Firefly Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Firefly Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	10% 7% 3 7% 5% 5% 6% 5% 7% 8% 38%	126809
	Night	10% 8% 3 8% 5% 5% 5% 4 6% 8% 39%	123555
Transparency	Day	40% 8% 13% 19% 15% 5%	130520
	Night	38% 8% 13% 20% 16% 5%	127913
Seeing	Day	19% 14% 24% 21% 14% 7%	16441
	Night	18% 12% 21% 28% 19% 3	26477

By Month

Percentage Cloud Cover SE (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	9% 6% 3 6% 4 4 4 4 6% 8% 47%	4056
	11th to 20th	Day	8% 5% 2 5% 4 4 5% 4 6% 5% 51%	4060
	21st to end	Day	12% 10% 4 6% 4 4 4 3 5% 6% 42%	4466
February	1st to 10th	Day	12% 8% 3 7% 5% 4 5% 3 6% 7% 40%	1880
	11th to 20th	Day	14% 11% 4 8% 4 4 4 4 5% 6% 36%	1948
	21st to end	Day	13% 7% 3 8% 5% 4 5% 4 5% 6% 40%	1650
March	1st to 10th	Day	14% 8% 3 8% 4 5% 4 3 5% 6% 38%	4837
	11th to 20th	Day	10% 7% 3 6% 3 4 4 3 5% 6% 49%	4980
	21st to end	Day	13% 8% 3 6% 4 4 4 4 5% 6% 43%	5465
April	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 4 6% 9% 41%	2200
	11th to 20th	Day	13% 7% 3 6% 5% 3 4 3 6% 8% 42%	2393
	21st to end	Day	11% 7% 2 7% 4 4 5% 4 6% 7% 43%	2600
May	1st to 10th	Day	9% 8% 3 10% 6% 6% 6% 5% 7% 8% 30%	5862
	11th to 20th	Day	14% 8% 3 9% 6% 6% 7% 5% 7% 9% 25%	5861
	21st to end	Day	9% 7% 3 8% 6% 6% 6% 5% 7% 10% 35%	6433
June	1st to 10th	Day	9% 7% 2 7% 7% 7% 7% 5% 8% 9% 32%	2600
	11th to 20th	Day	6% 6% 3 8% 7% 7% 6% 6% 9% 10% 33%	2600
	21st to end	Day	7% 6% 2 7% 6% 6% 7% 6% 8% 10% 34%	2600
July	1st to 10th	Day	4 5% 3 8% 6% 7% 7% 5% 9% 10% 35%	5900
	11th to 20th	Day	6% 6% 3 9% 7% 7% 8% 6% 9% 11% 27%	5900
	21st to end	Day	9% 8% 4 10% 8% 8% 8% 6% 9% 9% 22%	6424
August	1st to 10th	Day	10% 8% 3 9% 7% 7% 7% 6% 9% 11% 23%	4234
	11th to 20th	Day	7% 5% 3 7% 6% 6% 8% 6% 9% 11% 31%	4174
	21st to end	Day	5% 5% 2 6% 5% 5% 6% 5% 6% 9% 45%	4331
September	1st to 10th	Day	9% 6% 3 8% 6% 6% 6% 5% 6% 8% 37%	2200
	11th to 20th	Day	13% 7% 2 7% 7% 6% 6% 5% 6% 10% 32%	2200
	21st to end	Day	12% 7% 2 7% 5% 6% 5% 6% 6% 7% 36%	2200
October	1st to 10th	Day	10% 7% 3 6% 4 4 4 4 5% 6% 48%	2161
	11th to 20th	Day	8% 6% 2 7% 3 3 4 3 4 6% 54%	2000
	21st to end	Day	11% 6% 3 5% 4 3 3 3 5% 5% 52%	2200
November	1st to 10th	Day	7% 7% 2 5% 4 3 3 3 4 7% 54%	1962
	11th to 20th	Day	9% 6% 2 6% 5% 3 4 3 6% 6% 50%	1863
	21st to end	Day	12% 8% 3 6% 3 4 4 3 6% 6% 46%	1800
December	1st to 10th	Day	10% 7% 2 6% 4 5% 4 4 6% 6% 46%	2930
	11th to 20th	Day	11% 7% 4 8% 5% 4 5% 5% 7% 8% 37%	3373
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 5% 8% 38%	4466
January	1st to 10th	Night	10% 7% 3 10% 5% 5% 5% 4 6% 7% 38%	5889
	11th to 20th	Night	10% 6% 3 7% 5% 4 4 3 6% 8% 45%	5900
	21st to end	Night	11% 11% 3 7% 4 3 3 3 5% 6% 43%	6490
February	1st to 10th	Night	11% 9% 3 7% 4 4 5% 4 6% 7% 40%	2515
	11th to 20th	Night	16% 9% 3 6% 5% 5% 4 4 5% 6% 37%	2338
	21st to end	Night	13% 8% 3 8% 5% 5% 5% 3 6% 6% 37%	1980
