San Ignacio Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the San Ignacio Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	4 3 1 3 3 3 4 3 5% 7% 63%	134297
	Night	4 4 1 4 3 3 4 3 5% 7% 61%	132186
Transparency	Day	20% 3 7% 20% 27% 22%	129375
	Night	22% 3 7% 19% 28% 22%	128113
Seeing	Day	9% 9% 27% 29% 20% 6%	16341
	Night	7% 10% 34% 34% 14% 1	26335

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	3 9% 3 4 3 3 3 3 4 6% 58%	3845
	11th to 20th	Day	2 3 1 3 3 2 2 2 4 5% 73%	4100
	21st to end	Day	2 5% 2 6% 4 4 3 3 5% 6% 60%	4510
February	1st to 10th	Day	2 5% 2 4 4 2 2 4 4 6% 65%	1952
	11th to 20th	Day	3 5% 2 5% 3 3 3 2 5% 7% 63%	1949
	21st to end	Day	2 4 1 4 2 2 2 3 4 5% 69%	1650
March	1st to 10th	Day	2 4 1 3 3 3 4 2 4 6% 67%	4500
	11th to 20th	Day	8% 5% 1 3 3 2 2 3 4 6% 62%	5000
	21st to end	Day	2 4 1 4 3 3 3 4 5% 7% 62%	5809
April	1st to 10th	Day	3 4 1 3 2 2 3 3 4 7% 68%	2400
	11th to 20th	Day	4 4 1 3 2 3 3 3 4 6% 66%	2371
	21st to end	Day	3 3 1 2 2 2 3 3 4 6% 72%	2400
May	1st to 10th	Day	5% 3 1 3 2 3 4 4 5% 7% 62%	5440
	11th to 20th	Day	6% 3 1 2 2 2 3 3 4 6% 67%	5712
	21st to end	Day	5% 3 1 2 2 2 4 3 5% 6% 68%	6725
June	1st to 10th	Day	3 3 1 3 3 3 4 4 7% 9% 60%	2800
	11th to 20th	Day	2 2 1 3 3 2 3 3 5% 7% 70%	2800
	21st to end	Day	5% 3 1 4 3 4 3 5% 7% 8% 56%	2800
July	1st to 10th	Day	8% 4 1 4 4 4 5% 5% 8% 9% 49%	6339
	11th to 20th	Day	2 2 1 3 3 3 5% 5% 8% 12% 56%	6028
	21st to end	Day	4 3 1 5% 5% 5% 6% 6% 9% 12% 45%	6401
August	1st to 10th	Day	2 2 1 4 3 4 5% 4 7% 11% 57%	5358
	11th to 20th	Day	3 2 1 4 3 4 4 4 7% 11% 58%	5366
	21st to end	Day	3 3 1 5% 4 5% 6% 5% 8% 11% 50%	5984
September	1st to 10th	Day	6% 4 2 4 4 3 4 4 6% 8% 54%	2400
	11th to 20th	Day	7% 3 1 4 3 3 3 4 5% 7% 60%	2336
	21st to end	Day	6% 2 1 2 2 3 2 2 3 5% 72%	2090
October	1st to 10th	Day	5% 2 1 2 2 2 2 2 3 3 76%	3277
	11th to 20th	Day	5% 3 1 3 2 3 3 3 4 6% 67%	3150
	21st to end	Day	2 2 1 2 1 2 2 1 2 4 81%	3438
November	1st to 10th	Day	4 3 1 2 2 2 1 3 4 5% 74%	1819
	11th to 20th	Day	2 3 1 3 2 2 3 2 5% 6% 71%	1800
	21st to end	Day	5% 4 1 4 2 2 3 1 4 6% 66%	1800
December	1st to 10th	Day	2 3 1 4 2 1 2 2 4 6% 74%	2918
	11th to 20th	Day	2 2 1 2 2 2 3 3 5% 8% 70%	3026
	21st to end	Day	2 3 1 2 2 2 2 2 4 7% 72%	4004
January	1st to 10th	Night	2 6% 2 5% 3 3 4 2 5% 7% 60%	6161
	11th to 20th	Night	2 4 1 3 3 2 3 2 3 5% 73%	5880
	21st to end	Night	2 4 2 4 3 3 4 4 5% 7% 62%	6468
February	1st to 10th	Night	2 5% 2 4 3 2 3 3 5% 7% 64%	2445
	11th to 20th	Night	4 5% 2 4 3 3 3 3 5% 6% 62%	2338
	21st to end	Night	2 5% 1 3 2 2 3 3 4 6% 70%	1980
March	1st to 10th	Night	3 4 1 3 2 2 3 3 5% 6% 68%	5358
	11th to 20th	Night	10% 4 1 4 2 2 2 2 3 5% 64%	5000
	21st to end	Night	1 2 1 3 3 3 3 3 5% 7% 69%	5150
