Firetail Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Firetail Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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	14% 6% 2 6% 4 4 4 3 5% 7% 45%	127279
	Night	16% 6% 2 5% 3 3 3 3 4 7% 48%	124373
Transparency	Day	38% 4 9% 21% 18% 10%	131071
	Night	36% 4 10% 20% 18% 12%	128644
Seeing	Day	15% 8% 23% 30% 20% 4	16295
	Night	17% 7% 25% 34% 15% 1	26277

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	18% 9% 2 5% 3 3 2 2 3 4 49%	4086
	11th to 20th	Day	17% 6% 1 4 2 2 2 3 4 5% 53%	4080
	21st to end	Day	28% 6% 2 4 2 2 2 2 3 6% 41%	4874
February	1st to 10th	Day	20% 8% 2 4 4 2 3 2 4 4 45%	2000
	11th to 20th	Day	20% 7% 2 5% 4 3 3 3 3 6% 44%	1949
	21st to end	Day	20% 6% 3 4 3 2 4 2 4 5% 48%	1650
March	1st to 10th	Day	14% 6% 2 5% 3 3 3 2 4 6% 52%	4867
	11th to 20th	Day	16% 7% 3 6% 4 3 3 2 3 4 50%	4980
	21st to end	Day	19% 7% 2 5% 3 3 3 3 4 6% 45%	5462
April	1st to 10th	Day	14% 8% 3 8% 4 4 4 3 4 5% 43%	2200
	11th to 20th	Day	21% 6% 2 6% 3 3 3 3 5% 5% 43%	2176
	21st to end	Day	15% 8% 3 6% 5% 5% 4 3 4 5% 42%	2535
May	1st to 10th	Day	22% 8% 3 6% 4 4 4 4 5% 6% 34%	5862
	11th to 20th	Day	11% 5% 2 6% 4 4 4 3 6% 7% 46%	5862
	21st to end	Day	14% 7% 2 6% 5% 5% 6% 5% 7% 9% 35%	6434
June	1st to 10th	Day	7% 5% 2 5% 4 5% 5% 5% 7% 9% 47%	2600
	11th to 20th	Day	8% 6% 2 6% 5% 5% 5% 4 7% 9% 43%	2600
	21st to end	Day	7% 6% 3 7% 7% 6% 6% 6% 8% 9% 36%	2600
July	1st to 10th	Day	12% 7% 3 8% 6% 5% 5% 4 6% 8% 36%	5900
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 8% 11% 40%	5900
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 40%	6422
August	1st to 10th	Day	1 2 1 2 3 3 3 3 5% 8% 67%	4175
	11th to 20th	Day	4 2 1 3 4 4 5% 3 8% 11% 55%	3846
	21st to end	Day	5% 3 2 6% 5% 6% 5% 5% 8% 10% 45%	4129
September	1st to 10th	Day	10% 6% 2 8% 6% 5% 5% 4 6% 7% 42%	2200
	11th to 20th	Day	10% 7% 3 6% 5% 4 5% 5% 7% 9% 40%	2200
	21st to end	Day	8% 4 2 5% 4 5% 6% 4 5% 8% 50%	2200
October	1st to 10th	Day	6% 5% 2 5% 4 3 4 3 6% 6% 55%	2200
	11th to 20th	Day	12% 5% 2 4 3 4 3 3 5% 5% 52%	2080
	21st to end	Day	11% 5% 2 5% 4 3 3 2 4 5% 57%	2200
November	1st to 10th	Day	15% 7% 2 6% 4 3 3 3 5% 5% 46%	2058
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 5% 6% 50%	1964
	21st to end	Day	17% 7% 2 7% 4 3 3 2 4 4 44%	1890
December	1st to 10th	Day	28% 8% 3 5% 3 3 2 2 3 5% 38%	3030
	11th to 20th	Day	19% 7% 3 5% 4 4 3 3 4 5% 43%	3481
	21st to end	Day	16% 5% 1 4 3 2 3 2 3 4 57%	4587
January	1st to 10th	Night	21% 6% 2 4 2 2 2 2 3 5% 50%	5902
	11th to 20th	Night	13% 3 1 2 2 2 3 2 4 5% 62%	5900
	21st to end	Night	31% 6% 2 4 2 2 2 3 3 5% 40%	6145
February	1st to 10th	Night	20% 8% 2 5% 3 3 2 2 4 5% 47%	2400
	11th to 20th	Night	21% 8% 2 4 2 3 2 2 3 5% 48%	2338
	21st to end	Night	21% 6% 2 4 3 4 3 3 4 5% 44%	1980
