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 2024-02-25.

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 5% 5% 3 4 4 5% 7% 45%	78366
	Night	11% 6% 3 5% 4 3 4 4 5% 6% 49%	80988
Transparency	Day	38% 5% 12% 26% 16% 3	78089
	Night	36% 5% 11% 26% 19% 4	80785
Seeing	Day	14% 9% 29% 29% 16% 3	14612
	Night	13% 9% 33% 33% 11% 1	24687

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% 4 8% 6% 3 4 5% 6% 6% 36%	2847
	11th to 20th	Day	9% 9% 4 8% 7% 5% 6% 3 4 5% 40%	2850
	21st to end	Day	10% 12% 8% 9% 4 2 4 3 3 6% 38%	3135
February	1st to 10th	Day	13% 10% 4 7% 5% 3 4 3 4 6% 41%	1710
	11th to 20th	Day	10% 8% 4 7% 6% 3 4 4 4 7% 43%	1720
	21st to end	Day	10% 9% 3 6% 4 2 4 4 4 6% 46%	1537
March	1st to 10th	Day	10% 8% 3 6% 5% 3 4 5% 4 8% 44%	1980
	11th to 20th	Day	19% 8% 3 5% 4 3 4 4 4 5% 41%	1980
	21st to end	Day	17% 11% 4 7% 5% 3 4 4 5% 7% 33%	2178
April	1st to 10th	Day	14% 7% 2 5% 4 2 4 4 5% 7% 47%	2029
	11th to 20th	Day	14% 8% 3 5% 5% 3 5% 5% 4 7% 41%	2151
	21st to end	Day	18% 6% 3 6% 6% 3 4 4 5% 6% 39%	2340
May	1st to 10th	Day	17% 8% 4 5% 4 3 4 4 5% 6% 38%	2340
	11th to 20th	Day	16% 5% 3 5% 5% 3 5% 4 5% 7% 42%	2302
	21st to end	Day	15% 7% 4 5% 6% 4 4 5% 6% 9% 35%	2531
June	1st to 10th	Day	6% 3 1 4 4 3 5% 5% 8% 9% 51%	2340
	11th to 20th	Day	8% 4 2 4 5% 3 5% 5% 6% 8% 50%	2340
	21st to end	Day	7% 4 1 4 4 3 4 4 5% 9% 54%	2340
July	1st to 10th	Day	10% 5% 2 4 5% 2 5% 6% 8% 10% 44%	2340
	11th to 20th	Day	4 3 1 3 4 2 5% 5% 7% 9% 55%	2340
	21st to end	Day	9% 4 2 4 5% 3 5% 5% 7% 8% 48%	2546
August	1st to 10th	Day	8% 4 2 4 5% 3 5% 5% 7% 10% 47%	2328
	11th to 20th	Day	8% 4 2 4 5% 3 4 5% 6% 8% 52%	2301
	21st to end	Day	7% 3 2 4 4 3 4 5% 5% 8% 55%	2477
September	1st to 10th	Day	13% 4 2 4 3 2 3 3 5% 7% 53%	2092
	11th to 20th	Day	10% 5% 2 5% 5% 3 4 4 4 6% 51%	1980
	21st to end	Day	8% 4 2 4 5% 3 3 4 5% 6% 55%	1980
October	1st to 10th	Day	11% 6% 1 4 4 3 4 4 5% 8% 51%	1836
	11th to 20th	Day	11% 5% 2 4 3 2 3 3 4 5% 58%	1800
	21st to end	Day	15% 6% 2 5% 4 2 3 3 4 5% 51%	1980
November	1st to 10th	Day	8% 5% 2 4 3 2 4 4 5% 6% 57%	1796
	11th to 20th	Day	11% 5% 2 3 4 2 3 3 4 5% 56%	1710
	21st to end	Day	14% 7% 3 4 4 2 3 3 4 6% 51%	1596
December	1st to 10th	Day	9% 6% 3 5% 4 2 4 4 5% 8% 49%	1520
	11th to 20th	Day	11% 8% 4 7% 6% 4 5% 4 4 6% 42%	1959
	21st to end	Day	13% 12% 9% 12% 5% 3 4 4 4 6% 27%	3135
January	1st to 10th	Night	8% 9% 4 13% 7% 3 4 3 4 5% 41%	4108
	11th to 20th	Night	9% 8% 4 7% 6% 5% 7% 5% 4 5% 42%	4120
	21st to end	Night	15% 10% 4 8% 4 2 3 3 4 4 41%	4532
February	1st to 10th	Night	10% 11% 4 6% 5% 3 4 4 5% 6% 41%	2470
	11th to 20th	Night	12% 7% 3 7% 4 2 4 4 4 6% 46%	2365
	21st to end	Night	11% 5% 2 5% 4 3 4 4 4 5% 50%	1812
March	1st to 10th	Night	11% 6% 3 5% 4 2 4 5% 5% 6% 50%	1980
