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 2024-11-05.

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% 70%	104994
	Night	7% 3 1 3 2 2 2 2 4 5% 66%	102801
Transparency	Day	18% 4 7% 17% 24% 29%	104746
	Night	21% 4 8% 19% 23% 24%	102639
Seeing	Day	6% 4 15% 29% 30% 15%	15313
	Night	7% 3 18% 40% 28% 5%	24787

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	11% 9% 3 9% 7% 5% 4 4 6% 8% 33%	2539
	11th to 20th	Day	12% 8% 3 9% 6% 5% 5% 4 6% 7% 34%	2796
	21st to end	Day	17% 10% 3 7% 5% 5% 4 5% 5% 8% 31%	3146
February	1st to 10th	Day	13% 6% 2 5% 3 3 5% 4 5% 7% 49%	1771
	11th to 20th	Day	11% 6% 2 4 3 3 3 4 5% 7% 53%	1850
	21st to end	Day	14% 5% 2 6% 3 3 4 2 5% 6% 51%	1570
March	1st to 10th	Day	11% 6% 3 7% 4 5% 5% 4 7% 10% 38%	3138
	11th to 20th	Day	8% 3 2 3 2 2 3 2 4 7% 64%	3478
	21st to end	Day	13% 5% 2 5% 4 4 4 4 6% 7% 47%	4058
April	1st to 10th	Day	9% 4 1 4 3 3 3 3 4 6% 61%	2280
	11th to 20th	Day	7% 4 1 3 3 3 3 3 5% 6% 63%	2249
	21st to end	Day	5% 2 1 3 3 2 3 3 3 6% 69%	2280
May	1st to 10th	Day	4 2 1 2 1 1 2 1 2 4 80%	3846
	11th to 20th	Day	2 1 1 1 1 2 2 3 4 83%	4146
	21st to end	Day	2 1 1 1 1 1 1 3 4 84%	4813
June	1st to 10th	Day	2 2 1 2 2 2 2 2 3 6% 76%	2660
	11th to 20th	Day	1 1 1 1 1 1 1 2 2 89%	2660
	21st to end	Day	1 1 1 1 1 2 1 2 3 87%	2660
July	1st to 10th	Day	1 1 1 1 1 2 93%	4420
	11th to 20th	Day	1 1 1 1 1 1 1 2 3 3 85%	4420
	21st to end	Day	1 1 1 1 1 1 1 2 2 88%	4465
August	1st to 10th	Day	2 1 1 1 1 1 1 2 3 86%	3937
	11th to 20th	Day	1 1 1 1 1 1 2 92%	3724
	21st to end	Day	1 1 1 1 1 1 1 1 2 91%	4180
September	1st to 10th	Day	1 1 1 1 1 1 1 2 2 89%	2280
	11th to 20th	Day	2 1 2 1 1 2 2 2 4 84%	2248
	21st to end	Day	1 1 1 1 1 1 2 3 4 83%	2090
October	1st to 10th	Day	1 1 1 2 1 1 1 2 3 4 81%	3247
	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 5% 7% 62%	1724
	11th to 20th	Day	8% 4 2 4 3 4 4 3 4 6% 58%	1710
	21st to end	Day	8% 6% 2 3 3 3 3 3 4 8% 56%	1710
December	1st to 10th	Day	12% 6% 4 7% 4 3 5% 3 5% 8% 44%	1568
	11th to 20th	Day	19% 8% 4 6% 5% 4 4 3 5% 5% 35%	1928
	21st to end	Day	12% 10% 4 10% 6% 5% 5% 4 6% 8% 29%	2794
January	1st to 10th	Night	14% 8% 3 8% 4 4 3 3 4 7% 42%	4406
	11th to 20th	Night	16% 7% 3 8% 5% 5% 4 4 6% 7% 36%	4168
	21st to end	Night	19% 7% 3 6% 4 5% 4 3 4 8% 37%	4521
February	1st to 10th	Night	12% 6% 2 4 3 3 3 3 4 6% 54%	2406
	11th to 20th	Night	11% 4 1 3 3 3 3 2 5% 7% 57%	2218
	21st to end	Night	13% 5% 2 4 3 3 3 2 4 6% 55%	1884
March	1st to 10th	Night	13% 5% 2 6% 5% 4 4 4 6% 7% 44%	3736
	11th to 20th	Night	9% 4 2 4 3 2 2 2 4 6% 63%	3499
