First Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the First Light Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	2 4 2 4 3 3 3 3 5% 6% 64%	140337
	Night	3 5% 2 5% 3 3 4 3 5% 6% 60%	139905
Transparency	Day	20% 5% 8% 17% 25% 25%	135317
	Night	23% 5% 10% 19% 24% 18%	135906
Seeing	Day	6% 9% 23% 31% 22% 8%	16884
	Night	6% 5% 19% 40% 25% 5%	26419

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 7% 5% 4 5% 3 6% 7% 47%	4722
	11th to 20th	Day	5% 5% 1 5% 3 3 3 2 4 6% 63%	4740
	21st to end	Day	3 7% 3 6% 4 4 5% 5% 7% 10% 47%	5578
February	1st to 10th	Day	3 7% 3 6% 4 4 4 4 7% 7% 50%	2037
	11th to 20th	Day	3 7% 3 7% 5% 4 4 3 6% 9% 49%	2050
	21st to end	Day	3 8% 2 7% 4 4 5% 4 6% 8% 50%	1730
March	1st to 10th	Day	3 10% 3 9% 5% 5% 5% 4 7% 10% 39%	4618
	11th to 20th	Day	3 6% 2 6% 3 4 4 4 6% 7% 56%	4778
	21st to end	Day	2 9% 4 9% 5% 5% 5% 4 6% 6% 45%	6101
April	1st to 10th	Day	5% 7% 2 6% 4 4 4 4 5% 8% 51%	2520
	11th to 20th	Day	4 6% 3 5% 4 4 4 3 5% 6% 55%	2489
	21st to end	Day	4 8% 2 5% 3 3 3 3 6% 7% 55%	2567
May	1st to 10th	Day	3 4 1 4 3 2 3 3 4 6% 68%	6010
	11th to 20th	Day	2 3 1 4 3 3 3 3 4 6% 67%	5920
	21st to end	Day	3 3 1 4 3 3 4 3 4 5% 68%	6892
June	1st to 10th	Day	3 4 2 5% 4 3 4 3 6% 6% 61%	2800
	11th to 20th	Day	3 2 1 2 2 2 3 3 3 6% 74%	2800
	21st to end	Day	1 2 1 3 2 2 2 3 4 5% 76%	2800
July	1st to 10th	Day	1 1 1 2 2 2 2 2 2 86%	6319
	11th to 20th	Day	1 2 1 2 2 2 2 2 3 5% 77%	6320
	21st to end	Day	1 1 1 2 2 3 3 3 4 7% 73%	6880
August	1st to 10th	Day	1 2 2 2 2 2 2 2 4 81%	5657
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 86%	5364
	21st to end	Day	2 2 1 3 2 2 3 3 4 5% 74%	5984
September	1st to 10th	Day	2 2 1 2 2 2 3 2 3 4 78%	2400
	11th to 20th	Day	2 3 1 3 3 2 3 2 4 6% 72%	2275
	21st to end	Day	2 3 1 3 1 2 2 2 4 5% 75%	2200
October	1st to 10th	Day	2 1 1 3 2 2 3 2 4 6% 74%	3576
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 4 74%	3261
	21st to end	Day	3 2 1 3 3 3 4 3 5% 8% 66%	3274
November	1st to 10th	Day	4 5% 2 3 3 3 3 5% 6% 8% 57%	1854
	11th to 20th	Day	4 5% 2 4 3 3 3 3 6% 8% 59%	1890
	21st to end	Day	4 6% 2 6% 3 3 4 3 5% 6% 57%	1890
December	1st to 10th	Day	2 3 1 3 3 2 2 3 4 7% 69%	2841
	11th to 20th	Day	4 9% 3 7% 5% 4 5% 5% 7% 9% 41%	3108
	21st to end	Day	4 7% 3 8% 5% 5% 5% 4 7% 10% 41%	4092
January	1st to 10th	Night	3 8% 3 7% 4 4 5% 4 6% 9% 48%	8172
	11th to 20th	Night	5% 6% 2 5% 3 3 3 2 4 5% 63%	8220
	21st to end	Night	3 7% 3 6% 4 4 4 4 5% 7% 52%	8621
February	1st to 10th	Night	4 8% 3 7% 4 3 5% 4 6% 6% 51%	2580
	11th to 20th	Night	3 7% 2 6% 5% 6% 5% 4 6% 7% 50%	2458
	21st to end	Night	2 8% 3 7% 4 4 4 4 6% 7% 51%	2076
March	1st to 10th	Night	2 9% 4 9% 5% 5% 4 4 5% 6% 47%	5496
	11th to 20th	Night	3 6% 2 5% 4 4 4 4 6% 9% 54%	5437
	21st to end	Night	4 8% 2 5% 4 4 5% 4 7% 8% 48%	5116
