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 2025-04-26.

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% 7% 3 6% 4 4 4 3 5% 7% 47%	113603
	Night	10% 6% 2 6% 4 4 4 3 5% 6% 50%	115231
Transparency	Day	36% 5% 11% 26% 19% 4	117268
	Night	34% 4 11% 26% 21% 4	119682
Seeing	Day	14% 9% 29% 29% 15% 3	16195
	Night	13% 9% 33% 32% 11% 1	26219

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	14% 9% 3 9% 4 4 4 4 5% 6% 38%	4077
	11th to 20th	Day	7% 8% 4 10% 6% 7% 4 3 4 5% 42%	4080
	21st to end	Day	9% 10% 6% 8% 4 4 4 3 3 5% 43%	4488
February	1st to 10th	Day	13% 11% 4 8% 4 4 3 2 4 6% 40%	1800
	11th to 20th	Day	9% 10% 4 8% 6% 5% 4 3 4 6% 42%	1813
	21st to end	Day	9% 9% 3 6% 3 3 4 4 5% 6% 48%	1669
March	1st to 10th	Day	10% 8% 2 5% 4 3 3 2 3 7% 53%	4936
	11th to 20th	Day	9% 6% 2 5% 3 3 3 3 5% 6% 56%	4980
	21st to end	Day	11% 9% 5% 8% 5% 5% 4 3 5% 8% 38%	5462
April	1st to 10th	Day	14% 8% 2 5% 3 3 3 3 5% 7% 48%	2255
	11th to 20th	Day	13% 8% 3 6% 4 4 5% 4 4 7% 42%	2393
	21st to end	Day	18% 7% 3 7% 5% 4 4 3 5% 6% 39%	2536
May	1st to 10th	Day	16% 9% 3 7% 4 4 4 3 5% 6% 40%	4082
	11th to 20th	Day	15% 6% 3 6% 4 4 5% 3 5% 7% 42%	4082
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 8% 43%	4477
June	1st to 10th	Day	6% 3 1 5% 4 5% 4 5% 8% 9% 50%	2470
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 7% 7% 50%	2470
	21st to end	Day	7% 4 1 5% 4 4 4 4 5% 9% 53%	2470
July	1st to 10th	Day	11% 6% 2 6% 4 4 5% 4 7% 9% 42%	4120
	11th to 20th	Day	6% 3 1 4 3 4 5% 4 7% 10% 53%	4120
	21st to end	Day	6% 4 2 5% 5% 5% 6% 5% 8% 9% 45%	4465
August	1st to 10th	Day	7% 4 1 5% 4 4 5% 3 6% 8% 54%	4104
	11th to 20th	Day	12% 5% 2 5% 4 4 4 4 5% 7% 48%	4041
	21st to end	Day	5% 3 2 4 4 4 4 4 6% 8% 56%	4351
September	1st to 10th	Day	12% 4 2 5% 3 3 3 2 5% 6% 55%	2208
	11th to 20th	Day	9% 5% 2 6% 4 4 4 3 4 6% 51%	2090
	21st to end	Day	8% 4 1 5% 4 3 4 3 5% 6% 55%	2090
October	1st to 10th	Day	10% 6% 1 5% 3 3 4 3 5% 7% 53%	1937
	11th to 20th	Day	11% 6% 2 4 3 2 3 2 4 5% 60%	1900
	21st to end	Day	15% 6% 2 5% 3 2 3 2 4 5% 52%	2090
November	1st to 10th	Day	9% 5% 2 5% 2 3 4 3 5% 6% 56%	1892
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 4 5% 56%	1800
	21st to end	Day	14% 7% 2 4 4 2 3 2 5% 5% 51%	1680
December	1st to 10th	Day	6% 5% 2 4 3 3 3 3 5% 8% 59%	2610
	11th to 20th	Day	9% 7% 3 6% 4 4 3 3 3 5% 51%	3077
	21st to end	Day	18% 13% 8% 11% 4 3 4 3 4 5% 28%	4488
January	1st to 10th	Night	11% 9% 3 13% 5% 5% 3 3 4 5% 37%	5888
	11th to 20th	Night	7% 6% 3 7% 5% 6% 5% 3 4 6% 47%	5900
	21st to end	Night	13% 8% 3 8% 3 3 3 3 4 5% 45%	6490
February	1st to 10th	Night	10% 12% 3 7% 5% 4 4 4 6% 5% 41%	2600
	11th to 20th	Night	12% 9% 2 7% 4 3 5% 3 4 6% 45%	2492
	21st to end	Night	11% 6% 2 6% 4 4 4 3 5% 5% 50%	1960
March	1st to 10th	Night	12% 6% 3 5% 3 2 3 3 3 5% 55%	4919
