Sandlot Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sandlot Observatory clear sky chart from 2005-11-06 to 2026-08-30.

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% 3 6% 4 4 4 3 5% 7% 47%	143119
	Night	10% 6% 2 6% 4 4 4 3 5% 6% 50%	139466
Transparency	Day	36% 4 11% 27% 19% 4	146743
	Night	34% 4 11% 27% 20% 4	143896
Seeing	Day	14% 9% 30% 29% 15% 3	18196
	Night	13% 9% 33% 32% 11% 1	27940

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	13% 9% 3 8% 4 4 3 3 4 6% 43%	5289
	11th to 20th	Day	5% 7% 3 9% 6% 7% 4 3 5% 6% 45%	5310
	21st to end	Day	9% 11% 6% 8% 4 4 4 3 3 5% 44%	5812
February	1st to 10th	Day	12% 11% 4 8% 4 4 3 2 4 7% 41%	1890
	11th to 20th	Day	9% 9% 4 8% 6% 5% 4 3 4 6% 42%	1906
	21st to end	Day	9% 9% 3 6% 3 3 4 4 5% 6% 49%	1750
March	1st to 10th	Day	10% 8% 2 5% 4 3 3 2 3 6% 53%	5046
	11th to 20th	Day	9% 6% 2 5% 3 3 3 3 4 6% 57%	5090
	21st to end	Day	11% 9% 5% 8% 4 5% 4 3 5% 8% 38%	5583
April	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 5% 7% 47%	2368
	11th to 20th	Day	13% 7% 3 6% 4 4 5% 4 4 7% 43%	2514
	21st to end	Day	18% 7% 3 7% 5% 4 4 3 5% 6% 38%	2730
May	1st to 10th	Day	16% 7% 2 6% 4 4 4 3 5% 6% 45%	7617
	11th to 20th	Day	11% 5% 2 5% 3 3 4 3 5% 7% 50%	7627
	21st to end	Day	20% 6% 2 6% 4 4 4 3 6% 7% 37%	8392
June	1st to 10th	Day	7% 3 1 5% 4 5% 4 5% 8% 9% 49%	2730
	11th to 20th	Day	8% 5% 2 5% 4 3 5% 4 6% 7% 50%	2730
	21st to end	Day	7% 4 1 5% 4 4 4 4 5% 9% 51%	2730
July	1st to 10th	Day	8% 5% 2 6% 4 4 6% 4 7% 9% 46%	7680
	11th to 20th	Day	7% 4 1 5% 3 4 4 4 6% 9% 53%	7680
	21st to end	Day	9% 5% 2 7% 4 5% 6% 5% 7% 9% 41%	8337
August	1st to 10th	Day	6% 4 2 5% 4 4 5% 3 6% 8% 54%	4364
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 8% 48%	4301
	21st to end	Day	6% 4 2 4 4 4 4 4 6% 8% 55%	4602
September	1st to 10th	Day	12% 4 2 5% 3 3 3 2 5% 6% 54%	2324
	11th to 20th	Day	9% 5% 2 6% 4 5% 4 4 5% 7% 50%	2200
	21st to end	Day	8% 5% 1 5% 4 4 4 3 5% 6% 54%	2200
October	1st to 10th	Day	10% 5% 1 5% 3 4 4 3 5% 7% 53%	2039
	11th to 20th	Day	11% 5% 2 5% 3 3 3 2 4 5% 59%	2000
	21st to end	Day	16% 6% 1 5% 3 2 3 2 4 5% 51%	2200
November	1st to 10th	Day	9% 5% 2 5% 2 3 4 3 5% 6% 58%	1988
	11th to 20th	Day	11% 6% 2 4 3 3 3 3 4 5% 56%	1890
	21st to end	Day	14% 7% 2 5% 4 2 3 2 5% 5% 50%	1764
December	1st to 10th	Day	6% 5% 2 5% 3 3 3 3 5% 8% 58%	2690
	11th to 20th	Day	9% 6% 3 6% 4 4 3 3 4 5% 52%	3159
	21st to end	Day	18% 13% 8% 11% 4 3 4 3 4 5% 28%	4587
January	1st to 10th	Night	11% 9% 3 12% 5% 4 3 2 4 5% 40%	7634
	11th to 20th	Night	6% 6% 2 6% 5% 5% 5% 3 4 6% 52%	7680
	21st to end	Night	13% 9% 3 8% 4 4 4 3 4 5% 43%	8402
February	1st to 10th	Night	10% 12% 3 7% 5% 4 4 4 6% 5% 42%	2730
	11th to 20th	Night	12% 9% 2 7% 4 3 4 3 4 6% 45%	2620
	21st to end	Night	11% 5% 2 6% 4 4 4 3 5% 5% 51%	2055
