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 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	10% 11% 4 9% 6% 6% 6% 5% 6% 7% 31%	142216
	Night	10% 13% 4 10% 6% 6% 5% 4 6% 7% 28%	139492
Transparency	Day	51% 5% 14% 21% 8%	141910
	Night	55% 5% 13% 19% 7%	139381
Seeing	Day	17% 9% 25% 27% 17% 6%	18861
	Night	18% 7% 22% 32% 18% 3	26454

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	7% 19% 8% 16% 8% 6% 6% 4 4 6% 15%	3989
	11th to 20th	Day	8% 13% 4 8% 6% 6% 7% 5% 7% 7% 30%	4119
	21st to end	Day	10% 19% 6% 12% 6% 5% 5% 5% 5% 6% 19%	4529
February	1st to 10th	Day	13% 14% 5% 12% 7% 5% 5% 4 5% 6% 24%	1654
	11th to 20th	Day	7% 15% 6% 11% 7% 5% 5% 4 7% 7% 26%	1845
	21st to end	Day	7% 15% 6% 12% 8% 6% 6% 4 5% 6% 26%	1557
March	1st to 10th	Day	5% 14% 5% 11% 8% 7% 6% 5% 9% 6% 23%	4594
	11th to 20th	Day	5% 13% 5% 11% 7% 7% 7% 6% 8% 9% 22%	5110
	21st to end	Day	7% 11% 4 8% 5% 6% 6% 4 7% 7% 35%	5728
April	1st to 10th	Day	7% 13% 5% 10% 7% 6% 6% 5% 6% 7% 28%	2583
	11th to 20th	Day	9% 15% 5% 9% 7% 5% 5% 5% 5% 6% 28%	2698
	21st to end	Day	11% 13% 4 8% 6% 6% 5% 4 6% 8% 29%	2882
May	1st to 10th	Day	6% 8% 4 8% 5% 5% 6% 4 7% 8% 39%	8249
	11th to 20th	Day	8% 10% 4 11% 7% 7% 7% 4 6% 7% 29%	8757
	21st to end	Day	10% 10% 4 10% 7% 7% 7% 5% 7% 8% 24%	9944
June	1st to 10th	Day	11% 7% 3 11% 7% 5% 6% 5% 6% 6% 32%	3360
	11th to 20th	Day	14% 9% 3 9% 6% 5% 6% 4 7% 9% 28%	3360
	21st to end	Day	11% 6% 3 8% 7% 6% 6% 5% 6% 8% 33%	3360
July	1st to 10th	Day	9% 6% 2 6% 5% 5% 6% 4 7% 7% 43%	7315
	11th to 20th	Day	9% 5% 2 6% 4 4 5% 5% 7% 9% 45%	6930
	21st to end	Day	8% 4 2 7% 6% 6% 6% 5% 7% 10% 39%	7526
August	1st to 10th	Day	4 4 2 4 4 4 4 4 6% 9% 55%	4781
	11th to 20th	Day	12% 7% 2 6% 3 4 4 4 6% 8% 43%	4317
	21st to end	Day	12% 6% 3 8% 5% 6% 5% 5% 6% 8% 37%	4664
September	1st to 10th	Day	11% 6% 2 7% 5% 5% 6% 5% 7% 9% 38%	2459
	11th to 20th	Day	12% 8% 3 7% 5% 5% 5% 5% 6% 8% 36%	2352
	21st to end	Day	18% 10% 3 9% 5% 5% 5% 4 6% 8% 27%	2138
October	1st to 10th	Day	18% 12% 4 10% 6% 6% 6% 4 6% 7% 21%	3280
	11th to 20th	Day	14% 10% 4 9% 6% 5% 5% 5% 7% 9% 25%	3239
	21st to end	Day	10% 15% 4 9% 6% 5% 6% 5% 7% 7% 26%	2888
November	1st to 10th	Day	18% 17% 4 10% 6% 5% 3 3 5% 6% 23%	1648
	11th to 20th	Day	11% 18% 5% 12% 7% 7% 5% 4 6% 6% 18%	1654
	21st to end	Day	12% 17% 7% 10% 8% 8% 6% 4 5% 7% 16%	1307
December	1st to 10th	Day	23% 17% 4 10% 5% 6% 8% 4 5% 5% 13%	2020
	11th to 20th	Day	13% 21% 7% 17% 9% 5% 5% 3 5% 5% 11%	2322
	21st to end	Day	7% 18% 8% 15% 7% 7% 6% 5% 4 5% 18%	3058
January	1st to 10th	Night	9% 20% 7% 16% 8% 7% 6% 4 5% 5% 13%	8947
	11th to 20th	Night	9% 10% 4 8% 6% 6% 6% 5% 7% 8% 31%	8779
	21st to end	Night	12% 17% 5% 10% 7% 5% 5% 5% 8% 7% 19%	9651
February	1st to 10th	Night	12% 17% 5% 11% 8% 5% 4 4 4 6% 24%	2962
	11th to 20th	Night	9% 13% 5% 10% 7% 6% 5% 4 7% 8% 27%	2662