April	1st to 10th	Night	3 3 1 3 3 3 2 3 5% 6% 68%	2000
	11th to 20th	Night	5% 4 1 3 3 3 3 2 3 7% 66%	1977
	21st to end	Night	5% 3 1 2 2 2 3 2 4 6% 70%	2000
May	1st to 10th	Night	9% 4 1 4 3 3 3 3 4 6% 59%	4540
	11th to 20th	Night	7% 3 1 3 2 3 3 3 5% 7% 64%	4201
	21st to end	Night	6% 3 1 3 2 2 3 3 4 5% 69%	4155
June	1st to 10th	Night	4 4 2 4 3 4 5% 5% 9% 9% 50%	1600
	11th to 20th	Night	2 1 1 3 2 3 3 3 6% 8% 68%	1600
	21st to end	Night	5% 5% 2 4 4 5% 4 5% 7% 10% 48%	1600
July	1st to 10th	Night	7% 5% 2 5% 4 4 5% 4 7% 9% 49%	3640
	11th to 20th	Night	3 3 2 5% 4 5% 6% 6% 9% 14% 43%	3933
	21st to end	Night	6% 5% 3 7% 5% 5% 6% 5% 9% 12% 36%	4472
August	1st to 10th	Night	3 3 2 6% 6% 6% 8% 7% 9% 12% 39%	4454
	11th to 20th	Night	3 3 1 5% 5% 5% 5% 5% 9% 12% 46%	4483
	21st to end	Night	5% 5% 2 7% 6% 6% 7% 6% 9% 11% 37%	4993
September	1st to 10th	Night	6% 4 2 5% 3 3 4 4 7% 8% 53%	2000
	11th to 20th	Night	7% 4 1 4 3 4 4 3 4 6% 58%	2018
	21st to end	Night	6% 3 1 3 2 2 3 2 4 6% 67%	2090
October	1st to 10th	Night	6% 3 1 3 2 3 3 2 3 4 71%	3672
	11th to 20th	Night	8% 3 1 2 2 2 2 2 4 5% 70%	3778
	21st to end	Night	3 2 1 2 2 1 2 2 4 5% 76%	4120
November	1st to 10th	Night	3 3 1 3 2 2 2 3 3 6% 73%	2458
	11th to 20th	Night	2 3 1 3 2 2 2 3 4 6% 73%	2600
	21st to end	Night	6% 4 2 4 3 2 2 2 4 6% 65%	2600
December	1st to 10th	Night	2 4 2 4 3 2 3 3 4 6% 67%	4267
	11th to 20th	Night	2 2 1 3 3 2 3 3 5% 8% 69%	5181
	21st to end	Night	2 4 1 3 2 3 3 3 5% 6% 68%	6974

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	29% 2 2 20% 33% 14%	3846
	11th to 20th	Day	16% 1 5% 22% 31% 24%	4100
	21st to end	Day	26% 2 4 17% 31% 21%	4510
February	1st to 10th	Day	20% 3 6% 17% 32% 22%	1952
	11th to 20th	Day	24% 3 3 20% 29% 21%	1959
	21st to end	Day	19% 2 1 12% 36% 30%	1650
March	1st to 10th	Day	19% 2 1 8% 30% 39%	4500
	11th to 20th	Day	25% 2 2 15% 29% 27%	5000
	21st to end	Day	21% 3 3 15% 37% 20%	5809
April	1st to 10th	Day	17% 3 3 17% 33% 27%	2400
	11th to 20th	Day	20% 2 3 19% 26% 29%	2374
	21st to end	Day	15% 2 6% 19% 35% 23%	2400
May	1st to 10th	Day	21% 3 10% 20% 22% 25%	5440
	11th to 20th	Day	19% 2 4 13% 25% 37%	5716
	21st to end	Day	17% 2 8% 16% 16% 40%	6726
June	1st to 10th	Day	20% 4 11% 23% 21% 22%	2800
	11th to 20th	Day	14% 3 4 16% 25% 39%	2800
	21st to end	Day	23% 4 9% 21% 23% 19%	2800
July	1st to 10th	Day	30% 6% 12% 20% 16% 16%	6271
	11th to 20th	Day	18% 3 19% 33% 21% 5%	5955
	21st to end	Day	28% 4 20% 29% 17% 3	6217
August	1st to 10th	Day	20% 4 17% 30% 19% 11%	3901
	11th to 20th	Day	20% 3 10% 25% 31% 11%	3882
	21st to end	Day	26% 5% 19% 25% 21% 4	4311
September	1st to 10th	Day	27% 4 10% 23% 24% 13%	2400
	11th to 20th	Day	25% 3 8% 22% 25% 18%	2336
	21st to end	Day	18% 1 3 22% 36% 19%	2090
