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 2025-03-12.

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	12% 7% 3 6% 4 4 5% 4 6% 8% 41%	109254
	Night	12% 7% 2 6% 4 4 4 3 5% 7% 45%	109792
Transparency	Day	38% 7% 11% 20% 19% 6%	113074
	Night	36% 6% 10% 20% 21% 7%	114110
Seeing	Day	16% 10% 23% 24% 18% 9%	14929
	Night	15% 8% 23% 30% 21% 4	25023

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	15% 10% 5% 6% 3 3 3 3 3 4 45%	4056
	11th to 20th	Day	12% 6% 5% 7% 3 3 2 3 4 5% 51%	4241
	21st to end	Day	22% 9% 3 6% 4 3 3 3 3 5% 39%	4972
February	1st to 10th	Day	17% 11% 3 6% 4 4 4 3 4 6% 38%	2000
	11th to 20th	Day	24% 8% 3 5% 3 3 5% 3 4 5% 39%	1948
	21st to end	Day	19% 8% 3 7% 5% 3 4 3 4 5% 38%	1650
March	1st to 10th	Day	22% 9% 3 6% 4 4 4 3 5% 6% 34%	3220
	11th to 20th	Day	14% 9% 3 7% 3 4 4 4 5% 6% 41%	3354
	21st to end	Day	16% 6% 3 6% 4 4 5% 4 5% 7% 38%	3995
April	1st to 10th	Day	11% 7% 2 6% 4 4 4 3 5% 7% 46%	2280
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 3 5% 7% 44%	2254
	21st to end	Day	8% 4 2 5% 5% 5% 4 4 5% 8% 49%	2280
May	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 4 7% 9% 42%	4036
	11th to 20th	Day	15% 7% 2 7% 4 4 4 4 6% 9% 38%	4081
	21st to end	Day	8% 6% 2 7% 4 4 6% 4 7% 9% 42%	4476
June	1st to 10th	Day	10% 7% 2 8% 6% 5% 6% 6% 9% 10% 32%	2470
	11th to 20th	Day	5% 4 2 5% 5% 6% 6% 5% 8% 11% 43%	2470
	21st to end	Day	8% 5% 2 6% 6% 6% 6% 5% 8% 11% 40%	2470
July	1st to 10th	Day	6% 6% 2 9% 7% 8% 8% 7% 8% 12% 28%	4120
	11th to 20th	Day	8% 5% 3 7% 6% 7% 8% 7% 9% 11% 29%	4120
	21st to end	Day	10% 8% 4 10% 7% 6% 8% 6% 9% 10% 20%	4377
August	1st to 10th	Day	4 2 1 4 4 5% 5% 4 8% 11% 50%	3785
	11th to 20th	Day	6% 4 2 6% 5% 5% 5% 4 7% 10% 46%	3729
	21st to end	Day	6% 4 2 6% 5% 5% 5% 4 7% 8% 49%	4180
September	1st to 10th	Day	10% 6% 2 8% 5% 5% 5% 4 6% 8% 41%	2228
	11th to 20th	Day	13% 7% 2 6% 4 4 5% 3 6% 6% 43%	2090
	21st to end	Day	10% 6% 2 6% 5% 4 4 3 5% 7% 49%	2090
October	1st to 10th	Day	9% 4 2 5% 4 4 4 3 4 7% 54%	2090
	11th to 20th	Day	9% 5% 2 5% 3 3 3 3 4 8% 56%	2090
	21st to end	Day	12% 4 2 5% 4 3 3 3 4 5% 55%	1971
November	1st to 10th	Day	12% 7% 2 5% 4 3 3 3 4 5% 52%	1762
	11th to 20th	Day	13% 6% 2 5% 3 3 4 3 5% 6% 52%	1800
	21st to end	Day	14% 6% 2 6% 4 4 4 3 5% 7% 46%	1800
December	1st to 10th	Day	20% 7% 2 5% 4 3 3 3 4 6% 43%	2930
	11th to 20th	Day	15% 9% 3 7% 5% 4 4 3 4 5% 42%	3373
	21st to end	Day	19% 10% 3 6% 4 3 3 2 5% 6% 39%	4466
January	1st to 10th	Night	15% 10% 4 6% 3 3 3 2 4 4 44%	5889
	11th to 20th	Night	16% 9% 4 6% 3 3 3 3 3 5% 46%	5716
	21st to end	Night	19% 11% 2 5% 3 3 3 3 4 4 43%	5984
February	1st to 10th	Night	17% 7% 2 7% 4 4 4 2 4 6% 43%	2400
	11th to 20th	Night	20% 8% 3 6% 4 4 3 2 4 4 42%	2338
	21st to end	Night	17% 7% 3 5% 4 4 4 3 6% 6% 40%	1980
March	1st to 10th	Night	23% 11% 3 6% 4 4 3 3 4 6% 35%	3853
