Forillon National Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Forillon National Park clear sky chart from 2005-11-06 to 2026-03-18.

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	16% 10% 3 8% 5% 5% 5% 4 6% 7% 31%	131246
	Night	16% 11% 4 9% 5% 5% 4 4 5% 6% 30%	130493
Transparency	Day	52% 6% 14% 20% 8% 1	134698
	Night	57% 6% 12% 18% 7% 1	135133
Seeing	Day	22% 11% 27% 25% 11% 2	18407
	Night	24% 11% 25% 26% 12% 2	26024

By Month

Percentage Cloud Cover NE (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	12% 21% 6% 11% 8% 6% 4 4 7% 7% 16%	4093
	11th to 20th	Day	12% 15% 6% 16% 5% 4 4 5% 6% 7% 19%	4084
	21st to end	Day	4 8% 5% 11% 6% 7% 7% 5% 7% 8% 31%	5234
February	1st to 10th	Day	9% 11% 4 10% 7% 5% 5% 4 7% 7% 31%	1890
	11th to 20th	Day	10% 17% 4 11% 7% 5% 4 3 6% 7% 27%	1843
	21st to end	Day	10% 10% 4 10% 6% 6% 4 4 5% 6% 34%	1557
March	1st to 10th	Day	17% 14% 4 11% 4 5% 5% 4 5% 5% 26%	4881
	11th to 20th	Day	18% 17% 5% 11% 5% 5% 5% 4 4 5% 21%	5057
	21st to end	Day	21% 16% 5% 9% 4 4 4 3 4 5% 25%	5498
April	1st to 10th	Day	19% 12% 3 9% 6% 5% 4 4 5% 7% 28%	2400
	11th to 20th	Day	17% 12% 3 7% 5% 3 5% 4 5% 5% 34%	2550
	21st to end	Day	25% 9% 3 7% 4 5% 4 3 4 6% 30%	2600
May	1st to 10th	Day	23% 11% 4 8% 5% 4 4 3 5% 5% 28%	6317
	11th to 20th	Day	18% 9% 4 8% 5% 4 4 3 5% 7% 34%	6318
	21st to end	Day	22% 9% 2 7% 4 4 4 4 5% 6% 33%	7263
June	1st to 10th	Day	22% 9% 2 7% 5% 4 5% 3 5% 6% 31%	3000
	11th to 20th	Day	18% 7% 2 6% 5% 4 4 4 6% 7% 37%	3000
	21st to end	Day	23% 9% 3 6% 6% 4 6% 4 6% 6% 28%	3000
July	1st to 10th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 9% 36%	6820
	11th to 20th	Day	10% 6% 3 8% 5% 6% 6% 5% 7% 8% 35%	6820
	21st to end	Day	12% 6% 2 6% 4 4 4 4 5% 8% 44%	7155
August	1st to 10th	Day	15% 6% 3 7% 5% 5% 6% 4 7% 8% 34%	4377
	11th to 20th	Day	15% 5% 2 7% 5% 6% 5% 5% 7% 9% 35%	4175
	21st to end	Day	13% 4 2 5% 4 4 4 4 5% 8% 48%	4614
September	1st to 10th	Day	18% 6% 2 5% 4 4 4 4 5% 7% 40%	2400
	11th to 20th	Day	19% 6% 2 6% 3 3 4 3 5% 6% 44%	2329
	21st to end	Day	18% 6% 2 5% 4 5% 4 3 6% 7% 40%	2135
October	1st to 10th	Day	17% 5% 3 7% 4 4 4 4 7% 7% 38%	2000
	11th to 20th	Day	22% 9% 3 8% 4 4 4 4 4 6% 32%	2000
	21st to end	Day	23% 10% 2 7% 5% 5% 4 4 5% 6% 28%	2036
November	1st to 10th	Day	17% 12% 3 9% 5% 5% 5% 4 5% 6% 28%	1645
	11th to 20th	Day	20% 12% 5% 8% 6% 5% 6% 4 3 6% 24%	1680