October	1st to 10th	Day	15% 1 4 12% 31% 36%	3277
	11th to 20th	Day	21% 2 7% 16% 26% 28%	3150
	21st to end	Day	11% 1 2 16% 37% 34%	3438
November	1st to 10th	Day	15% 1 4 18% 34% 28%	1819
	11th to 20th	Day	16% 2 3 20% 36% 23%	1800
	21st to end	Day	22% 2 4 17% 28% 28%	1800
December	1st to 10th	Day	14% 2 3 26% 36% 19%	2918
	11th to 20th	Day	14% 2 3 24% 37% 20%	3024
	21st to end	Day	14% 2 3 25% 33% 23%	4004
January	1st to 10th	Night	25% 2 3 22% 38% 11%	6161
	11th to 20th	Night	17% 1 6% 21% 34% 21%	5880
	21st to end	Night	22% 1 5% 19% 34% 18%	6468
February	1st to 10th	Night	20% 3 4 21% 29% 22%	2445
	11th to 20th	Night	23% 4 4 20% 29% 21%	2338
	21st to end	Night	17% 2 2 15% 32% 32%	1980
March	1st to 10th	Night	17% 2 2 11% 31% 38%	5358
	11th to 20th	Night	26% 1 2 15% 30% 26%	5000
	21st to end	Night	15% 3 3 20% 38% 22%	5150
April	1st to 10th	Night	17% 3 4 15% 34% 27%	2000
	11th to 20th	Night	21% 2 5% 16% 24% 31%	1977
	21st to end	Night	17% 3 7% 21% 29% 24%	2000
May	1st to 10th	Night	26% 3 8% 17% 17% 29%	4537
	11th to 20th	Night	21% 2 6% 15% 21% 34%	4202
	21st to end	Night	18% 3 9% 11% 11% 48%	4156
June	1st to 10th	Night	27% 5% 13% 19% 16% 21%	1600
	11th to 20th	Night	15% 3 8% 16% 20% 39%	1600
	21st to end	Night	29% 4 14% 18% 18% 18%	1600
July	1st to 10th	Night	31% 5% 14% 22% 16% 12%	3600
	11th to 20th	Night	28% 5% 22% 25% 16% 4	3883
	21st to end	Night	38% 5% 19% 22% 15% 2	4344
August	1st to 10th	Night	34% 6% 16% 21% 14% 11%	3235
	11th to 20th	Night	26% 4 12% 23% 26% 10%	3243
	21st to end	Night	35% 5% 20% 21% 15% 4	3597
September	1st to 10th	Night	27% 5% 12% 25% 21% 11%	2000
	11th to 20th	Night	28% 4 10% 17% 21% 20%	2018
	21st to end	Night	20% 2 6% 22% 30% 20%	2090
October	1st to 10th	Night	18% 2 4 12% 30% 34%	3672
	11th to 20th	Night	20% 2 7% 13% 25% 32%	3778
	21st to end	Night	13% 1 3 16% 36% 31%	4120
November	1st to 10th	Night	16% 1 3 17% 35% 28%	2458
	11th to 20th	Night	15% 1 3 19% 38% 24%	2600
	21st to end	Night	22% 2 3 19% 28% 26%	2600
December	1st to 10th	Night	20% 2 4 27% 30% 17%	4267
	11th to 20th	Night	15% 1 3 18% 40% 22%	5182
	21st to end	Night	19% 2 3 21% 36% 20%	6974

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	11% 15% 37% 24% 11% 1	300
	11th to 20th	Day	17% 13% 39% 24% 4 3	300
	21st to end	Day	8% 15% 38% 24% 10% 4	330
February	1st to 10th	Day	8% 19% 37% 26% 9%	300
	11th to 20th	Day	14% 15% 45% 20% 4 1	300
	21st to end	Day	15% 21% 41% 18% 4 1	249
March	1st to 10th	Day	10% 15% 41% 24% 8% 1	300
	11th to 20th	Day	17% 17% 37% 22% 6% 2	500
	21st to end	Day	15% 23% 36% 19% 6% 1	550
April	1st to 10th	Day	10% 19% 40% 24% 6% 2	500
	11th to 20th	Day	13% 16% 45% 18% 8%	492
	21st to end	Day	10% 16% 41% 25% 7% 1	500
May	1st to 10th	Day	7% 15% 41% 25% 10% 2	503
