LEAR Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the LEAR Observatory clear sky chart from 2005-11-06 to 2024-11-12.

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% 7% 3 6% 4 4 4 4 5% 7% 52%	104793
	Night	5% 8% 3 7% 4 4 4 3 5% 7% 50%	103142
Transparency	Day	28% 9% 11% 18% 20% 14%	104558
	Night	30% 9% 12% 19% 19% 11%	102966
Seeing	Day	8% 15% 31% 31% 12% 2	14771
	Night	9% 9% 23% 36% 20% 3	25379

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	6% 15% 6% 16% 7% 4 4 3 5% 5% 28%	2858
	11th to 20th	Day	7% 20% 6% 14% 6% 4 3 3 4 6% 26%	2860
	21st to end	Day	7% 14% 5% 10% 5% 5% 5% 4 4 6% 33%	3146
February	1st to 10th	Day	7% 12% 4 9% 6% 5% 6% 5% 5% 6% 34%	1710
	11th to 20th	Day	7% 13% 4 10% 6% 4 5% 4 6% 6% 34%	1740
	21st to end	Day	5% 14% 5% 10% 5% 5% 4 4 6% 8% 33%	1570
March	1st to 10th	Day	2 11% 6% 13% 6% 5% 5% 5% 5% 7% 33%	3350
	11th to 20th	Day	5% 10% 4 7% 4 4 4 4 5% 8% 45%	3490
	21st to end	Day	6% 16% 5% 12% 7% 6% 5% 4 6% 7% 25%	3825
April	1st to 10th	Day	7% 10% 3 7% 5% 4 6% 4 6% 6% 43%	2100
	11th to 20th	Day	6% 10% 3 7% 6% 5% 4 4 6% 6% 43%	2403
	21st to end	Day	7% 9% 2 6% 5% 5% 4 3 5% 8% 47%	2470
May	1st to 10th	Day	7% 10% 3 7% 4 5% 5% 4 6% 8% 41%	4110
	11th to 20th	Day	4 4 1 3 3 3 4 3 6% 8% 59%	4071
	21st to end	Day	5% 6% 2 5% 4 4 5% 5% 6% 7% 51%	4477
June	1st to 10th	Day	3 4 2 5% 3 4 4 4 7% 8% 57%	2470
	11th to 20th	Day	4 4 1 4 3 3 3 3 5% 6% 64%	2470
	21st to end	Day	2 2 1 2 3 3 3 2 4 5% 72%	2470
July	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 78%	4110
	11th to 20th	Day	2 3 1 3 3 4 5% 4 7% 8% 60%	4110
	21st to end	Day	2 2 1 3 3 3 4 4 5% 7% 66%	4454
August	1st to 10th	Day	2 2 1 3 3 4 4 4 6% 7% 64%	4092
	11th to 20th	Day	2 2 1 4 3 5% 4 3 5% 7% 65%	4028
	21st to end	Day	2 2 1 3 2 3 3 3 4 6% 70%	4493
September	1st to 10th	Day	2 2 1 3 2 2 3 3 4 6% 71%	2135
	11th to 20th	Day	3 2 1 3 3 3 4 4 4 7% 65%	2090
	21st to end	Day	5% 3 1 3 2 2 2 2 4 6% 70%	2090
October	1st to 10th	Day	4 4 1 3 2 3 3 2 3 4 70%	3340
	11th to 20th	Day	7% 5% 1 4 3 3 3 2 5% 7% 61%	3150
	21st to end	Day	6% 6% 2 4 3 3 4 3 4 6% 59%	3418
November	1st to 10th	Day	7% 7% 3 6% 3 3 5% 4 6% 7% 48%	1764
	11th to 20th	Day	9% 8% 2 6% 5% 3 4 4 5% 7% 46%	1719
	21st to end	Day	4 12% 3 8% 5% 5% 4 3 6% 7% 42%	1609
December	1st to 10th	Day	8% 10% 4 9% 4 3 5% 3 5% 8% 40%	1520
	11th to 20th	Day	8% 11% 4 10% 6% 4 7% 4 6% 9% 31%	1935
	21st to end	Day	6% 12% 5% 11% 6% 4 5% 3 6% 8% 33%	3146
January	1st to 10th	Night	6% 13% 6% 15% 6% 5% 4 4 4 7% 31%	4097
	11th to 20th	Night	10% 19% 5% 13% 5% 4 4 3 4 7% 25%	4110
	21st to end	Night	6% 13% 4 10% 6% 6% 6% 4 6% 9% 31%	4521
February	1st to 10th	Night	6% 11% 4 9% 6% 5% 5% 4 6% 8% 36%	2470
	11th to 20th	Night	6% 12% 3 8% 5% 5% 5% 3 5% 7% 40%	2323
	21st to end	Night	5% 12% 4 9% 5% 4 5% 4 5% 7% 39%	1884