	21st to end	Day	20% 16% 5% 10% 6% 4 5% 4 5% 5% 23%	1680
December	1st to 10th	Day	12% 14% 6% 12% 5% 5% 5% 4 5% 5% 27%	2554
	11th to 20th	Day	17% 12% 4 12% 7% 5% 4 3 5% 6% 26%	2688
	21st to end	Day	14% 13% 4 14% 8% 5% 6% 3 5% 6% 22%	3553
January	1st to 10th	Night	14% 18% 5% 13% 7% 6% 5% 3 5% 6% 17%	8831
	11th to 20th	Night	13% 14% 7% 14% 5% 5% 4 4 5% 7% 22%	8831
	21st to end	Night	4 9% 5% 10% 6% 7% 5% 5% 8% 9% 32%	8944
February	1st to 10th	Night	10% 12% 4 9% 7% 5% 5% 5% 6% 6% 32%	2730
	11th to 20th	Night	13% 15% 5% 10% 5% 5% 4 4 5% 6% 28%	2663
	21st to end	Night	11% 14% 4 8% 6% 5% 4 5% 5% 6% 33%	2249
March	1st to 10th	Night	16% 14% 4 9% 6% 5% 5% 3 4 6% 28%	5174
	11th to 20th	Night	21% 17% 5% 10% 6% 5% 5% 3 4 5% 20%	5057
	21st to end	Night	23% 14% 4 10% 6% 4 3 3 4 6% 23%	5433
April	1st to 10th	Night	23% 10% 3 9% 5% 4 4 4 6% 5% 27%	2000
	11th to 20th	Night	20% 11% 3 6% 4 4 4 4 5% 6% 34%	1798
	21st to end	Night	24% 10% 3 6% 4 3 4 3 4 5% 34%	1800
May	1st to 10th	Night	24% 11% 3 7% 5% 4 4 3 3 5% 32%	3642
	11th to 20th	Night	19% 8% 3 6% 4 4 4 3 5% 6% 37%	3596
	21st to end	Night	18% 8% 3 8% 5% 5% 4 4 6% 7% 32%	3676
June	1st to 10th	Night	25% 8% 3 7% 5% 4 4 4 5% 5% 31%	1400
	11th to 20th	Night	20% 6% 2 7% 5% 4 3 4 4 6% 39%	1400
	21st to end	Night	21% 7% 3 7% 5% 5% 5% 5% 5% 7% 30%	1400
July	1st to 10th	Night	14% 8% 2 7% 5% 4 5% 4 7% 8% 36%	3160
	11th to 20th	Night	11% 6% 2 6% 5% 4 6% 5% 8% 9% 38%	3160
	21st to end	Night	9% 6% 2 7% 5% 4 4 3 6% 7% 45%	3673
August	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 6% 38%	2774
	11th to 20th	Night	14% 6% 2 6% 4 4 5% 4 6% 8% 41%	2872
	21st to end	Night	14% 5% 2 5% 3 3 4 3 6% 7% 47%	3284
September	1st to 10th	Night	19% 5% 2 5% 4 4 4 3 5% 6% 44%	2000
	11th to 20th	Night	18% 5% 2 5% 3 4 3 3 4 7% 46%	2070
	21st to end	Night	19% 6% 2 5% 4 3 4 3 5% 5% 45%	2264
October	1st to 10th	Night	19% 7% 3 5% 5% 4 4 3 5% 6% 40%	2400
	11th to 20th	Night	22% 9% 2 6% 5% 4 3 4 5% 6% 34%	2400
	21st to end	Night	22% 9% 2 7% 4 5% 4 3 5% 7% 33%	2800
November	1st to 10th	Night	20% 11% 3 7% 4 3 4 4 5% 6% 32%	2873
	11th to 20th	Night	22% 13% 4 9% 5% 4 4 4 5% 6% 24%	2940
	21st to end	Night	21% 15% 5% 10% 6% 4 4 3 5% 6% 21%	2940
December	1st to 10th	Night	14% 13% 5% 13% 5% 5% 4 4 5% 5% 28%	4851
	11th to 20th	Night	17% 13% 4 11% 6% 5% 4 3 6% 6% 25%	5741
	21st to end	Night	18% 14% 5% 13% 7% 5% 4 4 4 7% 19%	7667