	11th to 20th	Night	15% 7% 3 5% 4 2 4 4 5% 7% 45%	1980
	21st to end	Night	20% 9% 3 5% 4 3 4 3 4 6% 38%	2178
April	1st to 10th	Night	10% 5% 2 4 5% 3 4 4 4 7% 52%	1931
	11th to 20th	Night	14% 6% 2 4 5% 3 4 3 4 7% 47%	1754
	21st to end	Night	16% 6% 2 4 5% 3 4 5% 5% 6% 44%	1620
May	1st to 10th	Night	16% 7% 3 5% 4 3 5% 4 5% 8% 41%	1620
	11th to 20th	Night	13% 6% 3 4 3 3 4 3 5% 8% 50%	1593
	21st to end	Night	10% 5% 3 5% 4 3 4 4 7% 9% 46%	1745
June	1st to 10th	Night	5% 2 1 3 5% 3 4 4 5% 10% 57%	1620
	11th to 20th	Night	6% 3 2 4 4 3 5% 7% 6% 10% 52%	1620
	21st to end	Night	6% 3 1 2 4 2 3 5% 5% 9% 62%	1620
July	1st to 10th	Night	7% 4 2 3 4 3 4 5% 5% 8% 53%	1620
	11th to 20th	Night	2 2 1 2 3 2 4 5% 6% 10% 61%	1620
	21st to end	Night	7% 3 2 5% 5% 2 5% 5% 7% 10% 50%	1772
August	1st to 10th	Night	4 2 2 2 6% 3 5% 4 8% 11% 53%	1611
	11th to 20th	Night	6% 4 2 4 4 2 4 4 5% 8% 57%	1600
	21st to end	Night	6% 3 1 4 4 2 3 3 4 8% 62%	1877
September	1st to 10th	Night	12% 4 2 4 3 2 3 3 4 6% 55%	1864
	11th to 20th	Night	9% 3 2 5% 3 2 4 4 5% 7% 56%	1980
	21st to end	Night	6% 4 2 5% 3 3 4 4 4 7% 59%	1980
October	1st to 10th	Night	11% 4 2 4 4 3 3 4 4 6% 54%	2106
	11th to 20th	Night	13% 5% 1 4 3 2 3 2 4 5% 59%	2160
	21st to end	Night	14% 6% 2 4 3 1 3 3 4 5% 54%	2376
November	1st to 10th	Night	8% 4 2 4 3 2 3 3 3 6% 62%	2281
	11th to 20th	Night	9% 5% 2 4 4 2 3 4 4 6% 57%	2470
	21st to end	Night	14% 6% 2 3 3 2 3 3 4 5% 55%	2578
December	1st to 10th	Night	9% 6% 3 5% 4 3 4 5% 5% 7% 48%	2660
	11th to 20th	Night	10% 7% 3 6% 4 3 4 4 5% 6% 48%	3233
	21st to end	Night	17% 13% 6% 9% 5% 3 4 3 3 5% 33%	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	48% 5% 11% 23% 12% 1	2848
	11th to 20th	Day	49% 3 16% 20% 12% 1	2850
	21st to end	Day	49% 3 12% 21% 14% 1	3135
February	1st to 10th	Day	46% 4 11% 25% 11% 3	1710
	11th to 20th	Day	43% 3 11% 26% 14% 3	1729
	21st to end	Day	40% 3 10% 25% 17% 4	1537
March	1st to 10th	Day	40% 5% 11% 24% 16% 4	1980
	11th to 20th	Day	46% 3 9% 23% 16% 2	1980
	21st to end	Day	52% 4 14% 20% 8% 2	2178
April	1st to 10th	Day	37% 4 13% 24% 19% 2	2029
	11th to 20th	Day	42% 5% 12% 24% 16% 1	2151
	21st to end	Day	46% 5% 12% 25% 12% 1	2340
May	1st to 10th	Day	46% 7% 12% 21% 14% 1	2340
	11th to 20th	Day	41% 6% 14% 25% 13%	2306
	21st to end	Day	46% 9% 13% 21% 9% 2	2531
June	1st to 10th	Day	26% 8% 17% 32% 16% 2	2340
	11th to 20th	Day	31% 7% 13% 28% 19% 2	2340
	21st to end	Day	27% 6% 14% 31% 19% 4	2340
July	1st to 10th	Day	33% 8% 14% 27% 15% 4	2276
	11th to 20th	Day	23% 8% 15% 29% 20% 5%	2271
	21st to end	Day	30% 6% 15% 31% 16% 2	2405
August	1st to 10th	Day	31% 8% 15% 29% 15% 2	2314
	11th to 20th	Day	29% 7% 11% 33% 17% 3	2301
	21st to end	Day	28% 6% 11% 26% 21% 7%	2477
September	1st to 10th	Day	32% 5% 12% 27% 18% 7%	2092
