Foothills Astronomical Society
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Foothills Astronomical Society clear sky chart from 2005-11-06 to 2026-07-31.

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% 5% 2 6% 4 4 5% 4 6% 8% 43%	138201
	Night	15% 5% 2 5% 4 4 4 4 5% 7% 45%	134824
Transparency	Day	25% 36% 8% 14% 12% 5%	141899
	Night	25% 36% 7% 13% 14% 5%	139198
Seeing	Day	18% 10% 22% 24% 17% 9%	17414
	Night	17% 7% 21% 31% 20% 4	27451

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% 6% 2 5% 2 3 2 3 3 5% 55%	5258
	11th to 20th	Day	9% 5% 1 4 2 2 3 2 3 2 66%	5280
	21st to end	Day	20% 6% 2 6% 4 3 3 3 4 5% 44%	5780
February	1st to 10th	Day	14% 5% 2 5% 4 3 4 3 5% 7% 46%	1968
	11th to 20th	Day	19% 6% 2 5% 4 3 3 4 5% 5% 45%	2048
	21st to end	Day	17% 6% 2 6% 5% 4 4 4 6% 5% 42%	1730
March	1st to 10th	Day	20% 6% 2 5% 3 3 4 3 5% 5% 44%	4990
	11th to 20th	Day	11% 5% 2 5% 3 4 3 3 4 6% 54%	5090
	21st to end	Day	19% 6% 1 4 3 3 3 3 5% 5% 47%	5586
April	1st to 10th	Day	12% 4 2 4 4 4 4 4 6% 7% 47%	2310
	11th to 20th	Day	12% 5% 1 5% 5% 4 5% 3 5% 7% 48%	2486
	21st to end	Day	13% 4 2 5% 4 4 5% 4 5% 6% 47%	2730
May	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 6% 9% 37%	7616
	11th to 20th	Day	15% 6% 2 6% 4 5% 5% 4 6% 8% 38%	7626
	21st to end	Day	16% 7% 3 8% 5% 5% 5% 5% 7% 9% 32%	8391
June	1st to 10th	Day	10% 6% 2 7% 6% 6% 7% 6% 9% 9% 33%	2730
	11th to 20th	Day	7% 5% 2 6% 5% 5% 7% 6% 9% 12% 38%	2730
	21st to end	Day	8% 5% 2 5% 5% 6% 7% 5% 9% 10% 39%	2730
July	1st to 10th	Day	6% 4 2 7% 6% 6% 7% 6% 8% 11% 37%	6030
	11th to 20th	Day	7% 4 2 8% 6% 7% 7% 5% 10% 12% 32%	6030
	21st to end	Day	9% 6% 3 8% 7% 7% 8% 7% 9% 11% 25%	6554
August	1st to 10th	Day	17% 7% 2 8% 5% 6% 6% 5% 8% 10% 27%	4234
	11th to 20th	Day	10% 5% 2 6% 5% 5% 7% 5% 8% 10% 36%	4174
	21st to end	Day	7% 3 1 6% 4 4 4 4 6% 8% 51%	4323
September	1st to 10th	Day	10% 5% 2 6% 5% 5% 6% 5% 6% 9% 42%	2200
	11th to 20th	Day	16% 7% 2 6% 4 4 5% 3 5% 8% 40%	2200
	21st to end	Day	17% 5% 2 6% 4 3 5% 4 5% 6% 42%	2200
October	1st to 10th	Day	16% 4 1 4 2 3 3 3 4 5% 54%	2150
	11th to 20th	Day	13% 4 1 3 2 2 2 2 4 5% 61%	2000
	21st to end	Day	15% 4 2 4 3 2 3 2 3 4 58%	2200
November	1st to 10th	Day	13% 5% 1 5% 3 3 3 2 3 5% 58%	1999
	11th to 20th	Day	13% 4 1 4 3 3 3 3 4 6% 56%	1890
	21st to end	Day	15% 3 1 4 3 4 4 3 5% 6% 53%	1890
December	1st to 10th	Day	14% 5% 2 5% 3 3 3 3 4 5% 53%	3020
	11th to 20th	Day	17% 4 1 4 4 3 4 3 5% 7% 46%	3463
	21st to end	Day	23% 4 1 4 2 3 3 3 4 6% 48%	4565
January	1st to 10th	Night	15% 4 1 3 2 2 2 2 3 5% 59%	7635
	11th to 20th	Night	10% 5% 1 4 3 3 3 2 4 5% 60%	7680
	21st to end	Night	18% 6% 2 4 3 3 3 3 4 5% 47%	8401
February	1st to 10th	Night	14% 5% 2 5% 3 4 3 3 5% 5% 50%	2649
	11th to 20th	Night	20% 6% 2 5% 3 3 4 3 5% 4 46%	2458
	21st to end	Night	15% 6% 2 5% 5% 4 4 3 5% 7% 44%	2076
