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 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	11% 11% 4 8% 7% 4 6% 6% 6% 7% 29%	77426
	Night	12% 13% 5% 9% 8% 4 6% 5% 6% 7% 27%	81586
Transparency	Day	53% 7% 14% 19% 6% 1	77122
	Night	56% 7% 13% 17% 6% 1	81470
Seeing	Day	17% 8% 24% 27% 17% 6%	15387
	Night	19% 7% 21% 32% 19% 3	23369

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% 6% 14% 12% 5% 8% 5% 4 4 15%	2137
	11th to 20th	Day	11% 18% 6% 8% 7% 3 5% 5% 5% 6% 27%	2199
	21st to end	Day	13% 22% 8% 12% 10% 4 6% 4 4 6% 12%	2442
February	1st to 10th	Day	13% 14% 6% 10% 9% 4 6% 4 5% 6% 24%	1495
	11th to 20th	Day	7% 14% 7% 10% 8% 4 5% 5% 6% 8% 26%	1612
	21st to end	Day	7% 13% 6% 10% 8% 5% 5% 6% 5% 6% 29%	1332
March	1st to 10th	Day	7% 13% 5% 9% 7% 4 6% 6% 8% 7% 28%	1781
	11th to 20th	Day	7% 14% 5% 8% 8% 4 6% 6% 7% 8% 27%	1980
	21st to end	Day	7% 13% 6% 7% 7% 5% 6% 6% 7% 7% 29%	2220
April	1st to 10th	Day	8% 13% 5% 10% 8% 5% 5% 6% 6% 7% 28%	2214
	11th to 20th	Day	10% 13% 7% 8% 8% 5% 5% 6% 5% 6% 26%	2308
	21st to end	Day	13% 12% 5% 7% 7% 4 5% 5% 6% 7% 28%	2470
May	1st to 10th	Day	12% 12% 5% 8% 7% 4 6% 5% 5% 6% 29%	2533
	11th to 20th	Day	13% 8% 4 8% 7% 4 7% 5% 7% 7% 32%	2656
	21st to end	Day	12% 9% 4 9% 8% 4 5% 6% 6% 7% 31%	3002
June	1st to 10th	Day	12% 7% 4 10% 8% 4 6% 6% 6% 6% 31%	2880
	11th to 20th	Day	14% 8% 4 8% 7% 4 6% 5% 6% 8% 28%	2880
	21st to end	Day	10% 5% 3 7% 8% 5% 7% 6% 6% 8% 35%	2880
July	1st to 10th	Day	12% 7% 3 6% 6% 4 6% 6% 6% 8% 35%	2853
	11th to 20th	Day	13% 6% 3 6% 6% 3 6% 6% 6% 8% 36%	2700
	21st to end	Day	9% 4 3 5% 8% 4 5% 6% 7% 8% 39%	2938
August	1st to 10th	Day	7% 5% 2 5% 6% 4 5% 6% 6% 10% 43%	2643
	11th to 20th	Day	9% 5% 2 4 5% 4 5% 6% 6% 8% 47%	2392
	21st to end	Day	13% 6% 3 8% 7% 5% 7% 6% 6% 8% 31%	2574
September	1st to 10th	Day	12% 6% 2 6% 7% 4 5% 6% 7% 8% 37%	2214
	11th to 20th	Day	11% 7% 4 6% 6% 3 6% 6% 6% 8% 37%	2118
	21st to end	Day	16% 10% 3 7% 6% 4 5% 6% 6% 8% 28%	1925
October	1st to 10th	Day	15% 10% 3 8% 6% 4 7% 5% 6% 7% 28%	1800
	11th to 20th	Day	16% 10% 4 9% 8% 4 5% 5% 6% 8% 24%	1792
	21st to end	Day	14% 16% 4 7% 7% 3 5% 6% 6% 6% 27%	1616
November	1st to 10th	Day	18% 16% 5% 8% 7% 4 3 3 5% 6% 24%	1488
	11th to 20th	Day	12% 17% 6% 11% 9% 6% 6% 5% 7% 6% 16%	1497
	21st to end	Day	13% 15% 7% 9% 9% 7% 6% 6% 5% 7% 16%	1183
December	1st to 10th	Day	13% 12% 5% 11% 9% 5% 9% 6% 6% 7% 18%	1140
	11th to 20th	Day	17% 17% 5% 10% 9% 5% 5% 5% 6% 7% 14%	1442
	21st to end	Day	9% 16% 5% 10% 9% 4 7% 6% 5% 5% 24%	2090
