Irondale
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irondale clear sky chart from 2005-11-06 to 2026-07-16.

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	13% 7% 3 7% 5% 4 5% 4 6% 8% 40%	137924
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	134805
Transparency	Day	39% 7% 11% 19% 18% 5%	141704
	Night	38% 6% 10% 20% 19% 7%	139100
Seeing	Day	16% 10% 24% 24% 18% 8%	16746
	Night	15% 8% 23% 30% 21% 4	26707

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	19% 10% 5% 6% 3 3 3 2 3 4 42%	5258
	11th to 20th	Day	11% 5% 4 6% 2 2 2 2 4 5% 57%	5517
	21st to end	Day	24% 9% 3 7% 4 4 3 3 3 5% 35%	6436
February	1st to 10th	Day	17% 11% 3 6% 4 3 4 3 4 6% 39%	2100
	11th to 20th	Day	23% 8% 3 4 3 3 5% 3 4 5% 39%	2048
	21st to end	Day	18% 8% 3 7% 5% 3 4 3 4 5% 39%	1730
March	1st to 10th	Day	19% 9% 3 6% 5% 4 4 4 5% 6% 36%	4580
	11th to 20th	Day	12% 7% 2 6% 4 4 4 4 5% 6% 46%	4894
	21st to end	Day	14% 6% 3 7% 4 4 4 4 5% 6% 41%	5853
April	1st to 10th	Day	11% 7% 2 7% 3 4 4 3 6% 8% 45%	2520
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 3 5% 7% 47%	2494
	21st to end	Day	8% 5% 2 6% 5% 5% 4 4 5% 9% 47%	2520
May	1st to 10th	Day	15% 7% 3 7% 5% 5% 6% 4 7% 8% 33%	7514
	11th to 20th	Day	13% 7% 3 8% 5% 5% 5% 5% 6% 9% 36%	7626
	21st to end	Day	15% 9% 3 9% 5% 5% 5% 4 6% 7% 31%	8391
June	1st to 10th	Day	10% 7% 2 8% 5% 5% 6% 6% 8% 10% 33%	2730
	11th to 20th	Day	6% 4 2 6% 5% 6% 6% 5% 8% 11% 40%	2730
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	2730
July	1st to 10th	Day	6% 5% 2 8% 6% 7% 7% 6% 9% 12% 33%	6030
	11th to 20th	Day	6% 5% 2 6% 6% 7% 8% 7% 10% 12% 31%	5965
	21st to end	Day	8% 6% 3 8% 7% 7% 8% 7% 10% 13% 23%	6293
August	1st to 10th	Day	5% 3 1 4 4 5% 5% 4 9% 11% 49%	3905
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 7% 10% 45%	3849
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 8% 48%	4312
September	1st to 10th	Day	10% 6% 2 8% 5% 5% 5% 4 6% 8% 42%	2345
	11th to 20th	Day	12% 7% 2 5% 4 4 5% 4 6% 7% 43%	2200
	21st to end	Day	9% 6% 2 6% 5% 4 4 3 6% 7% 49%	2200
October	1st to 10th	Day	10% 4 2 5% 4 4 4 3 4 7% 53%	2200
	11th to 20th	Day	8% 5% 2 4 3 3 3 3 4 8% 57%	2200
	21st to end	Day	13% 5% 2 5% 4 3 3 3 4 5% 54%	2074
November	1st to 10th	Day	12% 7% 2 5% 4 3 3 3 4 5% 53%	1852
	11th to 20th	Day	12% 6% 2 4 3 3 3 3 5% 6% 53%	1890
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 7% 45%	1890
December	1st to 10th	Day	20% 8% 2 5% 4 3 3 3 4 6% 43%	3020
	11th to 20th	Day	15% 8% 3 7% 5% 4 4 3 4 5% 42%	3463
	21st to end	Day	19% 10% 3 6% 4 3 3 2 5% 6% 39%	4565
January	1st to 10th	Night	17% 10% 3 6% 3 3 4 3 4 5% 42%	7635
	11th to 20th	Night	15% 9% 3 6% 3 3 3 3 3 5% 47%	7440
	21st to end	Night	20% 10% 3 7% 3 3 3 3 3 4 41%	7745
February	1st to 10th	Night	16% 7% 2 8% 4 4 4 2 4 6% 43%	2520
	11th to 20th	Night	20% 9% 3 6% 4 4 3 2 4 4 42%	2458
	21st to end	Night	18% 7% 3 5% 4 4 4 3 6% 5% 40%	2076