March	1st to 10th	Night	15% 11% 4 9% 4 4 4 4 5% 6% 34%	5014
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 48%	4980
	21st to end	Night	13% 9% 3 6% 5% 4 4 3 5% 7% 42%	5467
April	1st to 10th	Night	11% 8% 3 5% 5% 4 4 4 6% 8% 43%	2200
	11th to 20th	Night	9% 8% 2 6% 4 4 4 4 6% 7% 46%	1968
	21st to end	Night	9% 7% 3 7% 4 3 5% 3 5% 7% 46%	1800
May	1st to 10th	Night	10% 9% 3 9% 4 5% 5% 5% 8% 11% 32%	4033
	11th to 20th	Night	14% 11% 4 10% 5% 5% 5% 4 6% 8% 26%	4032
	21st to end	Night	9% 8% 4 8% 6% 5% 5% 4 7% 10% 34%	4421
June	1st to 10th	Night	7% 7% 3 7% 5% 6% 7% 5% 8% 13% 33%	1800
	11th to 20th	Night	5% 7% 2 5% 6% 5% 5% 5% 9% 13% 39%	1800
	21st to end	Night	5% 5% 2 6% 5% 6% 5% 5% 8% 10% 42%	1800
July	1st to 10th	Night	5% 6% 3 8% 6% 7% 6% 5% 8% 11% 34%	4058
	11th to 20th	Night	6% 5% 3 8% 7% 7% 7% 6% 10% 12% 30%	4057
	21st to end	Night	8% 9% 3 11% 7% 7% 7% 5% 7% 10% 26%	4427
August	1st to 10th	Night	7% 7% 2 8% 6% 7% 7% 5% 9% 11% 30%	2915
	11th to 20th	Night	7% 7% 3 10% 7% 6% 6% 6% 7% 10% 30%	2875
	21st to end	Night	5% 5% 2 6% 4 4 5% 4 7% 10% 46%	3515
September	1st to 10th	Night	9% 5% 2 7% 6% 6% 5% 5% 7% 9% 39%	2200
	11th to 20th	Night	12% 8% 3 9% 5% 5% 5% 5% 6% 7% 36%	2200
	21st to end	Night	10% 7% 3 8% 5% 4 5% 3 6% 7% 43%	2200
October	1st to 10th	Night	12% 6% 2 6% 4 5% 4 4 5% 7% 46%	2237
	11th to 20th	Night	9% 6% 2 7% 4 4 4 3 6% 6% 48%	2400
	21st to end	Night	9% 7% 3 6% 4 2 4 3 5% 5% 52%	2640
November	1st to 10th	Night	10% 6% 2 5% 4 5% 4 4 6% 6% 48%	2550
	11th to 20th	Night	10% 7% 3 6% 4 4 3 3 5% 7% 48%	2692
	21st to end	Night	12% 7% 2 6% 4 5% 4 4 5% 6% 44%	2600
December	1st to 10th	Night	11% 6% 3 6% 4 5% 5% 4 5% 7% 45%	4250
	11th to 20th	Night	13% 10% 4 10% 6% 5% 5% 4 6% 6% 32%	4822
	21st to end	Night	13% 9% 3 8% 5% 4 5% 4 6% 7% 36%	6490

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	31% 4 8% 23% 29% 4	4057
	11th to 20th	Day	29% 7% 9% 14% 34% 8%	4060
	21st to end	Day	38% 6% 8% 20% 21% 7%	4466
February	1st to 10th	Day	41% 5% 13% 21% 17% 3	1880
	11th to 20th	Day	45% 6% 11% 17% 17% 4	1958
	21st to end	Day	43% 7% 9% 15% 19% 6%	1650
March	1st to 10th	Day	43% 6% 7% 15% 20% 9%	4837
	11th to 20th	Day	33% 5% 7% 17% 20% 18%	4980
	21st to end	Day	40% 5% 9% 26% 15% 4	5465
April	1st to 10th	Day	36% 8% 13% 21% 18% 4	2200
	11th to 20th	Day	39% 6% 9% 20% 20% 5%	2393
	21st to end	Day	38% 7% 13% 25% 14% 2	2600
May	1st to 10th	Day	48% 9% 14% 19% 9% 1	5897
	11th to 20th	Day	51% 10% 16% 15% 8%	5865
	21st to end	Day	41% 10% 13% 19% 15% 2	6447
June	1st to 10th	Day	44% 13% 18% 17% 8% 1	2600
	11th to 20th	Day	41% 12% 18% 17% 10% 2	2600
	21st to end	Day	41% 12% 16% 18% 11% 2	2600
July	1st to 10th	Day	40% 11% 18% 22% 9%	5835
	11th to 20th	Day	46% 12% 25% 13% 3	5831
	21st to end	Day	53% 10% 18% 12% 6%	6240
August	1st to 10th	Day	49% 10% 18% 19% 3	4221
	11th to 20th	Day	41% 9% 23% 16% 9% 2	4213