March	1st to 10th	Night	18% 7% 2 5% 4 3 3 2 4 6% 44%	4975
	11th to 20th	Night	17% 8% 2 5% 3 3 3 2 3 5% 49%	4980
	21st to end	Night	23% 8% 2 4 3 3 3 3 4 5% 42%	5467
April	1st to 10th	Night	15% 8% 2 5% 4 4 3 3 5% 8% 44%	2200
	11th to 20th	Night	20% 7% 2 5% 4 2 3 3 4 6% 44%	2167
	21st to end	Night	21% 6% 3 5% 3 3 3 2 4 5% 44%	1861
May	1st to 10th	Night	26% 9% 2 6% 4 3 3 3 4 5% 34%	4053
	11th to 20th	Night	10% 5% 2 5% 4 4 4 4 6% 8% 48%	4053
	21st to end	Night	16% 7% 3 6% 4 4 5% 4 6% 9% 36%	4443
June	1st to 10th	Night	7% 4 2 4 3 3 3 3 5% 9% 57%	1800
	11th to 20th	Night	5% 4 2 5% 3 4 4 3 5% 10% 54%	1800
	21st to end	Night	7% 5% 1 5% 4 4 4 5% 6% 10% 49%	1800
July	1st to 10th	Night	11% 7% 2 6% 4 4 5% 4 6% 10% 41%	4078
	11th to 20th	Night	6% 4 2 5% 4 4 6% 5% 8% 11% 45%	4077
	21st to end	Night	10% 7% 2 6% 4 3 4 4 6% 10% 43%	4449
August	1st to 10th	Night	1 1 2 2 2 2 3 5% 9% 73%	2980
	11th to 20th	Night	3 3 1 3 3 4 4 3 8% 12% 56%	3213
	21st to end	Night	5% 4 2 4 3 4 4 4 7% 10% 54%	3772
September	1st to 10th	Night	8% 6% 3 5% 4 4 4 3 6% 8% 49%	2200
	11th to 20th	Night	10% 6% 2 7% 4 4 5% 3 5% 8% 46%	2200
	21st to end	Night	9% 5% 2 5% 4 3 4 3 5% 9% 51%	2200
October	1st to 10th	Night	6% 4 1 4 3 4 3 3 5% 7% 60%	2200
	11th to 20th	Night	14% 5% 2 4 3 3 3 2 4 5% 55%	2300
	21st to end	Night	15% 5% 2 4 2 2 2 1 3 4 61%	2640
November	1st to 10th	Night	14% 6% 2 5% 3 3 2 2 3 5% 53%	2460
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 4 6% 54%	2651
	21st to end	Night	19% 6% 2 4 3 3 3 3 5% 5% 47%	2730
December	1st to 10th	Night	26% 9% 3 5% 4 3 3 2 3 5% 37%	4380
	11th to 20th	Night	23% 9% 2 5% 3 3 2 2 3 3 47%	4946
	21st to end	Night	19% 6% 2 4 2 2 2 2 3 4 55%	6633

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	41% 2 4 19% 23% 11%	4087
	11th to 20th	Day	35% 2 4 16% 23% 20%	4080
	21st to end	Day	47% 2 3 10% 25% 13%	4874
February	1st to 10th	Day	44% 3 5% 13% 21% 16%	2000
	11th to 20th	Day	44% 2 6% 18% 19% 12%	1959
	21st to end	Day	41% 2 6% 16% 18% 16%	1650
March	1st to 10th	Day	36% 2 5% 20% 16% 21%	4867
	11th to 20th	Day	41% 2 4 8% 16% 28%	4980
	21st to end	Day	43% 3 7% 19% 20% 9%	5464
April	1st to 10th	Day	44% 2 7% 17% 17% 13%	2200
	11th to 20th	Day	45% 3 6% 17% 18% 12%	2176
	21st to end	Day	44% 3 6% 20% 17% 9%	2535
May	1st to 10th	Day	50% 4 9% 19% 14% 4	5900
	11th to 20th	Day	37% 3 11% 20% 21% 8%	5866
	21st to end	Day	44% 5% 9% 28% 13% 1	6447
June	1st to 10th	Day	32% 5% 13% 29% 18% 3	2600
	11th to 20th	Day	36% 4 11% 25% 19% 4	2600
	21st to end	Day	42% 6% 12% 25% 13% 3	2600
July	1st to 10th	Day	45% 6% 15% 28% 5% 1	5836
	11th to 20th	Day	34% 9% 17% 29% 11% 1	5831
	21st to end	Day	36% 8% 16% 25% 13% 3	6240
August	1st to 10th	Day	15% 4 11% 43% 26% 1	4161
	11th to 20th	Day	21% 6% 23% 37% 11% 2	3882
	21st to end	Day	30% 9% 18% 25% 17% 2	4128