	11th to 20th	Day	9% 10% 30% 31% 16% 4	653
	21st to end	Day	8% 9% 33% 33% 15% 2	789
June	1st to 10th	Day	6% 4 23% 35% 27% 6%	730
	11th to 20th	Day	4 3 29% 37% 23% 4	730
	21st to end	Day	8% 2 17% 37% 28% 8%	730
July	1st to 10th	Day	5% 1 11% 33% 38% 12%	721
	11th to 20th	Day	3 5% 32% 41% 20%	590
	21st to end	Day	5% 1 6% 30% 37% 22%	575
August	1st to 10th	Day	4 2 8% 33% 37% 16%	527
	11th to 20th	Day	5% 1 9% 37% 34% 15%	517
	21st to end	Day	4 1 10% 36% 35% 14%	583
September	1st to 10th	Day	6% 2 14% 38% 31% 10%	530
	11th to 20th	Day	5% 2 16% 40% 27% 10%	482
	21st to end	Day	9% 3 18% 34% 29% 7%	300
October	1st to 10th	Day	15% 4 22% 26% 26% 7%	300
	11th to 20th	Day	8% 7% 29% 31% 19% 6%	300
	21st to end	Day	11% 7% 32% 29% 17% 4	330
November	1st to 10th	Day	11% 16% 35% 21% 13% 3	300
	11th to 20th	Day	8% 11% 32% 33% 13% 4	300
	21st to end	Day	14% 14% 37% 22% 11% 2	300
December	1st to 10th	Day	10% 14% 35% 30% 9% 3	300
	11th to 20th	Day	11% 13% 45% 25% 5% 1	300
	21st to end	Day	14% 24% 35% 20% 6% 1	330
January	1st to 10th	Night	6% 18% 39% 27% 9% 1	925
	11th to 20th	Night	7% 16% 43% 27% 6% 1	800
	21st to end	Night	7% 17% 41% 29% 6% 1	880
February	1st to 10th	Night	7% 16% 44% 28% 5% 1	800
	11th to 20th	Night	8% 17% 48% 25% 3	784
	21st to end	Night	8% 17% 53% 19% 2	660
March	1st to 10th	Night	8% 15% 44% 28% 6%	800
	11th to 20th	Night	13% 12% 37% 30% 8%	800
	21st to end	Night	8% 15% 47% 26% 4	735
April	1st to 10th	Night	7% 17% 48% 26% 3	600
	11th to 20th	Night	9% 14% 46% 27% 5%	593
	21st to end	Night	8% 12% 45% 29% 6%	600
May	1st to 10th	Night	7% 9% 42% 37% 6%	600
	11th to 20th	Night	10% 7% 31% 39% 12% 1	591
	21st to end	Night	6% 7% 38% 38% 10% 1	650
June	1st to 10th	Night	7% 3 29% 45% 17%	600
	11th to 20th	Night	3 3 39% 45% 11%	600
	21st to end	Night	7% 3 23% 48% 19% 1	600
July	1st to 10th	Night	6% 1 10% 49% 32% 1	585
	11th to 20th	Night	4 1 8% 48% 36% 2	582
	21st to end	Night	6% 2 6% 43% 41% 3	657
August	1st to 10th	Night	6% 1 7% 40% 42% 5%	597
	11th to 20th	Night	7% 1 7% 44% 38% 3	594
	21st to end	Night	5% 1 9% 46% 37% 2	660
September	1st to 10th	Night	7% 1 13% 48% 30% 1	600
	11th to 20th	Night	10% 1 15% 51% 21% 1	594
	21st to end	Night	8% 2 25% 44% 20% 1	570
October	1st to 10th	Night	11% 3 26% 40% 19%	684
	11th to 20th	Night	6% 6% 35% 35% 16% 2	760
	21st to end	Night	9% 8% 32% 33% 16% 1	836
November	1st to 10th	Night	6% 8% 35% 37% 12% 1	898
	11th to 20th	Night	4 11% 38% 34% 12% 1	1000
	21st to end	Night	9% 13% 36% 30% 11% 2	1000
December	1st to 10th	Night	7% 15% 41% 30% 7%	1000
	11th to 20th	Night	8% 18% 42% 27% 5% 1	1000
	21st to end	Night	10% 18% 44% 23% 4	1100

Page generated at 2025-09-15 23:49:15. Version ac07

Copyright © 2025 Attilla Danko.