March	1st to 10th	Night	20% 10% 3 7% 4 4 4 3 5% 6% 34%	5493
	11th to 20th	Night	12% 7% 2 5% 3 3 3 4 5% 7% 50%	5284
	21st to end	Night	14% 7% 3 7% 5% 4 4 3 4 5% 44%	5359
April	1st to 10th	Night	11% 5% 2 5% 4 3 5% 4 6% 9% 46%	2100
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 7% 51%	2075
	21st to end	Night	10% 6% 2 5% 5% 3 3 3 5% 6% 51%	2100
May	1st to 10th	Night	15% 8% 3 7% 5% 5% 5% 4 5% 8% 35%	5339
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 36%	5234
	21st to end	Night	14% 7% 3 8% 5% 5% 6% 5% 6% 9% 32%	5761
June	1st to 10th	Night	6% 6% 2 7% 6% 6% 5% 5% 8% 11% 40%	1890
	11th to 20th	Night	5% 3 1 4 4 5% 5% 5% 9% 11% 49%	1890
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 7% 11% 44%	1890
July	1st to 10th	Night	5% 6% 3 7% 6% 6% 6% 5% 8% 12% 37%	4148
	11th to 20th	Night	6% 5% 2 6% 5% 6% 6% 5% 10% 14% 36%	4102
	21st to end	Night	6% 7% 3 8% 7% 6% 7% 7% 10% 11% 28%	4552
August	1st to 10th	Night	3 2 1 5% 3 4 5% 4 7% 10% 56%	3244
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 52%	3200
	21st to end	Night	6% 4 1 4 4 5% 5% 4 6% 8% 54%	3586
September	1st to 10th	Night	8% 5% 3 7% 6% 5% 5% 4 6% 8% 43%	2033
	11th to 20th	Night	10% 5% 2 6% 5% 4 4 4 5% 8% 46%	2200
	21st to end	Night	7% 5% 2 6% 5% 4 4 3 6% 7% 53%	2200
October	1st to 10th	Night	8% 6% 2 4 3 3 3 3 4 5% 60%	2200
	11th to 20th	Night	10% 4 2 4 3 3 3 3 4 6% 58%	2200
	21st to end	Night	13% 6% 2 5% 3 3 3 2 3 3 56%	2760
November	1st to 10th	Night	10% 5% 2 3 4 2 4 3 5% 5% 57%	2666
	11th to 20th	Night	11% 7% 2 4 3 3 2 3 4 5% 55%	2730
	21st to end	Night	13% 6% 2 5% 5% 4 4 4 5% 5% 48%	2730
December	1st to 10th	Night	19% 7% 2 5% 3 3 3 2 4 6% 47%	4380
	11th to 20th	Night	15% 8% 3 6% 4 4 3 2 4 5% 45%	4952
	21st to end	Night	19% 7% 3 7% 4 3 3 3 4 6% 40%	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	47% 3 6% 24% 17% 3	5259
	11th to 20th	Day	31% 3 5% 22% 32% 7%	5517
	21st to end	Day	53% 2 5% 13% 18% 8%	6436
February	1st to 10th	Day	48% 3 7% 19% 19% 4	2100
	11th to 20th	Day	48% 5% 4 18% 19% 7%	2058
	21st to end	Day	48% 4 6% 13% 21% 8%	1730
March	1st to 10th	Day	48% 5% 8% 13% 19% 6%	4580
	11th to 20th	Day	37% 4 7% 16% 21% 15%	4894
	21st to end	Day	42% 5% 9% 19% 20% 5%	5855
April	1st to 10th	Day	37% 5% 11% 21% 19% 8%	2520
	11th to 20th	Day	37% 5% 7% 20% 25% 7%	2494
	21st to end	Day	33% 7% 11% 24% 22% 3	2520
May	1st to 10th	Day	45% 9% 11% 21% 13% 2	7551
	11th to 20th	Day	42% 8% 16% 21% 12% 1	7630
	21st to end	Day	49% 9% 11% 19% 12% 1	8405
June	1st to 10th	Day	41% 9% 15% 18% 15% 1	2730
	11th to 20th	Day	33% 10% 18% 23% 15% 2	2730
	21st to end	Day	34% 10% 18% 23% 11% 4	2730
July	1st to 10th	Day	38% 13% 20% 21% 8% 1	5965
	11th to 20th	Day	37% 15% 27% 16% 4 1	5896
	21st to end	Day	45% 14% 20% 16% 3	6111
August	1st to 10th	Day	25% 9% 13% 34% 18% 1	3893
	11th to 20th	Day	31% 9% 17% 25% 15% 3	3885