January	1st to 10th	Night	9% 17% 7% 14% 10% 5% 7% 6% 6% 6% 14%	4823
	11th to 20th	Night	11% 14% 6% 9% 8% 4 5% 4 5% 6% 28%	4699
	21st to end	Night	16% 21% 7% 11% 9% 4 6% 4 4 5% 12%	5214
February	1st to 10th	Night	14% 17% 6% 9% 8% 3 4 4 4 6% 24%	2681
	11th to 20th	Night	10% 13% 5% 9% 8% 4 6% 6% 7% 7% 26%	2329
	21st to end	Night	8% 12% 6% 9% 7% 4 6% 6% 6% 6% 31%	1924
March	1st to 10th	Night	8% 14% 6% 9% 8% 5% 6% 5% 5% 6% 29%	2178
	11th to 20th	Night	7% 12% 6% 9% 7% 5% 6% 6% 6% 6% 29%	1979
	21st to end	Night	8% 13% 5% 9% 8% 4 6% 6% 6% 7% 29%	2136
April	1st to 10th	Night	8% 13% 5% 8% 7% 4 6% 6% 5% 7% 30%	1743
	11th to 20th	Night	10% 11% 3 8% 9% 5% 6% 7% 7% 7% 26%	1601
	21st to end	Night	10% 11% 3 6% 7% 4 6% 5% 6% 9% 34%	1490
May	1st to 10th	Night	14% 10% 4 8% 8% 4 7% 4 5% 6% 28%	1423
	11th to 20th	Night	13% 8% 4 7% 5% 4 6% 5% 7% 9% 31%	1239
	21st to end	Night	15% 8% 5% 7% 8% 5% 6% 6% 6% 6% 28%	1272
June	1st to 10th	Night	14% 8% 3 7% 7% 5% 7% 7% 7% 7% 28%	1080
	11th to 20th	Night	15% 7% 3 8% 8% 5% 6% 5% 6% 7% 31%	1080
	21st to end	Night	10% 6% 4 6% 6% 4 6% 7% 6% 10% 35%	1080
July	1st to 10th	Night	13% 5% 4 8% 6% 3 6% 7% 6% 8% 35%	1105
	11th to 20th	Night	13% 5% 2 5% 7% 4 6% 5% 6% 8% 41%	1260
	21st to end	Night	9% 6% 2 5% 6% 3 5% 6% 6% 9% 44%	1379
August	1st to 10th	Night	7% 4 3 5% 4 2 4 5% 7% 9% 51%	1296
	11th to 20th	Night	8% 5% 2 5% 5% 3 5% 4 6% 8% 51%	1504
	21st to end	Night	13% 7% 2 7% 7% 4 6% 6% 6% 8% 35%	1782
September	1st to 10th	Night	12% 7% 3 5% 6% 3 5% 5% 5% 9% 39%	1746
	11th to 20th	Night	10% 6% 3 6% 6% 4 6% 5% 5% 7% 42%	1840
	21st to end	Night	15% 10% 4 7% 7% 3 5% 5% 5% 7% 32%	2035
October	1st to 10th	Night	15% 9% 4 8% 7% 3 5% 5% 5% 7% 32%	2160
	11th to 20th	Night	17% 11% 4 7% 6% 3 6% 6% 6% 7% 27%	2165
	21st to end	Night	16% 12% 4 7% 7% 4 5% 5% 5% 7% 28%	2740
November	1st to 10th	Night	16% 13% 5% 10% 6% 3 6% 5% 6% 8% 22%	2590
	11th to 20th	Night	11% 15% 6% 9% 9% 4 6% 6% 6% 6% 21%	2680
	21st to end	Night	10% 18% 7% 9% 8% 4 6% 5% 6% 7% 19%	2977
December	1st to 10th	Night	10% 13% 5% 10% 9% 5% 7% 5% 6% 6% 23%	3040
	11th to 20th	Night	15% 17% 6% 11% 9% 5% 6% 5% 5% 6% 16%	3750
	21st to end	Night	9% 15% 6% 11% 10% 5% 7% 6% 5% 6% 21%	5566

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	70% 7% 8% 11% 4	2137
	11th to 20th	Day	59% 6% 13% 7% 11% 4	2220
	21st to end	Day	75% 5% 9% 8% 3	2442
February	1st to 10th	Day	61% 5% 12% 12% 10% 1	1495
	11th to 20th	Day	56% 6% 19% 14% 5% 1	1621
	21st to end	Day	56% 4 15% 14% 8% 2	1332
