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 2024-12-06.

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	17% 9% 3 8% 5% 5% 5% 4 6% 7% 32%	102093
	Night	17% 11% 3 8% 5% 5% 4 4 5% 6% 31%	97746
Transparency	Day	51% 5% 14% 21% 8% 1	105568
	Night	56% 4 13% 19% 7% 1	102365
Seeing	Day	23% 11% 28% 25% 11% 2	16785
	Night	25% 11% 25% 26% 11% 2	24250

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	13% 14% 4 12% 9% 5% 6% 5% 8% 7% 18%	2207
	11th to 20th	Day	10% 12% 5% 10% 5% 5% 5% 6% 8% 9% 24%	2210
	21st to end	Day	6% 8% 4 12% 6% 10% 6% 5% 6% 9% 29%	2818
February	1st to 10th	Day	10% 12% 4 9% 7% 4 5% 4 7% 7% 32%	1710
	11th to 20th	Day	10% 17% 4 11% 7% 5% 4 4 6% 7% 27%	1663
	21st to end	Day	10% 9% 4 10% 7% 7% 4 4 5% 6% 33%	1413
March	1st to 10th	Day	14% 13% 4 10% 4 5% 5% 4 5% 5% 31%	3317
	11th to 20th	Day	19% 19% 5% 11% 5% 6% 4 3 4 5% 18%	3480
	21st to end	Day	22% 15% 4 7% 4 4 4 3 4 5% 29%	3834
April	1st to 10th	Day	20% 12% 3 8% 5% 4 4 4 5% 7% 28%	2280
	11th to 20th	Day	17% 12% 3 7% 5% 3 5% 4 5% 6% 35%	2421
	21st to end	Day	24% 9% 3 7% 4 5% 4 3 5% 6% 30%	2470
May	1st to 10th	Day	25% 12% 4 8% 5% 4 4 3 5% 6% 26%	4411
	11th to 20th	Day	14% 8% 3 7% 4 4 4 4 6% 8% 38%	4398
	21st to end	Day	20% 10% 3 8% 5% 5% 5% 4 5% 6% 28%	5053
June	1st to 10th	Day	23% 9% 2 7% 5% 4 5% 3 5% 6% 31%	2850
	11th to 20th	Day	18% 7% 2 5% 5% 4 4 4 6% 7% 37%	2850
	21st to end	Day	24% 9% 3 6% 6% 4 6% 4 6% 6% 27%	2850
July	1st to 10th	Day	14% 6% 2 6% 5% 4 5% 4 7% 8% 39%	4760
	11th to 20th	Day	11% 6% 3 7% 6% 6% 6% 5% 8% 8% 35%	4760
	21st to end	Day	12% 6% 2 6% 4 5% 4 4 6% 8% 42%	4973
August	1st to 10th	Day	15% 6% 3 8% 5% 5% 6% 4 7% 8% 33%	4243
	11th to 20th	Day	15% 5% 2 7% 5% 6% 5% 5% 7% 9% 35%	4045
	21st to end	Day	13% 4 2 5% 4 3 4 3 5% 8% 49%	4473
September	1st to 10th	Day	19% 6% 2 6% 4 4 4 4 5% 7% 39%	2280
	11th to 20th	Day	20% 6% 2 5% 3 3 4 3 5% 6% 44%	2213
	21st to end	Day	19% 6% 2 5% 5% 5% 4 3 6% 6% 38%	2028
October	1st to 10th	Day	17% 5% 3 7% 5% 4 4 4 7% 7% 38%	1900
	11th to 20th	Day	23% 9% 2 7% 4 4 4 4 4 6% 32%	1900
	21st to end	Day	23% 11% 2 7% 5% 4 4 4 5% 6% 29%	1935
November	1st to 10th	Day	18% 12% 3 9% 5% 5% 5% 4 5% 6% 28%	1565
	11th to 20th	Day	21% 12% 5% 8% 6% 5% 5% 4 3 6% 24%	1600
	21st to end	Day	20% 16% 5% 10% 6% 4 4 4 5% 4 23%	1600
December	1st to 10th	Day	18% 13% 6% 12% 5% 5% 5% 4 6% 5% 21%	1484
	11th to 20th	Day	13% 14% 5% 14% 10% 5% 5% 4 5% 5% 18%	1668
	21st to end	Day	15% 13% 4 13% 6% 5% 6% 4 5% 6% 21%	2431
January	1st to 10th	Night	10% 12% 5% 16% 10% 6% 5% 4 7% 8% 16%	4748
	11th to 20th	Night	16% 13% 4 9% 6% 5% 5% 4 6% 7% 24%	4760
	21st to end	Night	7% 10% 4 9% 6% 8% 4 5% 7% 9% 30%	4840
February	1st to 10th	Night	10% 13% 4 9% 7% 5% 5% 4 6% 6% 32%	2470
	11th to 20th	Night	13% 15% 4 10% 6% 5% 5% 4 5% 6% 27%	2403