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	75% 5% 10% 8% 2	4094
	11th to 20th	Day	64% 4 15% 11% 6%	4110
	21st to end	Day	53% 9% 15% 17% 6%	5234
February	1st to 10th	Day	53% 6% 14% 18% 8% 1	1890
	11th to 20th	Day	59% 4 12% 18% 5%	1851
	21st to end	Day	50% 7% 8% 25% 9% 1	1557
March	1st to 10th	Day	62% 4 11% 14% 8%	4885
	11th to 20th	Day	66% 5% 8% 13% 6% 2	5057
	21st to end	Day	63% 6% 8% 17% 5% 1	5498
April	1st to 10th	Day	57% 5% 7% 19% 12% 1	2400
	11th to 20th	Day	50% 4 9% 22% 13% 1	2550
	21st to end	Day	54% 5% 12% 17% 11% 2	2600
May	1st to 10th	Day	56% 4 10% 19% 9% 1	6329
	11th to 20th	Day	50% 4 14% 23% 8% 1	6322
	21st to end	Day	49% 6% 11% 24% 9% 1	7263
June	1st to 10th	Day	52% 5% 13% 19% 10%	3000
	11th to 20th	Day	44% 7% 13% 24% 10% 1	3000
	21st to end	Day	54% 6% 13% 19% 9%	3000
July	1st to 10th	Day	42% 7% 16% 23% 11%	6745
	11th to 20th	Day	43% 9% 20% 22% 6%	6739
	21st to end	Day	37% 6% 21% 25% 10%	6951
August	1st to 10th	Day	42% 9% 21% 23% 5%	4363
	11th to 20th	Day	42% 8% 21% 22% 6% 1	4214
	21st to end	Day	34% 6% 18% 31% 12% 1	4614
September	1st to 10th	Day	41% 5% 17% 27% 9%	2400
	11th to 20th	Day	41% 5% 16% 28% 9%	2329
	21st to end	Day	42% 6% 12% 28% 11% 1	2135
October	1st to 10th	Day	44% 6% 16% 27% 6% 1	3260
	11th to 20th	Day	48% 7% 14% 24% 7%	3240
	21st to end	Day	47% 7% 15% 21% 10%	3268
November	1st to 10th	Day	57% 6% 13% 16% 8%	1645
	11th to 20th	Day	63% 5% 13% 13% 5%	1680
	21st to end	Day	68% 4 12% 11% 5%	1680
December	1st to 10th	Day	65% 5% 8% 17% 4	2554
	11th to 20th	Day	66% 4 11% 12% 8%	2688
	21st to end	Day	70% 6% 10% 11% 2 1	3553
January	1st to 10th	Night	74% 6% 10% 9% 1	8831
	11th to 20th	Night	67% 5% 10% 14% 4	8880
	21st to end	Night	54% 10% 14% 13% 9% 1	8944
February	1st to 10th	Night	55% 6% 10% 19% 9% 2	2730
	11th to 20th	Night	58% 5% 11% 18% 7%	2666
	21st to end	Night	52% 5% 10% 23% 10% 1	2249
March	1st to 10th	Night	60% 4 8% 18% 9% 1	5183
	11th to 20th	Night	69% 5% 8% 12% 5% 1	5058
	21st to end	Night	64% 4 7% 16% 7% 1	5433
April	1st to 10th	Night	59% 4 9% 15% 12% 1	2000
	11th to 20th	Night	52% 4 8% 24% 11% 1	1798
	21st to end	Night	55% 3 10% 18% 14% 2	1800
May	1st to 10th	Night	57% 4 10% 18% 8% 3	3647
	11th to 20th	Night	48% 4 11% 26% 11%	3597
	21st to end	Night	50% 6% 11% 22% 11% 1	3677
June	1st to 10th	Night	55% 4 10% 22% 9%	1400
	11th to 20th	Night	46% 6% 14% 19% 14% 1	1400
	21st to end	Night	53% 6% 14% 19% 8% 1	1400
July	1st to 10th	Night	45% 8% 15% 22% 11%	3125
	11th to 20th	Night	40% 8% 19% 29% 4	3118
	21st to end	Night	39% 7% 20% 24% 10%	3564
August	1st to 10th	Night	48% 5% 16% 23% 8%	2763
	11th to 20th	Night	40% 7% 21% 24% 7% 1	2899
	21st to end	Night	36% 6% 16% 29% 12%	3283
September	1st to 10th	Night	43% 5% 14% 28% 9% 1	2000
	11th to 20th	Night	42% 3 15% 28% 11%	2070
	21st to end	Night	41% 5% 12% 28% 13% 1	2264
October	1st to 10th	Night	49% 4 13% 27% 7% 1	3920
	11th to 20th	Night	46% 6% 13% 25% 9%	3899
	21st to end	Night	51% 7% 14% 22% 7%	4522
November	1st to 10th	Night	54% 5% 15% 19% 6% 1	2873
	11th to 20th	Night	63% 4 13% 14% 5% 1	2940
	21st to end	Night	68% 4 12% 11% 3	2940
December	1st to 10th	Night	64% 8% 8% 16% 4	4851
	11th to 20th	Night	66% 6% 9% 13% 6% 1	5742
	21st to end	Night	70% 5% 12% 12% 1	7667

