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 2024-02-15.

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% 5% 2 5% 5% 3 5% 5% 6% 7% 44%	78708
	Night	16% 5% 2 4 4 3 4 4 5% 7% 45%	80380
Transparency	Day	26% 34% 7% 15% 14% 4	78431
	Night	27% 33% 7% 14% 15% 4	80176
Seeing	Day	18% 9% 22% 24% 18% 10%	14070
	Night	17% 7% 21% 30% 21% 4	24235

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	17% 3 1 3 3 2 3 3 3 7% 55%	2836
	11th to 20th	Day	13% 7% 2 4 3 2 4 3 4 3 54%	2840
	21st to end	Day	25% 4 1 3 3 2 2 3 4 4 48%	3124
February	1st to 10th	Day	16% 5% 2 4 5% 2 4 4 5% 7% 46%	1780
	11th to 20th	Day	19% 5% 2 4 4 2 3 4 4 5% 46%	1788
	21st to end	Day	19% 6% 2 5% 5% 3 4 4 5% 5% 41%	1480
March	1st to 10th	Day	14% 4 1 5% 4 2 4 3 4 7% 52%	1952
	11th to 20th	Day	19% 5% 3 5% 4 4 5% 4 4 7% 41%	1980
	21st to end	Day	18% 6% 2 4 4 2 4 5% 6% 7% 42%	2178
April	1st to 10th	Day	12% 4 2 4 5% 3 4 6% 6% 7% 48%	1980
	11th to 20th	Day	13% 5% 2 5% 5% 2 5% 4 5% 7% 46%	2126
	21st to end	Day	14% 4 1 5% 4 3 5% 5% 5% 7% 48%	2340
May	1st to 10th	Day	9% 5% 2 5% 6% 4 6% 5% 5% 8% 44%	2340
	11th to 20th	Day	15% 6% 2 5% 6% 4 5% 6% 7% 8% 37%	2301
	21st to end	Day	13% 5% 2 5% 5% 4 5% 6% 7% 11% 37%	2531
June	1st to 10th	Day	10% 6% 3 6% 6% 4 7% 7% 9% 10% 33%	2340
	11th to 20th	Day	8% 4 2 5% 6% 4 7% 7% 8% 11% 38%	2340
	21st to end	Day	9% 4 2 4 6% 4 7% 6% 8% 10% 38%	2340
July	1st to 10th	Day	10% 5% 3 6% 8% 5% 7% 6% 9% 10% 31%	2340
	11th to 20th	Day	10% 5% 3 7% 6% 4 8% 7% 10% 10% 30%	2340
	21st to end	Day	8% 4 2 5% 7% 5% 7% 8% 9% 11% 33%	2547
August	1st to 10th	Day	13% 6% 2 7% 6% 5% 7% 7% 8% 11% 28%	2327
	11th to 20th	Day	14% 6% 2 5% 6% 4 7% 7% 8% 9% 31%	2303
	21st to end	Day	13% 5% 2 6% 6% 4 5% 6% 7% 9% 36%	2387
September	1st to 10th	Day	10% 5% 3 5% 6% 3 7% 5% 6% 9% 41%	1980
	11th to 20th	Day	16% 6% 2 5% 5% 3 5% 4 5% 8% 41%	1980
	21st to end	Day	17% 5% 2 5% 5% 3 4 5% 4 6% 43%	1980
October	1st to 10th	Day	17% 4 1 3 3 2 2 3 4 6% 53%	1936
	11th to 20th	Day	14% 4 2 3 2 2 2 3 4 6% 59%	1800
	21st to end	Day	14% 4 2 3 4 2 3 3 4 5% 58%	1980
November	1st to 10th	Day	12% 5% 1 3 3 2 3 3 3 5% 59%	1805
	11th to 20th	Day	14% 4 2 3 3 2 3 4 4 5% 55%	1710
	21st to end	Day	16% 3 1 3 3 3 4 4 5% 6% 53%	1710
December	1st to 10th	Day	19% 6% 2 5% 5% 3 4 4 5% 5% 42%	1710
	11th to 20th	Day	17% 3 1 3 3 2 3 3 5% 6% 53%	2153
	21st to end	Day	21% 4 2 4 4 2 3 4 5% 6% 46%	3124
January	1st to 10th	Night	18% 4 1 3 3 2 3 4 4 5% 53%	4109
	11th to 20th	Night	12% 4 2 4 4 3 5% 4 5% 6% 52%	4120
	21st to end	Night	25% 5% 2 4 3 2 3 4 3 5% 44%	4532
February	1st to 10th	Night	15% 5% 2 4 4 3 4 3 5% 5% 51%	2397
	11th to 20th	Night	20% 6% 2 4 3 2 4 4 4 4 46%	2153
	21st to end	Night	17% 5% 3 4 5% 3 4 5% 5% 7% 43%	1776
March	1st to 10th	Night	13% 6% 2 4 3 2 4 4 4 5% 50%	2006