March	1st to 10th	Day	52% 6% 12% 19% 10% 1	1781
	11th to 20th	Day	54% 6% 16% 17% 7%	1980
	21st to end	Day	53% 6% 14% 20% 6%	2220
April	1st to 10th	Day	55% 6% 11% 21% 7% 1	2214
	11th to 20th	Day	57% 6% 9% 20% 7% 1	2308
	21st to end	Day	55% 6% 12% 18% 9%	2470
May	1st to 10th	Day	54% 7% 11% 20% 7% 1	2533
	11th to 20th	Day	49% 7% 10% 22% 10% 1	2659
	21st to end	Day	51% 6% 12% 22% 8%	3003
June	1st to 10th	Day	51% 7% 11% 22% 9% 1	2880
	11th to 20th	Day	52% 8% 11% 21% 7%	2880
	21st to end	Day	45% 8% 13% 26% 8% 1	2880
July	1st to 10th	Day	46% 7% 14% 25% 8% 1	2775
	11th to 20th	Day	44% 8% 19% 23% 6%	2619
	21st to end	Day	38% 9% 19% 27% 7%	2775
August	1st to 10th	Day	35% 8% 22% 29% 6%	2625
	11th to 20th	Day	34% 7% 23% 30% 6%	2392
	21st to end	Day	47% 9% 18% 22% 4	2574
September	1st to 10th	Day	43% 8% 23% 21% 5%	2214
	11th to 20th	Day	43% 7% 17% 27% 4	2118
	21st to end	Day	53% 8% 15% 19% 4	1925
October	1st to 10th	Day	54% 7% 17% 18% 4	1800
	11th to 20th	Day	57% 7% 18% 17% 2	1792
	21st to end	Day	56% 7% 15% 17% 4	1616
November	1st to 10th	Day	61% 5% 13% 16% 5%	1488
	11th to 20th	Day	67% 8% 10% 11% 3	1497
	21st to end	Day	68% 6% 9% 14% 4	1183
December	1st to 10th	Day	65% 7% 9% 15% 3	1140
	11th to 20th	Day	70% 5% 10% 10% 5%	1444
	21st to end	Day	60% 7% 12% 12% 6% 1	2090
January	1st to 10th	Night	68% 8% 9% 13% 2	4823
	11th to 20th	Night	59% 6% 10% 8% 12% 6%	4740
	21st to end	Night	74% 7% 7% 8% 4	5214
February	1st to 10th	Night	63% 4 10% 11% 10% 1	2681
	11th to 20th	Night	56% 5% 17% 15% 6%	2331
	21st to end	Night	53% 5% 12% 20% 9% 1	1924
March	1st to 10th	Night	55% 5% 12% 18% 10% 1	2178
	11th to 20th	Night	54% 5% 13% 21% 7%	1980
	21st to end	Night	54% 6% 13% 22% 5%	2136
April	1st to 10th	Night	53% 7% 11% 20% 10%	1743
	11th to 20th	Night	54% 6% 11% 22% 7%	1601
	21st to end	Night	46% 7% 13% 23% 11%	1490
May	1st to 10th	Night	55% 7% 14% 18% 7%	1423
	11th to 20th	Night	48% 6% 14% 21% 9% 1	1239
	21st to end	Night	54% 7% 12% 18% 9%	1272
June	1st to 10th	Night	50% 9% 10% 22% 9% 1	1080
	11th to 20th	Night	52% 6% 12% 23% 8%	1080
	21st to end	Night	43% 8% 16% 26% 7%	1080
July	1st to 10th	Night	46% 9% 14% 22% 8% 1	1074
	11th to 20th	Night	42% 6% 21% 22% 7% 1	1218
	21st to end	Night	34% 9% 21% 26% 9%	1302
August	1st to 10th	Night	29% 7% 24% 28% 11%	1289
	11th to 20th	Night	32% 6% 20% 33% 9%	1504
	21st to end	Night	46% 7% 19% 22% 5%	1782
September	1st to 10th	Night	42% 7% 18% 27% 5%	1746
	11th to 20th	Night	41% 7% 15% 30% 8% 1	1840