March	1st to 10th	Night	3 10% 4 12% 7% 6% 6% 4 6% 8% 34%	3524
	11th to 20th	Night	4 8% 3 6% 4 4 5% 4 5% 8% 49%	3490
	21st to end	Night	6% 15% 6% 11% 6% 5% 4 4 5% 5% 34%	3828
April	1st to 10th	Night	10% 9% 3 6% 5% 3 4 3 5% 6% 47%	2078
	11th to 20th	Night	5% 7% 3 6% 4 4 6% 5% 6% 7% 47%	1723
	21st to end	Night	8% 7% 3 6% 4 4 4 4 5% 6% 49%	1710
May	1st to 10th	Night	5% 10% 2 8% 5% 4 4 4 6% 7% 42%	2860
	11th to 20th	Night	4 3 2 4 3 3 4 3 5% 7% 61%	2834
	21st to end	Night	7% 5% 2 5% 4 3 4 3 6% 8% 54%	3110
June	1st to 10th	Night	5% 4 1 5% 3 3 4 4 6% 8% 57%	1710
	11th to 20th	Night	4 3 1 4 2 3 3 3 5% 6% 66%	1710
	21st to end	Night	2 3 1 3 3 2 3 3 6% 7% 68%	1710
July	1st to 10th	Night	1 2 1 4 3 3 3 3 3 5% 72%	2860
	11th to 20th	Night	2 3 2 4 4 5% 5% 5% 7% 9% 54%	2860
	21st to end	Night	2 3 2 5% 4 3 4 4 6% 6% 60%	3109
August	1st to 10th	Night	2 2 1 4 3 3 5% 4 6% 9% 61%	2847
	11th to 20th	Night	2 2 2 5% 3 4 5% 4 5% 6% 62%	2811
	21st to end	Night	2 3 1 4 2 3 3 3 4 5% 70%	3172
September	1st to 10th	Night	3 3 1 3 3 3 3 3 4 5% 68%	2042
	11th to 20th	Night	4 3 1 2 3 3 4 3 4 7% 66%	2090
	21st to end	Night	5% 3 2 3 2 2 2 3 3 4 71%	2090
October	1st to 10th	Night	4 3 1 3 3 2 3 2 3 5% 72%	3635
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 62%	3778
	21st to end	Night	5% 6% 2 5% 3 3 3 3 5% 7% 59%	4137
November	1st to 10th	Night	7% 8% 3 6% 4 3 3 3 6% 7% 51%	2535
	11th to 20th	Night	8% 6% 2 5% 4 3 4 3 5% 8% 52%	2483
	21st to end	Night	6% 9% 3 6% 5% 4 5% 3 5% 8% 47%	2547
December	1st to 10th	Night	7% 10% 3 7% 4 4 4 3 6% 8% 44%	2660
	11th to 20th	Night	10% 12% 4 8% 5% 4 5% 3 6% 8% 35%	3271
	21st to end	Night	7% 12% 5% 11% 6% 5% 5% 4 6% 8% 31%	4523

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	50% 15% 14% 14% 5%	2859
	11th to 20th	Day	55% 12% 16% 12% 4 1	2860
	21st to end	Day	48% 13% 19% 13% 6% 1	3146
February	1st to 10th	Day	47% 12% 16% 15% 9% 1	1710
	11th to 20th	Day	45% 11% 15% 17% 11% 1	1749
	21st to end	Day	44% 13% 15% 17% 10% 1	1570
March	1st to 10th	Day	34% 19% 17% 20% 10% 1	3350
	11th to 20th	Day	30% 12% 17% 30% 10% 1	3490
	21st to end	Day	48% 16% 17% 12% 6% 1	3825
April	1st to 10th	Day	38% 11% 14% 18% 14% 5%	2100
	11th to 20th	Day	36% 10% 14% 21% 16% 2	2406
	21st to end	Day	31% 10% 14% 23% 20% 2	2470
May	1st to 10th	Day	30% 14% 13% 21% 17% 4	4110
	11th to 20th	Day	19% 8% 10% 30% 29% 4	4074
	21st to end	Day	23% 11% 13% 26% 23% 4	4477
June	1st to 10th	Day	21% 9% 11% 22% 28% 10%	2470
	11th to 20th	Day	20% 6% 8% 18% 29% 20%	2470
	21st to end	Day	13% 4 5% 12% 29% 36%	2470
July	1st to 10th	Day	9% 3 5% 7% 15% 61%	4046
	11th to 20th	Day	17% 5% 6% 11% 24% 36%	4041
	21st to end	Day	15% 6% 7% 10% 23% 40%	4314
August	1st to 10th	Day	15% 5% 6% 17% 28% 28%	4078
	11th to 20th	Day	17% 5% 6% 14% 33% 26%	4067
	21st to end	Day	14% 4 6% 15% 28% 33%	4492