	21st to end	Night	11% 14% 3 7% 6% 5% 4 5% 5% 6% 33%	2041
March	1st to 10th	Night	12% 11% 4 8% 7% 6% 6% 3 4 6% 32%	3518
	11th to 20th	Night	22% 18% 6% 10% 5% 4 4 3 4 4 20%	3479
	21st to end	Night	26% 13% 3 6% 5% 3 3 3 4 6% 28%	3797
April	1st to 10th	Night	23% 10% 3 8% 5% 4 4 4 6% 5% 27%	1900
	11th to 20th	Night	20% 11% 2 6% 4 4 4 4 5% 6% 35%	1707
	21st to end	Night	24% 10% 3 5% 5% 3 4 3 4 5% 34%	1710
May	1st to 10th	Night	26% 10% 3 7% 5% 5% 5% 3 3 5% 28%	2539
	11th to 20th	Night	15% 8% 2 5% 3 4 4 4 6% 7% 41%	2506
	21st to end	Night	15% 10% 4 10% 5% 6% 4 4 6% 7% 29%	2561
June	1st to 10th	Night	25% 8% 3 6% 5% 4 4 4 5% 5% 31%	1330
	11th to 20th	Night	20% 6% 1 7% 5% 4 3 4 4 6% 38%	1330
	21st to end	Night	21% 7% 3 7% 5% 5% 5% 5% 5% 7% 30%	1330
July	1st to 10th	Night	14% 7% 2 6% 4 3 5% 3 6% 7% 42%	2210
	11th to 20th	Night	13% 7% 2 6% 4 4 5% 4 7% 8% 39%	2210
	21st to end	Night	11% 6% 2 6% 4 4 4 3 6% 8% 46%	2559
August	1st to 10th	Night	14% 8% 3 7% 5% 5% 5% 4 6% 6% 37%	2688
	11th to 20th	Night	14% 6% 2 6% 5% 4 5% 4 6% 8% 40%	2782
	21st to end	Night	15% 5% 2 5% 3 3 4 3 6% 7% 47%	3183
September	1st to 10th	Night	20% 5% 2 5% 4 4 4 3 5% 6% 43%	1900
	11th to 20th	Night	19% 6% 1 5% 3 4 3 3 4 7% 46%	1966
	21st to end	Night	20% 6% 2 5% 4 3 4 3 5% 5% 44%	2151
October	1st to 10th	Night	19% 7% 3 5% 5% 4 4 3 5% 6% 40%	2280
	11th to 20th	Night	23% 8% 2 6% 5% 4 3 4 5% 5% 34%	2280
	21st to end	Night	23% 9% 2 6% 4 4 4 3 5% 7% 33%	2660
November	1st to 10th	Night	20% 11% 3 7% 4 3 4 4 5% 6% 32%	2733
	11th to 20th	Night	21% 13% 4 9% 5% 4 4 4 6% 7% 24%	2800
	21st to end	Night	22% 16% 5% 10% 6% 4 4 3 5% 6% 21%	2800
December	1st to 10th	Night	20% 14% 4 10% 6% 5% 5% 5% 6% 5% 21%	2808
	11th to 20th	Night	20% 15% 5% 10% 7% 5% 4 3 6% 6% 18%	3531
	21st to end	Night	19% 12% 5% 14% 6% 5% 5% 4 4 6% 20%	5236

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% 3 10% 10% 2	2208
	11th to 20th	Day	59% 3 16% 17% 5%	2210
	21st to end	Day	57% 4 15% 18% 5% 1	2818
February	1st to 10th	Day	52% 5% 15% 19% 8% 1	1710
	11th to 20th	Day	59% 4 12% 20% 5%	1671
	21st to end	Day	50% 7% 8% 26% 8% 1	1413
March	1st to 10th	Day	57% 3 14% 19% 7% 1	3321
	11th to 20th	Day	68% 4 7% 14% 6% 1	3480
	21st to end	Day	60% 4 9% 20% 6% 1	3834
April	1st to 10th	Day	56% 5% 7% 19% 12% 1	2280
	11th to 20th	Day	49% 4 10% 22% 14% 2	2421
	21st to end	Day	53% 5% 12% 17% 12% 2	2470
May	1st to 10th	Day	58% 4 9% 21% 7% 1	4426
	11th to 20th	Day	45% 4 15% 27% 8% 1	4402
	21st to end	Day	53% 6% 10% 22% 8% 1	5053
June	1st to 10th	Day	53% 5% 13% 19% 9%	2850
	11th to 20th	Day	44% 6% 13% 25% 10% 1	2850
	21st to end	Day	55% 6% 13% 19% 8%	2850
July	1st to 10th	Day	41% 7% 16% 24% 12%	4685
	11th to 20th	Day	43% 8% 19% 23% 7%	4679
	21st to end	Day	37% 7% 22% 24% 10%	4815
August	1st to 10th	Day	43% 9% 21% 22% 4	4229
	11th to 20th	Day	43% 8% 21% 22% 5% 1	4084
