Fort Saint James
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fort Saint James clear sky chart from 2005-11-06 to 2025-03-27.

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	10% 11% 4 9% 6% 6% 5% 4 6% 7% 31%	111838
	Night	11% 13% 5% 10% 6% 6% 5% 4 6% 7% 28%	116130
Transparency	Day	51% 6% 14% 21% 8% 1	111576
	Night	55% 6% 13% 18% 7% 1	116038
Seeing	Day	17% 9% 25% 27% 17% 6%	16882
	Night	18% 7% 21% 32% 19% 3	24848

By Month

Percentage Cloud Cover NW (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	8% 19% 8% 18% 8% 5% 6% 4 4 5% 15%	3069
	11th to 20th	Day	9% 15% 5% 9% 6% 6% 6% 4 6% 5% 29%	3159
	21st to end	Day	11% 21% 7% 11% 6% 4 5% 5% 4 6% 20%	3498
February	1st to 10th	Day	12% 14% 5% 12% 7% 5% 5% 4 6% 6% 24%	1575
	11th to 20th	Day	7% 15% 6% 11% 7% 5% 5% 4 7% 7% 26%	1755
	21st to end	Day	7% 14% 6% 12% 8% 6% 6% 4 5% 6% 27%	1485
March	1st to 10th	Day	5% 14% 5% 12% 8% 7% 6% 5% 8% 6% 24%	3219
	11th to 20th	Day	6% 13% 5% 11% 7% 7% 7% 5% 7% 8% 25%	3600
	21st to end	Day	5% 11% 4 8% 6% 6% 6% 4 6% 7% 38%	3973
April	1st to 10th	Day	7% 14% 5% 11% 7% 7% 6% 5% 5% 6% 28%	2337
	11th to 20th	Day	9% 15% 5% 9% 7% 5% 5% 5% 5% 6% 27%	2438
	21st to end	Day	12% 14% 4 8% 6% 6% 5% 4 6% 7% 28%	2608
May	1st to 10th	Day	8% 11% 4 9% 6% 5% 7% 5% 7% 8% 31%	4447
	11th to 20th	Day	10% 11% 4 11% 7% 6% 6% 4 7% 7% 28%	4696
	21st to end	Day	10% 10% 4 11% 7% 7% 6% 5% 7% 7% 26%	5316
June	1st to 10th	Day	12% 8% 3 11% 7% 5% 6% 5% 6% 6% 31%	3040
	11th to 20th	Day	14% 9% 3 9% 6% 5% 5% 4 7% 8% 28%	3040
	21st to end	Day	11% 6% 3 8% 7% 6% 6% 5% 6% 8% 34%	3040
July	1st to 10th	Day	8% 5% 2 5% 5% 4 5% 4 6% 7% 50%	5005
	11th to 20th	Day	7% 4 1 5% 4 3 5% 4 7% 8% 52%	4740
	21st to end	Day	10% 5% 2 8% 7% 6% 6% 6% 7% 9% 34%	5138
August	1st to 10th	Day	4 4 1 4 4 4 4 4 6% 9% 56%	4634
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 8% 44%	4182
	21st to end	Day	13% 6% 3 8% 5% 6% 5% 5% 6% 8% 36%	4521
September	1st to 10th	Day	11% 6% 2 8% 5% 5% 5% 5% 7% 8% 37%	2336
	11th to 20th	Day	11% 8% 3 7% 6% 5% 6% 5% 6% 8% 36%	2235
	21st to end	Day	17% 10% 3 9% 5% 5% 5% 4 6% 8% 27%	2031
October	1st to 10th	Day	19% 12% 4 10% 6% 6% 6% 4 6% 7% 21%	3180
	11th to 20th	Day	14% 10% 4 10% 6% 5% 5% 5% 7% 9% 24%	3140
	21st to end	Day	10% 15% 4 9% 6% 5% 6% 5% 7% 7% 26%	2799
November	1st to 10th	Day	18% 17% 4 9% 6% 5% 3 3 5% 6% 24%	1568
	11th to 20th	Day	11% 18% 5% 12% 7% 8% 5% 4 6% 6% 16%	1575
