Starr Mountain
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starr Mountain clear sky chart from 2005-11-06 to 2026-03-03.

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% 8% 3 7% 5% 4 5% 4 6% 7% 38%	131069
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 41%	130127
Transparency	Day	41% 8% 11% 20% 16% 3	134821
	Night	40% 7% 11% 20% 17% 4	134432
Seeing	Day	19% 12% 23% 23% 16% 7%	16486
	Night	18% 10% 23% 29% 18% 3	26594

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% 12% 3 6% 4 3 3 3 4 4 37%	5258
	11th to 20th	Day	13% 8% 7% 7% 3 3 2 2 3 4 48%	5280
	21st to end	Day	21% 12% 3 7% 4 3 3 3 4 6% 34%	5934
February	1st to 10th	Day	18% 10% 4 8% 5% 4 4 3 5% 5% 35%	2100
	11th to 20th	Day	22% 11% 3 8% 4 3 4 3 5% 6% 32%	2048
	21st to end	Day	20% 10% 3 8% 5% 3 3 4 4 5% 36%	1730
March	1st to 10th	Day	19% 8% 3 7% 5% 4 4 4 5% 6% 34%	4587
	11th to 20th	Day	13% 7% 3 6% 4 3 4 3 4 6% 48%	4980
	21st to end	Day	17% 8% 2 6% 4 4 4 3 4 7% 42%	5465
April	1st to 10th	Day	12% 7% 2 5% 5% 5% 5% 5% 6% 8% 41%	2305
	11th to 20th	Day	14% 7% 2 6% 4 3 4 3 5% 6% 45%	2374
	21st to end	Day	13% 7% 2 7% 5% 6% 4 3 5% 7% 41%	2573
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 7% 8% 32%	5862
	11th to 20th	Day	16% 8% 4 10% 6% 6% 6% 5% 6% 7% 26%	5861
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 6% 7% 37%	6433
June	1st to 10th	Day	9% 6% 2 6% 6% 5% 6% 6% 8% 10% 37%	2600
	11th to 20th	Day	6% 4 2 7% 6% 6% 6% 6% 8% 11% 40%	2600
	21st to end	Day	8% 5% 2 6% 6% 5% 6% 5% 8% 11% 38%	2600
July	1st to 10th	Day	6% 5% 2 6% 5% 6% 6% 5% 9% 11% 39%	5900
	11th to 20th	Day	8% 5% 2 7% 5% 6% 6% 6% 9% 12% 33%	5900
	21st to end	Day	11% 8% 3 9% 7% 7% 7% 6% 8% 10% 22%	6424
August	1st to 10th	Day	7% 5% 2 6% 5% 6% 6% 6% 7% 10% 39%	4234
	11th to 20th	Day	8% 5% 2 7% 5% 5% 5% 5% 8% 11% 39%	3954
	21st to end	Day	7% 4 2 6% 5% 5% 5% 5% 6% 7% 49%	4312
September	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 7% 8% 40%	2235
	11th to 20th	Day	14% 6% 2 7% 5% 5% 4 5% 6% 8% 40%	2200
	21st to end	Day	14% 5% 2 6% 4 4 4 4 5% 6% 46%	2200
October	1st to 10th	Day	9% 5% 2 5% 4 4 4 3 4 7% 53%	2200
	11th to 20th	Day	12% 5% 2 4 2 3 3 2 4 6% 56%	2085
	21st to end	Day	15% 5% 2 5% 3 3 4 3 4 6% 51%	2155
November	1st to 10th	Day	13% 5% 2 4 3 4 3 3 5% 5% 53%	1852
	11th to 20th	Day	13% 7% 2 5% 4 3 3 3 5% 7% 48%	1890
	21st to end	Day	16% 8% 3 4 3 3 5% 3 5% 6% 44%	1890
December	1st to 10th	Day	17% 8% 2 5% 3 4 4 3 5% 5% 44%	3020
	11th to 20th	Day	19% 14% 4 9% 5% 4 3 3 4 4 31%	3463
	21st to end	Day	16% 13% 4 7% 4 4 4 3 6% 7% 33%	4565
January	1st to 10th	Night	17% 10% 4 7% 3 3 3 2 4 5% 41%	7635
	11th to 20th	Night	19% 8% 3 6% 4 3 2 2 3 4 46%	7680
	21st to end	Night	19% 11% 3 6% 4 3 3 3 4 5% 39%	8244
February	1st to 10th	Night	16% 10% 4 7% 4 3 3 3 5% 6% 39%	2520
	11th to 20th	Night	23% 10% 3 7% 4 4 4 3 4 3 35%	2458