	11th to 20th	Day	33% 7% 9% 26% 23% 3	1980
	21st to end	Day	29% 6% 9% 27% 25% 3	1980
October	1st to 10th	Day	32% 5% 11% 28% 21% 3	1836
	11th to 20th	Day	31% 3 8% 25% 28% 6%	1800
	21st to end	Day	37% 3 9% 24% 19% 6%	1980
November	1st to 10th	Day	28% 5% 12% 29% 20% 6%	1796
	11th to 20th	Day	32% 3 10% 29% 23% 4	1710
	21st to end	Day	36% 3 10% 27% 22% 2	1596
December	1st to 10th	Day	35% 5% 13% 32% 15% 1	1520
	11th to 20th	Day	44% 4 10% 26% 14% 2	1956
	21st to end	Day	58% 6% 9% 18% 8% 1	3135
January	1st to 10th	Night	46% 4 13% 23% 13% 2	4108
	11th to 20th	Night	45% 4 11% 22% 16% 1	4120
	21st to end	Night	47% 4 13% 20% 14% 2	4532
February	1st to 10th	Night	45% 4 11% 22% 15% 3	2470
	11th to 20th	Night	40% 4 11% 25% 17% 4	2366
	21st to end	Night	37% 3 9% 23% 21% 7%	1812
March	1st to 10th	Night	36% 4 10% 25% 20% 6%	1980
	11th to 20th	Night	39% 5% 11% 24% 17% 4	1980
	21st to end	Night	48% 6% 12% 20% 13% 2	2178
April	1st to 10th	Night	34% 4 11% 24% 23% 4	1931
	11th to 20th	Night	39% 3 10% 27% 19% 3	1754
	21st to end	Night	40% 5% 12% 25% 17% 2	1620
May	1st to 10th	Night	42% 6% 13% 22% 16% 2	1620
	11th to 20th	Night	34% 6% 16% 26% 18% 1	1594
	21st to end	Night	33% 8% 19% 26% 12% 2	1746
June	1st to 10th	Night	22% 7% 20% 33% 15% 3	1620
	11th to 20th	Night	25% 7% 15% 32% 18% 3	1620
	21st to end	Night	21% 4 13% 35% 22% 4	1620
July	1st to 10th	Night	29% 5% 14% 31% 17% 4	1575
	11th to 20th	Night	18% 6% 14% 32% 24% 5%	1566
	21st to end	Night	28% 6% 12% 32% 21% 1	1673
August	1st to 10th	Night	24% 7% 19% 32% 14% 3	1601
	11th to 20th	Night	27% 6% 10% 32% 23% 2	1600
	21st to end	Night	23% 5% 11% 29% 25% 7%	1877
September	1st to 10th	Night	31% 4 9% 28% 19% 8%	1864
	11th to 20th	Night	28% 4 10% 26% 26% 5%	1980
	21st to end	Night	26% 4 8% 26% 31% 5%	1980
October	1st to 10th	Night	31% 4 9% 25% 24% 6%	2106
	11th to 20th	Night	30% 3 6% 29% 25% 8%	2160
	21st to end	Night	35% 3 8% 23% 26% 5%	2376
November	1st to 10th	Night	26% 4 10% 28% 24% 7%	2281
	11th to 20th	Night	29% 3 10% 29% 24% 4	2470
	21st to end	Night	33% 3 9% 30% 20% 4	2578
December	1st to 10th	Night	35% 4 14% 28% 18% 1	2660
	11th to 20th	Night	37% 4 9% 30% 17% 2	3235
	21st to end	Night	55% 5% 7% 21% 9% 2	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	19% 17% 33% 23% 8%	270
	11th to 20th	Day	21% 14% 43% 19% 2	270
	21st to end	Day	12% 11% 49% 23% 5%	297
February	1st to 10th	Day	23% 19% 36% 20% 2 1	270
	11th to 20th	Day	17% 16% 45% 17% 6%	270
	21st to end	Day	15% 23% 42% 15% 4	210
March	1st to 10th	Day	15% 17% 46% 18% 4	240
	11th to 20th	Day	26% 12% 38% 18% 6%	240
	21st to end	Day	29% 18% 41% 10% 2	264
April	1st to 10th	Day	18% 19% 35% 22% 6%	289
	11th to 20th	Day	19% 14% 36% 24% 5% 1	429
	21st to end	Day	22% 14% 36% 19% 8% 1	600
May	1st to 10th	Day	18% 14% 35% 23% 8% 1	603
