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 2025-01-22.

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	3 5% 2 5% 3 3 3 3 5% 6% 62%	108903
	Night	4 6% 2 5% 4 4 4 3 5% 6% 58%	109188
Transparency	Day	21% 5% 9% 17% 23% 25%	108644
	Night	24% 6% 10% 19% 22% 18%	109037
Seeing	Day	6% 9% 23% 31% 22% 8%	15134
	Night	6% 5% 18% 40% 26% 5%	24691

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	4 13% 5% 9% 5% 4 5% 4 7% 7% 36%	2619
	11th to 20th	Day	8% 9% 2 9% 5% 5% 5% 3 5% 7% 40%	2620
	21st to end	Day	5% 10% 4 8% 5% 5% 5% 5% 6% 9% 37%	3001
February	1st to 10th	Day	4 7% 3 6% 4 4 4 4 7% 7% 50%	1843
	11th to 20th	Day	3 8% 3 7% 4 4 4 3 6% 8% 50%	1850
	21st to end	Day	3 8% 2 7% 4 4 5% 4 6% 8% 49%	1570
March	1st to 10th	Day	3 9% 3 8% 6% 6% 5% 4 7% 10% 39%	3138
	11th to 20th	Day	2 6% 2 6% 3 3 3 3 5% 6% 61%	3256
	21st to end	Day	3 13% 5% 9% 5% 5% 5% 4 6% 7% 37%	4165
April	1st to 10th	Day	5% 7% 3 6% 5% 4 4 4 4 8% 51%	2280
	11th to 20th	Day	4 7% 3 5% 4 4 4 3 5% 6% 55%	2249
	21st to end	Day	4 8% 2 5% 3 3 3 3 6% 7% 56%	2323
May	1st to 10th	Day	3 5% 1 4 3 3 3 3 5% 7% 65%	4110
	11th to 20th	Day	3 4 1 3 3 3 3 3 4 5% 69%	4071
	21st to end	Day	4 4 1 5% 3 3 3 2 4 6% 63%	4803
June	1st to 10th	Day	4 4 2 5% 4 3 4 3 6% 6% 60%	2660
	11th to 20th	Day	3 2 1 2 2 2 3 3 4 6% 73%	2660
	21st to end	Day	2 2 1 3 2 2 2 3 4 4 76%	2660
July	1st to 10th	Day	1 1 1 2 1 2 2 2 3 84%	4420
	11th to 20th	Day	2 2 1 3 3 3 3 3 4 6% 71%	4420
	21st to end	Day	1 2 1 3 3 3 3 3 4 6% 71%	4790
August	1st to 10th	Day	2 2 1 3 2 2 3 2 3 5% 74%	4006
	11th to 20th	Day	1 1 2 1 2 2 2 3 4 84%	3724
	21st to end	Day	2 1 1 2 1 1 2 2 2 3 83%	4180
September	1st to 10th	Day	2 2 1 2 2 2 2 2 3 4 79%	2280
	11th to 20th	Day	2 3 1 3 3 2 3 2 4 6% 72%	2161
	21st to end	Day	2 3 1 3 1 2 2 2 4 5% 75%	2090
October	1st to 10th	Day	2 1 1 3 2 2 3 2 4 6% 74%	3467
	11th to 20th	Day	3 3 1 3 2 2 2 2 4 4 74%	3161
	21st to end	Day	3 2 1 3 3 3 4 3 5% 8% 66%	3173
November	1st to 10th	Day	4 5% 2 3 4 3 3 5% 6% 8% 57%	1764
	11th to 20th	Day	3 5% 2 3 3 3 3 4 6% 8% 60%	1800
	21st to end	Day	5% 7% 2 6% 4 3 4 3 5% 6% 56%	1800
December	1st to 10th	Day	3 3 1 3 3 2 2 3 4 7% 69%	2757
	11th to 20th	Day	4 9% 3 7% 5% 4 5% 5% 7% 9% 41%	3028
	21st to end	Day	4 7% 3 8% 5% 5% 5% 4 7% 10% 42%	4004
January	1st to 10th	Night	3 11% 4 9% 5% 4 5% 4 5% 7% 42%	4546
	11th to 20th	Night	8% 11% 4 9% 5% 4 4 3 6% 6% 41%	4560
	21st to end	Night	5% 11% 3 8% 5% 4 5% 4 6% 7% 42%	4681
February	1st to 10th	Night	4 8% 3 6% 4 3 4 4 6% 6% 51%	2335
	11th to 20th	Night	3 7% 2 5% 4 5% 5% 4 6% 7% 51%	2218
	21st to end	Night	2 9% 3 7% 4 4 4 4 6% 7% 50%	1884
March	1st to 10th	Night	2 9% 4 9% 6% 5% 6% 4 6% 7% 43%	3736
	11th to 20th	Night	3 6% 2 5% 4 4 3 3 5% 8% 56%	3719
	21st to end	Night	6% 11% 3 6% 4 4 5% 4 6% 8% 43%	3488