	21st to end	Day	12% 17% 7% 10% 8% 7% 6% 4 6% 7% 15%	1245
December	1st to 10th	Day	23% 17% 4 10% 5% 6% 8% 4 5% 5% 13%	1960
	11th to 20th	Day	13% 21% 6% 17% 9% 5% 4 3 4 5% 11%	2262
	21st to end	Day	7% 18% 8% 15% 7% 7% 6% 4 4 5% 18%	2992
January	1st to 10th	Night	8% 18% 7% 17% 8% 7% 6% 4 5% 6% 15%	6905
	11th to 20th	Night	11% 12% 5% 9% 6% 6% 5% 4 6% 7% 30%	6739
	21st to end	Night	14% 18% 6% 11% 6% 5% 5% 5% 7% 7% 17%	7458
February	1st to 10th	Night	12% 18% 5% 11% 8% 5% 4 4 4 6% 25%	2821
	11th to 20th	Night	9% 13% 4 10% 7% 5% 5% 4 7% 8% 27%	2532
	21st to end	Night	8% 14% 5% 10% 6% 6% 5% 5% 6% 6% 30%	2145
March	1st to 10th	Night	5% 15% 6% 13% 7% 6% 5% 5% 5% 5% 28%	3870
	11th to 20th	Night	5% 13% 5% 11% 8% 8% 6% 5% 6% 6% 27%	3599
	21st to end	Night	6% 10% 3 6% 4 4 4 4 5% 7% 46%	3818
April	1st to 10th	Night	8% 13% 4 10% 6% 5% 6% 4 5% 7% 31%	1839
	11th to 20th	Night	9% 12% 3 10% 8% 6% 5% 6% 7% 7% 27%	1691
	21st to end	Night	10% 12% 3 7% 5% 5% 6% 4 6% 9% 33%	1572
May	1st to 10th	Night	11% 10% 3 8% 6% 5% 5% 4 6% 9% 33%	2509
	11th to 20th	Night	11% 11% 4 9% 5% 6% 6% 6% 8% 9% 25%	2199
	21st to end	Night	13% 12% 5% 9% 7% 7% 5% 5% 6% 6% 26%	2258
June	1st to 10th	Night	13% 9% 3 8% 6% 7% 7% 4 8% 7% 28%	1140
	11th to 20th	Night	15% 8% 3 9% 7% 6% 4 4 7% 6% 31%	1140
	21st to end	Night	10% 7% 3 8% 6% 6% 6% 5% 7% 10% 33%	1140
July	1st to 10th	Night	8% 3 2 6% 4 4 4 4 5% 6% 53%	1953
	11th to 20th	Night	7% 3 1 3 4 4 4 4 6% 8% 54%	2220
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 6% 9% 41%	2414
August	1st to 10th	Night	4 3 1 3 2 2 3 3 6% 8% 66%	2296
	11th to 20th	Night	9% 6% 2 7% 4 4 4 3 7% 8% 45%	2647
	21st to end	Night	15% 8% 3 7% 5% 5% 4 4 5% 7% 36%	3146
September	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 9% 40%	1843
	11th to 20th	Night	10% 7% 3 7% 6% 5% 5% 4 5% 7% 41%	1943
	21st to end	Night	16% 10% 4 8% 6% 4 4 4 5% 6% 32%	2149
October	1st to 10th	Night	18% 13% 4 9% 5% 5% 5% 4 5% 7% 26%	3800
	11th to 20th	Night	12% 11% 4 10% 6% 6% 6% 4 7% 8% 26%	3794
	21st to end	Night	13% 14% 4 9% 6% 5% 6% 5% 5% 6% 29%	4750
November	1st to 10th	Night	16% 13% 4 11% 5% 5% 5% 4 6% 8% 22%	2730
	11th to 20th	Night	11% 17% 5% 11% 7% 6% 6% 5% 6% 6% 20%	2822
	21st to end	Night	10% 20% 6% 11% 7% 6% 5% 4 6% 7% 18%	3134
December	1st to 10th	Night	17% 17% 5% 11% 7% 7% 5% 4 6% 6% 16%	5220
	11th to 20th	Night	15% 20% 6% 14% 8% 6% 5% 3 4 5% 13%	5930
	21st to end	Night	7% 18% 7% 14% 9% 7% 6% 4 5% 6% 17%	7964