	11th to 20th	Night	18% 6% 3 5% 4 3 3 4 5% 8% 43%	1980
	21st to end	Night	16% 5% 2 5% 5% 3 5% 4 6% 6% 43%	2178
April	1st to 10th	Night	13% 5% 1 4 4 2 4 4 5% 7% 50%	1980
	11th to 20th	Night	14% 5% 2 4 4 2 3 3 6% 7% 50%	1796
	21st to end	Night	13% 4 2 4 4 2 4 4 5% 6% 52%	1620
May	1st to 10th	Night	12% 4 2 5% 5% 4 5% 5% 6% 7% 46%	1620
	11th to 20th	Night	15% 5% 2 6% 6% 4 6% 6% 6% 8% 37%	1592
	21st to end	Night	13% 4 3 5% 4 4 6% 5% 6% 10% 40%	1744
June	1st to 10th	Night	10% 5% 3 6% 7% 3 6% 7% 8% 13% 32%	1620
	11th to 20th	Night	11% 5% 2 5% 5% 3 5% 6% 7% 10% 40%	1620
	21st to end	Night	10% 5% 2 5% 6% 4 6% 6% 6% 10% 40%	1620
July	1st to 10th	Night	10% 6% 2 6% 7% 5% 8% 8% 7% 10% 29%	1620
	11th to 20th	Night	8% 4 3 5% 6% 5% 7% 7% 8% 12% 34%	1620
	21st to end	Night	7% 4 3 5% 6% 4 7% 6% 11% 13% 31%	1771
August	1st to 10th	Night	14% 5% 2 5% 5% 3 6% 8% 8% 10% 33%	1611
	11th to 20th	Night	14% 5% 2 7% 6% 3 6% 6% 7% 10% 34%	1598
	21st to end	Night	13% 6% 3 7% 6% 4 6% 6% 7% 9% 34%	1945
September	1st to 10th	Night	12% 5% 2 5% 5% 4 6% 5% 6% 9% 41%	1980
	11th to 20th	Night	16% 6% 2 7% 6% 4 4 5% 5% 7% 37%	1980
	21st to end	Night	18% 5% 2 4 5% 2 3 4 5% 7% 45%	1980
October	1st to 10th	Night	19% 4 1 4 4 3 3 3 4 5% 51%	2024
	11th to 20th	Night	16% 4 1 3 3 2 3 4 4 6% 53%	2160
	21st to end	Night	14% 4 1 3 3 2 3 3 3 5% 59%	2376
November	1st to 10th	Night	16% 4 1 3 4 1 4 3 4 4 55%	2268
	11th to 20th	Night	15% 5% 2 3 3 3 3 3 3 6% 54%	2470
	21st to end	Night	15% 4 1 3 4 2 4 4 5% 7% 50%	2470
December	1st to 10th	Night	19% 6% 2 5% 4 2 4 4 4 6% 44%	2470
	11th to 20th	Night	16% 5% 2 3 3 2 3 4 3 6% 54%	3042
	21st to end	Night	22% 5% 2 4 4 2 4 4 5% 6% 43%	4532

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	24% 23% 4 22% 21% 6%	2837
	11th to 20th	Day	27% 23% 4 12% 28% 5%	2840
	21st to end	Day	36% 20% 2 14% 22% 5%	3124
February	1st to 10th	Day	27% 33% 6% 14% 16% 4	1780
	11th to 20th	Day	26% 35% 5% 14% 14% 5%	1798
	21st to end	Day	29% 35% 5% 13% 12% 6%	1480
March	1st to 10th	Day	21% 34% 4 17% 17% 6%	1952
	11th to 20th	Day	31% 35% 8% 10% 11% 4	1980
	21st to end	Day	29% 37% 8% 15% 11% 1	2178
April	1st to 10th	Day	21% 35% 6% 16% 16% 5%	1980
	11th to 20th	Day	23% 33% 5% 13% 17% 8%	2126
	21st to end	Day	25% 36% 6% 17% 13% 3	2340
May	1st to 10th	Day	28% 36% 9% 13% 11% 3	2340
	11th to 20th	Day	30% 36% 11% 11% 9% 2	2305
	21st to end	Day	26% 37% 9% 18% 10% 1	2531
June	1st to 10th	Day	28% 37% 11% 17% 6% 1	2340
	11th to 20th	Day	26% 37% 13% 14% 8% 1	2340
	21st to end	Day	26% 38% 10% 13% 11% 2	2340
July	1st to 10th	Day	31% 38% 11% 9% 8% 3	2275
	11th to 20th	Day	29% 39% 13% 9% 8% 2	2271
	21st to end	Day	26% 39% 16% 12% 6%	2405
August	1st to 10th	Day	29% 38% 12% 17% 5%	2314
	11th to 20th	Day	33% 36% 10% 14% 7%	2303
	21st to end	Day	30% 37% 11% 15% 5% 2	2387
September	1st to 10th	Day	26% 36% 9% 14% 11% 3	1980
