Lee DaSilva Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Lee DaSilva Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	5% 3 1 3 2 2 2 2 3 5% 71%	134773
	Night	7% 3 1 3 2 2 3 2 4 5% 67%	132028
Transparency	Day	18% 4 7% 18% 26% 27%	129789
	Night	21% 4 9% 19% 25% 23%	127990
Seeing	Day	6% 4 15% 29% 30% 15%	16511
	Night	7% 3 18% 40% 27% 4	25962

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	8% 7% 3 7% 6% 5% 4 4 6% 9% 41%	3639
	11th to 20th	Day	8% 6% 2 7% 5% 4 4 3 4 5% 53%	4008
	21st to end	Day	13% 8% 2 6% 5% 5% 4 4 4 7% 43%	4510
February	1st to 10th	Day	13% 5% 2 5% 3 3 4 4 5% 7% 48%	1865
	11th to 20th	Day	11% 6% 2 4 4 3 3 4 5% 7% 52%	1950
	21st to end	Day	13% 5% 2 6% 3 4 4 3 5% 6% 50%	1650
March	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 7% 9% 41%	4518
	11th to 20th	Day	11% 4 2 4 3 3 4 3 4 7% 55%	4988
	21st to end	Day	11% 5% 2 4 3 3 4 4 5% 8% 52%	5810
April	1st to 10th	Day	9% 4 1 4 2 3 3 3 5% 6% 61%	2400
	11th to 20th	Day	7% 4 1 3 3 3 3 3 5% 6% 64%	2369
	21st to end	Day	5% 2 1 3 3 2 3 3 4 6% 69%	2400
May	1st to 10th	Day	3 2 1 2 1 1 1 1 2 4 81%	5499
	11th to 20th	Day	1 1 1 1 1 1 2 3 4 84%	5942
	21st to end	Day	2 1 1 1 1 1 1 2 4 86%	6902
June	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 76%	2800
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	2800
	21st to end	Day	1 1 1 1 1 2 1 2 2 88%	2800
July	1st to 10th	Day	1 1 1 2 95%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 90%	6320
	21st to end	Day	1 1 1 1 1 1 2 3 88%	6412
August	1st to 10th	Day	1 1 1 1 1 1 1 1 2 90%	5563
	11th to 20th	Day	1 1 1 2 95%	5364
	21st to end	Day	1 1 1 1 1 1 1 2 4 87%	5984
September	1st to 10th	Day	1 1 1 1 1 1 1 2 2 90%	2400
	11th to 20th	Day	2 1 2 1 1 2 1 2 4 84%	2366
	21st to end	Day	1 1 1 1 1 1 2 3 4 83%	2200
October	1st to 10th	Day	1 1 1 2 1 1 1 2 3 4 81%	3280
	11th to 20th	Day	2 1 1 2 2 2 2 2 3 4 79%	3161
	21st to end	Day	2 2 2 3 2 3 3 3 4 6% 69%	3448
November	1st to 10th	Day	6% 3 1 4 3 3 3 3 4 7% 63%	1778
	11th to 20th	Day	8% 4 1 4 3 4 4 3 4 6% 58%	1800
	21st to end	Day	9% 6% 2 4 4 3 3 3 4 8% 54%	1800
December	1st to 10th	Day	7% 6% 3 6% 4 4 5% 4 6% 9% 46%	2696
	11th to 20th	Day	15% 7% 3 7% 6% 5% 5% 4 6% 7% 35%	3028
	21st to end	Day	14% 10% 4 10% 7% 5% 5% 4 7% 8% 26%	4004
January	1st to 10th	Night	11% 6% 2 6% 4 3 3 3 4 9% 49%	6306
	11th to 20th	Night	11% 5% 2 5% 3 3 3 3 4 5% 54%	5951
	21st to end	Night	14% 5% 2 5% 4 4 4 3 4 7% 48%	6468
February	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 6% 53%	2532
	11th to 20th	Night	12% 4 1 4 3 3 3 2 5% 7% 56%	2338
	21st to end	Night	13% 5% 2 4 3 3 3 2 4 6% 55%	1980
March	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 6% 7% 44%	5376
