5%	490
July	1st to 10th	Day	9% 1 14% 35% 30% 10%	485
	11th to 20th	Day	7% 2 14% 34% 32% 10%	484
	21st to end	Day	8% 4 19% 34% 27% 8%	532
August	1st to 10th	Day	12% 3 20% 34% 23% 9%	486
	11th to 20th	Day	13% 2 18% 32% 25% 10%	479
	21st to end	Day	9% 3 18% 29% 28% 14%	539
September	1st to 10th	Day	15% 3 19% 26% 22% 15%	490
	11th to 20th	Day	15% 5% 18% 31% 19% 12%	490
	21st to end	Day	24% 9% 24% 25% 14% 5%	490
October	1st to 10th	Day	19% 5% 22% 29% 17% 8%	490
	11th to 20th	Day	15% 13% 23% 30% 16% 3	490
	21st to end	Day	15% 12% 30% 25% 13% 5%	539
November	1st to 10th	Day	17% 9% 31% 26% 13% 4	468
	11th to 20th	Day	18% 17% 31% 25% 10% 1	400
	21st to end	Day	18% 18% 32% 21% 11% 1	277
December	1st to 10th	Day	23% 21% 32% 18% 7% 1	200
	11th to 20th	Day	24% 14% 33% 22% 8%	200
	21st to end	Day	22% 16% 31% 18% 11% 1	220
January	1st to 10th	Night	19% 19% 38% 21% 3	800
	11th to 20th	Night	20% 18% 33% 24% 5%	800
	21st to end	Night	20% 14% 40% 25% 1	880
February	1st to 10th	Night	21% 13% 41% 23% 3	800
	11th to 20th	Night	27% 18% 38% 16% 2	783
	21st to end	Night	22% 17% 38% 22% 2	660
March	1st to 10th	Night	19% 16% 36% 25% 4	800
	11th to 20th	Night	20% 16% 36% 23% 5% 1	800
	21st to end	Night	19% 14% 41% 22% 4	880
April	1st to 10th	Night	17% 19% 39% 23% 3	800
	11th to 20th	Night	16% 12% 36% 26% 9%	754
	21st to end	Night	18% 9% 36% 31% 6%	760
May	1st to 10th	Night	17% 11% 35% 29% 7% 1	721
	11th to 20th	Night	15% 6% 31% 35% 12% 1	515
	21st to end	Night	15% 2 21% 40% 20% 1	411
June	1st to 10th	Night	12% 3 20% 45% 18% 2	380
	11th to 20th	Night	11% 2 21% 46% 19% 2	380
	21st to end	Night	10% 3 20% 47% 19% 1	380
July	1st to 10th	Night	10% 2 15% 45% 26% 2	370
	11th to 20th	Night	6% 2 13% 40% 36% 4	541
	21st to end	Night	8% 3 19% 42% 24% 4	624
August	1st to 10th	Night	9% 2 15% 43% 27% 4	567
	11th to 20th	Night	11% 2 17% 41% 26% 4	701
	21st to end	Night	11% 2 14% 37% 31% 5%	836
September	1st to 10th	Night	13% 2 16% 37% 27% 4	760
	11th to 20th	Night	13% 4 18% 32% 30% 4	760
	21st to end	Night	25% 6% 19% 32% 17% 1	760
October	1st to 10th	Night	17% 4 22% 28% 24% 4	760
	11th to 20th	Night	16% 9% 25% 33% 16% 1	760
	21st to end	Night	17% 8% 28% 30% 15% 2	836
November	1st to 10th	Night	17% 7% 28% 32% 13% 2	780
	11th to 20th	Night	19% 10% 31% 30% 10%	800
	21st to end	Night	13% 14% 30% 30% 11% 1	800
December	1st to 10th	Night	21% 13% 33% 25% 8% 1	800
	11th to 20th	Night	19% 10% 37% 29% 6%	800
	21st to end	Night	19% 13% 32% 30% 6%	880

Page generated at 2025-04-08 09:17:30. Version ac07

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