	11th to 20th	Day	19% 9% 29% 28% 12% 3	625
	21st to end	Day	17% 8% 30% 24% 17% 4	679
June	1st to 10th	Day	5% 5% 21% 36% 25% 8%	630
	11th to 20th	Day	10% 3 26% 39% 18% 4	630
	21st to end	Day	8% 6% 30% 39% 15% 2	630
July	1st to 10th	Day	10% 2 13% 36% 29% 9%	621
	11th to 20th	Day	4 2 17% 41% 28% 8%	618
	21st to end	Day	9% 2 14% 40% 28% 6%	684
August	1st to 10th	Day	8% 2 16% 40% 28% 5%	626
	11th to 20th	Day	8% 2 16% 40% 29% 5%	615
	21st to end	Day	10% 1 14% 33% 33% 9%	596
September	1st to 10th	Day	13% 1 18% 35% 27% 5%	382
	11th to 20th	Day	5% 3 21% 40% 28% 4	270
	21st to end	Day	14% 3 27% 37% 17% 2	270
October	1st to 10th	Day	13% 8% 32% 25% 14% 8%	270
	11th to 20th	Day	9% 15% 43% 26% 7%	270
	21st to end	Day	25% 15% 34% 20% 5% 2	297
November	1st to 10th	Day	16% 9% 34% 30% 8% 2	270
	11th to 20th	Day	19% 15% 34% 22% 9% 1	270
	21st to end	Day	22% 11% 39% 19% 9%	270
December	1st to 10th	Day	9% 13% 41% 27% 7% 3	270
	11th to 20th	Day	17% 15% 37% 23% 7%	270
	21st to end	Day	18% 21% 40% 19% 2	297
January	1st to 10th	Night	12% 13% 38% 30% 7% 1	950
	11th to 20th	Night	15% 12% 38% 28% 5% 1	950
	21st to end	Night	14% 13% 40% 29% 5%	1045
February	1st to 10th	Night	17% 14% 41% 26% 3	950
	11th to 20th	Night	16% 13% 39% 29% 3	891
	21st to end	Night	16% 14% 41% 25% 5%	592
March	1st to 10th	Night	14% 13% 42% 26% 5%	540
	11th to 20th	Night	18% 11% 38% 27% 6%	540
	21st to end	Night	25% 10% 38% 22% 4	594
April	1st to 10th	Night	12% 14% 41% 28% 5%	540
	11th to 20th	Night	19% 13% 36% 27% 5%	533
	21st to end	Night	20% 9% 40% 23% 7% 1	540
May	1st to 10th	Night	19% 7% 36% 31% 6% 1	540
	11th to 20th	Night	16% 7% 33% 33% 11%	531
	21st to end	Night	11% 5% 32% 33% 17% 2	584
June	1st to 10th	Night	6% 3 23% 41% 25% 2	540
	11th to 20th	Night	6% 2 31% 42% 18% 1	540
	21st to end	Night	5% 4 32% 45% 14% 1	540
July	1st to 10th	Night	8% 2 14% 44% 30% 2	525
	11th to 20th	Night	2 2 16% 50% 26% 3	522
	21st to end	Night	8% 16% 44% 30% 2	591
August	1st to 10th	Night	4 1 18% 45% 30% 1	537
	11th to 20th	Night	7% 2 14% 46% 29% 1	534
	21st to end	Night	7% 1 15% 43% 32% 3	594
September	1st to 10th	Night	12% 2 20% 45% 19% 2	540
	11th to 20th	Night	9% 2 23% 40% 22% 4	540
	21st to end	Night	7% 3 27% 41% 19% 2	540
October	1st to 10th	Night	13% 5% 29% 33% 20% 1	666
	11th to 20th	Night	14% 9% 42% 27% 7%	720
	21st to end	Night	19% 11% 34% 28% 8% 1	792
November	1st to 10th	Night	10% 10% 34% 32% 12%	801
	11th to 20th	Night	12% 13% 37% 30% 7%	950
	21st to end	Night	18% 10% 35% 31% 6%	950
December	1st to 10th	Night	14% 13% 38% 29% 6% 1	950
	11th to 20th	Night	17% 11% 39% 29% 4	950
	21st to end	Night	19% 15% 36% 26% 4	1045

Page generated at 2024-02-22 05:05:47. Version ac07

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