	21st to end	Day	33% 6% 18% 31% 12% 1	4473
September	1st to 10th	Day	42% 5% 16% 26% 10%	2280
	11th to 20th	Day	41% 5% 16% 28% 9%	2213
	21st to end	Day	44% 6% 12% 27% 10% 1	2028
October	1st to 10th	Day	44% 7% 17% 26% 6% 1	3160
	11th to 20th	Day	48% 7% 14% 24% 6%	3140
	21st to end	Day	47% 7% 15% 21% 10%	3167
November	1st to 10th	Day	57% 6% 14% 16% 8% 1	1565
	11th to 20th	Day	63% 5% 14% 13% 5%	1600
	21st to end	Day	68% 4 13% 10% 5%	1600
December	1st to 10th	Day	69% 4 12% 11% 3	1484
	11th to 20th	Day	75% 2 9% 10% 3	1668
	21st to end	Day	68% 5% 11% 11% 3 1	2431
January	1st to 10th	Night	73% 3 12% 11% 1	4748
	11th to 20th	Night	66% 3 10% 17% 4	4760
	21st to end	Night	58% 4 15% 15% 7% 2	4840
February	1st to 10th	Night	55% 5% 10% 19% 10% 2	2470
	11th to 20th	Night	58% 5% 11% 18% 8%	2406
	21st to end	Night	52% 5% 10% 23% 10% 1	2041
March	1st to 10th	Night	53% 3 11% 24% 8% 1	3527
	11th to 20th	Night	69% 3 7% 14% 6% 1	3480
	21st to end	Night	59% 3 8% 19% 9% 2	3797
April	1st to 10th	Night	59% 4 9% 16% 12% 1	1900
	11th to 20th	Night	52% 4 8% 25% 11% 1	1707
	21st to end	Night	55% 3 9% 18% 14% 2	1710
May	1st to 10th	Night	61% 3 10% 17% 8% 1	2544
	11th to 20th	Night	43% 4 14% 29% 9%	2507
	21st to end	Night	54% 5% 11% 21% 8% 1	2562
June	1st to 10th	Night	55% 4 10% 22% 9%	1330
	11th to 20th	Night	46% 6% 14% 19% 14% 1	1330
	21st to end	Night	53% 5% 15% 19% 7% 1	1330
July	1st to 10th	Night	42% 7% 16% 21% 13%	2175
	11th to 20th	Night	41% 7% 19% 27% 5%	2168
	21st to end	Night	38% 7% 20% 24% 10%	2478
August	1st to 10th	Night	49% 5% 17% 22% 8%	2677
	11th to 20th	Night	41% 7% 21% 23% 7% 1	2809
	21st to end	Night	36% 6% 16% 29% 13%	3182
September	1st to 10th	Night	44% 5% 13% 28% 10% 1	1900
	11th to 20th	Night	43% 3 14% 28% 11%	1966
	21st to end	Night	42% 5% 12% 28% 12% 1	2151
October	1st to 10th	Night	49% 4 12% 27% 7% 1	3800
	11th to 20th	Night	46% 6% 13% 26% 9%	3779
	21st to end	Night	50% 7% 14% 22% 7%	4382
November	1st to 10th	Night	55% 5% 15% 18% 6% 1	2733
	11th to 20th	Night	63% 4 13% 14% 5% 1	2800
	21st to end	Night	70% 4 12% 11% 3	2800
December	1st to 10th	Night	71% 4 11% 11% 2	2808
	11th to 20th	Night	75% 3 8% 11% 3	3532
	21st to end	Night	70% 3 14% 11% 2	5236

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	29% 15% 29% 23% 2 1	180
	11th to 20th	Day	32% 18% 26% 17% 6% 1	180
	21st to end	Day	18% 17% 34% 20% 10% 2	321
February	1st to 10th	Day	22% 16% 34% 16% 9% 2	370
	11th to 20th	Day	19% 21% 35% 19% 5% 1	366
	21st to end	Day	18% 15% 31% 23% 10% 2	307
March	1st to 10th	Day	21% 15% 36% 19% 7% 2	439
	11th to 20th	Day	23% 14% 29% 23% 9% 2	460
	21st to end	Day	26% 18% 30% 20% 6% 2	506
April	1st to 10th	Day	27% 19% 27% 20% 6%	460
	11th to 20th	Day	23% 13% 34% 21% 8% 2	452
	21st to end	Day	26% 10% 27% 25% 9% 2	460
May	1st to 10th	Day	28% 10% 25% 24% 10% 3	639