	11th to 20th	Night	10% 5% 2 5% 4 3 3 3 4 6% 53%	5009
	21st to end	Night	15% 5% 2 5% 3 3 3 3 4 7% 51%	5151
April	1st to 10th	Night	8% 4 1 4 3 2 3 3 5% 7% 61%	2000
	11th to 20th	Night	6% 2 1 4 3 2 3 3 4 6% 67%	1979
	21st to end	Night	5% 2 1 2 2 2 2 2 4 6% 71%	2000
May	1st to 10th	Night	2 1 1 2 1 1 2 2 4 4 81%	4496
	11th to 20th	Night	2 1 1 1 1 1 2 2 4 85%	3950
	21st to end	Night	1 1 1 1 1 1 1 2 3 89%	3972
June	1st to 10th	Night	2 2 1 2 2 1 2 2 3 5% 78%	1600
	11th to 20th	Night	1 1 1 1 1 2 3 89%	1600
	21st to end	Night	1 1 1 1 1 1 1 2 2 89%	1600
July	1st to 10th	Night	1 1 1 95%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 2 88%	3640
	21st to end	Night	1 1 1 1 1 1 1 2 3 88%	4444
August	1st to 10th	Night	1 1 1 1 1 1 1 2 2 89%	4256
	11th to 20th	Night	1 1 1 1 1 2 94%	4506
	21st to end	Night	2 1 1 1 1 1 1 2 3 87%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 2 3 90%	2000
	11th to 20th	Night	2 1 1 1 1 2 2 1 1 3 85%	2032
	21st to end	Night	1 1 1 1 1 1 2 2 2 3 84%	2200
October	1st to 10th	Night	2 1 1 1 1 2 2 3 4 84%	3762
	11th to 20th	Night	2 1 1 2 2 1 2 2 3 4 81%	3777
	21st to end	Night	3 2 1 3 1 2 2 1 3 5% 77%	4121
November	1st to 10th	Night	7% 3 1 3 2 2 3 2 4 6% 66%	2518
	11th to 20th	Night	6% 3 1 4 2 3 3 3 4 7% 65%	2600
	21st to end	Night	10% 3 2 4 3 3 3 3 5% 7% 58%	2600
December	1st to 10th	Night	8% 4 2 5% 3 3 4 3 5% 9% 54%	4481
	11th to 20th	Night	13% 6% 2 6% 5% 5% 4 3 7% 11% 37%	5176
	21st to end	Night	13% 7% 3 9% 6% 6% 6% 4 6% 8% 31%	6952

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	37% 12% 21% 17% 11% 2	3639
	11th to 20th	Day	34% 6% 10% 15% 23% 12%	4008
	21st to end	Day	40% 6% 10% 15% 25% 3	4510
February	1st to 10th	Day	34% 7% 12% 25% 16% 5%	1865
	11th to 20th	Day	32% 6% 12% 25% 20% 6%	1960
	21st to end	Day	35% 6% 12% 27% 18% 3	1650
March	1st to 10th	Day	37% 8% 15% 19% 19% 2	4518
	11th to 20th	Day	29% 4 16% 30% 13% 8%	4988
	21st to end	Day	30% 6% 14% 32% 15% 3	5810
April	1st to 10th	Day	25% 4 12% 29% 22% 9%	2400
	11th to 20th	Day	22% 4 12% 26% 26% 11%	2372
	21st to end	Day	17% 4 11% 30% 30% 8%	2400
May	1st to 10th	Day	12% 2 6% 24% 39% 18%	5499
	11th to 20th	Day	7% 2 4 28% 43% 17%	5945
	21st to end	Day	7% 2 4 21% 48% 19%	6904
June	1st to 10th	Day	11% 3 8% 21% 33% 24%	2800
	11th to 20th	Day	4 1 4 13% 37% 40%	2800
	21st to end	Day	6% 2 1 8% 27% 57%	2800
July	1st to 10th	Day	2 1 4 14% 80%	6251
	11th to 20th	Day	5% 1 2 3 12% 77%	6245
	21st to end	Day	4 1 1 10% 34% 50%	6226
August	1st to 10th	Day	7% 1 1 5% 25% 61%	4047
	11th to 20th	Day	3 1 1 4 24% 67%	3880
	21st to end	Day	5% 1 2 7% 28% 58%	4311
September	1st to 10th	Day	5% 1 3 8% 28% 55%	2400
	11th to 20th	Day	8% 2 3 18% 39% 31%	2366
	21st to end	Day	8% 2 4 17% 38% 32%	2200