March	1st to 10th	Night	19% 5% 2 5% 3 3 4 3 4 6% 45%	5081
	11th to 20th	Night	11% 4 1 3 3 3 3 3 5% 8% 57%	5090
	21st to end	Night	17% 4 2 5% 4 4 4 4 5% 6% 45%	5588
April	1st to 10th	Night	13% 5% 1 4 4 4 4 3 5% 7% 49%	2310
	11th to 20th	Night	12% 5% 2 5% 3 2 3 3 6% 7% 52%	2098
	21st to end	Night	12% 5% 2 5% 4 3 3 3 6% 6% 51%	1890
May	1st to 10th	Night	13% 6% 2 6% 4 5% 5% 5% 8% 10% 36%	5237
	11th to 20th	Night	17% 6% 2 7% 4 4 5% 4 6% 7% 38%	5234
	21st to end	Night	18% 7% 3 7% 4 5% 4 4 6% 9% 34%	5761
June	1st to 10th	Night	9% 5% 3 7% 6% 5% 6% 6% 8% 12% 32%	1890
	11th to 20th	Night	11% 5% 2 5% 4 5% 5% 5% 7% 11% 39%	1890
	21st to end	Night	8% 5% 2 6% 6% 5% 6% 4 7% 10% 41%	1890
July	1st to 10th	Night	8% 6% 3 8% 6% 7% 8% 6% 8% 10% 30%	4148
	11th to 20th	Night	9% 5% 3 8% 6% 6% 7% 6% 9% 13% 28%	4147
	21st to end	Night	12% 8% 4 9% 6% 7% 7% 6% 9% 10% 23%	4517
August	1st to 10th	Night	16% 8% 3 7% 5% 5% 6% 5% 7% 10% 27%	2915
	11th to 20th	Night	13% 6% 3 9% 6% 5% 6% 5% 7% 10% 30%	2875
	21st to end	Night	8% 5% 2 7% 4 5% 5% 5% 7% 8% 44%	3535
September	1st to 10th	Night	12% 6% 2 6% 4 5% 5% 4 6% 8% 41%	2200
	11th to 20th	Night	17% 6% 2 8% 5% 5% 4 4 6% 8% 35%	2200
	21st to end	Night	19% 5% 2 5% 5% 3 4 3 5% 7% 43%	2200
October	1st to 10th	Night	17% 4 1 5% 3 4 4 3 4 6% 50%	2250
	11th to 20th	Night	14% 4 2 4 2 3 3 3 4 5% 55%	2400
	21st to end	Night	15% 4 1 4 3 2 3 2 3 5% 58%	2640
November	1st to 10th	Night	16% 4 2 4 3 2 4 2 4 5% 54%	2514
	11th to 20th	Night	14% 5% 1 4 3 3 3 2 3 6% 55%	2730
	21st to end	Night	14% 4 1 4 3 3 4 2 6% 7% 51%	2730
December	1st to 10th	Night	13% 5% 2 5% 3 3 3 3 4 5% 55%	4380
	11th to 20th	Night	19% 5% 2 5% 3 3 4 3 4 5% 46%	4952
	21st to end	Night	22% 4 1 4 3 3 3 3 4 5% 49%	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	28% 20% 6% 16% 24% 6%	5259
	11th to 20th	Day	18% 20% 3 9% 34% 16%	5280
	21st to end	Day	36% 22% 3 14% 18% 8%	5780
February	1st to 10th	Day	15% 59% 3 12% 8% 3	1968
	11th to 20th	Day	19% 58% 3 8% 9% 2	2058
	21st to end	Day	18% 60% 2 5% 12% 3	1730
March	1st to 10th	Day	35% 28% 4 11% 12% 11%	4990
	11th to 20th	Day	20% 28% 3 10% 21% 18%	5090
	21st to end	Day	28% 28% 3 19% 17% 5%	5586
April	1st to 10th	Day	16% 60% 5% 8% 9% 2	2310
	11th to 20th	Day	16% 60% 3 10% 8% 3	2486
	21st to end	Day	15% 59% 5% 12% 7% 1	2730
May	1st to 10th	Day	34% 24% 13% 19% 9% 1	7651
	11th to 20th	Day	34% 24% 11% 19% 10% 2	7630
	21st to end	Day	40% 25% 10% 12% 11% 2	8405
June	1st to 10th	Day	18% 61% 7% 10% 4 1	2730
	11th to 20th	Day	13% 62% 10% 11% 5%	2730
	21st to end	Day	15% 60% 9% 10% 5% 1	2730
July	1st to 10th	Day	27% 32% 15% 19% 7%	5965
	11th to 20th	Day	30% 31% 24% 13% 1	5961
	21st to end	Day	38% 31% 12% 13% 5% 1	6370
August	1st to 10th	Day	34% 38% 11% 14% 2	4221
	11th to 20th	Day	20% 40% 18% 12% 9% 1	4213
	21st to end	Day	14% 37% 8% 24% 16% 1	4323