	11th to 20th	Night	11% 7% 2 5% 3 4 4 4 5% 8% 47%	3633
	21st to end	Night	15% 8% 3 8% 5% 4 5% 3 5% 5% 40%	3663
April	1st to 10th	Night	11% 5% 2 5% 4 3 5% 4 5% 9% 46%	1900
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 5% 7% 49%	1875
	21st to end	Night	10% 5% 2 5% 5% 4 3 3 5% 7% 52%	1900
May	1st to 10th	Night	9% 6% 3 7% 5% 5% 5% 4 6% 9% 42%	2859
	11th to 20th	Night	15% 7% 2 7% 4 4 4 4 6% 8% 38%	2812
	21st to end	Night	5% 3 1 5% 4 5% 6% 6% 8% 12% 46%	3079
June	1st to 10th	Night	5% 5% 2 7% 6% 6% 6% 5% 8% 11% 39%	1710
	11th to 20th	Night	3 3 1 4 4 5% 5% 5% 8% 11% 52%	1710
	21st to end	Night	5% 4 2 6% 5% 5% 5% 5% 7% 11% 45%	1710
July	1st to 10th	Night	5% 6% 3 8% 6% 7% 7% 6% 8% 11% 33%	2840
	11th to 20th	Night	7% 5% 2 7% 5% 6% 5% 6% 9% 12% 35%	2840
	21st to end	Night	9% 8% 4 10% 7% 7% 6% 6% 8% 9% 26%	3170
August	1st to 10th	Night	3 2 1 4 3 4 4 4 8% 10% 56%	3144
	11th to 20th	Night	5% 4 2 5% 4 4 5% 4 6% 9% 53%	3100
	21st to end	Night	5% 4 1 4 3 4 5% 4 6% 8% 55%	3476
September	1st to 10th	Night	8% 5% 3 7% 6% 5% 5% 4 6% 8% 44%	1931
	11th to 20th	Night	11% 6% 2 6% 5% 5% 4 4 5% 8% 45%	2090
	21st to end	Night	7% 5% 2 6% 5% 3 4 3 6% 6% 53%	2090
October	1st to 10th	Night	8% 5% 2 4 3 3 3 3 4 6% 61%	2090
	11th to 20th	Night	10% 5% 2 4 3 3 3 3 4 6% 57%	2090
	21st to end	Night	13% 6% 2 5% 3 3 3 2 3 4 56%	2622
November	1st to 10th	Night	10% 5% 2 4 4 2 4 3 5% 5% 56%	2536
	11th to 20th	Night	11% 7% 2 4 3 3 2 3 5% 6% 54%	2600
	21st to end	Night	13% 6% 2 5% 5% 4 4 4 5% 5% 48%	2600
December	1st to 10th	Night	19% 7% 2 5% 3 3 3 2 4 6% 47%	4250
	11th to 20th	Night	16% 8% 3 7% 4 4 3 3 4 5% 45%	4822
	21st to end	Night	19% 8% 3 7% 4 3 3 3 4 6% 39%	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	43% 4 5% 24% 22% 3	4057
	11th to 20th	Day	36% 3 6% 23% 28% 4	4241
	21st to end	Day	49% 3 6% 15% 19% 9%	4972
February	1st to 10th	Day	49% 3 7% 19% 18% 5%	2000
	11th to 20th	Day	48% 5% 4 17% 19% 7%	1958
	21st to end	Day	48% 4 6% 13% 21% 7%	1650
March	1st to 10th	Day	49% 6% 9% 11% 18% 7%	3220
	11th to 20th	Day	41% 5% 8% 17% 20% 8%	3354
	21st to end	Day	43% 7% 10% 16% 19% 5%	3995
April	1st to 10th	Day	37% 5% 11% 19% 19% 8%	2280
	11th to 20th	Day	39% 5% 7% 19% 22% 8%	2254
	21st to end	Day	31% 7% 10% 25% 24% 3	2280
May	1st to 10th	Day	32% 11% 13% 25% 16% 2	4074
	11th to 20th	Day	40% 8% 14% 26% 11% 1	4085
	21st to end	Day	34% 9% 13% 27% 16% 1	4489
June	1st to 10th	Day	42% 9% 15% 18% 14% 1	2470
	11th to 20th	Day	31% 10% 17% 25% 16% 2	2470
	21st to end	Day	35% 10% 18% 22% 11% 4	2470
July	1st to 10th	Day	42% 14% 21% 18% 5% 1	4055
	11th to 20th	Day	41% 13% 19% 20% 5% 1	4051
	21st to end	Day	53% 13% 14% 15% 5%	4237
August	1st to 10th	Day	24% 9% 13% 35% 19% 1	3773
	11th to 20th	Day	31% 9% 17% 25% 16% 3	3765
	21st to end	Day	29% 9% 15% 21% 22% 4	4179