October	1st to 10th	Day	8% 2 4 13% 25% 48%	3280
	11th to 20th	Day	11% 2 5% 23% 40% 20%	3161
	21st to end	Day	17% 2 5% 19% 39% 18%	3448
November	1st to 10th	Day	22% 4 12% 26% 27% 9%	1778
	11th to 20th	Day	25% 6% 14% 29% 22% 4	1800
	21st to end	Day	30% 5% 10% 25% 24% 7%	1800
December	1st to 10th	Day	33% 8% 11% 25% 19% 4	2696
	11th to 20th	Day	47% 7% 11% 20% 13% 2	3028
	21st to end	Day	52% 10% 16% 15% 6% 1	4004
January	1st to 10th	Night	32% 10% 19% 24% 13% 1	6306
	11th to 20th	Night	31% 6% 12% 15% 27% 8%	5951
	21st to end	Night	37% 5% 10% 20% 22% 6%	6468
February	1st to 10th	Night	33% 5% 10% 25% 20% 8%	2532
	11th to 20th	Night	27% 4 11% 27% 24% 6%	2339
	21st to end	Night	31% 4 12% 29% 20% 5%	1980
March	1st to 10th	Night	36% 7% 12% 20% 21% 3	5376
	11th to 20th	Night	32% 5% 13% 27% 16% 7%	5009
	21st to end	Night	33% 5% 15% 27% 16% 3	5151
April	1st to 10th	Night	24% 4 12% 31% 21% 9%	2000
	11th to 20th	Night	19% 4 11% 29% 27% 10%	1979
	21st to end	Night	16% 5% 10% 30% 33% 6%	2000
May	1st to 10th	Night	9% 2 7% 29% 36% 16%	4493
	11th to 20th	Night	7% 1 4 26% 47% 14%	3951
	21st to end	Night	6% 1 4 21% 54% 14%	3972
June	1st to 10th	Night	12% 3 7% 22% 34% 22%	1600
	11th to 20th	Night	5% 1 4 16% 35% 40%	1600
	21st to end	Night	4 1 1 8% 25% 60%	1600
July	1st to 10th	Night	2 1 2 13% 83%	3600
	11th to 20th	Night	6% 1 1 3 12% 78%	3592
	21st to end	Night	5% 1 1 7% 35% 51%	4318
August	1st to 10th	Night	7% 1 1 6% 21% 64%	3090
	11th to 20th	Night	3 1 2 3 20% 71%	3256
	21st to end	Night	6% 1 1 6% 24% 63%	3608
September	1st to 10th	Night	4 1 2 7% 26% 60%	2000
	11th to 20th	Night	9% 2 2 14% 40% 33%	2032
	21st to end	Night	7% 2 3 14% 36% 38%	2200
October	1st to 10th	Night	8% 2 3 10% 22% 55%	3762
	11th to 20th	Night	9% 2 5% 21% 39% 24%	3777
	21st to end	Night	13% 2 3 19% 41% 22%	4121
November	1st to 10th	Night	20% 4 8% 25% 27% 15%	2518
	11th to 20th	Night	19% 4 12% 28% 27% 9%	2600
	21st to end	Night	26% 3 10% 25% 27% 8%	2600
December	1st to 10th	Night	27% 8% 13% 26% 20% 7%	4481
	11th to 20th	Night	40% 8% 16% 18% 15% 2	5176
	21st to end	Night	47% 10% 17% 19% 6% 2	6952

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	23% 6% 18% 29% 20% 6%	200
	11th to 20th	Day	13% 4 13% 30% 26% 15%	360
	21st to end	Day	15% 3 14% 25% 27% 16%	440
February	1st to 10th	Day	16% 7% 15% 26% 21% 16%	433
	11th to 20th	Day	13% 6% 18% 30% 22% 11%	495
	21st to end	Day	10% 7% 20% 31% 26% 6%	414
March	1st to 10th	Day	13% 7% 21% 29% 24% 7%	500
	11th to 20th	Day	13% 9% 23% 24% 20% 11%	500
	21st to end	Day	14% 11% 21% 28% 18% 8%	550
April	1st to 10th	Day	11% 8% 23% 28% 19% 11%	500
	11th to 20th	Day	8% 9% 24% 32% 18% 9%	492
	21st to end	Day	4 8% 22% 31% 20% 15%	500