	11th to 20th	Day	23% 11% 24% 26% 11% 5%	674
	21st to end	Day	25% 13% 28% 24% 8% 1	735
June	1st to 10th	Day	27% 9% 26% 27% 10% 2	680
	11th to 20th	Day	21% 8% 20% 32% 15% 4	680
	21st to end	Day	26% 6% 28% 24% 15% 1	680
July	1st to 10th	Day	19% 4 26% 35% 14% 2	670
	11th to 20th	Day	16% 4 25% 35% 18% 2	668
	21st to end	Day	18% 6% 18% 33% 20% 4	740
August	1st to 10th	Day	16% 5% 22% 29% 25% 2	572
	11th to 20th	Day	15% 4 20% 36% 20% 4	479
	21st to end	Day	16% 7% 26% 32% 16% 4	539
September	1st to 10th	Day	21% 6% 32% 26% 12% 3	490
	11th to 20th	Day	24% 10% 26% 25% 12% 3	490
	21st to end	Day	21% 12% 34% 21% 9% 3	457
October	1st to 10th	Day	17% 15% 30% 22% 12% 4	390
	11th to 20th	Day	28% 14% 30% 16% 11% 2	390
	21st to end	Day	26% 17% 30% 19% 7% 1	429
November	1st to 10th	Day	27% 14% 30% 21% 7% 1	395
	11th to 20th	Day	29% 17% 28% 17% 7% 2	400
	21st to end	Day	35% 17% 25% 20% 3	400
December	1st to 10th	Day	24% 11% 37% 22% 5% 1	309
	11th to 20th	Day	28% 23% 31% 14% 3 1	180
	21st to end	Day	26% 24% 29% 17% 4 1	198
January	1st to 10th	Night	25% 17% 29% 22% 6% 1	850
	11th to 20th	Night	23% 18% 30% 22% 6%	850
	21st to end	Night	18% 16% 32% 24% 9% 1	881
February	1st to 10th	Night	19% 14% 30% 29% 8% 1	760
	11th to 20th	Night	23% 14% 33% 22% 6% 1	743
	21st to end	Night	20% 13% 29% 23% 11% 3	628
March	1st to 10th	Night	21% 13% 31% 23% 9% 1	760
	11th to 20th	Night	23% 13% 29% 24% 9% 2	760
	21st to end	Night	26% 13% 26% 24% 9% 1	829
April	1st to 10th	Night	30% 11% 25% 25% 9%	570
	11th to 20th	Night	25% 9% 28% 26% 10% 2	564
	21st to end	Night	29% 8% 19% 31% 10% 3	570
May	1st to 10th	Night	26% 6% 21% 29% 15% 3	570
	11th to 20th	Night	22% 6% 23% 29% 19% 2	562
	21st to end	Night	21% 9% 26% 32% 11% 2	506
June	1st to 10th	Night	27% 5% 21% 29% 17% 2	380
	11th to 20th	Night	24% 5% 15% 34% 19% 3	380
	21st to end	Night	25% 3 16% 36% 18% 2	380
July	1st to 10th	Night	19% 4 12% 39% 24% 3	370
	11th to 20th	Night	18% 4 14% 41% 22% 2	368
	21st to end	Night	20% 3 12% 37% 25% 2	510
August	1st to 10th	Night	17% 4 12% 39% 24% 3	568
	11th to 20th	Night	21% 2 13% 35% 26% 3	562
	21st to end	Night	18% 5% 17% 31% 24% 4	627
September	1st to 10th	Night	23% 5% 23% 32% 15% 2	570
	11th to 20th	Night	22% 7% 21% 33% 14% 2	636
	21st to end	Night	23% 8% 26% 28% 14% 1	760
October	1st to 10th	Night	26% 8% 27% 26% 11% 3	760
	11th to 20th	Night	28% 11% 25% 24% 12% 1	760
	21st to end	Night	27% 12% 27% 25% 9%	873
November	1st to 10th	Night	28% 14% 27% 23% 7% 1	880
	11th to 20th	Night	31% 14% 28% 19% 6% 2	900
	21st to end	Night	33% 16% 27% 19% 5% 1	900
December	1st to 10th	Night	28% 15% 32% 19% 5%	878
	11th to 20th	Night	29% 22% 29% 16% 4	850
	21st to end	Night	29% 17% 28% 19% 5% 1	935

Page generated at 2024-12-03 06:45:10. Version ac07

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