	21st to end	Day	34% 10% 13% 32% 10% 1	4331
September	1st to 10th	Day	41% 11% 13% 17% 15% 3	2200
	11th to 20th	Day	46% 10% 12% 18% 13% 2	2200
	21st to end	Day	44% 9% 14% 16% 14% 4	2200
October	1st to 10th	Day	28% 6% 7% 20% 27% 12%	3525
	11th to 20th	Day	22% 4 7% 21% 24% 21%	3240
	21st to end	Day	28% 3 8% 25% 21% 15%	3538
November	1st to 10th	Day	31% 4 11% 20% 26% 8%	1962
	11th to 20th	Day	33% 5% 8% 22% 25% 7%	1863
	21st to end	Day	36% 6% 8% 24% 22% 5%	1800
December	1st to 10th	Day	37% 5% 6% 19% 27% 6%	2930
	11th to 20th	Day	40% 5% 12% 20% 19% 4	3370
	21st to end	Day	41% 5% 12% 25% 14% 3	4466
January	1st to 10th	Night	34% 6% 12% 22% 22% 5%	5889
	11th to 20th	Night	31% 6% 8% 19% 27% 9%	5900
	21st to end	Night	40% 4 8% 18% 22% 8%	6490
February	1st to 10th	Night	40% 6% 13% 20% 18% 3	2515
	11th to 20th	Night	44% 6% 12% 18% 16% 5%	2339
	21st to end	Night	44% 7% 11% 16% 18% 5%	1980
March	1st to 10th	Night	44% 8% 8% 13% 20% 7%	5014
	11th to 20th	Night	30% 7% 8% 20% 20% 15%	4980
	21st to end	Night	42% 6% 11% 24% 12% 5%	5467
April	1st to 10th	Night	36% 7% 15% 21% 18% 3	2200
	11th to 20th	Night	34% 7% 11% 23% 20% 4	1968
	21st to end	Night	36% 9% 14% 24% 15% 2	1800
May	1st to 10th	Night	43% 11% 15% 24% 7% 1	4060
	11th to 20th	Night	49% 11% 17% 17% 6%	4033
	21st to end	Night	40% 11% 18% 21% 10% 1	4429
June	1st to 10th	Night	39% 14% 19% 19% 8% 1	1800
	11th to 20th	Night	33% 14% 23% 22% 9% 1	1800
	21st to end	Night	33% 13% 19% 20% 13% 2	1800
July	1st to 10th	Night	40% 11% 19% 23% 6%	4015
	11th to 20th	Night	41% 15% 28% 13% 3 1	4006
	21st to end	Night	49% 12% 20% 13% 5%	4297
August	1st to 10th	Night	43% 12% 26% 15% 4	2904
	11th to 20th	Night	44% 10% 23% 15% 8%	2902
	21st to end	Night	31% 9% 17% 33% 8% 1	3515
September	1st to 10th	Night	39% 11% 15% 19% 13% 3	2200
	11th to 20th	Night	45% 9% 14% 16% 15% 1	2200
	21st to end	Night	39% 10% 12% 21% 16% 3	2200
October	1st to 10th	Night	30% 5% 10% 20% 24% 10%	3653
	11th to 20th	Night	24% 5% 6% 20% 24% 20%	3899
	21st to end	Night	30% 4 6% 23% 22% 14%	4252
November	1st to 10th	Night	34% 7% 10% 20% 21% 7%	2550
	11th to 20th	Night	34% 5% 9% 20% 24% 8%	2692
	21st to end	Night	37% 6% 8% 23% 21% 6%	2600
December	1st to 10th	Night	35% 7% 11% 20% 23% 4	4250
	11th to 20th	Night	45% 8% 9% 18% 16% 3	4824
	21st to end	Night	42% 6% 14% 19% 17% 2	6490

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	37% 26% 28% 7% 2	300
	11th to 20th	Day	39% 23% 23% 13% 2	300
	21st to end	Day	29% 32% 28% 8% 2	330
February	1st to 10th	Day	27% 29% 30% 12% 1	300
	11th to 20th	Day	46% 29% 19% 6%	300
	21st to end	Day	39% 30% 22% 8% 1	249
March	1st to 10th	Day	28% 22% 28% 16% 5%	450
	11th to 20th	Day	28% 25% 27% 16% 4 1	500
	21st to end	Day	29% 23% 27% 16% 5%	550
April	1st to 10th	Day	26% 21% 26% 19% 8% 1	500
	11th to 20th	Day	22% 22% 25% 20% 10% 3	492
	21st to end	Day	20% 16% 28% 20% 12% 4	500