April	1st to 10th	Night	4 6% 2 6% 5% 4 5% 4 6% 7% 53%	2100
	11th to 20th	Night	4 7% 2 4 4 3 4 4 5% 6% 57%	2079
	21st to end	Night	6% 7% 3 5% 3 3 3 3 4 5% 59%	2050
May	1st to 10th	Night	4 4 1 4 3 3 3 3 4 5% 66%	4190
	11th to 20th	Night	3 3 1 3 3 2 3 3 5% 5% 69%	4128
	21st to end	Night	3 4 2 5% 3 3 4 3 4 6% 63%	3979
June	1st to 10th	Night	3 3 1 4 3 4 4 4 6% 6% 61%	1600
	11th to 20th	Night	3 3 1 3 2 3 4 2 5% 5% 71%	1600
	21st to end	Night	2 2 1 2 2 3 4 2 6% 5% 71%	1600
July	1st to 10th	Night	1 1 1 2 2 2 2 2 3 4 81%	3640
	11th to 20th	Night	2 3 2 5% 5% 4 4 3 5% 5% 61%	3640
	21st to end	Night	2 3 1 3 4 4 4 3 6% 7% 64%	3970
August	1st to 10th	Night	3 2 1 3 2 2 3 3 4 5% 72%	4155
	11th to 20th	Night	1 1 1 3 2 2 3 3 4 4 76%	4506
	21st to end	Night	3 3 1 4 3 3 3 2 4 5% 70%	5015
September	1st to 10th	Night	2 2 1 3 2 3 3 2 4 5% 74%	2000
	11th to 20th	Night	3 4 1 3 3 3 3 3 4 5% 68%	2125
	21st to end	Night	3 3 1 2 2 2 2 2 3 4 77%	2200
October	1st to 10th	Night	3 3 1 4 3 2 4 2 4 4 71%	3622
	11th to 20th	Night	2 2 1 3 3 2 3 2 3 5% 74%	3897
	21st to end	Night	2 2 1 3 2 3 3 3 4 6% 73%	4528
November	1st to 10th	Night	4 5% 1 4 3 3 3 2 4 7% 63%	2665
	11th to 20th	Night	4 4 1 5% 3 3 3 3 5% 8% 60%	2730
	21st to end	Night	4 6% 2 5% 4 4 4 3 5% 8% 56%	2730
December	1st to 10th	Night	3 4 1 4 3 3 3 3 4 6% 66%	4558
	11th to 20th	Night	4 7% 3 7% 4 5% 4 4 7% 12% 44%	5316
	21st to end	Night	4 8% 4 10% 7% 6% 6% 5% 7% 8% 35%	7106

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	30% 9% 22% 25% 13%	4723
	11th to 20th	Day	23% 5% 11% 19% 33% 10%	4740
	21st to end	Day	28% 7% 11% 24% 22% 8%	5578
February	1st to 10th	Day	29% 7% 12% 26% 18% 8%	2037
	11th to 20th	Day	29% 7% 10% 26% 22% 5%	2060
	21st to end	Day	28% 8% 14% 24% 19% 6%	1730
March	1st to 10th	Day	33% 8% 11% 22% 18% 7%	4618
	11th to 20th	Day	24% 7% 12% 30% 25% 3	4778
	21st to end	Day	33% 8% 13% 20% 20% 6%	6101
April	1st to 10th	Day	30% 6% 11% 21% 23% 9%	2520
	11th to 20th	Day	27% 6% 11% 20% 25% 12%	2492
	21st to end	Day	27% 5% 10% 25% 27% 6%	2567
May	1st to 10th	Day	18% 4 7% 20% 41% 10%	6010
	11th to 20th	Day	16% 4 6% 21% 35% 17%	5923
	21st to end	Day	17% 4 6% 14% 36% 23%	6894
June	1st to 10th	Day	21% 6% 7% 17% 30% 20%	2800
	11th to 20th	Day	13% 3 5% 13% 32% 34%	2800
	21st to end	Day	11% 2 5% 10% 26% 46%	2800
July	1st to 10th	Day	8% 1 2 5% 13% 72%	6251
	11th to 20th	Day	11% 1 4 4 10% 70%	6245
	21st to end	Day	11% 3 4 10% 25% 48%	6681
August	1st to 10th	Day	13% 2 3 7% 22% 52%	4117
	11th to 20th	Day	7% 1 3 5% 15% 69%	3880
	21st to end	Day	9% 1 3 6% 27% 54%	4311
September	1st to 10th	Day	11% 3 4 8% 26% 50%	2400
	11th to 20th	Day	15% 3 5% 14% 30% 33%	2275
	21st to end	Day	13% 2 6% 13% 30% 36%	2200
October	1st to 10th	Day	14% 3 5% 10% 32% 37%	3576