September	1st to 10th	Day	15% 58% 7% 10% 8% 1	2200
	11th to 20th	Day	19% 58% 6% 9% 7% 2	2200
	21st to end	Day	20% 57% 7% 10% 6%	2200
October	1st to 10th	Day	15% 36% 5% 14% 22% 7%	3501
	11th to 20th	Day	8% 36% 3 11% 21% 21%	3240
	21st to end	Day	18% 35% 2 17% 14% 14%	3538
November	1st to 10th	Day	20% 53% 3 10% 11% 4	1999
	11th to 20th	Day	15% 54% 3 11% 14% 4	1890
	21st to end	Day	15% 54% 5% 11% 13% 3	1890
December	1st to 10th	Day	21% 35% 4 14% 20% 7%	3020
	11th to 20th	Day	26% 33% 6% 11% 18% 6%	3460
	21st to end	Day	30% 26% 5% 24% 14% 2	4565
January	1st to 10th	Night	24% 20% 7% 19% 24% 6%	7636
	11th to 20th	Night	21% 21% 2 12% 33% 11%	7680
	21st to end	Night	31% 22% 4 15% 18% 9%	8401
February	1st to 10th	Night	15% 59% 4 11% 9% 2	2649
	11th to 20th	Night	20% 58% 4 7% 9% 2	2459
	21st to end	Night	18% 59% 3 7% 10% 3	2076
March	1st to 10th	Night	33% 29% 4 10% 19% 5%	5081
	11th to 20th	Night	17% 28% 3 12% 24% 16%	5090
	21st to end	Night	29% 28% 4 18% 15% 7%	5588
April	1st to 10th	Night	16% 59% 5% 8% 10% 2	2310
	11th to 20th	Night	15% 60% 3 11% 8% 3	2098
	21st to end	Night	15% 60% 6% 11% 8% 1	1890
May	1st to 10th	Night	33% 25% 11% 21% 8% 2	5264
	11th to 20th	Night	34% 24% 11% 20% 9% 1	5235
	21st to end	Night	41% 24% 9% 16% 8% 2	5771
June	1st to 10th	Night	18% 61% 7% 8% 5% 2	1890
	11th to 20th	Night	15% 62% 9% 9% 5%	1890
	21st to end	Night	15% 60% 10% 9% 7%	1890
July	1st to 10th	Night	33% 32% 15% 16% 4	4105
	11th to 20th	Night	34% 32% 23% 10% 1	4096
	21st to end	Night	42% 31% 12% 11% 3 1	4387
August	1st to 10th	Night	36% 39% 14% 10% 2	2904
	11th to 20th	Night	29% 41% 14% 10% 6%	2902
	21st to end	Night	19% 39% 7% 24% 11% 1	3535
September	1st to 10th	Night	17% 59% 7% 8% 8% 1	2200
	11th to 20th	Night	21% 58% 7% 7% 6% 1	2200
	21st to end	Night	21% 58% 7% 9% 5%	2200
October	1st to 10th	Night	19% 39% 6% 12% 18% 5%	3679
	11th to 20th	Night	10% 35% 3 12% 22% 18%	3899
	21st to end	Night	19% 36% 2 12% 18% 12%	4252
November	1st to 10th	Night	20% 53% 4 10% 10% 4	2514
	11th to 20th	Night	15% 54% 4 8% 13% 6%	2730
	21st to end	Night	15% 54% 4 11% 14% 2	2730
December	1st to 10th	Night	21% 35% 3 15% 19% 6%	4380
	11th to 20th	Night	30% 32% 5% 11% 15% 6%	4954
	21st to end	Night	30% 26% 7% 19% 16% 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	22% 22% 36% 16% 4	330
	11th to 20th	Day	27% 21% 32% 15% 5%	330
	21st to end	Day	23% 20% 39% 15% 3	363
February	1st to 10th	Day	21% 20% 36% 18% 6%	330
	11th to 20th	Day	37% 21% 29% 13%	330
	21st to end	Day	32% 25% 29% 11% 4	273
March	1st to 10th	Day	23% 14% 32% 22% 8% 1	509
	11th to 20th	Day	26% 16% 30% 22% 5% 1	540
	21st to end	Day	21% 19% 31% 22% 6%	594
April	1st to 10th	Day	20% 17% 28% 23% 10% 1	540
	11th to 20th	Day	16% 17% 28% 23% 14% 3	532
	21st to end	Day	19% 9% 28% 26% 12% 6%	540
May	1st to 10th	Day	15% 14% 30% 25% 11% 5%	543