	11th to 20th	Day	29% 36% 7% 12% 15% 2	1980
	21st to end	Day	27% 36% 4 14% 13% 6%	1980
October	1st to 10th	Day	20% 35% 4 15% 21% 5%	1936
	11th to 20th	Day	21% 35% 5% 17% 17% 6%	1800
	21st to end	Day	20% 36% 4 14% 20% 7%	1980
November	1st to 10th	Day	14% 39% 6% 12% 21% 8%	1805
	11th to 20th	Day	20% 38% 4 17% 16% 5%	1710
	21st to end	Day	22% 38% 4 14% 17% 5%	1710
December	1st to 10th	Day	26% 39% 4 14% 13% 3	1710
	11th to 20th	Day	23% 30% 6% 15% 18% 7%	2150
	21st to end	Day	28% 23% 4 21% 20% 3	3124
January	1st to 10th	Night	26% 23% 4 17% 25% 5%	4109
	11th to 20th	Night	26% 23% 4 16% 25% 6%	4120
	21st to end	Night	39% 20% 4 12% 19% 5%	4532
February	1st to 10th	Night	26% 32% 6% 14% 17% 5%	2397
	11th to 20th	Night	27% 35% 4 13% 17% 5%	2154
	21st to end	Night	28% 35% 6% 11% 16% 5%	1776
March	1st to 10th	Night	22% 35% 4 13% 18% 7%	2006
	11th to 20th	Night	28% 35% 7% 13% 13% 3	1980
	21st to end	Night	28% 36% 6% 16% 12% 2	2178
April	1st to 10th	Night	22% 34% 7% 15% 18% 4	1980
	11th to 20th	Night	21% 34% 5% 16% 19% 6%	1796
	21st to end	Night	23% 35% 7% 17% 15% 3	1620
May	1st to 10th	Night	25% 36% 11% 13% 13% 1	1620
	11th to 20th	Night	33% 37% 8% 12% 9% 1	1593
	21st to end	Night	28% 37% 10% 17% 7% 1	1745
June	1st to 10th	Night	27% 37% 13% 15% 7%	1620
	11th to 20th	Night	28% 36% 11% 16% 8% 1	1620
	21st to end	Night	26% 39% 10% 11% 10% 4	1620
July	1st to 10th	Night	33% 39% 10% 9% 7% 2	1575
	11th to 20th	Night	28% 40% 14% 10% 6% 2	1566
	21st to end	Night	27% 40% 16% 11% 5%	1672
August	1st to 10th	Night	28% 38% 14% 14% 6%	1600
	11th to 20th	Night	32% 36% 12% 10% 9% 1	1598
	21st to end	Night	34% 37% 12% 12% 4 1	1945
September	1st to 10th	Night	29% 37% 8% 13% 10% 3	1980
	11th to 20th	Night	31% 36% 7% 11% 14% 1	1980
	21st to end	Night	27% 35% 4 14% 15% 5%	1980
October	1st to 10th	Night	24% 35% 5% 13% 20% 3	2024
	11th to 20th	Night	24% 35% 4 17% 16% 5%	2160
	21st to end	Night	22% 35% 3 15% 19% 6%	2376
November	1st to 10th	Night	17% 39% 4 12% 19% 9%	2268
	11th to 20th	Night	23% 38% 3 12% 18% 5%	2470
	21st to end	Night	23% 38% 3 14% 17% 5%	2470
December	1st to 10th	Night	24% 38% 6% 12% 15% 4	2470
	11th to 20th	Night	24% 32% 7% 12% 21% 5%	3044
	21st to end	Night	32% 23% 5% 19% 17% 3	4532

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% 35% 17% 4	270
	11th to 20th	Day	26% 20% 32% 16% 5%	270
	21st to end	Day	23% 19% 41% 13% 4	297
February	1st to 10th	Day	23% 20% 33% 17% 7%	270
	11th to 20th	Day	37% 21% 32% 10%	252
	21st to end	Day	31% 26% 29% 10% 4	198
March	1st to 10th	Day	21% 15% 35% 21% 8% 1	392
	11th to 20th	Day	26% 14% 32% 22% 5% 1	420
	21st to end	Day	23% 18% 28% 23% 6%	462
April	1st to 10th	Day	19% 17% 28% 23% 11% 2	420
	11th to 20th	Day	16% 18% 27% 21% 16% 3	412
	21st to end	Day	21% 9% 26% 25% 13% 7%	420