	21st to end	Day	30% 9% 15% 21% 22% 4	4311
September	1st to 10th	Day	38% 9% 14% 17% 17% 6%	2345
	11th to 20th	Day	39% 8% 10% 20% 19% 4	2200
	21st to end	Day	34% 6% 10% 21% 22% 6%	2200
October	1st to 10th	Day	24% 4 6% 12% 33% 21%	3590
	11th to 20th	Day	18% 3 5% 15% 32% 28%	3568
	21st to end	Day	24% 3 6% 19% 35% 12%	3324
November	1st to 10th	Day	35% 3 7% 17% 27% 11%	1852
	11th to 20th	Day	33% 4 7% 19% 27% 11%	1890
	21st to end	Day	38% 5% 7% 18% 24% 7%	1890
December	1st to 10th	Day	44% 3 5% 17% 26% 5%	3020
	11th to 20th	Day	46% 4 7% 19% 18% 6%	3460
	21st to end	Day	45% 5% 6% 24% 19% 1	4565
January	1st to 10th	Night	44% 4 7% 24% 18% 2	7636
	11th to 20th	Night	40% 3 5% 17% 26% 9%	7440
	21st to end	Night	48% 2 4 16% 23% 7%	7745
February	1st to 10th	Night	43% 4 6% 18% 21% 8%	2520
	11th to 20th	Night	47% 4 5% 18% 17% 9%	2459
	21st to end	Night	43% 5% 7% 14% 20% 10%	2076
March	1st to 10th	Night	50% 4 7% 14% 17% 7%	5493
	11th to 20th	Night	34% 4 8% 15% 22% 17%	5284
	21st to end	Night	42% 5% 7% 22% 18% 7%	5359
April	1st to 10th	Night	35% 5% 13% 20% 19% 8%	2100
	11th to 20th	Night	33% 5% 8% 20% 24% 11%	2075
	21st to end	Night	33% 6% 11% 23% 21% 5%	2100
May	1st to 10th	Night	45% 9% 12% 16% 16% 3	5366
	11th to 20th	Night	44% 8% 14% 21% 11% 1	5235
	21st to end	Night	44% 10% 12% 19% 12% 1	5771
June	1st to 10th	Night	35% 11% 14% 20% 18% 2	1890
	11th to 20th	Night	25% 10% 21% 26% 16% 2	1890
	21st to end	Night	31% 10% 17% 23% 15% 4	1890
July	1st to 10th	Night	37% 11% 22% 21% 8% 1	4105
	11th to 20th	Night	32% 15% 32% 17% 4 1	4051
	21st to end	Night	45% 14% 22% 15% 4	4420
August	1st to 10th	Night	20% 8% 15% 39% 16% 2	3232
	11th to 20th	Night	27% 9% 17% 29% 15% 3	3230
	21st to end	Night	26% 8% 15% 21% 26% 4	3586
September	1st to 10th	Night	36% 9% 16% 17% 17% 6%	2033
	11th to 20th	Night	36% 9% 10% 20% 21% 5%	2200
	21st to end	Night	30% 6% 10% 23% 24% 7%	2200
October	1st to 10th	Night	23% 3 6% 16% 29% 22%	3590
	11th to 20th	Night	18% 3 5% 17% 29% 29%	3571
	21st to end	Night	23% 3 4 22% 32% 16%	4460
November	1st to 10th	Night	30% 4 9% 18% 26% 13%	2666
	11th to 20th	Night	31% 3 7% 18% 28% 13%	2730
	21st to end	Night	37% 5% 7% 18% 24% 9%	2730
December	1st to 10th	Night	40% 4 5% 20% 26% 5%	4380
	11th to 20th	Night	43% 4 9% 16% 25% 5%	4954
	21st to end	Night	44% 4 7% 25% 18% 2	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	28% 23% 30% 17% 3	330
	11th to 20th	Day	25% 24% 29% 15% 6%	330
	21st to end	Day	26% 26% 33% 12% 2 1	363
February	1st to 10th	Day	25% 23% 37% 13% 2	330
	11th to 20th	Day	38% 17% 32% 12% 2	330
	21st to end	Day	29% 21% 38% 10% 1	273
March	1st to 10th	Day	25% 22% 32% 17% 4	330
	11th to 20th	Day	24% 17% 36% 16% 6% 1	448
	21st to end	Day	20% 18% 33% 22% 7%	594
April	1st to 10th	Day	21% 13% 33% 22% 10% 1	540
	11th to 20th	Day	17% 13% 33% 23% 12% 1	532
	21st to end	Day	12% 12% 31% 31% 11% 2	540