	21st to end	Night	8% 14% 6% 11% 6% 5% 5% 5% 6% 6% 29%	2249
March	1st to 10th	Night	5% 14% 6% 14% 7% 6% 6% 5% 6% 6% 27%	5511
	11th to 20th	Night	4 14% 5% 11% 7% 7% 7% 5% 6% 7% 27%	5109
	21st to end	Night	9% 11% 3 7% 4 4 4 4 5% 6% 42%	5489
April	1st to 10th	Night	7% 13% 4 10% 6% 6% 7% 5% 6% 7% 30%	2033
	11th to 20th	Night	8% 12% 3 10% 7% 6% 5% 6% 7% 7% 29%	1871
	21st to end	Night	9% 11% 2 7% 5% 5% 6% 4 7% 9% 34%	1737
May	1st to 10th	Night	10% 8% 2 6% 5% 5% 6% 4 7% 9% 37%	4666
	11th to 20th	Night	9% 10% 4 8% 5% 6% 7% 6% 8% 10% 27%	4105
	21st to end	Night	12% 12% 5% 10% 7% 6% 5% 4 5% 6% 28%	4226
June	1st to 10th	Night	12% 8% 2 8% 6% 7% 7% 4 7% 7% 31%	1260
	11th to 20th	Night	14% 8% 3 8% 7% 6% 4 5% 7% 6% 32%	1260
	21st to end	Night	10% 7% 3 8% 6% 6% 6% 5% 8% 9% 32%	1260
July	1st to 10th	Night	9% 5% 2 6% 4 4 4 4 6% 7% 49%	2862
	11th to 20th	Night	9% 4 2 5% 5% 4 5% 4 6% 8% 47%	3250
	21st to end	Night	8% 5% 2 5% 5% 5% 5% 4 7% 10% 44%	3538
August	1st to 10th	Night	4 3 1 3 2 2 3 3 6% 8% 65%	2369
	11th to 20th	Night	9% 6% 2 7% 4 5% 4 3 7% 8% 45%	2732
	21st to end	Night	15% 7% 3 7% 5% 5% 4 4 5% 7% 37%	3245
September	1st to 10th	Night	11% 7% 3 6% 4 5% 5% 4 6% 8% 41%	1940
	11th to 20th	Night	10% 7% 3 7% 6% 5% 5% 4 5% 7% 41%	2045
	21st to end	Night	16% 11% 4 8% 6% 4 4 4 5% 6% 31%	2262
October	1st to 10th	Night	17% 13% 4 9% 5% 5% 5% 4 5% 7% 26%	3920
	11th to 20th	Night	12% 11% 4 10% 6% 6% 6% 4 7% 8% 27%	3915
	21st to end	Night	13% 14% 4 9% 6% 5% 6% 5% 5% 6% 28%	4903
November	1st to 10th	Night	16% 13% 4 11% 5% 5% 6% 4 6% 8% 22%	2870
	11th to 20th	Night	11% 17% 5% 11% 8% 6% 6% 5% 6% 6% 20%	2963
	21st to end	Night	9% 20% 6% 11% 7% 6% 5% 4 6% 7% 18%	3291
December	1st to 10th	Night	17% 17% 5% 11% 7% 7% 5% 4 6% 6% 17%	5380
	11th to 20th	Night	14% 20% 6% 14% 8% 6% 5% 3 4 5% 13%	6090
	21st to end	Night	7% 18% 7% 14% 9% 7% 6% 4 5% 6% 17%	8140

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% 7% 12% 6%	3989
	11th to 20th	Day	53% 5% 17% 14% 8% 2	4140
	21st to end	Day	65% 3 13% 15% 4	4530
February	1st to 10th	Day	61% 4 11% 12% 11% 1	1654
	11th to 20th	Day	56% 5% 17% 15% 6%	1854
	21st to end	Day	60% 4 14% 14% 7% 2	1557
March	1st to 10th	Day	57% 6% 9% 15% 12%	4594
	11th to 20th	Day	56% 7% 10% 17% 9%	5110
	21st to end	Day	47% 5% 11% 26% 11%	5728
April	1st to 10th	Day	54% 6% 11% 21% 7% 1	2583
	11th to 20th	Day	56% 5% 10% 20% 9% 1	2698
	21st to end	Day	53% 6% 12% 20% 9%	2882
May	1st to 10th	Day	41% 7% 14% 27% 10% 1	8249
	11th to 20th	Day	54% 5% 8% 23% 10%	8760
	21st to end	Day	56% 6% 11% 17% 9% 1	9945
June	1st to 10th	Day	50% 6% 10% 22% 11% 1	3360
	11th to 20th	Day	52% 7% 11% 22% 7%	3360
	21st to end	Day	48% 7% 13% 25% 7% 1	3360
July	1st to 10th	Day	41% 4 12% 27% 16%	7237
	11th to 20th	Day	37% 4 22% 30% 6%	6849
	21st to end	Day	41% 5% 19% 28% 8%	7319
August	1st to 10th	Day	27% 5% 16% 41% 11%	4763