May	1st to 10th	Day	17% 21% 26% 20% 10% 5%	503
	11th to 20th	Day	20% 13% 23% 23% 14% 7%	527
	21st to end	Day	16% 9% 25% 21% 17% 11%	573
June	1st to 10th	Day	13% 8% 26% 26% 18% 9%	530
	11th to 20th	Day	9% 6% 25% 24% 24% 13%	530
	21st to end	Day	10% 8% 24% 27% 18% 13%	530
July	1st to 10th	Day	8% 3 17% 26% 28% 17%	525
	11th to 20th	Day	7% 4 15% 28% 25% 21%	524
	21st to end	Day	9% 4 12% 29% 28% 17%	576
August	1st to 10th	Day	19% 6% 19% 22% 18% 17%	526
	11th to 20th	Day	10% 4 20% 27% 23% 16%	519
	21st to end	Day	10% 4 18% 25% 27% 16%	583
September	1st to 10th	Day	9% 5% 20% 29% 25% 13%	530
	11th to 20th	Day	15% 6% 18% 22% 27% 11%	530
	21st to end	Day	15% 8% 23% 28% 22% 5%	530
October	1st to 10th	Day	15% 10% 22% 28% 18% 6%	530
	11th to 20th	Day	15% 16% 24% 26% 15% 4	530
	21st to end	Day	22% 15% 24% 25% 12% 2	583
November	1st to 10th	Day	20% 14% 29% 25% 9% 2	440
	11th to 20th	Day	27% 21% 30% 17% 4 1	321
	21st to end	Day	25% 19% 32% 17% 6% 1	300
December	1st to 10th	Day	30% 26% 30% 12% 2	300
	11th to 20th	Day	26% 20% 33% 16% 5% 1	300
	21st to end	Day	28% 21% 32% 14% 4 1	330
January	1st to 10th	Night	28% 20% 25% 22% 5%	800
	11th to 20th	Night	24% 20% 28% 24% 5%	800
	21st to end	Night	21% 21% 31% 24% 3	880
February	1st to 10th	Night	21% 18% 28% 29% 4	800
	11th to 20th	Night	29% 24% 25% 21% 1	783
	21st to end	Night	24% 23% 30% 21% 3	660
March	1st to 10th	Night	21% 20% 28% 28% 3	800
	11th to 20th	Night	23% 16% 30% 23% 7% 1	800
	21st to end	Night	21% 18% 29% 23% 7%	880
April	1st to 10th	Night	21% 20% 25% 27% 7% 1	800
	11th to 20th	Night	18% 17% 26% 26% 11% 1	694
	21st to end	Night	15% 13% 21% 35% 17% 1	600
May	1st to 10th	Night	15% 10% 24% 32% 18% 1	600
	11th to 20th	Night	16% 7% 21% 30% 22% 2	591
	21st to end	Night	12% 4 18% 26% 32% 7%	650
June	1st to 10th	Night	11% 5% 14% 34% 30% 6%	600
	11th to 20th	Night	8% 3 15% 32% 33% 10%	600
	21st to end	Night	6% 3 11% 33% 41% 6%	600
July	1st to 10th	Night	9% 2 10% 26% 46% 7%	585
	11th to 20th	Night	8% 3 9% 21% 47% 12%	582
	21st to end	Night	8% 2 7% 26% 46% 11%	657
August	1st to 10th	Night	11% 1 13% 25% 40% 10%	597
	11th to 20th	Night	12% 3 13% 25% 38% 9%	594
	21st to end	Night	11% 1 10% 28% 42% 9%	715
September	1st to 10th	Night	9% 3 15% 29% 37% 6%	800
	11th to 20th	Night	15% 4 16% 25% 33% 6%	800
	21st to end	Night	17% 6% 16% 33% 26% 4	800
October	1st to 10th	Night	15% 9% 19% 26% 24% 7%	800
	11th to 20th	Night	16% 12% 22% 31% 18% 2	800
	21st to end	Night	18% 13% 18% 34% 15% 2	880
November	1st to 10th	Night	17% 9% 24% 32% 15% 3	820
	11th to 20th	Night	20% 13% 23% 33% 11% 1	829
	21st to end	Night	20% 15% 21% 31% 12% 1	800
December	1st to 10th	Night	25% 18% 22% 28% 6%	800
	11th to 20th	Night	20% 15% 26% 31% 7% 1	800
	21st to end	Night	26% 16% 24% 29% 5%	880

Page generated at 2025-11-15 00:56:00. Version ac07

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