	21st to end	Night	17% 5% 2 4 3 3 3 3 4 6% 48%	3594
April	1st to 10th	Night	9% 4 1 4 3 2 3 3 5% 7% 60%	1900
	11th to 20th	Night	6% 2 1 4 3 2 2 3 4 6% 68%	1879
	21st to end	Night	5% 2 1 2 2 2 2 2 4 6% 72%	1900
May	1st to 10th	Night	3 2 1 2 1 1 1 2 3 4 81%	3130
	11th to 20th	Night	2 1 1 1 1 1 1 2 3 4 84%	2752
	21st to end	Night	2 1 1 1 1 1 1 1 2 3 86%	2762
June	1st to 10th	Night	2 2 1 2 2 1 2 2 3 5% 77%	1520
	11th to 20th	Night	1 1 1 1 1 2 3 89%	1520
	21st to end	Night	1 1 1 1 1 1 2 3 89%	1520
July	1st to 10th	Night	1 1 1 2 1 94%	2540
	11th to 20th	Night	2 2 1 2 1 1 2 1 2 3 83%	2540
	21st to end	Night	1 1 1 1 1 1 1 2 3 86%	3095
August	1st to 10th	Night	1 1 1 1 1 1 2 2 3 85%	3004
	11th to 20th	Night	1 1 1 1 1 1 2 2 91%	3126
	21st to end	Night	2 1 1 1 1 1 1 2 91%	3498
September	1st to 10th	Night	1 1 1 1 1 1 2 3 90%	1900
	11th to 20th	Night	2 1 1 1 2 2 1 1 3 85%	1930
	21st to end	Night	1 1 1 1 1 2 2 2 3 85%	2090
October	1st to 10th	Night	2 1 1 1 1 2 1 3 4 84%	3728
	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 3 2 3 2 4 6% 65%	2452
	11th to 20th	Night	6% 3 1 4 2 2 2 3 4 7% 65%	2470
	21st to end	Night	9% 4 2 3 3 2 3 3 5% 7% 60%	2470
December	1st to 10th	Night	12% 5% 3 6% 3 3 3 2 4 6% 52%	2607
	11th to 20th	Night	16% 7% 2 6% 5% 4 4 3 7% 8% 37%	3276
	21st to end	Night	14% 6% 3 8% 6% 5% 5% 4 6% 7% 35%	4862

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% 13% 18% 17% 6% 1	2539
	11th to 20th	Day	48% 8% 13% 17% 10% 4	2796
	21st to end	Day	50% 9% 13% 17% 10% 1	3146
February	1st to 10th	Day	33% 7% 12% 25% 16% 5%	1771
	11th to 20th	Day	31% 5% 11% 25% 21% 6%	1860
	21st to end	Day	35% 6% 12% 27% 18% 3	1570
March	1st to 10th	Day	37% 10% 18% 19% 15% 2	3138
	11th to 20th	Day	22% 4 13% 33% 16% 11%	3478
	21st to end	Day	33% 7% 14% 26% 16% 3	4058
April	1st to 10th	Day	26% 4 12% 28% 22% 9%	2280
	11th to 20th	Day	22% 4 12% 25% 26% 11%	2252
	21st to end	Day	17% 4 11% 29% 31% 8%	2280
May	1st to 10th	Day	11% 2 6% 24% 36% 21%	3846
	11th to 20th	Day	7% 2 5% 23% 43% 19%	4149
	21st to end	Day	8% 2 5% 27% 38% 20%	4814
June	1st to 10th	Day	12% 3 8% 21% 32% 25%	2660
	11th to 20th	Day	4 2 4 14% 38% 38%	2660
	21st to end	Day	6% 2 2 8% 26% 56%	2660
July	1st to 10th	Day	3 1 1 3 11% 81%	4351
	11th to 20th	Day	7% 1 2 5% 13% 72%	4345
	21st to end	Day	5% 1 2 5% 25% 61%	4324
August	1st to 10th	Day	7% 1 2 5% 25% 60%	3922
	11th to 20th	Day	3 1 1 4 24% 67%	3760
	21st to end	Day	5% 1 2 7% 27% 59%	4179
September	1st to 10th	Day	5% 1 3 7% 26% 58%	2280
	11th to 20th	Day	8% 2 3 17% 39% 32%	2248
	21st to end	Day	7% 2 3 17% 38% 33%	2090