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	71% 5% 8% 10% 6%	3069
	11th to 20th	Day	58% 6% 13% 13% 8% 3	3180
	21st to end	Day	67% 4 12% 12% 5%	3498
February	1st to 10th	Day	60% 5% 11% 12% 11% 1	1575
	11th to 20th	Day	56% 5% 18% 15% 6% 1	1764
	21st to end	Day	59% 4 14% 14% 8% 2	1485
March	1st to 10th	Day	55% 8% 8% 16% 12%	3219
	11th to 20th	Day	52% 9% 12% 21% 6%	3600
	21st to end	Day	43% 7% 13% 25% 10%	3973
April	1st to 10th	Day	55% 7% 10% 20% 8% 1	2337
	11th to 20th	Day	56% 6% 9% 20% 8% 1	2438
	21st to end	Day	55% 6% 12% 18% 9%	2608
May	1st to 10th	Day	44% 11% 13% 22% 9% 1	4447
	11th to 20th	Day	53% 8% 8% 19% 11% 1	4699
	21st to end	Day	52% 9% 11% 20% 8% 1	5317
June	1st to 10th	Day	50% 7% 10% 22% 9% 1	3040
	11th to 20th	Day	52% 8% 11% 22% 7%	3040
	21st to end	Day	47% 7% 13% 25% 8% 1	3040
July	1st to 10th	Day	34% 4 10% 31% 21% 1	4927
	11th to 20th	Day	30% 5% 24% 33% 8%	4659
	21st to end	Day	45% 6% 14% 24% 11%	4976
August	1st to 10th	Day	26% 5% 16% 41% 12%	4616
	11th to 20th	Day	39% 5% 16% 33% 7%	4224
	21st to end	Day	46% 6% 24% 20% 3	4520
September	1st to 10th	Day	42% 8% 23% 22% 5%	2336
	11th to 20th	Day	45% 7% 17% 27% 4	2235
	21st to end	Day	54% 8% 14% 19% 4	2031
October	1st to 10th	Day	64% 5% 14% 12% 4	3180
	11th to 20th	Day	57% 5% 19% 16% 3	3140
	21st to end	Day	57% 5% 20% 13% 6% 1	2799
November	1st to 10th	Day	61% 5% 13% 16% 5%	1568
	11th to 20th	Day	67% 8% 10% 12% 3	1575
	21st to end	Day	68% 5% 9% 14% 3	1245
December	1st to 10th	Day	74% 5% 7% 11% 2	1960
	11th to 20th	Day	76% 3 9% 8% 4	2264
	21st to end	Day	66% 6% 12% 11% 5% 1	2992
January	1st to 10th	Night	68% 6% 9% 12% 5% 1	6905
	11th to 20th	Night	56% 5% 12% 13% 10% 4	6780
	21st to end	Night	66% 5% 14% 12% 3	7458
February	1st to 10th	Night	62% 4 10% 11% 11% 1	2821
	11th to 20th	Night	54% 5% 18% 17% 6%	2534
	21st to end	Night	54% 5% 12% 20% 9% 1	2145
March	1st to 10th	Night	53% 8% 8% 18% 12% 1	3870
	11th to 20th	Night	54% 8% 9% 22% 7%	3600
	21st to end	Night	37% 6% 12% 30% 15%	3818
April	1st to 10th	Night	51% 7% 11% 20% 10% 1	1839
	11th to 20th	Night	52% 6% 11% 22% 8%	1691
	21st to end	Night	46% 7% 12% 23% 11%	1572
May	1st to 10th	Night	44% 9% 16% 22% 9%	2509
	11th to 20th	Night	51% 8% 14% 20% 7% 1	2199
	21st to end	Night	55% 7% 11% 21% 6%	2258
June	1st to 10th	Night	49% 9% 10% 21% 10% 1	1140
	11th to 20th	Night	52% 6% 11% 23% 7%	1140
	21st to end	Night	45% 7% 16% 25% 7%	1140
July	1st to 10th	Night	32% 6% 8% 31% 22% 2	1922
	11th to 20th	Night	29% 4 22% 38% 8%	2178
	21st to end	Night	40% 6% 22% 19% 14%	2337
August	1st to 10th	Night	17% 4 18% 41% 19%	2289
	11th to 20th	Night	37% 5% 15% 33% 10%	2671
	21st to end	Night	48% 6% 23% 20% 3	3146
September	1st to 10th	Night	42% 6% 18% 28% 6%	1843
	11th to 20th	Night	42% 6% 14% 29% 8% 1	1943
	21st to end	Night	52% 7% 13% 19% 8%	2149
October	1st to 10th	Night	59% 5% 16% 17% 3	3800
	11th to 20th	Night	57% 4 15% 21% 3	3794
	21st to end	Night	57% 4 16% 19% 4	4750
November	1st to 10th	Night	60% 7% 12% 16% 5%	2730
	11th to 20th	Night	63% 8% 12% 13% 4	2822
	21st to end	Night	65% 6% 12% 12% 4	3134
December	1st to 10th	Night	69% 5% 10% 14% 3	5220
	11th to 20th	Night	73% 4 11% 9% 3	5927
	21st to end	Night	66% 6% 11% 12% 4	7964