March	1st to 10th	Night	13% 7% 3 5% 3 2 3 3 3 5% 55%	5029
	11th to 20th	Night	7% 4 1 4 3 2 3 3 7% 7% 58%	5090
	21st to end	Night	13% 7% 3 6% 5% 4 4 3 5% 7% 43%	5588
April	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 4 6% 51%	2252
	11th to 20th	Night	13% 6% 2 5% 4 4 3 3 4 7% 48%	2050
	21st to end	Night	16% 7% 2 5% 4 4 4 4 5% 6% 43%	1890
May	1st to 10th	Night	15% 7% 2 5% 3 3 4 3 5% 6% 47%	5266
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 5% 7% 52%	5265
	21st to end	Night	19% 6% 2 5% 3 3 4 3 5% 6% 46%	5793
June	1st to 10th	Night	6% 3 1 4 5% 4 4 4 5% 10% 54%	1890
	11th to 20th	Night	6% 4 2 4 3 4 4 5% 7% 9% 52%	1890
	21st to end	Night	6% 3 1 3 3 3 3 4 5% 9% 59%	1890
July	1st to 10th	Night	7% 5% 2 5% 3 4 4 4 6% 8% 53%	5308
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 9% 57%	5307
	21st to end	Night	8% 4 2 6% 5% 4 6% 4 7% 10% 45%	5770
August	1st to 10th	Night	2 2 1 3 5% 4 5% 4 7% 9% 58%	3016
	11th to 20th	Night	8% 6% 2 5% 4 4 4 3 6% 7% 52%	2978
	21st to end	Night	5% 3 2 5% 3 3 3 3 5% 8% 61%	3497
September	1st to 10th	Night	12% 5% 2 5% 3 3 3 3 4 6% 56%	2072
	11th to 20th	Night	9% 4 2 5% 3 4 3 3 5% 7% 55%	2200
	21st to end	Night	6% 4 2 6% 2 3 4 3 4 6% 58%	2200
October	1st to 10th	Night	10% 4 2 4 3 3 3 3 4 6% 56%	2340
	11th to 20th	Night	12% 5% 1 4 3 3 2 2 4 5% 59%	2400
	21st to end	Night	15% 7% 2 4 3 2 3 2 4 5% 53%	2640
November	1st to 10th	Night	9% 4 2 4 3 3 3 3 3 5% 61%	2528
	11th to 20th	Night	8% 5% 2 4 3 3 3 3 4 6% 57%	2730
	21st to end	Night	13% 7% 2 4 3 2 3 2 4 5% 54%	2850
December	1st to 10th	Night	6% 5% 2 5% 3 4 4 3 4 6% 58%	4720
	11th to 20th	Night	10% 7% 3 6% 4 4 4 4 5% 6% 49%	5263
	21st to end	Night	17% 12% 5% 9% 4 4 4 3 4 5% 34%	6633

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	44% 4 10% 28% 13% 2	5290
	11th to 20th	Day	41% 2 12% 22% 18% 4	5310
	21st to end	Day	45% 3 11% 23% 16% 3	5812
February	1st to 10th	Day	45% 4 11% 26% 12% 3	1890
	11th to 20th	Day	44% 3 11% 25% 14% 3	1915
	21st to end	Day	37% 3 9% 25% 20% 5%	1750
March	1st to 10th	Day	34% 3 8% 21% 23% 11%	5046
	11th to 20th	Day	29% 3 9% 22% 28% 10%	5090
	21st to end	Day	45% 5% 13% 30% 7% 1	5585
April	1st to 10th	Day	38% 4 12% 24% 20% 3	2368
	11th to 20th	Day	41% 5% 11% 22% 18% 2	2514
	21st to end	Day	47% 5% 12% 25% 11% 1	2730
May	1st to 10th	Day	41% 5% 8% 25% 19% 1	7655
	11th to 20th	Day	33% 5% 10% 24% 24% 4	7631
	21st to end	Day	46% 5% 10% 23% 15% 1	8405
June	1st to 10th	Day	28% 8% 17% 29% 16% 2	2730
	11th to 20th	Day	31% 6% 13% 28% 19% 2	2730
	21st to end	Day	28% 6% 15% 29% 18% 4	2730
July	1st to 10th	Day	34% 6% 11% 28% 20% 2	7616
	11th to 20th	Day	27% 5% 12% 29% 19% 7%	7611
	21st to end	Day	37% 6% 12% 33% 10% 1	8155
August	1st to 10th	Day	29% 5% 11% 27% 22% 5%	4350
	11th to 20th	Day	36% 5% 15% 27% 16% 2	4340
	21st to end	Day	27% 6% 14% 31% 17% 5%	4601