May	1st to 10th	Day	4 6% 25% 35% 19% 10%	503
	11th to 20th	Day	3 6% 24% 33% 25% 9%	528
	21st to end	Day	4 5% 17% 31% 31% 12%	573
June	1st to 10th	Day	2 3 20% 28% 30% 17%	530
	11th to 20th	Day	2 3 19% 34% 31% 11%	530
	21st to end	Day	2 12% 34% 33% 18%	530
July	1st to 10th	Day	7% 32% 41% 19%	526
	11th to 20th	Day	1 3 24% 44% 27%	524
	21st to end	Day	1 1 5% 27% 44% 23%	575
August	1st to 10th	Day	4 1 7% 35% 35% 19%	527
	11th to 20th	Day	1 1 9% 28% 43% 19%	517
	21st to end	Day	1 1 7% 26% 42% 23%	583
September	1st to 10th	Day	1 2 7% 33% 39% 19%	530
	11th to 20th	Day	3 2 17% 31% 32% 15%	530
	21st to end	Day	2 2 13% 27% 37% 18%	530
October	1st to 10th	Day	2 3 16% 27% 30% 22%	503
	11th to 20th	Day	3 4 13% 24% 34% 21%	490
	21st to end	Day	4 3 13% 24% 35% 21%	539
November	1st to 10th	Day	6% 4 12% 24% 32% 22%	314
	11th to 20th	Day	4 4 10% 32% 34% 16%	300
	21st to end	Day	11% 3 12% 27% 34% 13%	300
December	1st to 10th	Day	12% 4 13% 31% 25% 15%	225
	11th to 20th	Day	15% 5% 12% 30% 23% 17%	200
	21st to end	Day	19% 5% 15% 30% 24% 7%	220
January	1st to 10th	Night	18% 2 13% 37% 24% 7%	800
	11th to 20th	Night	11% 3 11% 39% 29% 7%	800
	21st to end	Night	17% 2 10% 37% 29% 5%	880
February	1st to 10th	Night	15% 4 13% 37% 25% 7%	800
	11th to 20th	Night	12% 5% 16% 34% 27% 6%	784
	21st to end	Night	13% 4 13% 44% 22% 3	660
March	1st to 10th	Night	13% 5% 17% 38% 24% 3	800
	11th to 20th	Night	12% 4 21% 33% 25% 4	800
	21st to end	Night	11% 5% 25% 37% 20% 2	880
April	1st to 10th	Night	9% 6% 27% 33% 23% 2	800
	11th to 20th	Night	5% 7% 30% 35% 20% 3	791
	21st to end	Night	5% 8% 28% 35% 21% 3	800
May	1st to 10th	Night	2 6% 33% 38% 18% 3	775
	11th to 20th	Night	5% 5% 35% 42% 13%	546
	21st to end	Night	3 5% 34% 43% 15%	433
June	1st to 10th	Night	3 6% 38% 44% 8% 1	400
	11th to 20th	Night	1 6% 33% 48% 13%	400
	21st to end	Night	1 1 31% 53% 14%	400
July	1st to 10th	Night	1 25% 57% 18%	390
	11th to 20th	Night	2 1 13% 61% 24%	388
	21st to end	Night	1 15% 48% 33% 3	642
August	1st to 10th	Night	2 1 18% 49% 27% 2	693
	11th to 20th	Night	18% 50% 29% 2	792
	21st to end	Night	1 1 10% 45% 41% 2	880
September	1st to 10th	Night	1 11% 44% 40% 3	800
	11th to 20th	Night	2 2 20% 41% 32% 4	800
	21st to end	Night	2 2 15% 37% 38% 6%	800
October	1st to 10th	Night	3 2 16% 35% 38% 7%	772
	11th to 20th	Night	3 3 11% 38% 37% 9%	760
	21st to end	Night	4 2 11% 34% 36% 13%	836
November	1st to 10th	Night	8% 3 11% 31% 37% 9%	780
	11th to 20th	Night	7% 2 10% 46% 29% 8%	800
	21st to end	Night	10% 2 11% 39% 32% 7%	800
December	1st to 10th	Night	14% 2 10% 40% 27% 7%	800
	11th to 20th	Night	15% 2 12% 40% 27% 5%	800
	21st to end	Night	16% 4 15% 42% 20% 3	880

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