	11th to 20th	Day	15% 3 5% 15% 42% 21%	3261
	21st to end	Day	16% 3 7% 23% 37% 14%	3274
November	1st to 10th	Day	21% 6% 12% 24% 25% 12%	1854
	11th to 20th	Day	21% 6% 12% 24% 29% 7%	1890
	21st to end	Day	27% 5% 9% 26% 26% 6%	1890
December	1st to 10th	Day	17% 4 10% 27% 31% 11%	2841
	11th to 20th	Day	34% 9% 16% 21% 16% 4	3108
	21st to end	Day	33% 10% 21% 23% 12% 1	4092
January	1st to 10th	Night	30% 8% 20% 31% 10% 1	8172
	11th to 20th	Night	23% 5% 11% 18% 31% 12%	8220
	21st to end	Night	30% 6% 10% 23% 26% 7%	8621
February	1st to 10th	Night	30% 6% 13% 25% 20% 6%	2580
	11th to 20th	Night	30% 7% 12% 24% 22% 5%	2459
	21st to end	Night	28% 7% 14% 25% 18% 7%	2076
March	1st to 10th	Night	33% 8% 9% 22% 19% 10%	5496
	11th to 20th	Night	22% 7% 12% 32% 23% 4	5437
	21st to end	Night	27% 9% 16% 27% 17% 4	5116
April	1st to 10th	Night	28% 7% 12% 23% 21% 8%	2100
	11th to 20th	Night	26% 6% 10% 25% 22% 11%	2079
	21st to end	Night	27% 6% 10% 24% 27% 6%	2050
May	1st to 10th	Night	19% 5% 9% 20% 38% 10%	4187
	11th to 20th	Night	16% 5% 6% 20% 37% 17%	4129
	21st to end	Night	18% 5% 8% 14% 36% 18%	3979
June	1st to 10th	Night	21% 6% 9% 19% 27% 19%	1600
	11th to 20th	Night	16% 3 6% 13% 32% 30%	1600
	21st to end	Night	13% 3 7% 10% 24% 43%	1600
July	1st to 10th	Night	9% 2 3 5% 12% 69%	3600
	11th to 20th	Night	23% 2 4 5% 10% 56%	3592
	21st to end	Night	18% 3 6% 14% 22% 36%	3857
August	1st to 10th	Night	19% 3 4 9% 18% 47%	3015
	11th to 20th	Night	11% 2 4 6% 16% 61%	3256
	21st to end	Night	12% 2 3 6% 26% 50%	3608
September	1st to 10th	Night	14% 3 4 8% 25% 47%	2000
	11th to 20th	Night	17% 5% 6% 13% 26% 34%	2125
	21st to end	Night	13% 3 5% 12% 31% 37%	2200
October	1st to 10th	Night	17% 2 6% 11% 26% 38%	3622
	11th to 20th	Night	13% 3 5% 13% 41% 24%	3897
	21st to end	Night	13% 3 6% 22% 43% 14%	4528
November	1st to 10th	Night	22% 5% 11% 23% 26% 13%	2665
	11th to 20th	Night	21% 5% 12% 25% 31% 6%	2730
	21st to end	Night	26% 5% 9% 25% 27% 7%	2730
December	1st to 10th	Night	20% 4 8% 24% 31% 12%	4558
	11th to 20th	Night	30% 8% 17% 23% 18% 4	5316
	21st to end	Night	41% 11% 13% 21% 13% 1	7106

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% 14% 25% 30% 15% 5%	220
	11th to 20th	Day	8% 9% 20% 30% 18% 15%	220
	21st to end	Day	8% 6% 19% 32% 24% 10%	377
February	1st to 10th	Day	9% 13% 26% 28% 17% 7%	507
	11th to 20th	Day	8% 15% 26% 30% 19% 3	535
	21st to end	Day	8% 11% 32% 32% 16% 1	446
March	1st to 10th	Day	11% 14% 25% 33% 14% 3	540
	11th to 20th	Day	9% 13% 26% 29% 17% 6%	540
	21st to end	Day	10% 15% 28% 30% 14% 2	594
April	1st to 10th	Day	11% 11% 24% 29% 19% 5%	540
	11th to 20th	Day	9% 16% 27% 28% 15% 4	532
	21st to end	Day	8% 13% 29% 29% 15% 6%	540
May	1st to 10th	Day	6% 17% 31% 26% 15% 5%	543
	11th to 20th	Day	8% 13% 27% 29% 17% 7%	553