September	1st to 10th	Day	32% 4 12% 26% 19% 7%	2324
	11th to 20th	Day	32% 6% 9% 28% 22% 3	2200
	21st to end	Day	30% 6% 9% 27% 25% 4	2200
October	1st to 10th	Day	20% 3 6% 21% 40% 9%	3312
	11th to 20th	Day	19% 2 5% 30% 34% 11%	3240
	21st to end	Day	31% 3 9% 27% 26% 5%	3538
November	1st to 10th	Day	28% 4 12% 31% 20% 6%	1988
	11th to 20th	Day	32% 2 9% 29% 22% 4	1890
	21st to end	Day	37% 2 10% 28% 20% 2	1764
December	1st to 10th	Day	27% 4 11% 40% 18% 1	2690
	11th to 20th	Day	36% 3 10% 34% 16% 1	3156
	21st to end	Day	59% 5% 8% 20% 7% 1	4587
January	1st to 10th	Night	48% 3 11% 24% 13% 1	7635
	11th to 20th	Night	35% 3 9% 27% 22% 4	7680
	21st to end	Night	44% 3 11% 20% 17% 5%	8402
February	1st to 10th	Night	44% 4 11% 23% 15% 3	2730
	11th to 20th	Night	41% 4 11% 25% 16% 3	2621
	21st to end	Night	35% 4 8% 23% 23% 8%	2055
March	1st to 10th	Night	34% 3 6% 25% 27% 6%	5029
	11th to 20th	Night	25% 4 7% 21% 34% 10%	5090
	21st to end	Night	40% 6% 14% 26% 11% 2	5588
April	1st to 10th	Night	36% 4 10% 23% 23% 5%	2252
	11th to 20th	Night	38% 4 10% 25% 19% 5%	2050
	21st to end	Night	40% 6% 12% 25% 15% 2	1890
May	1st to 10th	Night	37% 5% 10% 26% 19% 3	5293
	11th to 20th	Night	33% 5% 11% 25% 21% 6%	5266
	21st to end	Night	40% 5% 11% 26% 17% 1	5805
June	1st to 10th	Night	25% 7% 20% 30% 16% 3	1890
	11th to 20th	Night	26% 6% 15% 31% 18% 3	1890
	21st to end	Night	22% 5% 15% 34% 21% 3	1890
July	1st to 10th	Night	29% 4 13% 28% 22% 3	5265
	11th to 20th	Night	24% 6% 14% 29% 21% 6%	5256
	21st to end	Night	34% 5% 13% 37% 11%	5641
August	1st to 10th	Night	22% 6% 17% 26% 21% 7%	3006
	11th to 20th	Night	32% 5% 14% 28% 19% 2	3005
	21st to end	Night	23% 4 14% 37% 17% 5%	3497
September	1st to 10th	Night	31% 4 9% 27% 20% 8%	2072
	11th to 20th	Night	29% 4 11% 28% 24% 5%	2200
	21st to end	Night	27% 4 8% 25% 31% 5%	2200
October	1st to 10th	Night	20% 3 5% 18% 46% 7%	3821
	11th to 20th	Night	21% 2 5% 28% 31% 13%	3899
	21st to end	Night	32% 2 7% 28% 28% 3	4252
November	1st to 10th	Night	27% 4 10% 29% 24% 6%	2528
	11th to 20th	Night	29% 3 10% 29% 24% 5%	2730
	21st to end	Night	34% 3 9% 30% 19% 4	2850
December	1st to 10th	Night	27% 3 14% 34% 20% 1	4720
	11th to 20th	Night	35% 3 11% 29% 18% 3	5265
	21st to end	Night	54% 4 8% 24% 9% 1	6633

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	18% 15% 35% 24% 7%	330
	11th to 20th	Day	18% 17% 45% 18% 2	330
	21st to end	Day	12% 10% 50% 24% 4	363
February	1st to 10th	Day	20% 20% 37% 20% 3 1	330
	11th to 20th	Day	19% 17% 42% 17% 5%	330
	21st to end	Day	13% 24% 42% 17% 4	273
March	1st to 10th	Day	18% 16% 45% 17% 4	330
	11th to 20th	Day	25% 14% 36% 20% 5%	330
	21st to end	Day	25% 21% 41% 10% 2 1	363
April	1st to 10th	Day	22% 19% 34% 21% 5%	388
	11th to 20th	Day	20% 16% 39% 21% 4 1	552
	21st to end	Day	23% 14% 37% 19% 7% 1	750