	11th to 20th	Day	40% 4 16% 33% 7%	4359
	21st to end	Day	45% 6% 23% 21% 4	4663
September	1st to 10th	Day	41% 7% 22% 23% 5%	2459
	11th to 20th	Day	45% 7% 18% 26% 4	2352
	21st to end	Day	55% 8% 14% 19% 4	2138
October	1st to 10th	Day	63% 5% 14% 13% 4	3280
	11th to 20th	Day	57% 5% 19% 17% 3	3239
	21st to end	Day	57% 5% 20% 13% 5% 1	2888
November	1st to 10th	Day	62% 5% 13% 16% 5%	1648
	11th to 20th	Day	66% 8% 10% 12% 4	1654
	21st to end	Day	67% 5% 9% 14% 4	1307
December	1st to 10th	Day	74% 5% 8% 11% 2	2020
	11th to 20th	Day	76% 3 9% 8% 4	2324
	21st to end	Day	66% 5% 12% 11% 5% 1	3058
January	1st to 10th	Night	72% 5% 7% 11% 5% 1	8948
	11th to 20th	Night	51% 4 16% 17% 9% 3	8820
	21st to end	Night	63% 4 14% 16% 3	9651
February	1st to 10th	Night	63% 4 10% 11% 11% 1	2962
	11th to 20th	Night	54% 4 17% 17% 7%	2664
	21st to end	Night	55% 5% 12% 19% 8% 1	2249
March	1st to 10th	Night	56% 6% 7% 18% 13% 1	5511
	11th to 20th	Night	54% 7% 7% 20% 11%	5110
	21st to end	Night	43% 4 10% 29% 14%	5489
April	1st to 10th	Night	52% 6% 11% 21% 10% 1	2033
	11th to 20th	Night	52% 6% 11% 22% 9%	1871
	21st to end	Night	46% 7% 13% 24% 11%	1737
May	1st to 10th	Night	42% 5% 17% 27% 8%	4663
	11th to 20th	Night	51% 5% 11% 27% 6%	4105
	21st to end	Night	58% 4 9% 19% 9%	4230
June	1st to 10th	Night	47% 9% 10% 22% 11% 1	1260
	11th to 20th	Night	52% 5% 11% 24% 8% 1	1260
	21st to end	Night	46% 7% 16% 25% 7%	1260
July	1st to 10th	Night	37% 5% 11% 28% 17% 1	2831
	11th to 20th	Night	37% 3 22% 32% 6%	3208
	21st to end	Night	37% 4 26% 23% 10%	3440
August	1st to 10th	Night	18% 4 19% 41% 19%	2362
	11th to 20th	Night	38% 5% 15% 33% 10%	2756
	21st to end	Night	47% 6% 22% 20% 5%	3245
September	1st to 10th	Night	40% 6% 18% 30% 6%	1940
	11th to 20th	Night	42% 6% 15% 29% 8%	2045
	21st to end	Night	53% 7% 13% 19% 8%	2262
October	1st to 10th	Night	59% 5% 16% 17% 3	3920
	11th to 20th	Night	56% 4 15% 21% 3	3915
	21st to end	Night	57% 4 16% 18% 4	4903
November	1st to 10th	Night	60% 7% 12% 16% 5%	2870
	11th to 20th	Night	63% 7% 12% 13% 4	2963
	21st to end	Night	65% 6% 12% 13% 4	3291
December	1st to 10th	Night	69% 4 10% 14% 3	5380
	11th to 20th	Night	73% 4 11% 9% 3	6087
	21st to end	Night	67% 6% 11% 12% 4	8140

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	20% 8% 23% 31% 12% 5%	220
	11th to 20th	Day	22% 8% 22% 29% 15% 3	220
	21st to end	Day	19% 9% 24% 29% 15% 5%	242
February	1st to 10th	Day	21% 13% 28% 22% 11% 5%	315
	11th to 20th	Day	16% 10% 32% 24% 14% 4	425
	21st to end	Day	20% 14% 30% 21% 10% 6%	355
March	1st to 10th	Day	15% 15% 27% 21% 16% 6%	493
	11th to 20th	Day	14% 12% 31% 26% 14% 2	540
	21st to end	Day	17% 13% 26% 24% 14% 5%	594
April	1st to 10th	Day	17% 12% 28% 26% 11% 5%	540
	11th to 20th	Day	17% 12% 29% 22% 13% 7%	532
	21st to end	Day	17% 13% 27% 26% 13% 4	540
May	1st to 10th	Day	16% 10% 30% 26% 17% 1	559