October	1st to 10th	Day	8% 2 4 12% 25% 49%	3247
	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%	1724
	11th to 20th	Day	25% 6% 14% 27% 23% 4	1710
	21st to end	Day	29% 5% 10% 25% 24% 7%	1710
December	1st to 10th	Day	38% 6% 11% 19% 18% 7%	1568
	11th to 20th	Day	50% 6% 10% 19% 12% 3	1928
	21st to end	Day	49% 10% 16% 16% 8% 1	2794
January	1st to 10th	Night	41% 9% 17% 23% 9% 1	4406
	11th to 20th	Night	44% 9% 16% 16% 9% 6%	4168
	21st to end	Night	44% 6% 14% 21% 11% 3	4521
February	1st to 10th	Night	32% 5% 11% 26% 19% 8%	2406
	11th to 20th	Night	26% 4 11% 27% 25% 7%	2219
	21st to end	Night	32% 4 12% 29% 19% 4	1884
March	1st to 10th	Night	35% 9% 15% 20% 18% 4	3736
	11th to 20th	Night	23% 4 14% 28% 21% 9%	3499
	21st to end	Night	36% 7% 13% 26% 15% 3	3594
April	1st to 10th	Night	24% 4 11% 30% 21% 9%	1900
	11th to 20th	Night	19% 4 11% 29% 27% 10%	1879
	21st to end	Night	14% 5% 10% 30% 34% 7%	1900
May	1st to 10th	Night	9% 2 8% 28% 36% 17%	3130
	11th to 20th	Night	7% 2 5% 22% 45% 19%	2753
	21st to end	Night	7% 2 5% 27% 45% 14%	2762
June	1st to 10th	Night	12% 3 8% 22% 33% 22%	1520
	11th to 20th	Night	5% 2 4 16% 36% 38%	1520
	21st to end	Night	4 1 2 8% 26% 60%	1520
July	1st to 10th	Night	2 1 3 12% 82%	2500
	11th to 20th	Night	9% 1 2 4 14% 71%	2492
	21st to end	Night	6% 1 1 5% 25% 63%	2998
August	1st to 10th	Night	7% 1 2 6% 21% 64%	2995
	11th to 20th	Night	3 1 2 3 19% 72%	3156
	21st to end	Night	5% 1 1 6% 24% 63%	3498
September	1st to 10th	Night	4 1 2 6% 24% 62%	1900
	11th to 20th	Night	8% 1 2 14% 41% 34%	1930
	21st to end	Night	7% 1 3 13% 37% 39%	2090
October	1st to 10th	Night	8% 2 3 10% 22% 56%	3728
	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	21% 4 9% 25% 27% 15%	2452
	11th to 20th	Night	19% 4 12% 27% 27% 10%	2470
	21st to end	Night	25% 3 11% 25% 28% 9%	2470
December	1st to 10th	Night	34% 5% 10% 22% 17% 12%	2607
	11th to 20th	Night	44% 7% 12% 18% 14% 4	3276
	21st to end	Night	43% 10% 16% 19% 9% 2	4862

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% 6% 18% 29% 17% 5%	180
	11th to 20th	Day	14% 4 13% 29% 25% 15%	332
	21st to end	Day	16% 3 15% 25% 27% 15%	407
February	1st to 10th	Day	16% 6% 14% 25% 22% 17%	399
	11th to 20th	Day	13% 6% 18% 30% 22% 11%	455
	21st to end	Day	10% 8% 21% 31% 24% 6%	382
March	1st to 10th	Day	13% 7% 21% 29% 23% 7%	460
	11th to 20th	Day	12% 9% 23% 24% 20% 12%	460
	21st to end	Day	14% 11% 22% 28% 19% 7%	506
April	1st to 10th	Day	11% 8% 24% 27% 18% 11%	460
	11th to 20th	Day	9% 9% 24% 30% 19% 9%	452
	21st to end	Day	4 8% 22% 31% 21% 14%	460
May	1st to 10th	Day	4 6% 26% 36% 19% 9%	463