	11th to 20th	Night	7% 4 1 4 3 2 3 3 7% 7% 58%	4980
	21st to end	Night	13% 8% 3 6% 5% 4 4 3 5% 7% 42%	5467
April	1st to 10th	Night	11% 5% 2 6% 4 4 4 3 4 6% 51%	2145
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 4 7% 48%	1951
	21st to end	Night	17% 7% 2 5% 4 4 4 4 5% 6% 43%	1760
May	1st to 10th	Night	14% 8% 2 5% 4 4 4 4 5% 7% 42%	2823
	11th to 20th	Night	15% 7% 2 5% 3 3 4 3 6% 8% 45%	2823
	21st to end	Night	10% 5% 1 4 3 3 3 3 5% 7% 54%	3090
June	1st to 10th	Night	5% 3 1 4 5% 4 4 4 5% 10% 56%	1710
	11th to 20th	Night	5% 3 1 5% 3 4 4 5% 7% 10% 52%	1710
	21st to end	Night	6% 3 1 3 3 3 3 4 5% 9% 60%	1710
July	1st to 10th	Night	10% 7% 2 6% 3 3 4 3 4 6% 50%	2850
	11th to 20th	Night	3 2 1 3 3 3 5% 4 7% 10% 60%	2850
	21st to end	Night	6% 3 2 5% 4 4 6% 4 8% 9% 49%	3098
August	1st to 10th	Night	2 2 1 3 5% 4 5% 4 7% 9% 57%	2836
	11th to 20th	Night	9% 6% 2 5% 4 4 4 3 6% 7% 51%	2798
	21st to end	Night	5% 3 2 5% 3 3 3 3 5% 8% 61%	3308
September	1st to 10th	Night	11% 4 2 4 3 3 3 3 4 6% 57%	1968
	11th to 20th	Night	9% 3 2 5% 3 3 3 3 5% 7% 56%	2090
	21st to end	Night	6% 4 2 6% 2 3 4 3 4 6% 59%	2090
October	1st to 10th	Night	11% 4 2 4 3 3 3 3 4 6% 56%	2223
	11th to 20th	Night	12% 5% 1 4 3 3 2 1 4 5% 60%	2280
	21st to end	Night	14% 6% 2 4 3 2 3 2 4 5% 54%	2508
November	1st to 10th	Night	10% 4 2 4 3 3 3 2 3 5% 61%	2405
	11th to 20th	Night	9% 6% 2 4 4 3 3 3 4 6% 57%	2600
	21st to end	Night	13% 7% 2 4 3 2 3 3 4 5% 56%	2714
December	1st to 10th	Night	6% 5% 2 4 3 4 4 3 4 6% 59%	4580
	11th to 20th	Night	10% 7% 3 6% 4 4 4 4 5% 6% 49%	5125
	21st to end	Night	18% 12% 5% 9% 4 4 4 3 4 5% 34%	6490

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	46% 5% 10% 24% 12% 1	4078
	11th to 20th	Day	45% 3 11% 20% 18% 3	4080
	21st to end	Day	45% 3 11% 21% 17% 3	4488
February	1st to 10th	Day	47% 4 11% 25% 10% 3	1800
	11th to 20th	Day	44% 3 11% 25% 14% 3	1822
	21st to end	Day	38% 3 10% 26% 19% 5%	1669
March	1st to 10th	Day	34% 3 8% 22% 23% 11%	4936
	11th to 20th	Day	29% 4 9% 21% 28% 10%	4980
	21st to end	Day	45% 5% 13% 29% 7% 1	5464
April	1st to 10th	Day	37% 4 12% 24% 20% 3	2255
	11th to 20th	Day	42% 5% 11% 23% 17% 1	2393
	21st to end	Day	46% 5% 12% 25% 11% 1	2536
May	1st to 10th	Day	46% 6% 9% 22% 15% 2	4120
	11th to 20th	Day	39% 6% 13% 24% 16%	4086
	21st to end	Day	38% 7% 11% 27% 15% 2	4489
June	1st to 10th	Day	26% 8% 17% 31% 16% 2	2470
	11th to 20th	Day	31% 7% 13% 29% 19% 2	2470
	21st to end	Day	27% 6% 14% 30% 19% 4	2470
July	1st to 10th	Day	38% 6% 11% 22% 19% 4	4056
	11th to 20th	Day	25% 6% 11% 34% 20% 3	4051
	21st to end	Day	30% 7% 15% 30% 16% 2	4325
August	1st to 10th	Day	29% 5% 11% 27% 22% 6%	4090
	11th to 20th	Day	36% 5% 15% 26% 16% 2	4080
	21st to end	Day	26% 6% 14% 31% 18% 5%	4350
September	1st to 10th	Day	31% 5% 12% 26% 20% 7%	2208