May	1st to 10th	Day	15% 11% 28% 29% 13% 4	543
	11th to 20th	Day	10% 7% 28% 28% 19% 7%	567
	21st to end	Day	14% 6% 21% 28% 17% 13%	617
June	1st to 10th	Day	13% 4 19% 29% 23% 12%	570
	11th to 20th	Day	8% 3 15% 29% 25% 19%	570
	21st to end	Day	10% 3 11% 27% 31% 18%	570
July	1st to 10th	Day	5% 2 10% 26% 33% 23%	565
	11th to 20th	Day	8% 1 10% 21% 33% 27%	544
	21st to end	Day	7% 1 10% 23% 35% 23%	576
August	1st to 10th	Day	10% 2 10% 24% 33% 20%	526
	11th to 20th	Day	9% 2 8% 27% 35% 18%	519
	21st to end	Day	14% 3 11% 25% 30% 17%	583
September	1st to 10th	Day	9% 3 13% 33% 30% 12%	530
	11th to 20th	Day	13% 3 14% 28% 32% 10%	530
	21st to end	Day	12% 4 18% 31% 29% 7%	530
October	1st to 10th	Day	11% 7% 26% 31% 19% 6%	530
	11th to 20th	Day	11% 7% 28% 35% 18% 1	530
	21st to end	Day	20% 11% 26% 30% 13% 1	393
November	1st to 10th	Day	19% 10% 34% 25% 9% 3	330
	11th to 20th	Day	15% 15% 41% 22% 6% 1	330
	21st to end	Day	15% 21% 37% 19% 8% 1	330
December	1st to 10th	Day	32% 15% 30% 20% 3	330
	11th to 20th	Day	24% 16% 38% 15% 6% 1	330
	21st to end	Day	29% 16% 32% 18% 5%	363
January	1st to 10th	Night	23% 14% 34% 25% 4	840
	11th to 20th	Night	22% 17% 29% 25% 7%	840
	21st to end	Night	20% 14% 36% 26% 3	924
February	1st to 10th	Night	21% 12% 34% 30% 4	840
	11th to 20th	Night	26% 13% 33% 25% 3	823
	21st to end	Night	24% 14% 35% 24% 3	692
March	1st to 10th	Night	20% 13% 30% 31% 6%	840
	11th to 20th	Night	17% 12% 36% 26% 9% 1	840
	21st to end	Night	16% 13% 34% 28% 9%	824
April	1st to 10th	Night	13% 12% 32% 30% 12% 1	630
	11th to 20th	Night	15% 9% 31% 30% 14% 1	623
	21st to end	Night	13% 9% 26% 32% 18% 2	630
May	1st to 10th	Night	11% 7% 27% 32% 20% 3	630
	11th to 20th	Night	11% 3 25% 33% 24% 4	621
	21st to end	Night	9% 3 14% 34% 35% 6%	683
June	1st to 10th	Night	8% 3 12% 39% 32% 6%	630
	11th to 20th	Night	7% 1 10% 29% 40% 12%	630
	21st to end	Night	6% 2 10% 28% 42% 12%	630
July	1st to 10th	Night	6% 1 7% 28% 44% 14%	615
	11th to 20th	Night	7% 1 5% 22% 44% 21%	597
	21st to end	Night	7% 1 8% 25% 46% 14%	657
August	1st to 10th	Night	6% 1 4 28% 47% 15%	597
	11th to 20th	Night	8% 1 7% 28% 45% 10%	594
	21st to end	Night	12% 1 8% 28% 39% 11%	660
September	1st to 10th	Night	9% 2 12% 30% 39% 9%	633
	11th to 20th	Night	12% 3 12% 29% 39% 5%	800
	21st to end	Night	9% 2 15% 33% 38% 4	800
October	1st to 10th	Night	10% 4 22% 32% 26% 6%	800
	11th to 20th	Night	11% 6% 24% 36% 20% 3	800
	21st to end	Night	16% 8% 20% 36% 19% 2	880
November	1st to 10th	Night	14% 7% 20% 37% 19% 3	820
	11th to 20th	Night	13% 9% 30% 34% 13% 1	840
	21st to end	Night	16% 9% 29% 33% 11% 1	840
December	1st to 10th	Night	23% 11% 28% 30% 8% 1	840
	11th to 20th	Night	24% 10% 29% 30% 8%	840
	21st to end	Night	25% 12% 29% 27% 7%	924

Page generated at 2026-07-13 02:14:53. Version ac07

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