September	1st to 10th	Day	41% 5% 9% 20% 16% 9%	2200
	11th to 20th	Day	38% 7% 12% 21% 18% 5%	2200
	21st to end	Day	32% 4 10% 22% 18% 13%	2200
October	1st to 10th	Day	18% 2 6% 13% 25% 35%	3590
	11th to 20th	Day	25% 3 7% 20% 25% 20%	3372
	21st to end	Day	28% 3 5% 19% 30% 15%	3538
November	1st to 10th	Day	40% 2 7% 19% 19% 12%	2058
	11th to 20th	Day	36% 3 8% 18% 21% 15%	1964
	21st to end	Day	44% 3 5% 16% 19% 13%	1890
December	1st to 10th	Day	52% 2 5% 18% 14% 9%	3030
	11th to 20th	Day	44% 4 7% 12% 21% 13%	3479
	21st to end	Day	34% 2 4 17% 27% 17%	4587
January	1st to 10th	Night	39% 2 4 19% 22% 15%	5902
	11th to 20th	Night	27% 1 4 19% 29% 20%	5900
	21st to end	Night	50% 2 3 10% 20% 15%	6145
February	1st to 10th	Night	41% 3 5% 13% 20% 18%	2400
	11th to 20th	Night	42% 2 5% 17% 19% 15%	2338
	21st to end	Night	43% 2 5% 16% 18% 16%	1980
March	1st to 10th	Night	43% 2 5% 17% 16% 17%	4975
	11th to 20th	Night	41% 2 6% 11% 14% 26%	4980
	21st to end	Night	46% 3 7% 17% 18% 10%	5467
April	1st to 10th	Night	39% 4 9% 18% 16% 14%	2200
	11th to 20th	Night	43% 3 8% 14% 17% 15%	2167
	21st to end	Night	44% 3 8% 19% 16% 10%	1861
May	1st to 10th	Night	53% 4 7% 17% 14% 6%	4080
	11th to 20th	Night	35% 4 11% 22% 20% 9%	4054
	21st to end	Night	44% 5% 13% 27% 10% 1	4452
June	1st to 10th	Night	26% 4 15% 31% 21% 2	1800
	11th to 20th	Night	26% 4 15% 30% 19% 5%	1800
	21st to end	Night	31% 6% 19% 27% 14% 3	1800
July	1st to 10th	Night	38% 9% 22% 27% 4	4035
	11th to 20th	Night	30% 9% 21% 29% 10%	4026
	21st to end	Night	35% 7% 21% 23% 11% 3	4320
August	1st to 10th	Night	11% 4 17% 48% 20%	2970
	11th to 20th	Night	20% 6% 31% 33% 8% 1	3243
	21st to end	Night	24% 10% 23% 27% 15% 2	3772
September	1st to 10th	Night	34% 5% 16% 21% 16% 8%	2200
	11th to 20th	Night	36% 5% 15% 24% 15% 5%	2200
	21st to end	Night	30% 5% 15% 22% 17% 12%	2200
October	1st to 10th	Night	18% 2 7% 16% 21% 35%	3590
	11th to 20th	Night	23% 2 9% 18% 29% 19%	3734
	21st to end	Night	26% 2 6% 19% 30% 16%	4252
November	1st to 10th	Night	36% 3 7% 18% 22% 14%	2460
	11th to 20th	Night	32% 3 8% 19% 21% 17%	2651
	21st to end	Night	40% 2 8% 17% 18% 14%	2730
December	1st to 10th	Night	52% 3 5% 15% 14% 11%	4380
	11th to 20th	Night	45% 2 5% 9% 21% 18%	4947
	21st to end	Night	36% 2 3 14% 28% 17%	6633

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	25% 16% 34% 19% 5%	303
	11th to 20th	Day	27% 17% 27% 21% 6% 1	300
	21st to end	Day	23% 16% 35% 20% 5%	330
February	1st to 10th	Day	33% 12% 30% 20% 5%	300
	11th to 20th	Day	30% 18% 32% 16% 2 1	300
	21st to end	Day	29% 19% 39% 13% 1	249
March	1st to 10th	Day	27% 16% 41% 14% 2	300
	11th to 20th	Day	20% 19% 43% 14% 4	300
	21st to end	Day	24% 15% 39% 18% 5%	330
April	1st to 10th	Day	21% 18% 39% 19% 2	300
	11th to 20th	Day	23% 16% 35% 21% 4	294
	21st to end	Day	20% 14% 38% 21% 6%	635