	11th to 20th	Day	32% 7% 9% 27% 23% 3	2090
	21st to end	Day	29% 6% 9% 27% 25% 4	2090
October	1st to 10th	Day	20% 3 6% 21% 40% 9%	3210
	11th to 20th	Day	18% 2 5% 30% 34% 11%	3140
	21st to end	Day	30% 3 9% 27% 26% 5%	3428
November	1st to 10th	Day	29% 4 12% 29% 20% 6%	1892
	11th to 20th	Day	32% 3 9% 28% 23% 5%	1800
	21st to end	Day	36% 3 11% 28% 21% 2	1680
December	1st to 10th	Day	26% 4 11% 40% 18% 1	2610
	11th to 20th	Day	36% 3 9% 34% 16% 1	3074
	21st to end	Day	59% 5% 8% 19% 7% 1	4488
January	1st to 10th	Night	49% 4 11% 23% 12% 1	5888
	11th to 20th	Night	39% 4 9% 24% 20% 4	5900
	21st to end	Night	42% 3 10% 19% 18% 7%	6490
February	1st to 10th	Night	45% 4 11% 23% 15% 3	2600
	11th to 20th	Night	41% 4 11% 24% 16% 3	2493
	21st to end	Night	35% 4 8% 23% 22% 8%	1960
March	1st to 10th	Night	34% 3 6% 25% 27% 6%	4919
	11th to 20th	Night	25% 4 7% 21% 34% 10%	4980
	21st to end	Night	41% 6% 14% 26% 10% 2	5467
April	1st to 10th	Night	35% 4 11% 23% 23% 5%	2145
	11th to 20th	Night	38% 3 10% 26% 19% 4	1951
	21st to end	Night	40% 6% 12% 24% 16% 2	1760
May	1st to 10th	Night	40% 8% 14% 21% 14% 3	2850
	11th to 20th	Night	37% 7% 14% 26% 16%	2824
	21st to end	Night	29% 6% 14% 31% 18% 1	3099
June	1st to 10th	Night	23% 7% 19% 32% 15% 3	1710
	11th to 20th	Night	25% 7% 15% 32% 18% 3	1710
	21st to end	Night	21% 5% 14% 35% 22% 4	1710
July	1st to 10th	Night	35% 4 11% 24% 20% 5%	2805
	11th to 20th	Night	20% 6% 13% 34% 25% 3	2796
	21st to end	Night	28% 6% 17% 31% 16% 1	2999
August	1st to 10th	Night	22% 6% 17% 26% 21% 8%	2826
	11th to 20th	Night	32% 6% 14% 27% 20% 2	2825
	21st to end	Night	23% 4 14% 37% 18% 4	3308
September	1st to 10th	Night	30% 4 9% 27% 21% 9%	1968
	11th to 20th	Night	28% 4 10% 27% 25% 5%	2090
	21st to end	Night	27% 4 8% 25% 31% 5%	2090
October	1st to 10th	Night	20% 3 5% 18% 46% 8%	3704
	11th to 20th	Night	20% 2 4 28% 32% 14%	3779
	21st to end	Night	31% 2 7% 29% 28% 3	4120
November	1st to 10th	Night	27% 4 10% 28% 24% 6%	2405
	11th to 20th	Night	30% 3 9% 28% 24% 5%	2600
	21st to end	Night	33% 3 10% 31% 20% 4	2714
December	1st to 10th	Night	26% 3 15% 34% 21% 1	4580
	11th to 20th	Night	36% 3 11% 29% 18% 3	5127
	21st to end	Night	55% 4 8% 24% 8% 1	6490

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	20% 15% 33% 24% 8%	300
	11th to 20th	Day	20% 16% 44% 19% 2	300
	21st to end	Day	12% 10% 50% 24% 4	330
February	1st to 10th	Day	22% 20% 36% 20% 2 1	300
	11th to 20th	Day	19% 17% 42% 16% 5%	300
	21st to end	Day	13% 24% 43% 16% 4	249
March	1st to 10th	Day	16% 16% 46% 18% 4	300
	11th to 20th	Day	24% 15% 37% 19% 5%	300
	21st to end	Day	26% 20% 41% 11% 2 1	330
April	1st to 10th	Day	21% 19% 35% 20% 5%	355
	11th to 20th	Day	20% 15% 38% 22% 5% 1	511
	21st to end	Day	22% 14% 36% 20% 8% 1	676