September	1st to 10th	Day	39% 9% 14% 16% 17% 6%	2228
	11th to 20th	Day	40% 8% 10% 19% 19% 4	2090
	21st to end	Day	35% 6% 9% 22% 21% 7%	2090
October	1st to 10th	Day	23% 4 6% 12% 33% 22%	3480
	11th to 20th	Day	18% 3 5% 15% 31% 28%	3458
	21st to end	Day	24% 3 6% 19% 36% 13%	3221
November	1st to 10th	Day	35% 3 7% 17% 27% 11%	1762
	11th to 20th	Day	33% 4 7% 18% 27% 12%	1800
	21st to end	Day	38% 6% 7% 17% 25% 8%	1800
December	1st to 10th	Day	43% 3 5% 17% 27% 5%	2930
	11th to 20th	Day	47% 4 7% 19% 18% 5%	3370
	21st to end	Day	45% 5% 6% 23% 20% 1	4466
January	1st to 10th	Night	43% 4 5% 23% 23% 3	5889
	11th to 20th	Night	42% 3 5% 19% 24% 8%	5716
	21st to end	Night	46% 3 4 15% 26% 6%	5984
February	1st to 10th	Night	43% 4 6% 18% 21% 8%	2400
	11th to 20th	Night	47% 4 5% 17% 18% 10%	2339
	21st to end	Night	43% 6% 7% 15% 20% 9%	1980
March	1st to 10th	Night	50% 5% 8% 14% 15% 7%	3853
	11th to 20th	Night	33% 6% 10% 18% 22% 11%	3633
	21st to end	Night	44% 6% 7% 19% 19% 5%	3663
April	1st to 10th	Night	35% 5% 12% 20% 20% 8%	1900
	11th to 20th	Night	35% 5% 8% 19% 22% 12%	1875
	21st to end	Night	31% 6% 10% 24% 23% 6%	1900
May	1st to 10th	Night	34% 11% 12% 19% 20% 3	2886
	11th to 20th	Night	40% 10% 15% 23% 11% 2	2813
	21st to end	Night	24% 12% 15% 29% 17% 3	3087
June	1st to 10th	Night	36% 11% 14% 20% 17% 1	1710
	11th to 20th	Night	22% 9% 22% 28% 16% 3	1710
	21st to end	Night	31% 10% 17% 22% 17% 4	1710
July	1st to 10th	Night	41% 11% 23% 17% 6% 1	2795
	11th to 20th	Night	34% 14% 25% 21% 5% 1	2786
	21st to end	Night	50% 13% 16% 14% 6% 1	3069
August	1st to 10th	Night	20% 8% 15% 39% 17% 2	3132
	11th to 20th	Night	26% 9% 17% 30% 15% 3	3130
	21st to end	Night	26% 8% 15% 21% 26% 4	3476
September	1st to 10th	Night	36% 9% 16% 17% 17% 6%	1931
	11th to 20th	Night	37% 9% 10% 19% 21% 5%	2090
	21st to end	Night	30% 6% 10% 24% 23% 8%	2090
October	1st to 10th	Night	22% 3 6% 16% 29% 23%	3480
	11th to 20th	Night	18% 3 5% 17% 29% 29%	3461
	21st to end	Night	22% 3 4 22% 32% 16%	4322
November	1st to 10th	Night	30% 4 9% 18% 25% 13%	2536
	11th to 20th	Night	32% 3 6% 17% 28% 13%	2600
	21st to end	Night	37% 5% 7% 18% 24% 9%	2600
December	1st to 10th	Night	39% 4 5% 20% 27% 5%	4250
	11th to 20th	Night	43% 4 9% 16% 25% 4	4824
	21st to end	Night	44% 4 7% 24% 18% 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	26% 24% 30% 17% 3	300
	11th to 20th	Day	26% 23% 29% 15% 6%	300
	21st to end	Day	25% 25% 35% 12% 3 1	330
February	1st to 10th	Day	26% 21% 36% 14% 2	300
	11th to 20th	Day	40% 16% 31% 12% 2	300
	21st to end	Day	29% 22% 40% 8% 1	249
March	1st to 10th	Day	27% 23% 30% 15% 4	300
	11th to 20th	Day	23% 17% 36% 17% 7% 1	379
	21st to end	Day	22% 18% 32% 20% 8%	506
April	1st to 10th	Day	21% 13% 32% 23% 10% 1	460
	11th to 20th	Day	18% 14% 31% 23% 12% 1	452
	21st to end	Day	12% 12% 32% 30% 12% 2	460