	21st to end	Night	22% 9% 3 7% 4 4 4 3 4 7% 34%	2076
March	1st to 10th	Night	19% 11% 2 7% 4 4 4 3 5% 7% 34%	5311
	11th to 20th	Night	13% 6% 1 4 3 3 4 4 6% 7% 49%	4980
	21st to end	Night	13% 8% 3 7% 4 5% 4 3 5% 6% 42%	5467
April	1st to 10th	Night	13% 5% 1 5% 4 4 4 4 6% 8% 46%	2110
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 8% 48%	1975
	21st to end	Night	14% 6% 2 5% 4 4 4 3 5% 7% 45%	1825
May	1st to 10th	Night	14% 7% 2 8% 6% 6% 5% 5% 7% 9% 32%	4033
	11th to 20th	Night	19% 9% 3 8% 6% 5% 5% 4 5% 7% 28%	4032
	21st to end	Night	14% 7% 2 7% 5% 4 5% 4 6% 9% 37%	4421
June	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 5% 7% 11% 38%	1800
	11th to 20th	Night	5% 3 2 4 5% 4 5% 6% 7% 13% 46%	1800
	21st to end	Night	6% 4 2 6% 5% 6% 5% 4 7% 11% 44%	1800
July	1st to 10th	Night	8% 5% 2 7% 5% 6% 6% 5% 8% 12% 35%	4058
	11th to 20th	Night	8% 5% 2 7% 5% 5% 5% 4 8% 12% 38%	4057
	21st to end	Night	10% 9% 3 8% 6% 7% 7% 6% 9% 12% 24%	4427
August	1st to 10th	Night	7% 5% 2 7% 6% 5% 5% 4 7% 10% 43%	2915
	11th to 20th	Night	8% 5% 2 7% 4 4 5% 4 6% 9% 45%	3090
	21st to end	Night	5% 4 2 5% 4 4 5% 5% 7% 11% 49%	3586
September	1st to 10th	Night	11% 5% 3 6% 5% 4 5% 4 7% 7% 42%	2143
	11th to 20th	Night	12% 6% 3 7% 4 4 4 4 6% 9% 42%	2200
	21st to end	Night	12% 7% 2 5% 4 4 5% 3 5% 6% 48%	2200
October	1st to 10th	Night	12% 5% 1 4 3 3 4 2 3 6% 57%	2200
	11th to 20th	Night	14% 3 2 4 3 4 3 2 4 5% 56%	2313
	21st to end	Night	14% 6% 2 4 2 3 3 3 4 5% 54%	2680
November	1st to 10th	Night	12% 7% 2 5% 3 3 3 2 5% 5% 52%	2666
	11th to 20th	Night	13% 7% 2 5% 3 3 3 3 5% 6% 50%	2730
	21st to end	Night	16% 8% 3 5% 4 4 4 3 5% 5% 44%	2730
December	1st to 10th	Night	18% 9% 2 6% 4 3 4 3 4 5% 42%	4380
	11th to 20th	Night	20% 11% 4 7% 4 3 3 3 3 5% 35%	4952
	21st to end	Night	16% 11% 4 7% 5% 4 4 3 5% 6% 34%	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	49% 5% 10% 20% 16% 1	5259
	11th to 20th	Day	41% 4 5% 14% 31% 5%	5280
	21st to end	Day	52% 4 7% 19% 14% 5%	5934
February	1st to 10th	Day	50% 6% 8% 18% 16% 3	2100
	11th to 20th	Day	54% 4 7% 17% 15% 3	2058
	21st to end	Day	51% 5% 8% 12% 19% 5%	1730
March	1st to 10th	Day	48% 5% 9% 13% 18% 7%	4587
	11th to 20th	Day	37% 4 7% 16% 23% 12%	4980
	21st to end	Day	43% 5% 8% 28% 15% 1	5465
April	1st to 10th	Day	39% 9% 11% 22% 17% 3	2305
	11th to 20th	Day	39% 5% 9% 19% 24% 5%	2374
	21st to end	Day	41% 7% 11% 26% 13% 2	2573
May	1st to 10th	Day	47% 10% 13% 20% 10% 1	5897
	11th to 20th	Day	53% 10% 12% 17% 7% 1	5865
	21st to end	Day	44% 9% 10% 21% 14% 2	6447
June	1st to 10th	Day	38% 12% 16% 22% 10% 2	2600
	11th to 20th	Day	35% 11% 18% 23% 12% 2	2600
	21st to end	Day	36% 13% 18% 18% 13% 3	2600
July	1st to 10th	Day	34% 12% 20% 23% 10% 1	5835
	11th to 20th	Day	38% 13% 26% 17% 4 1	5831
	21st to end	Day	53% 12% 16% 15% 4	6240
August	1st to 10th	Day	35% 11% 15% 30% 8% 1	4221
	11th to 20th	Day	34% 11% 18% 23% 12% 1	3993