May	1st to 10th	Day	14% 13% 31% 24% 13% 5%	423
	11th to 20th	Day	17% 5% 23% 27% 19% 9%	447
	21st to end	Day	16% 5% 15% 27% 22% 14%	485
June	1st to 10th	Day	11% 4 18% 30% 22% 14%	450
	11th to 20th	Day	11% 4 18% 24% 23% 19%	450
	21st to end	Day	12% 4 14% 31% 24% 15%	450
July	1st to 10th	Day	10% 2 8% 24% 31% 25%	445
	11th to 20th	Day	8% 2 9% 27% 30% 24%	444
	21st to end	Day	7% 2 8% 27% 30% 26%	488
August	1st to 10th	Day	18% 2 13% 23% 23% 21%	446
	11th to 20th	Day	13% 2 11% 25% 29% 19%	439
	21st to end	Day	15% 2 12% 19% 31% 21%	495
September	1st to 10th	Day	12% 2 12% 24% 29% 21%	450
	11th to 20th	Day	18% 3 10% 20% 32% 16%	450
	21st to end	Day	20% 4 18% 26% 26% 7%	450
October	1st to 10th	Day	22% 5% 15% 26% 22% 10%	450
	11th to 20th	Day	16% 10% 22% 30% 19% 4	450
	21st to end	Day	20% 6% 22% 33% 15% 4	495
November	1st to 10th	Day	20% 11% 24% 26% 16% 4	403
	11th to 20th	Day	19% 18% 31% 24% 7% 2	270
	21st to end	Day	13% 15% 34% 25% 11% 2	270
December	1st to 10th	Day	27% 16% 31% 19% 7%	270
	11th to 20th	Day	21% 16% 32% 23% 7%	270
	21st to end	Day	25% 15% 34% 18% 6% 1	297
January	1st to 10th	Night	21% 13% 32% 26% 7%	760
	11th to 20th	Night	19% 13% 31% 31% 5%	760
	21st to end	Night	19% 12% 35% 31% 3	836
February	1st to 10th	Night	17% 12% 29% 38% 4	760
	11th to 20th	Night	24% 12% 37% 24% 3	722
	21st to end	Night	19% 13% 37% 27% 4	592
March	1st to 10th	Night	16% 12% 31% 35% 5%	720
	11th to 20th	Night	21% 12% 29% 29% 8% 1	720
	21st to end	Night	19% 10% 32% 31% 8% 1	792
April	1st to 10th	Night	17% 13% 29% 30% 12%	720
	11th to 20th	Night	18% 11% 26% 31% 15% 1	658
	21st to end	Night	14% 6% 23% 36% 19% 1	540
May	1st to 10th	Night	14% 7% 24% 34% 21% 1	540
	11th to 20th	Night	18% 4 17% 32% 26% 3	531
	21st to end	Night	16% 1 8% 32% 36% 7%	584
June	1st to 10th	Night	12% 1 10% 32% 38% 6%	540
	11th to 20th	Night	14% 2 10% 27% 33% 14%	540
	21st to end	Night	9% 2 10% 29% 39% 10%	540
July	1st to 10th	Night	12% 2 5% 21% 50% 10%	525
	11th to 20th	Night	9% 1 6% 21% 49% 14%	522
	21st to end	Night	9% 1 5% 29% 43% 13%	591
August	1st to 10th	Night	17% 1 8% 22% 41% 12%	537
	11th to 20th	Night	15% 3 4 25% 42% 11%	534
	21st to end	Night	14% 1 6% 25% 42% 11%	663
September	1st to 10th	Night	13% 1 8% 28% 40% 10%	720
	11th to 20th	Night	17% 3 12% 24% 36% 8%	720
	21st to end	Night	19% 3 13% 33% 28% 3	720
October	1st to 10th	Night	19% 4 16% 24% 29% 8%	720
	11th to 20th	Night	19% 6% 22% 30% 22% 2	720
	21st to end	Night	16% 7% 19% 38% 18% 2	792
November	1st to 10th	Night	18% 5% 21% 34% 18% 3	740
	11th to 20th	Night	17% 8% 26% 36% 12% 1	760
	21st to end	Night	17% 9% 26% 32% 15% 1	760
December	1st to 10th	Night	23% 10% 25% 32% 10%	760
	11th to 20th	Night	19% 7% 27% 38% 9% 1	760
	21st to end	Night	26% 8% 27% 33% 7%	836

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