	11th to 20th	Day	4 5% 22% 33% 26% 10%	488
	21st to end	Day	4 5% 17% 30% 32% 13%	529
June	1st to 10th	Day	2 3 21% 28% 30% 16%	490
	11th to 20th	Day	2 3 18% 33% 32% 12%	490
	21st to end	Day	1 13% 34% 33% 18%	490
July	1st to 10th	Day	7% 31% 41% 20%	486
	11th to 20th	Day	1 3 24% 44% 28%	484
	21st to end	Day	1 1 5% 26% 44% 24%	531
August	1st to 10th	Day	4 1 7% 35% 36% 18%	487
	11th to 20th	Day	1 1 9% 28% 42% 19%	477
	21st to end	Day	1 7% 26% 43% 22%	539
September	1st to 10th	Day	1 2 6% 32% 40% 19%	490
	11th to 20th	Day	2 2 18% 31% 32% 14%	490
	21st to end	Day	2 2 12% 26% 38% 20%	490
October	1st to 10th	Day	2 3 16% 27% 30% 22%	490
	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% 3 12% 25% 34% 20%	296
	11th to 20th	Day	5% 3 9% 30% 35% 18%	270
	21st to end	Day	9% 3 12% 28% 35% 14%	270
December	1st to 10th	Day	13% 3 14% 33% 25% 12%	203
	11th to 20th	Day	14% 4 12% 31% 23% 16%	180
	21st to end	Day	20% 6% 13% 29% 24% 8%	198
January	1st to 10th	Night	19% 2 12% 36% 24% 8%	760
	11th to 20th	Night	11% 3 11% 38% 29% 7%	760
	21st to end	Night	17% 2 10% 36% 29% 5%	836
February	1st to 10th	Night	15% 3 12% 37% 26% 7%	760
	11th to 20th	Night	12% 5% 15% 34% 28% 6%	744
	21st to end	Night	14% 4 13% 44% 21% 4	628
March	1st to 10th	Night	13% 5% 17% 38% 24% 3	760
	11th to 20th	Night	12% 4 20% 33% 26% 4	760
	21st to end	Night	11% 5% 25% 37% 20% 1	836
April	1st to 10th	Night	9% 6% 27% 32% 23% 2	760
	11th to 20th	Night	5% 7% 30% 34% 20% 3	751
	21st to end	Night	5% 8% 29% 34% 21% 3	760
May	1st to 10th	Night	2 7% 33% 38% 18% 2	736
	11th to 20th	Night	5% 4 34% 43% 14%	519
	21st to end	Night	3 5% 33% 44% 15%	411
June	1st to 10th	Night	3 6% 39% 43% 8% 1	380
	11th to 20th	Night	1 5% 32% 48% 14%	380
	21st to end	Night	1 1 31% 53% 14%	380
July	1st to 10th	Night	1 24% 56% 19%	370
	11th to 20th	Night	2 1 13% 59% 25%	368
	21st to end	Night	1 14% 47% 35% 3	610
August	1st to 10th	Night	2 1 19% 49% 27% 2	658
	11th to 20th	Night	18% 49% 30% 3	752
	21st to end	Night	1 1 10% 44% 42% 2	836
September	1st to 10th	Night	1 10% 44% 41% 3	760
	11th to 20th	Night	2 2 20% 41% 32% 4	760
	21st to end	Night	2 2 14% 36% 39% 7%	760
October	1st to 10th	Night	3 2 16% 35% 38% 7%	760
	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% 4 11% 31% 37% 9%	760
	11th to 20th	Night	7% 2 9% 45% 30% 8%	760
	21st to end	Night	9% 2 11% 39% 32% 7%	760
December	1st to 10th	Night	14% 2 10% 41% 26% 7%	760
	11th to 20th	Night	15% 2 11% 40% 27% 4	760
	21st to end	Night	16% 4 15% 41% 21% 3	836

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