May	1st to 10th	Day	12% 10% 27% 31% 15% 5%	463
	11th to 20th	Day	9% 6% 26% 29% 20% 8%	487
	21st to end	Day	11% 5% 21% 29% 19% 15%	529
June	1st to 10th	Day	12% 4 18% 29% 24% 13%	490
	11th to 20th	Day	7% 4 14% 28% 27% 21%	490
	21st to end	Day	11% 4 11% 28% 31% 17%	490
July	1st to 10th	Day	6% 2 9% 26% 32% 24%	485
	11th to 20th	Day	8% 1 10% 20% 32% 29%	484
	21st to end	Day	8% 2 11% 22% 33% 24%	532
August	1st to 10th	Day	9% 1 10% 24% 34% 21%	486
	11th to 20th	Day	9% 2 8% 28% 36% 16%	479
	21st to end	Day	14% 3 9% 25% 31% 18%	539
September	1st to 10th	Day	10% 3 12% 31% 31% 13%	490
	11th to 20th	Day	14% 4 14% 28% 31% 9%	490
	21st to end	Day	13% 3 17% 30% 29% 8%	490
October	1st to 10th	Day	10% 6% 26% 31% 21% 6%	490
	11th to 20th	Day	12% 7% 28% 34% 19% 1	490
	21st to end	Day	18% 11% 25% 31% 14% 1	359
November	1st to 10th	Day	18% 10% 35% 24% 9% 3	300
	11th to 20th	Day	17% 16% 40% 20% 6% 1	300
	21st to end	Day	14% 20% 36% 20% 8% 1	300
December	1st to 10th	Day	30% 15% 31% 21% 3	300
	11th to 20th	Day	25% 16% 38% 15% 5% 1	300
	21st to end	Day	30% 16% 32% 17% 6%	330
January	1st to 10th	Night	22% 14% 34% 25% 4	800
	11th to 20th	Night	22% 17% 29% 25% 7%	800
	21st to end	Night	19% 14% 36% 26% 4	880
February	1st to 10th	Night	21% 11% 33% 30% 5%	800
	11th to 20th	Night	26% 13% 32% 25% 3	783
	21st to end	Night	23% 14% 36% 25% 2	660
March	1st to 10th	Night	21% 13% 30% 30% 6%	800
	11th to 20th	Night	17% 11% 36% 26% 9% 1	767
	21st to end	Night	17% 13% 33% 27% 10%	746
April	1st to 10th	Night	13% 12% 32% 29% 12% 1	570
	11th to 20th	Night	16% 9% 30% 29% 13% 2	563
	21st to end	Night	12% 9% 27% 31% 18% 2	570
May	1st to 10th	Night	10% 7% 26% 32% 21% 3	570
	11th to 20th	Night	11% 3 24% 32% 26% 4	561
	21st to end	Night	7% 2 14% 34% 37% 6%	617
June	1st to 10th	Night	8% 2 12% 39% 33% 6%	570
	11th to 20th	Night	4 1 11% 29% 41% 14%	570
	21st to end	Night	6% 1 11% 28% 42% 12%	570
July	1st to 10th	Night	6% 1 8% 28% 45% 13%	555
	11th to 20th	Night	7% 1 5% 22% 44% 21%	552
	21st to end	Night	7% 1 8% 26% 44% 14%	624
August	1st to 10th	Night	5% 1 4 27% 48% 15%	567
	11th to 20th	Night	9% 1 7% 28% 45% 9%	564
	21st to end	Night	12% 1 8% 27% 41% 11%	627
September	1st to 10th	Night	9% 2 12% 29% 39% 9%	601
	11th to 20th	Night	13% 3 12% 28% 39% 5%	760
	21st to end	Night	9% 1 15% 33% 39% 4	760
October	1st to 10th	Night	10% 4 21% 33% 27% 6%	760
	11th to 20th	Night	11% 6% 24% 35% 20% 3	760
	21st to end	Night	15% 7% 20% 36% 19% 2	836
November	1st to 10th	Night	13% 7% 20% 37% 19% 3	780
	11th to 20th	Night	14% 9% 30% 34% 12% 1	800
	21st to end	Night	17% 9% 29% 33% 12% 1	800
December	1st to 10th	Night	22% 11% 28% 30% 8% 1	800
	11th to 20th	Night	24% 9% 30% 29% 8%	800
	21st to end	Night	25% 12% 29% 26% 7%	880

Page generated at 2025-03-09 07:08:43. Version ac07

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