	21st to end	Day	8% 10% 21% 29% 23% 9%	573
June	1st to 10th	Day	5% 9% 24% 26% 24% 12%	530
	11th to 20th	Day	6% 12% 28% 28% 18% 8%	530
	21st to end	Day	3 8% 23% 33% 23% 10%	530
July	1st to 10th	Day	3 6% 24% 29% 26% 11%	526
	11th to 20th	Day	2 4 17% 36% 28% 12%	524
	21st to end	Day	2 3 20% 33% 29% 13%	575
August	1st to 10th	Day	4 4 22% 34% 24% 12%	527
	11th to 20th	Day	2 4 21% 35% 24% 14%	517
	21st to end	Day	3 4 20% 33% 29% 12%	583
September	1st to 10th	Day	3 5% 19% 34% 28% 12%	530
	11th to 20th	Day	6% 7% 21% 34% 23% 9%	530
	21st to end	Day	4 6% 20% 37% 28% 5%	530
October	1st to 10th	Day	5% 8% 23% 30% 25% 9%	530
	11th to 20th	Day	4 9% 22% 33% 24% 6%	530
	21st to end	Day	4 10% 19% 31% 26% 9%	413
November	1st to 10th	Day	6% 5% 25% 32% 26% 8%	330
	11th to 20th	Day	4 14% 20% 32% 20% 11%	330
	21st to end	Day	11% 7% 18% 30% 24% 10%	330
December	1st to 10th	Day	7% 6% 16% 34% 25% 12%	267
	11th to 20th	Day	5% 9% 24% 30% 23% 10%	220
	21st to end	Day	17% 11% 17% 29% 20% 6%	242
January	1st to 10th	Night	8% 7% 25% 34% 19% 8%	840
	11th to 20th	Night	6% 4 21% 39% 23% 6%	840
	21st to end	Night	10% 5% 18% 42% 20% 6%	924
February	1st to 10th	Night	8% 5% 23% 39% 21% 5%	840
	11th to 20th	Night	6% 8% 23% 40% 18% 4	824
	21st to end	Night	6% 7% 25% 45% 15% 3	692
March	1st to 10th	Night	7% 8% 27% 39% 17% 3	840
	11th to 20th	Night	5% 7% 24% 37% 22% 5%	840
	21st to end	Night	9% 7% 26% 39% 17% 2	924
April	1st to 10th	Night	7% 8% 21% 43% 19% 3	840
	11th to 20th	Night	7% 8% 22% 40% 19% 4	831
	21st to end	Night	9% 8% 22% 38% 19% 5%	790
May	1st to 10th	Night	5% 8% 27% 41% 17% 2	630
	11th to 20th	Night	9% 6% 21% 39% 23% 2	609
	21st to end	Night	5% 6% 18% 40% 29% 3	440
June	1st to 10th	Night	3 2 19% 43% 30% 4	400
	11th to 20th	Night	4 3 19% 47% 26% 2	400
	21st to end	Night	3 1 16% 50% 28% 3	400
July	1st to 10th	Night	2 1 13% 53% 29% 2	390
	11th to 20th	Night	4 1 11% 47% 36% 2	388
	21st to end	Night	5% 1 9% 47% 37% 2	438
August	1st to 10th	Night	4 2 15% 45% 31% 2	643
	11th to 20th	Night	2 2 13% 40% 36% 7%	792
	21st to end	Night	3 1 9% 43% 39% 5%	880
September	1st to 10th	Night	3 3 10% 40% 37% 8%	800
	11th to 20th	Night	4 3 11% 39% 33% 10%	800
	21st to end	Night	5% 2 11% 41% 30% 10%	800
October	1st to 10th	Night	5% 5% 15% 41% 29% 6%	800
	11th to 20th	Night	4 3 18% 39% 30% 7%	800
	21st to end	Night	4 4 18% 37% 28% 10%	880
November	1st to 10th	Night	6% 3 17% 38% 28% 7%	820
	11th to 20th	Night	6% 3 19% 44% 23% 5%	840
	21st to end	Night	6% 6% 21% 39% 23% 6%	840
December	1st to 10th	Night	7% 5% 20% 42% 19% 6%	840
	11th to 20th	Night	10% 5% 22% 37% 23% 3	840
	21st to end	Night	12% 8% 25% 37% 16% 2	924

Page generated at 2026-05-13 22:59:00. Version ac07

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