April	1st to 10th	Night	5% 6% 2 6% 5% 3 5% 4 5% 7% 52%	1900
	11th to 20th	Night	4 7% 2 4 4 3 4 3 5% 6% 57%	1879
	21st to end	Night	5% 7% 2 5% 3 3 3 3 4 5% 59%	1855
May	1st to 10th	Night	4 5% 2 5% 4 3 3 3 4 6% 62%	2860
	11th to 20th	Night	4 3 1 3 2 2 2 3 4 5% 71%	2834
	21st to end	Night	4 4 2 6% 4 4 4 3 4 5% 58%	2769
June	1st to 10th	Night	3 3 1 4 3 4 4 4 6% 6% 61%	1520
	11th to 20th	Night	3 3 1 3 2 3 4 2 5% 5% 70%	1520
	21st to end	Night	2 2 1 2 2 3 4 2 6% 5% 71%	1520
July	1st to 10th	Night	1 1 1 2 2 2 2 2 4 4 80%	2540
	11th to 20th	Night	3 4 2 6% 5% 5% 4 4 5% 6% 57%	2540
	21st to end	Night	3 4 2 3 4 4 4 3 6% 7% 60%	2761
August	1st to 10th	Night	4 3 1 4 2 3 4 4 5% 7% 63%	2930
	11th to 20th	Night	2 1 1 3 2 2 2 2 3 4 78%	3126
	21st to end	Night	2 2 1 2 1 2 2 2 3 3 80%	3498
September	1st to 10th	Night	2 2 1 3 2 3 3 1 4 4 75%	1900
	11th to 20th	Night	3 4 1 3 3 3 3 3 4 5% 69%	2019
	21st to end	Night	3 3 1 2 1 1 2 2 3 4 78%	2090
October	1st to 10th	Night	3 3 1 3 3 2 4 2 3 4 71%	3511
	11th to 20th	Night	2 2 1 3 3 2 3 2 3 5% 74%	3777
	21st to end	Night	2 2 1 3 2 3 3 2 4 6% 73%	4387
November	1st to 10th	Night	5% 5% 1 4 3 3 3 2 4 7% 62%	2535
	11th to 20th	Night	3 4 1 5% 3 3 3 3 5% 8% 61%	2600
	21st to end	Night	4 6% 2 5% 4 4 4 3 5% 8% 55%	2600
December	1st to 10th	Night	3 4 1 4 3 3 3 3 5% 6% 65%	4422
	11th to 20th	Night	4 7% 3 7% 4 5% 5% 4 7% 12% 43%	5176
	21st to end	Night	5% 8% 4 10% 7% 6% 6% 5% 7% 8% 34%	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% 20% 22% 9% 1	2619
	11th to 20th	Day	39% 8% 17% 21% 11% 4	2620
	21st to end	Day	37% 10% 18% 24% 10% 1	3001
February	1st to 10th	Day	29% 8% 13% 27% 17% 7%	1843
	11th to 20th	Day	30% 7% 10% 26% 22% 5%	1860
	21st to end	Day	29% 8% 14% 25% 20% 4	1570
March	1st to 10th	Day	31% 11% 14% 25% 13% 6%	3138
	11th to 20th	Day	22% 6% 12% 33% 24% 3	3256
	21st to end	Day	37% 10% 15% 20% 14% 4	4165
April	1st to 10th	Day	31% 6% 11% 22% 21% 9%	2280
	11th to 20th	Day	27% 6% 12% 20% 23% 10%	2252
	21st to end	Day	26% 5% 10% 27% 26% 6%	2323
May	1st to 10th	Day	17% 5% 9% 25% 35% 10%	4110
	11th to 20th	Day	16% 4 7% 17% 35% 21%	4074
	21st to end	Day	20% 5% 7% 19% 38% 11%	4804
June	1st to 10th	Day	22% 6% 7% 16% 29% 20%	2660
	11th to 20th	Day	13% 3 5% 14% 33% 32%	2660
	21st to end	Day	11% 2 5% 10% 25% 46%	2660
July	1st to 10th	Day	8% 2 2 6% 11% 72%	4351
	11th to 20th	Day	14% 2 5% 6% 13% 61%	4345
	21st to end	Day	13% 4 4 9% 20% 49%	4639
August	1st to 10th	Day	14% 2 3 7% 22% 52%	3990
	11th to 20th	Day	7% 2 3 5% 14% 69%	3760
	21st to end	Day	9% 1 3 6% 26% 55%	4179
September	1st to 10th	Day	10% 3 3 7% 25% 52%	2280
	11th to 20th	Day	14% 3 5% 14% 30% 34%	2161
	21st to end	Day	13% 2 6% 12% 30% 36%	2090
October	1st to 10th	Day	14% 3 5% 9% 32% 38%	3467