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	30% 15% 26% 24% 5% 1	220
	11th to 20th	Day	30% 18% 24% 17% 8% 3	220
	21st to end	Day	16% 17% 35% 22% 9% 1	379
February	1st to 10th	Day	20% 16% 34% 20% 9% 2	430
	11th to 20th	Day	18% 21% 34% 19% 5% 2	426
	21st to end	Day	18% 13% 31% 24% 11% 2	355
March	1st to 10th	Day	21% 15% 36% 20% 6% 2	515
	11th to 20th	Day	25% 14% 30% 22% 8% 1	528
	21st to end	Day	26% 17% 29% 20% 6% 2	550
April	1st to 10th	Day	27% 19% 27% 22% 6%	500
	11th to 20th	Day	23% 13% 33% 21% 8% 2	492
	21st to end	Day	27% 12% 26% 24% 9% 2	500
May	1st to 10th	Day	28% 10% 25% 25% 10% 3	688
	11th to 20th	Day	23% 11% 23% 27% 11% 5%	724
	21st to end	Day	25% 13% 27% 25% 10% 1	790
June	1st to 10th	Day	26% 8% 25% 28% 10% 2	730
	11th to 20th	Day	20% 8% 20% 33% 15% 4	730
	21st to end	Day	25% 6% 28% 25% 15% 1	730
July	1st to 10th	Day	19% 4 25% 35% 15% 2	720
	11th to 20th	Day	15% 5% 24% 36% 18% 2	718
	21st to end	Day	18% 6% 20% 33% 19% 4	795
August	1st to 10th	Day	15% 5% 21% 29% 26% 3	616
	11th to 20th	Day	15% 4 19% 35% 21% 5%	519
	21st to end	Day	16% 7% 24% 32% 17% 4	583
September	1st to 10th	Day	20% 6% 32% 25% 13% 4	530
	11th to 20th	Day	23% 10% 25% 25% 14% 3	530
	21st to end	Day	19% 12% 35% 21% 10% 3	494
October	1st to 10th	Day	17% 15% 29% 23% 13% 4	420
	11th to 20th	Day	27% 13% 29% 17% 13% 2	420
	21st to end	Day	25% 16% 29% 19% 9% 2	462
November	1st to 10th	Day	27% 15% 30% 20% 7% 1	425
	11th to 20th	Day	28% 17% 27% 18% 8% 2	430
	21st to end	Day	34% 17% 26% 20% 3	430
December	1st to 10th	Day	23% 12% 38% 23% 4 1	346
	11th to 20th	Day	27% 21% 31% 16% 4	220
	21st to end	Day	24% 24% 28% 18% 7% 1	242
January	1st to 10th	Night	26% 16% 28% 22% 6% 1	950
	11th to 20th	Night	23% 17% 29% 22% 8% 1	950
	21st to end	Night	17% 17% 33% 24% 9% 1	978
February	1st to 10th	Night	18% 13% 31% 28% 8% 1	840
	11th to 20th	Night	23% 14% 32% 23% 7% 1	823
	21st to end	Night	20% 13% 30% 23% 11% 3	692
March	1st to 10th	Night	22% 13% 32% 23% 9% 1	840
	11th to 20th	Night	23% 14% 30% 24% 9% 2	829
	21st to end	Night	26% 13% 26% 24% 10% 1	872
April	1st to 10th	Night	29% 12% 25% 25% 9%	600
	11th to 20th	Night	25% 8% 28% 27% 10% 2	594
	21st to end	Night	30% 9% 19% 31% 10% 3	600
May	1st to 10th	Night	26% 6% 21% 29% 15% 3	600
	11th to 20th	Night	23% 7% 22% 28% 18% 2	592
	21st to end	Night	21% 8% 25% 33% 12% 2	533
June	1st to 10th	Night	27% 5% 20% 30% 17% 2	400
	11th to 20th	Night	24% 5% 15% 35% 19% 3	400
	21st to end	Night	25% 3 16% 37% 17% 2	400
July	1st to 10th	Night	18% 4 13% 39% 24% 3	390
	11th to 20th	Night	17% 4 13% 41% 22% 2	388
	21st to end	Night	19% 3 12% 39% 24% 2	537
August	1st to 10th	Night	17% 4 12% 38% 25% 4	598
	11th to 20th	Night	20% 2 14% 35% 27% 4	592
	21st to end	Night	17% 5% 17% 32% 25% 4	660
September	1st to 10th	Night	22% 5% 23% 32% 16% 2	600
	11th to 20th	Night	21% 7% 21% 33% 15% 2	670
	21st to end	Night	23% 7% 26% 28% 14% 2	800
October	1st to 10th	Night	25% 8% 27% 26% 11% 3	800
	11th to 20th	Night	27% 11% 24% 24% 13% 1	800
	21st to end	Night	26% 12% 26% 25% 10%	921
November	1st to 10th	Night	28% 15% 28% 23% 7% 1	930
	11th to 20th	Night	31% 14% 28% 19% 6% 2	950
	21st to end	Night	32% 15% 27% 20% 5% 1	950
December	1st to 10th	Night	27% 15% 33% 19% 5%	950
	11th to 20th	Night	28% 22% 30% 16% 5%	950
	21st to end	Night	28% 17% 27% 20% 7% 1	1045

Page generated at 2026-03-15 02:23:54. Version ac07

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