	21st to end	Day	32% 10% 11% 32% 14% 1	4311
September	1st to 10th	Day	38% 10% 14% 17% 16% 5%	2235
	11th to 20th	Day	40% 11% 10% 22% 16% 2	2200
	21st to end	Day	38% 8% 13% 18% 18% 5%	2200
October	1st to 10th	Day	25% 6% 7% 19% 34% 10%	3590
	11th to 20th	Day	21% 4 5% 22% 31% 16%	3377
	21st to end	Day	26% 5% 7% 28% 25% 9%	3457
November	1st to 10th	Day	33% 4 10% 18% 29% 6%	1852
	11th to 20th	Day	35% 6% 6% 23% 25% 5%	1890
	21st to end	Day	40% 5% 9% 21% 21% 4	1890
December	1st to 10th	Day	42% 5% 6% 20% 25% 3	3020
	11th to 20th	Day	56% 4 7% 16% 14% 3	3460
	21st to end	Day	48% 5% 12% 24% 9% 1	4565
January	1st to 10th	Night	45% 4 13% 21% 15% 1	7636
	11th to 20th	Night	43% 4 4 17% 26% 6%	7680
	21st to end	Night	48% 4 8% 17% 18% 6%	8244
February	1st to 10th	Night	46% 4 9% 19% 18% 3	2520
	11th to 20th	Night	54% 5% 6% 16% 14% 4	2459
	21st to end	Night	51% 6% 6% 17% 15% 4	2076
March	1st to 10th	Night	49% 6% 8% 14% 19% 4	5311
	11th to 20th	Night	33% 6% 8% 19% 22% 12%	4980
	21st to end	Night	42% 6% 9% 24% 16% 3	5467
April	1st to 10th	Night	34% 7% 13% 21% 21% 4	2110
	11th to 20th	Night	35% 6% 10% 22% 22% 6%	1975
	21st to end	Night	37% 7% 13% 26% 16% 1	1825
May	1st to 10th	Night	45% 10% 16% 19% 9% 1	4060
	11th to 20th	Night	53% 10% 14% 16% 8%	4033
	21st to end	Night	40% 11% 13% 23% 12% 1	4429
June	1st to 10th	Night	37% 12% 16% 22% 11% 2	1800
	11th to 20th	Night	27% 11% 21% 26% 13% 1	1800
	21st to end	Night	32% 10% 19% 22% 12% 4	1800
July	1st to 10th	Night	39% 10% 20% 23% 7% 1	4015
	11th to 20th	Night	36% 11% 27% 20% 4 2	4006
	21st to end	Night	49% 13% 18% 15% 4 1	4297
August	1st to 10th	Night	34% 9% 17% 31% 10%	2904
	11th to 20th	Night	32% 10% 22% 24% 11% 1	3117
	21st to end	Night	27% 9% 15% 32% 15% 1	3586
September	1st to 10th	Night	40% 8% 16% 17% 14% 5%	2143
	11th to 20th	Night	37% 11% 13% 20% 19% 1	2200
	21st to end	Night	35% 8% 12% 20% 20% 6%	2200
October	1st to 10th	Night	25% 5% 8% 20% 29% 14%	3590
	11th to 20th	Night	22% 3 6% 19% 33% 16%	3760
	21st to end	Night	23% 4 4 28% 29% 12%	4316
November	1st to 10th	Night	34% 4 9% 21% 23% 8%	2666
	11th to 20th	Night	36% 5% 7% 20% 25% 8%	2730
	21st to end	Night	42% 5% 6% 22% 22% 4	2730
December	1st to 10th	Night	45% 5% 6% 18% 24% 2	4380
	11th to 20th	Night	51% 5% 7% 16% 19% 2	4954
	21st to end	Night	49% 6% 11% 22% 11% 1	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	31% 24% 32% 10% 3	330
	11th to 20th	Day	31% 26% 29% 10% 4	330
	21st to end	Day	26% 30% 32% 10% 2	363
February	1st to 10th	Day	26% 24% 36% 12% 2	330
	11th to 20th	Day	40% 28% 23% 8% 1	330
	21st to end	Day	38% 25% 29% 8% 1	273
March	1st to 10th	Day	30% 22% 27% 17% 4	341
	11th to 20th	Day	29% 21% 28% 16% 5% 1	500
	21st to end	Day	28% 19% 31% 16% 6%	550
April	1st to 10th	Day	23% 21% 28% 19% 9% 1	500
	11th to 20th	Day	22% 16% 31% 21% 7% 2	492
	21st to end	Day	20% 13% 28% 25% 10% 3	500