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% 7% 22% 32% 13% 6%	200
	11th to 20th	Day	24% 8% 22% 28% 16% 3	200
	21st to end	Day	18% 9% 24% 29% 15% 5%	220
February	1st to 10th	Day	20% 14% 27% 22% 11% 6%	290
	11th to 20th	Day	16% 9% 32% 24% 14% 4	395
	21st to end	Day	19% 14% 29% 21% 10% 6%	331
March	1st to 10th	Day	14% 15% 26% 22% 17% 7%	457
	11th to 20th	Day	14% 11% 31% 27% 15% 2	500
	21st to end	Day	19% 13% 24% 24% 14% 5%	531
April	1st to 10th	Day	17% 13% 29% 25% 10% 5%	460
	11th to 20th	Day	18% 10% 28% 24% 13% 7%	452
	21st to end	Day	19% 13% 28% 25% 11% 3	460
May	1st to 10th	Day	19% 11% 28% 25% 16% 2	477
	11th to 20th	Day	18% 7% 24% 29% 18% 6%	675
	21st to end	Day	15% 8% 25% 30% 19% 4	825
June	1st to 10th	Day	17% 9% 24% 31% 15% 4	870
	11th to 20th	Day	18% 6% 25% 30% 16% 4	870
	21st to end	Day	13% 5% 23% 31% 20% 7%	870
July	1st to 10th	Day	14% 7% 19% 30% 24% 6%	828
	11th to 20th	Day	14% 7% 21% 27% 24% 7%	668
	21st to end	Day	11% 7% 22% 30% 22% 9%	739
August	1st to 10th	Day	11% 5% 16% 30% 26% 12%	630
	11th to 20th	Day	12% 6% 24% 28% 21% 8%	477
	21st to end	Day	14% 8% 29% 27% 16% 7%	539
September	1st to 10th	Day	16% 8% 22% 29% 20% 5%	490
	11th to 20th	Day	18% 5% 23% 24% 22% 8%	490
	21st to end	Day	25% 11% 29% 21% 11% 3	490
October	1st to 10th	Day	23% 10% 21% 25% 15% 5%	490
	11th to 20th	Day	23% 11% 28% 21% 13% 3	485
	21st to end	Day	21% 10% 27% 25% 15% 2	253
November	1st to 10th	Day	29% 12% 23% 23% 11% 4	200
	11th to 20th	Day	27% 6% 22% 26% 17% 4	200
	21st to end	Day	21% 10% 28% 24% 10% 9%	200
December	1st to 10th	Day	20% 4 24% 30% 19% 5%	200
	11th to 20th	Day	16% 7% 24% 31% 20% 4	200
	21st to end	Day	25% 3 17% 31% 18% 6%	220
January	1st to 10th	Night	21% 9% 21% 36% 12% 2	961
	11th to 20th	Night	20% 10% 25% 30% 13% 1	900
	21st to end	Night	22% 8% 21% 31% 16% 1	990
February	1st to 10th	Night	23% 9% 21% 29% 16% 3	857
	11th to 20th	Night	15% 7% 24% 36% 16% 2	784
	21st to end	Night	15% 9% 24% 32% 16% 4	660
March	1st to 10th	Night	16% 9% 23% 33% 18% 3	800
	11th to 20th	Night	13% 9% 25% 36% 15% 3	800
	21st to end	Night	16% 7% 20% 29% 24% 5%	862
April	1st to 10th	Night	15% 7% 20% 31% 22% 4	699
	11th to 20th	Night	15% 7% 20% 30% 25% 3	563
	21st to end	Night	16% 8% 25% 28% 22% 1	432
May	1st to 10th	Night	19% 7% 22% 35% 15% 3	380
	11th to 20th	Night	16% 6% 23% 31% 23% 1	374
	21st to end	Night	22% 5% 17% 29% 25% 2	411
June	1st to 10th	Night	16% 6% 24% 34% 18% 2	380
	11th to 20th	Night	18% 4 24% 28% 21% 4	380
	21st to end	Night	13% 4 22% 36% 22% 3	380
July	1st to 10th	Night	17% 4 18% 31% 27% 2	370
	11th to 20th	Night	15% 3 20% 30% 28% 3	368
	21st to end	Night	12% 3 22% 34% 25% 4	416
August	1st to 10th	Night	9% 4 13% 33% 33% 8%	378
	11th to 20th	Night	12% 4 18% 34% 26% 6%	461
	21st to end	Night	18% 3 17% 33% 25% 4	627
September	1st to 10th	Night	15% 4 19% 31% 29% 2	703
	11th to 20th	Night	12% 5% 15% 31% 31% 6%	760
	21st to end	Night	23% 8% 21% 32% 14% 3	760
October	1st to 10th	Night	22% 7% 19% 30% 18% 4	760
	11th to 20th	Night	23% 7% 22% 28% 18% 2	760
	21st to end	Night	25% 8% 22% 31% 14%	836
November	1st to 10th	Night	24% 9% 22% 27% 16% 2	780
	11th to 20th	Night	21% 10% 24% 31% 12% 1	812
	21st to end	Night	21% 8% 24% 30% 14% 3	1034
December	1st to 10th	Night	20% 6% 19% 36% 16% 3	1100
	11th to 20th	Night	21% 8% 21% 33% 15% 2	1100
	21st to end	Night	18% 6% 24% 34% 15% 3	1210

Page generated at 2025-03-24 06:04:27. Version ac07

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