	11th to 20th	Day	19% 7% 23% 27% 17% 8%	567
	21st to end	Day	18% 7% 18% 27% 19% 11%	617
June	1st to 10th	Day	11% 5% 20% 29% 21% 13%	570
	11th to 20th	Day	9% 5% 20% 26% 22% 18%	570
	21st to end	Day	10% 4 14% 31% 24% 17%	570
July	1st to 10th	Day	8% 3 8% 27% 29% 25%	565
	11th to 20th	Day	9% 2 9% 25% 30% 25%	564
	21st to end	Day	8% 3 10% 27% 28% 23%	616
August	1st to 10th	Day	21% 3 13% 24% 21% 18%	526
	11th to 20th	Day	13% 2 12% 26% 29% 19%	519
	21st to end	Day	14% 3 12% 20% 30% 22%	583
September	1st to 10th	Day	11% 3 14% 25% 29% 18%	530
	11th to 20th	Day	19% 4 11% 20% 30% 17%	530
	21st to end	Day	21% 5% 19% 25% 24% 7%	530
October	1st to 10th	Day	19% 4 15% 27% 24% 10%	530
	11th to 20th	Day	14% 11% 21% 30% 21% 4	530
	21st to end	Day	21% 7% 22% 31% 15% 4	583
November	1st to 10th	Day	20% 11% 25% 26% 14% 3	477
	11th to 20th	Day	17% 16% 35% 23% 7% 2	330
	21st to end	Day	15% 17% 33% 23% 11% 2	330
December	1st to 10th	Day	26% 16% 33% 18% 6%	330
	11th to 20th	Day	22% 15% 33% 23% 7%	330
	21st to end	Day	26% 15% 31% 19% 7% 1	363
January	1st to 10th	Night	20% 14% 33% 27% 6%	840
	11th to 20th	Night	19% 13% 33% 31% 5%	840
	21st to end	Night	19% 11% 36% 31% 3	924
February	1st to 10th	Night	17% 12% 29% 38% 3	840
	11th to 20th	Night	24% 13% 37% 24% 3	823
	21st to end	Night	18% 14% 37% 26% 4	692
March	1st to 10th	Night	16% 12% 31% 35% 6%	840
	11th to 20th	Night	20% 13% 29% 30% 7% 1	840
	21st to end	Night	18% 11% 33% 30% 8%	924
April	1st to 10th	Night	16% 13% 29% 30% 11%	840
	11th to 20th	Night	16% 11% 26% 33% 13% 1	768
	21st to end	Night	14% 6% 24% 37% 19% 1	630
May	1st to 10th	Night	14% 6% 24% 35% 20% 1	630
	11th to 20th	Night	18% 5% 17% 33% 25% 3	621
	21st to end	Night	17% 2 11% 31% 33% 6%	683
June	1st to 10th	Night	11% 1 12% 34% 37% 5%	630
	11th to 20th	Night	13% 3 10% 29% 33% 12%	630
	21st to end	Night	8% 2 10% 29% 42% 10%	630
July	1st to 10th	Night	11% 2 5% 21% 50% 11%	615
	11th to 20th	Night	9% 1 5% 20% 49% 15%	612
	21st to end	Night	10% 2 6% 28% 42% 12%	687
August	1st to 10th	Night	19% 2 8% 22% 39% 11%	597
	11th to 20th	Night	16% 4 4 26% 40% 11%	594
	21st to end	Night	14% 2 7% 25% 43% 11%	737
September	1st to 10th	Night	13% 1 8% 31% 38% 9%	800
	11th to 20th	Night	18% 3 12% 24% 36% 8%	800
	21st to end	Night	20% 3 13% 33% 28% 3	800
October	1st to 10th	Night	17% 4 16% 26% 29% 8%	800
	11th to 20th	Night	17% 6% 21% 32% 22% 2	800
	21st to end	Night	16% 7% 19% 37% 18% 2	880
November	1st to 10th	Night	18% 5% 21% 36% 17% 3	820
	11th to 20th	Night	16% 8% 26% 37% 11% 1	840
	21st to end	Night	16% 10% 27% 33% 14% 1	840
December	1st to 10th	Night	23% 10% 25% 32% 9%	840
	11th to 20th	Night	19% 7% 27% 38% 8%	840
	21st to end	Night	25% 8% 26% 34% 7%	924

Page generated at 2026-07-28 03:25:19. Version ac07

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