September	1st to 10th	Day	14% 5% 7% 16% 28% 30%	2135
	11th to 20th	Day	17% 8% 9% 18% 26% 21%	2090
	21st to end	Day	17% 5% 8% 20% 31% 19%	2090
October	1st to 10th	Day	18% 5% 6% 13% 38% 21%	3340
	11th to 20th	Day	23% 4 8% 28% 28% 9%	3150
	21st to end	Day	25% 7% 11% 31% 23% 4	3418
November	1st to 10th	Day	33% 9% 14% 21% 20% 3	1764
	11th to 20th	Day	35% 9% 13% 28% 14% 1	1719
	21st to end	Day	39% 9% 16% 23% 11% 2	1609
December	1st to 10th	Day	42% 11% 17% 19% 9% 2	1520
	11th to 20th	Day	48% 11% 15% 18% 8%	1933
	21st to end	Day	43% 12% 17% 16% 11%	3146
January	1st to 10th	Night	43% 16% 16% 17% 7%	4097
	11th to 20th	Night	55% 13% 15% 12% 5% 1	4110
	21st to end	Night	44% 14% 21% 14% 7% 1	4521
February	1st to 10th	Night	42% 12% 19% 18% 8% 1	2470
	11th to 20th	Night	41% 8% 14% 20% 15% 1	2324
	21st to end	Night	39% 10% 14% 23% 11% 2	1884
March	1st to 10th	Night	37% 15% 14% 21% 12% 1	3524
	11th to 20th	Night	26% 12% 15% 30% 15% 2	3490
	21st to end	Night	42% 15% 13% 17% 12% 2	3828
April	1st to 10th	Night	34% 9% 15% 21% 16% 6%	2078
	11th to 20th	Night	31% 10% 13% 22% 21% 3	1723
	21st to end	Night	32% 9% 15% 20% 21% 2	1710
May	1st to 10th	Night	27% 14% 13% 22% 21% 2	2860
	11th to 20th	Night	18% 9% 10% 28% 28% 6%	2835
	21st to end	Night	23% 9% 13% 27% 26% 3	3110
June	1st to 10th	Night	23% 8% 11% 21% 27% 10%	1710
	11th to 20th	Night	17% 7% 8% 18% 30% 20%	1710
	21st to end	Night	15% 5% 5% 10% 27% 38%	1710
July	1st to 10th	Night	14% 3 4 9% 11% 60%	2815
	11th to 20th	Night	21% 5% 6% 12% 25% 32%	2806
	21st to end	Night	22% 4 7% 11% 23% 33%	3011
August	1st to 10th	Night	16% 5% 8% 15% 27% 29%	2838
	11th to 20th	Night	19% 5% 8% 14% 29% 24%	2838
	21st to end	Night	16% 4 7% 10% 29% 34%	3172
September	1st to 10th	Night	17% 5% 7% 15% 25% 32%	2042
	11th to 20th	Night	18% 5% 9% 17% 27% 24%	2090
	21st to end	Night	17% 4 6% 16% 32% 24%	2090
October	1st to 10th	Night	15% 4 6% 16% 35% 24%	3635
	11th to 20th	Night	22% 5% 6% 28% 30% 9%	3778
	21st to end	Night	23% 6% 9% 35% 23% 4	4137
November	1st to 10th	Night	31% 8% 12% 23% 22% 5%	2535
	11th to 20th	Night	28% 10% 15% 27% 16% 3	2483
	21st to end	Night	33% 10% 13% 27% 15% 3	2547
December	1st to 10th	Night	37% 9% 17% 24% 10% 3	2660
	11th to 20th	Night	44% 9% 16% 20% 10%	3272
	21st to end	Night	46% 12% 16% 16% 10% 1	4523

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% 20% 23% 23% 15% 1	270
	11th to 20th	Day	9% 20% 35% 23% 11% 2	270
	21st to end	Day	10% 15% 32% 30% 11% 2	297
February	1st to 10th	Day	14% 27% 30% 19% 9%	270
	11th to 20th	Day	15% 18% 36% 22% 8% 1	345
	21st to end	Day	12% 23% 37% 23% 4 1	382
March	1st to 10th	Day	10% 25% 35% 22% 7% 1	460
	11th to 20th	Day	10% 20% 36% 27% 7%	460
	21st to end	Day	17% 24% 34% 22% 3	506
April	1st to 10th	Day	13% 21% 31% 26% 8% 1	460
	11th to 20th	Day	15% 25% 33% 21% 6% 1	452