May	1st to 10th	Day	18% 14% 36% 23% 8% 1	753
	11th to 20th	Day	17% 11% 29% 28% 12% 3	775
	21st to end	Day	18% 9% 29% 26% 15% 3	844
June	1st to 10th	Day	7% 6% 23% 35% 22% 7%	780
	11th to 20th	Day	9% 4 29% 37% 16% 4	780
	21st to end	Day	8% 7% 32% 38% 14% 2	780
July	1st to 10th	Day	9% 3 16% 35% 28% 8%	771
	11th to 20th	Day	5% 2 18% 40% 26% 8%	768
	21st to end	Day	10% 2 16% 38% 27% 6%	849
August	1st to 10th	Day	7% 3 21% 38% 26% 5%	776
	11th to 20th	Day	8% 3 18% 40% 27% 5%	765
	21st to end	Day	10% 1 15% 35% 30% 8%	736
September	1st to 10th	Day	12% 1 17% 37% 27% 5%	454
	11th to 20th	Day	5% 3 21% 40% 27% 4	330
	21st to end	Day	15% 4 23% 37% 17% 4	330
October	1st to 10th	Day	12% 6% 32% 28% 15% 7%	330
	11th to 20th	Day	9% 14% 40% 28% 9% 1	330
	21st to end	Day	26% 15% 34% 20% 4 2	363
November	1st to 10th	Day	17% 12% 36% 27% 6% 2	330
	11th to 20th	Day	19% 15% 34% 22% 9% 1	330
	21st to end	Day	22% 12% 39% 18% 9%	330
December	1st to 10th	Day	9% 13% 44% 27% 5% 2	330
	11th to 20th	Day	15% 15% 40% 22% 6%	330
	21st to end	Day	19% 20% 37% 20% 4	363
January	1st to 10th	Night	12% 13% 39% 29% 6% 1	1050
	11th to 20th	Night	14% 13% 40% 27% 5% 1	1050
	21st to end	Night	14% 13% 41% 28% 4	1155
February	1st to 10th	Night	16% 14% 41% 26% 3	1050
	11th to 20th	Night	17% 14% 38% 27% 3	986
	21st to end	Night	15% 13% 41% 26% 5%	671
March	1st to 10th	Night	15% 13% 42% 26% 4	630
	11th to 20th	Night	17% 11% 40% 26% 5%	630
	21st to end	Night	23% 11% 40% 22% 4	693
April	1st to 10th	Night	14% 14% 42% 26% 5%	630
	11th to 20th	Night	18% 16% 37% 26% 4	623
	21st to end	Night	20% 10% 41% 22% 6%	630
May	1st to 10th	Night	18% 7% 37% 31% 6% 1	630
	11th to 20th	Night	15% 9% 33% 31% 12%	621
	21st to end	Night	13% 5% 31% 34% 15% 2	683
June	1st to 10th	Night	7% 3 24% 41% 23% 2	630
	11th to 20th	Night	6% 4 31% 42% 16% 1	630
	21st to end	Night	5% 4 33% 44% 12% 1	630
July	1st to 10th	Night	8% 2 17% 43% 28% 2	615
	11th to 20th	Night	2 3 17% 48% 26% 3	612
	21st to end	Night	8% 1 17% 44% 28% 2	690
August	1st to 10th	Night	4 2 20% 46% 27% 1	627
	11th to 20th	Night	7% 2 15% 47% 27% 2	624
	21st to end	Night	7% 1 14% 46% 30% 2	687
September	1st to 10th	Night	12% 3 19% 45% 19% 2	600
	11th to 20th	Night	8% 2 23% 42% 22% 4	600
	21st to end	Night	7% 4 26% 40% 21% 2	600
October	1st to 10th	Night	12% 4 30% 34% 19% 1	740
	11th to 20th	Night	14% 9% 42% 28% 7%	800
	21st to end	Night	19% 11% 33% 28% 7% 1	880
November	1st to 10th	Night	11% 11% 34% 32% 11%	888
	11th to 20th	Night	12% 14% 38% 30% 7%	1050
	21st to end	Night	18% 11% 35% 29% 6%	1050
December	1st to 10th	Night	13% 12% 39% 29% 6% 1	1050
	11th to 20th	Night	17% 11% 40% 28% 4	1050
	21st to end	Night	18% 14% 35% 28% 4	1155

Page generated at 2026-08-27 03:07:11. Version ac07

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