	11th to 20th	Day	17% 8% 23% 28% 18% 6%	775
	21st to end	Day	15% 9% 25% 29% 18% 4	945
June	1st to 10th	Day	16% 9% 24% 32% 15% 4	990
	11th to 20th	Day	17% 7% 24% 31% 17% 5%	990
	21st to end	Day	13% 5% 22% 31% 21% 8%	990
July	1st to 10th	Day	14% 9% 19% 29% 23% 5%	944
	11th to 20th	Day	14% 7% 20% 28% 25% 6%	768
	21st to end	Day	10% 7% 21% 29% 22% 10%	845
August	1st to 10th	Day	11% 5% 17% 31% 25% 11%	677
	11th to 20th	Day	12% 6% 25% 28% 20% 8%	517
	21st to end	Day	13% 8% 27% 25% 16% 11%	583
September	1st to 10th	Day	15% 7% 21% 28% 22% 8%	530
	11th to 20th	Day	19% 5% 23% 25% 21% 8%	530
	21st to end	Day	26% 11% 28% 21% 11% 3	530
October	1st to 10th	Day	23% 11% 22% 25% 15% 4	530
	11th to 20th	Day	23% 12% 28% 21% 13% 3	524
	21st to end	Day	22% 10% 27% 24% 15% 2	276
November	1st to 10th	Day	28% 12% 25% 22% 10% 3	220
	11th to 20th	Day	25% 6% 24% 27% 15% 4	220
	21st to end	Day	20% 10% 28% 24% 11% 8%	220
December	1st to 10th	Day	20% 5% 25% 29% 17% 5%	220
	11th to 20th	Day	15% 7% 27% 29% 18% 4	220
	21st to end	Day	24% 4 20% 30% 17% 5%	242
January	1st to 10th	Night	21% 10% 21% 35% 11% 2	1014
	11th to 20th	Night	19% 10% 26% 30% 13% 1	950
	21st to end	Night	22% 8% 21% 32% 16% 1	1045
February	1st to 10th	Night	22% 9% 22% 30% 15% 3	903
	11th to 20th	Night	15% 7% 24% 35% 16% 3	824
	21st to end	Night	16% 9% 24% 33% 15% 4	692
March	1st to 10th	Night	16% 9% 24% 32% 17% 3	840
	11th to 20th	Night	13% 9% 25% 35% 14% 2	840
	21st to end	Night	16% 8% 20% 29% 24% 4	924
April	1st to 10th	Night	14% 7% 20% 33% 22% 4	773
	11th to 20th	Night	14% 8% 20% 32% 23% 3	623
	21st to end	Night	15% 8% 25% 30% 21% 1	477
May	1st to 10th	Night	17% 7% 22% 36% 15% 2	420
	11th to 20th	Night	15% 6% 23% 31% 24% 1	414
	21st to end	Night	22% 5% 17% 30% 24% 2	455
June	1st to 10th	Night	15% 5% 24% 35% 18% 2	420
	11th to 20th	Night	18% 5% 24% 28% 22% 4	420
	21st to end	Night	13% 4 21% 36% 22% 3	420
July	1st to 10th	Night	16% 4 18% 33% 27% 2	410
	11th to 20th	Night	15% 4 20% 30% 28% 3	408
	21st to end	Night	12% 2 22% 34% 26% 4	458
August	1st to 10th	Night	9% 4 13% 35% 33% 8%	398
	11th to 20th	Night	12% 4 18% 35% 25% 6%	486
	21st to end	Night	17% 3 16% 32% 26% 5%	660
September	1st to 10th	Night	14% 4 18% 31% 30% 3	740
	11th to 20th	Night	13% 5% 16% 32% 30% 6%	800
	21st to end	Night	23% 8% 22% 31% 13% 3	800
October	1st to 10th	Night	22% 7% 20% 31% 18% 4	800
	11th to 20th	Night	22% 7% 23% 29% 18% 2	800
	21st to end	Night	25% 8% 23% 31% 13%	880
November	1st to 10th	Night	24% 10% 23% 27% 15% 2	820
	11th to 20th	Night	21% 10% 23% 32% 12% 1	853
	21st to end	Night	20% 8% 24% 31% 14% 3	1091
December	1st to 10th	Night	20% 7% 20% 35% 15% 3	1160
	11th to 20th	Night	21% 9% 22% 33% 14% 2	1160
	21st to end	Night	18% 7% 24% 34% 14% 3	1276

Page generated at 2026-07-28 00:25:20. Version ac07

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