May	1st to 10th	Day	18% 14% 36% 23% 8% 1	653
	11th to 20th	Day	18% 9% 29% 29% 12% 3	675
	21st to end	Day	17% 9% 30% 25% 16% 4	734
June	1st to 10th	Day	5% 5% 23% 36% 24% 8%	680
	11th to 20th	Day	10% 3 27% 39% 17% 4	680
	21st to end	Day	8% 6% 31% 39% 14% 2	680
July	1st to 10th	Day	10% 3 15% 35% 28% 9%	671
	11th to 20th	Day	5% 2 19% 41% 26% 8%	668
	21st to end	Day	9% 2 15% 39% 28% 7%	739
August	1st to 10th	Day	8% 2 18% 40% 28% 5%	676
	11th to 20th	Day	9% 3 17% 40% 27% 5%	665
	21st to end	Day	9% 1 15% 34% 32% 9%	645
September	1st to 10th	Day	12% 1 17% 36% 28% 6%	418
	11th to 20th	Day	5% 3 21% 40% 28% 4	300
	21st to end	Day	14% 4 25% 37% 17% 3	300
October	1st to 10th	Day	12% 7% 31% 27% 15% 8%	300
	11th to 20th	Day	8% 14% 41% 27% 9% 1	300
	21st to end	Day	23% 15% 35% 20% 5% 2	330
November	1st to 10th	Day	18% 11% 34% 28% 7% 2	300
	11th to 20th	Day	20% 16% 32% 22% 9% 1	300
	21st to end	Day	22% 11% 39% 18% 10%	300
December	1st to 10th	Day	8% 13% 43% 27% 6% 3	300
	11th to 20th	Day	16% 16% 39% 23% 6%	300
	21st to end	Day	20% 21% 37% 19% 3	330
January	1st to 10th	Night	12% 12% 39% 30% 7% 1	1000
	11th to 20th	Night	15% 12% 39% 28% 5% 1	1000
	21st to end	Night	13% 12% 41% 29% 5%	1100
February	1st to 10th	Night	17% 15% 40% 26% 3	1000
	11th to 20th	Night	17% 14% 39% 28% 3	938
	21st to end	Night	15% 13% 41% 26% 5%	640
March	1st to 10th	Night	14% 13% 42% 27% 5%	600
	11th to 20th	Night	18% 11% 39% 26% 6%	600
	21st to end	Night	24% 11% 39% 22% 4	660
April	1st to 10th	Night	13% 14% 42% 26% 5%	600
	11th to 20th	Night	18% 15% 36% 26% 5%	593
	21st to end	Night	20% 9% 41% 22% 7% 1	587
May	1st to 10th	Night	18% 7% 37% 30% 6% 1	570
	11th to 20th	Night	15% 8% 32% 33% 12%	561
	21st to end	Night	11% 5% 32% 33% 16% 2	617
June	1st to 10th	Night	6% 3 24% 41% 24% 2	570
	11th to 20th	Night	6% 3 32% 42% 18% 1	570
	21st to end	Night	5% 4 33% 44% 13% 1	570
July	1st to 10th	Night	9% 2 16% 43% 28% 2	555
	11th to 20th	Night	2 3 17% 50% 25% 3	552
	21st to end	Night	8% 1 16% 43% 30% 2	624
August	1st to 10th	Night	4 1 18% 46% 29% 1	567
	11th to 20th	Night	8% 2 15% 46% 28% 1	564
	21st to end	Night	7% 1 15% 44% 31% 3	627
September	1st to 10th	Night	12% 2 20% 45% 19% 2	570
	11th to 20th	Night	8% 2 24% 41% 22% 4	570
	21st to end	Night	7% 4 27% 41% 19% 2	570
October	1st to 10th	Night	12% 4 29% 34% 19% 1	703
	11th to 20th	Night	13% 9% 42% 28% 8%	760
	21st to end	Night	18% 12% 33% 29% 8% 1	836
November	1st to 10th	Night	11% 11% 33% 33% 12%	845
	11th to 20th	Night	12% 14% 37% 29% 7%	1000
	21st to end	Night	17% 11% 36% 30% 7%	1000
December	1st to 10th	Night	13% 12% 38% 30% 6% 1	1000
	11th to 20th	Night	17% 11% 40% 28% 4	1000
	21st to end	Night	19% 15% 35% 27% 4	1100

Page generated at 2025-04-23 08:25:02. Version ac07

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