May	1st to 10th	Day	20% 13% 37% 24% 5%	703
	11th to 20th	Day	17% 11% 31% 31% 8% 2	725
	21st to end	Day	18% 7% 28% 34% 13% 1	789
June	1st to 10th	Day	8% 3 20% 41% 24% 4	730
	11th to 20th	Day	9% 3 20% 45% 21% 2	730
	21st to end	Day	8% 2 15% 39% 28% 7%	730
July	1st to 10th	Day	10% 3 10% 36% 32% 8%	721
	11th to 20th	Day	6% 1 8% 36% 42% 7%	718
	21st to end	Day	6% 2 9% 36% 40% 8%	794
August	1st to 10th	Day	4 5% 36% 43% 12%	693
	11th to 20th	Day	5% 1 4 40% 40% 9%	517
	21st to end	Day	5% 1 6% 30% 40% 18%	478
September	1st to 10th	Day	14% 1 13% 26% 36% 11%	330
	11th to 20th	Day	6% 8% 33% 42% 12%	330
	21st to end	Day	9% 1 10% 32% 33% 14%	330
October	1st to 10th	Day	5% 3 14% 40% 31% 6%	330
	11th to 20th	Day	10% 6% 21% 42% 17% 4	330
	21st to end	Day	18% 13% 24% 27% 14% 3	363
November	1st to 10th	Day	18% 9% 28% 28% 12% 4	330
	11th to 20th	Day	19% 10% 30% 29% 9% 2	330
	21st to end	Day	25% 17% 31% 16% 9% 2	330
December	1st to 10th	Day	22% 11% 34% 24% 6% 3	330
	11th to 20th	Day	23% 15% 28% 26% 7%	330
	21st to end	Day	23% 10% 37% 22% 6% 1	363
January	1st to 10th	Night	21% 10% 28% 32% 8%	1006
	11th to 20th	Night	22% 11% 30% 29% 7%	1000
	21st to end	Night	24% 10% 28% 31% 7%	950
February	1st to 10th	Night	27% 11% 29% 27% 6%	800
	11th to 20th	Night	29% 14% 31% 23% 3	784
	21st to end	Night	26% 14% 37% 21% 1	660
March	1st to 10th	Night	24% 14% 34% 26% 3	632
	11th to 20th	Night	23% 12% 35% 26% 4	600
	21st to end	Night	22% 11% 37% 26% 3	660
April	1st to 10th	Night	19% 13% 39% 25% 4	600
	11th to 20th	Night	23% 12% 38% 25% 3	593
	21st to end	Night	23% 10% 37% 24% 6% 1	600
May	1st to 10th	Night	20% 10% 41% 26% 4	600
	11th to 20th	Night	17% 7% 35% 33% 8%	591
	21st to end	Night	16% 8% 32% 34% 10%	650
June	1st to 10th	Night	8% 4 26% 42% 21% 1	600
	11th to 20th	Night	7% 3 33% 43% 14%	600
	21st to end	Night	9% 2 19% 43% 25% 3	600
July	1st to 10th	Night	9% 3 14% 43% 28% 3	585
	11th to 20th	Night	4 2 14% 51% 28% 2	582
	21st to end	Night	7% 2 10% 46% 31% 3	657
August	1st to 10th	Night	2 10% 51% 33% 4	597
	11th to 20th	Night	4 1 10% 50% 30% 3	594
	21st to end	Night	7% 1 7% 40% 40% 5%	660
September	1st to 10th	Night	9% 1 9% 40% 36% 5%	600
	11th to 20th	Night	11% 1 8% 40% 36% 3	600
	21st to end	Night	11% 2 13% 36% 35% 4	600
October	1st to 10th	Night	8% 2 12% 46% 29% 4	600
	11th to 20th	Night	14% 3 21% 39% 20% 2	700
	21st to end	Night	17% 8% 21% 33% 18% 2	880
November	1st to 10th	Night	15% 7% 19% 38% 19% 1	820
	11th to 20th	Night	16% 7% 25% 36% 15% 1	971
	21st to end	Night	22% 10% 26% 29% 11% 1	1050
December	1st to 10th	Night	23% 8% 27% 30% 11% 1	1050
	11th to 20th	Night	27% 9% 24% 32% 8%	1050
	21st to end	Night	22% 8% 28% 32% 9%	1155

Page generated at 2025-12-30 00:14:10. Version ac07

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