May	1st to 10th	Day	17% 16% 31% 23% 9% 5%	503
	11th to 20th	Day	15% 9% 26% 27% 16% 7%	527
	21st to end	Day	16% 8% 22% 25% 17% 11%	573
June	1st to 10th	Day	10% 8% 21% 30% 23% 8%	530
	11th to 20th	Day	9% 4 16% 29% 28% 14%	530
	21st to end	Day	10% 4 19% 28% 23% 17%	530
July	1st to 10th	Day	10% 4 13% 27% 30% 16%	525
	11th to 20th	Day	10% 4 10% 27% 31% 18%	524
	21st to end	Day	13% 3 11% 24% 32% 18%	576
August	1st to 10th	Day	16% 5% 12% 27% 21% 19%	526
	11th to 20th	Day	13% 4 14% 28% 27% 14%	519
	21st to end	Day	13% 3 13% 25% 31% 15%	583
September	1st to 10th	Day	12% 3 16% 26% 28% 13%	530
	11th to 20th	Day	16% 5% 13% 26% 25% 15%	530
	21st to end	Day	17% 6% 15% 31% 26% 6%	530
October	1st to 10th	Day	12% 5% 23% 31% 21% 7%	530
	11th to 20th	Day	15% 11% 28% 26% 19% 2	530
	21st to end	Day	22% 10% 25% 26% 15% 2	538
November	1st to 10th	Day	22% 15% 28% 22% 13% 1	330
	11th to 20th	Day	18% 18% 35% 23% 5%	330
	21st to end	Day	21% 18% 39% 13% 9%	330
December	1st to 10th	Day	30% 21% 28% 18% 3	330
	11th to 20th	Day	29% 15% 33% 17% 5%	330
	21st to end	Day	25% 25% 28% 17% 4	363
January	1st to 10th	Night	25% 20% 30% 22% 3	840
	11th to 20th	Night	27% 18% 30% 21% 5%	840
	21st to end	Night	23% 17% 33% 24% 2	924
February	1st to 10th	Night	22% 15% 33% 27% 3	840
	11th to 20th	Night	30% 22% 29% 18% 1	823
	21st to end	Night	30% 18% 30% 20% 2	692
March	1st to 10th	Night	22% 16% 32% 26% 4	809
	11th to 20th	Night	23% 17% 30% 23% 6% 1	800
	21st to end	Night	22% 15% 30% 25% 8%	880
April	1st to 10th	Night	15% 20% 27% 29% 8% 1	710
	11th to 20th	Night	15% 13% 34% 27% 10% 1	593
	21st to end	Night	16% 8% 29% 37% 11% 1	600
May	1st to 10th	Night	11% 8% 30% 34% 15% 2	600
	11th to 20th	Night	16% 7% 20% 31% 21% 4	591
	21st to end	Night	12% 4 14% 35% 29% 6%	650
June	1st to 10th	Night	9% 5% 14% 38% 32% 4	600
	11th to 20th	Night	6% 2 13% 34% 36% 10%	600
	21st to end	Night	9% 1 9% 35% 39% 8%	600
July	1st to 10th	Night	12% 2 8% 27% 44% 7%	585
	11th to 20th	Night	11% 1 7% 27% 43% 11%	582
	21st to end	Night	10% 2 8% 27% 45% 9%	657
August	1st to 10th	Night	11% 1 8% 28% 44% 8%	597
	11th to 20th	Night	9% 3 10% 35% 39% 4	594
	21st to end	Night	11% 2 10% 30% 40% 7%	660
September	1st to 10th	Night	12% 3 10% 31% 38% 6%	743
	11th to 20th	Night	15% 3 12% 33% 33% 5%	800
	21st to end	Night	15% 4 18% 37% 25% 2	800
October	1st to 10th	Night	13% 6% 20% 31% 25% 5%	800
	11th to 20th	Night	16% 8% 25% 32% 18% 1	800
	21st to end	Night	17% 9% 24% 34% 14% 2	880
November	1st to 10th	Night	16% 8% 22% 36% 16% 2	820
	11th to 20th	Night	16% 15% 29% 29% 11% 1	840
	21st to end	Night	21% 13% 28% 27% 10% 1	840
December	1st to 10th	Night	26% 15% 25% 27% 7%	840
	11th to 20th	Night	24% 12% 30% 27% 7%	840
	21st to end	Night	25% 15% 26% 29% 4	924

Page generated at 2026-02-28 01:40:19. Version ac07

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