	11th to 20th	Day	14% 3 5% 15% 42% 21%	3161
	21st to end	Day	16% 3 7% 23% 37% 14%	3173
November	1st to 10th	Day	21% 7% 12% 23% 26% 12%	1764
	11th to 20th	Day	20% 6% 12% 24% 31% 7%	1800
	21st to end	Day	28% 5% 10% 24% 26% 6%	1800
December	1st to 10th	Day	17% 4 10% 26% 32% 11%	2757
	11th to 20th	Day	34% 9% 16% 21% 16% 3	3028
	21st to end	Day	33% 10% 21% 24% 11% 1	4004
January	1st to 10th	Night	34% 11% 19% 25% 10% 1	4546
	11th to 20th	Night	39% 8% 18% 18% 13% 4	4560
	21st to end	Night	37% 10% 15% 25% 12% 2	4681
February	1st to 10th	Night	31% 6% 13% 26% 19% 5%	2335
	11th to 20th	Night	29% 6% 13% 24% 23% 5%	2219
	21st to end	Night	30% 8% 14% 25% 18% 6%	1884
March	1st to 10th	Night	32% 10% 12% 23% 15% 8%	3736
	11th to 20th	Night	22% 8% 12% 31% 23% 4	3719
	21st to end	Night	31% 10% 17% 26% 13% 3	3488
April	1st to 10th	Night	29% 8% 12% 25% 20% 7%	1900
	11th to 20th	Night	27% 6% 10% 26% 21% 10%	1879
	21st to end	Night	26% 6% 11% 25% 27% 6%	1855
May	1st to 10th	Night	19% 6% 9% 24% 32% 10%	2860
	11th to 20th	Night	14% 5% 6% 16% 37% 22%	2835
	21st to end	Night	22% 7% 10% 18% 34% 8%	2769
June	1st to 10th	Night	20% 6% 10% 19% 26% 19%	1520
	11th to 20th	Night	17% 3 6% 13% 33% 28%	1520
	21st to end	Night	14% 3 7% 11% 23% 43%	1520
July	1st to 10th	Night	8% 2 4 6% 11% 68%	2500
	11th to 20th	Night	26% 3 4 6% 12% 49%	2492
	21st to end	Night	21% 4 6% 11% 15% 42%	2674
August	1st to 10th	Night	20% 3 4 9% 17% 47%	2922
	11th to 20th	Night	11% 2 4 6% 16% 61%	3156
	21st to end	Night	11% 2 3 6% 26% 51%	3498
September	1st to 10th	Night	13% 3 4 7% 24% 49%	1900
	11th to 20th	Night	16% 5% 6% 13% 26% 34%	2019
	21st to end	Night	12% 3 4 12% 31% 38%	2090
October	1st to 10th	Night	17% 2 6% 10% 25% 39%	3511
	11th to 20th	Night	13% 3 5% 13% 41% 25%	3777
	21st to end	Night	13% 3 6% 22% 43% 14%	4387
November	1st to 10th	Night	23% 5% 10% 23% 26% 13%	2535
	11th to 20th	Night	21% 5% 12% 24% 32% 7%	2600
	21st to end	Night	27% 6% 9% 24% 27% 7%	2600
December	1st to 10th	Night	21% 4 8% 23% 31% 12%	4422
	11th to 20th	Night	30% 8% 18% 23% 18% 4	5176
	21st to end	Night	41% 11% 13% 21% 13% 1	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	12% 12% 26% 31% 16% 6%	200
	11th to 20th	Day	9% 11% 22% 30% 17% 13%	200
	21st to end	Day	9% 6% 16% 31% 26% 11%	321
February	1st to 10th	Day	8% 11% 27% 29% 16% 8%	433
	11th to 20th	Day	6% 13% 25% 31% 22% 3	455
	21st to end	Day	8% 12% 29% 32% 18% 1	382
March	1st to 10th	Day	10% 13% 25% 33% 15% 3	460
	11th to 20th	Day	8% 13% 25% 30% 18% 7%	460
	21st to end	Day	10% 15% 28% 30% 14% 2	506
April	1st to 10th	Day	12% 12% 26% 27% 18% 6%	460
	11th to 20th	Day	9% 16% 26% 29% 15% 4	452
	21st to end	Day	8% 12% 29% 28% 16% 7%	460
May	1st to 10th	Day	6% 17% 32% 24% 15% 5%	463
	11th to 20th	Day	8% 13% 26% 28% 17% 7%	488