	21st to end	Day	13% 23% 28% 26% 10% 1	460
May	1st to 10th	Day	9% 25% 35% 22% 9% 1	463
	11th to 20th	Day	11% 16% 32% 29% 10% 3	488
	21st to end	Day	8% 13% 33% 29% 14% 3	529
June	1st to 10th	Day	4 13% 31% 36% 12% 3	490
	11th to 20th	Day	6% 14% 36% 31% 12% 1	490
	21st to end	Day	2 11% 35% 40% 10% 1	490
July	1st to 10th	Day	1 6% 30% 45% 15% 3	486
	11th to 20th	Day	3 6% 33% 40% 15% 4	484
	21st to end	Day	2 2 25% 49% 18% 4	531
August	1st to 10th	Day	4 6% 29% 43% 16% 2	487
	11th to 20th	Day	3 7% 29% 42% 16% 4	477
	21st to end	Day	3 7% 27% 40% 18% 4	539
September	1st to 10th	Day	3 7% 27% 39% 19% 5%	490
	11th to 20th	Day	5% 11% 30% 35% 16% 2	490
	21st to end	Day	4 14% 23% 37% 19% 3	490
October	1st to 10th	Day	5% 16% 28% 26% 20% 4	405
	11th to 20th	Day	8% 14% 28% 24% 22% 3	300
	21st to end	Day	9% 14% 34% 30% 13%	330
November	1st to 10th	Day	14% 14% 29% 27% 13% 2	300
	11th to 20th	Day	10% 17% 28% 29% 12% 5%	273
	21st to end	Day	12% 14% 24% 27% 20% 2	270
December	1st to 10th	Day	11% 15% 29% 29% 13% 3	270
	11th to 20th	Day	14% 11% 31% 34% 7% 2	270
	21st to end	Day	19% 17% 28% 29% 8%	297
January	1st to 10th	Night	14% 11% 22% 33% 17% 4	760
	11th to 20th	Night	12% 10% 24% 33% 17% 5%	760
	21st to end	Night	13% 10% 21% 34% 18% 4	836
February	1st to 10th	Night	10% 10% 29% 34% 14% 3	760
	11th to 20th	Night	11% 10% 28% 34% 15% 2	744
	21st to end	Night	10% 13% 30% 33% 13% 2	628
March	1st to 10th	Night	11% 13% 29% 33% 11% 4	760
	11th to 20th	Night	9% 10% 26% 35% 18% 3	760
	21st to end	Night	9% 16% 29% 33% 10% 3	836
April	1st to 10th	Night	14% 14% 26% 31% 13% 2	748
	11th to 20th	Night	7% 14% 30% 36% 11% 1	563
	21st to end	Night	9% 11% 26% 35% 16% 2	570
May	1st to 10th	Night	8% 9% 30% 36% 15% 2	570
	11th to 20th	Night	7% 7% 22% 35% 25% 4	561
	21st to end	Night	8% 8% 25% 34% 21% 4	617
June	1st to 10th	Night	5% 10% 18% 40% 25% 2	570
	11th to 20th	Night	4 10% 25% 39% 20% 2	570
	21st to end	Night	4 5% 22% 40% 26% 4	570
July	1st to 10th	Night	2 4 21% 37% 34% 2	555
	11th to 20th	Night	3 3 18% 45% 27% 3	552
	21st to end	Night	3 2 13% 36% 44% 1	624
August	1st to 10th	Night	4 2 17% 43% 31% 2	567
	11th to 20th	Night	3 2 18% 45% 29% 3	564
	21st to end	Night	2 4 18% 43% 30% 3	627
September	1st to 10th	Night	5% 4 15% 41% 30% 5%	712
	11th to 20th	Night	5% 6% 18% 38% 27% 6%	760
	21st to end	Night	7% 5% 17% 33% 29% 9%	760
October	1st to 10th	Night	7% 7% 22% 35% 25% 4	760
	11th to 20th	Night	6% 6% 21% 36% 24% 5%	760
	21st to end	Night	7% 8% 25% 33% 22% 4	836
November	1st to 10th	Night	11% 11% 18% 36% 20% 4	780
	11th to 20th	Night	9% 8% 27% 36% 16% 5%	764
	21st to end	Night	11% 8% 23% 33% 20% 5%	837
December	1st to 10th	Night	11% 9% 27% 35% 17% 2	950
	11th to 20th	Night	17% 9% 25% 35% 12% 2	950
	21st to end	Night	18% 13% 25% 32% 11% 1	838

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