	21st to end	Day	9% 11% 20% 29% 23% 9%	529
June	1st to 10th	Day	5% 10% 25% 24% 24% 11%	490
	11th to 20th	Day	6% 12% 28% 28% 18% 8%	490
	21st to end	Day	2 8% 24% 34% 22% 10%	490
July	1st to 10th	Day	3 5% 23% 30% 27% 12%	486
	11th to 20th	Day	2 4 17% 36% 28% 12%	484
	21st to end	Day	2 2 20% 34% 29% 12%	531
August	1st to 10th	Day	4 4 22% 33% 24% 13%	487
	11th to 20th	Day	3 5% 21% 34% 24% 13%	477
	21st to end	Day	3 4 20% 33% 28% 12%	539
September	1st to 10th	Day	3 4 19% 33% 28% 12%	490
	11th to 20th	Day	6% 7% 22% 33% 23% 9%	490
	21st to end	Day	4 6% 20% 37% 29% 5%	490
October	1st to 10th	Day	4 7% 23% 30% 26% 9%	490
	11th to 20th	Day	4 8% 23% 33% 25% 7%	490
	21st to end	Day	3 10% 20% 31% 26% 10%	378
November	1st to 10th	Day	6% 5% 26% 32% 25% 7%	300
	11th to 20th	Day	3 13% 20% 31% 22% 12%	300
	21st to end	Day	12% 7% 17% 30% 24% 10%	300
December	1st to 10th	Day	8% 6% 16% 34% 25% 12%	243
	11th to 20th	Day	5% 9% 24% 30% 24% 9%	200
	21st to end	Day	15% 10% 18% 30% 20% 6%	220
January	1st to 10th	Night	8% 5% 24% 34% 20% 8%	800
	11th to 20th	Night	6% 4 22% 38% 23% 7%	800
	21st to end	Night	10% 5% 16% 40% 21% 7%	840
February	1st to 10th	Night	7% 5% 22% 39% 22% 5%	760
	11th to 20th	Night	6% 7% 20% 42% 20% 5%	744
	21st to end	Night	6% 7% 22% 45% 16% 3	628
March	1st to 10th	Night	7% 8% 25% 39% 18% 3	760
	11th to 20th	Night	5% 7% 21% 38% 24% 6%	760
	21st to end	Night	10% 7% 25% 38% 18% 2	836
April	1st to 10th	Night	8% 8% 21% 42% 19% 3	760
	11th to 20th	Night	7% 8% 21% 39% 20% 5%	751
	21st to end	Night	8% 8% 22% 36% 21% 5%	715
May	1st to 10th	Night	5% 9% 26% 40% 17% 2	570
	11th to 20th	Night	8% 6% 20% 38% 25% 2	561
	21st to end	Night	5% 6% 18% 39% 29% 3	418
June	1st to 10th	Night	3 2 19% 42% 30% 4	380
	11th to 20th	Night	4 3 18% 46% 27% 2	380
	21st to end	Night	3 1 15% 49% 28% 3	380
July	1st to 10th	Night	2 12% 53% 31% 2	370
	11th to 20th	Night	4 1 11% 46% 36% 2	368
	21st to end	Night	5% 1 9% 45% 38% 2	416
August	1st to 10th	Night	4 3 14% 44% 32% 2	610
	11th to 20th	Night	2 2 12% 40% 36% 7%	752
	21st to end	Night	2 1 9% 43% 39% 5%	836
September	1st to 10th	Night	3 3 9% 39% 38% 8%	760
	11th to 20th	Night	4 3 11% 38% 33% 10%	760
	21st to end	Night	5% 2 10% 41% 31% 11%	760
October	1st to 10th	Night	5% 4 15% 40% 30% 6%	760
	11th to 20th	Night	4 3 16% 38% 31% 7%	760
	21st to end	Night	4 4 17% 37% 29% 11%	836
November	1st to 10th	Night	7% 3 16% 38% 28% 8%	780
	11th to 20th	Night	6% 3 18% 44% 24% 5%	800
	21st to end	Night	6% 6% 20% 39% 24% 6%	800
December	1st to 10th	Night	7% 6% 18% 43% 20% 7%	800
	11th to 20th	Night	11% 5% 21% 37% 23% 3	800
	21st to end	Night	11% 8% 25% 38% 16% 2	880

Page generated at 2025-01-19 00:06:45. Version ac07

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