	21st to end	Night	51% 8% 13% 20% 7%	2035
October	1st to 10th	Night	51% 7% 18% 19% 5%	2160
	11th to 20th	Night	56% 6% 16% 18% 3	2165
	21st to end	Night	55% 6% 14% 21% 4	2740
November	1st to 10th	Night	60% 7% 12% 15% 5%	2590
	11th to 20th	Night	63% 8% 12% 13% 4	2680
	21st to end	Night	64% 6% 12% 13% 4	2977
December	1st to 10th	Night	61% 7% 12% 16% 4	3040
	11th to 20th	Night	70% 6% 11% 10% 3	3747
	21st to end	Night	62% 8% 11% 13% 6%	5566

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% 21% 31% 13% 6%	180
	11th to 20th	Day	26% 8% 21% 25% 17% 3	180
	21st to end	Day	19% 9% 23% 29% 14% 6%	198
February	1st to 10th	Day	22% 15% 27% 20% 10% 5%	265
	11th to 20th	Day	17% 10% 34% 23% 13% 3	348
	21st to end	Day	18% 12% 31% 24% 10% 6%	280
March	1st to 10th	Day	14% 15% 26% 23% 17% 5%	385
	11th to 20th	Day	15% 11% 29% 26% 16% 2	420
	21st to end	Day	20% 14% 24% 24% 14% 5%	462
April	1st to 10th	Day	17% 12% 29% 25% 11% 5%	420
	11th to 20th	Day	18% 10% 29% 23% 12% 8%	412
	21st to end	Day	20% 14% 28% 25% 11% 3	420
May	1st to 10th	Day	20% 11% 28% 24% 15% 2	436
	11th to 20th	Day	17% 6% 24% 29% 18% 6%	625
	21st to end	Day	15% 7% 23% 31% 20% 5%	765
June	1st to 10th	Day	17% 9% 24% 31% 16% 4	810
	11th to 20th	Day	17% 6% 26% 31% 16% 4	810
	21st to end	Day	12% 5% 24% 31% 20% 7%	810
July	1st to 10th	Day	15% 7% 19% 30% 23% 5%	770
	11th to 20th	Day	15% 7% 22% 27% 24% 6%	618
	21st to end	Day	11% 6% 21% 31% 22% 9%	684
August	1st to 10th	Day	12% 5% 16% 30% 25% 11%	583
	11th to 20th	Day	12% 6% 24% 27% 21% 8%	437
	21st to end	Day	14% 8% 27% 26% 16% 7%	495
September	1st to 10th	Day	17% 8% 21% 29% 20% 5%	450
	11th to 20th	Day	18% 5% 22% 24% 22% 9%	450
	21st to end	Day	23% 11% 29% 22% 12% 4	450
October	1st to 10th	Day	22% 10% 21% 25% 17% 5%	450
	11th to 20th	Day	23% 11% 27% 22% 14% 4	446
	21st to end	Day	23% 9% 27% 23% 17% 2	230
November	1st to 10th	Day	29% 11% 23% 23% 10% 4	180
	11th to 20th	Day	28% 4 22% 25% 17% 4	180
	21st to end	Day	22% 8% 26% 25% 10% 9%	180
December	1st to 10th	Day	18% 2 23% 30% 21% 6%	180
	11th to 20th	Day	13% 7% 23% 31% 21% 4	180
	21st to end	Day	27% 3 16% 31% 17% 6%	198
January	1st to 10th	Night	21% 9% 21% 35% 12% 2	908
	11th to 20th	Night	21% 10% 24% 31% 14% 1	850
	21st to end	Night	23% 8% 20% 31% 16% 1	935
February	1st to 10th	Night	24% 9% 20% 30% 15% 3	812
	11th to 20th	Night	17% 7% 23% 36% 14% 3	722
	21st to end	Night	16% 8% 23% 33% 16% 4	592
March	1st to 10th	Night	16% 9% 23% 31% 18% 3	720
	11th to 20th	Night	14% 9% 24% 35% 15% 3	720
	21st to end	Night	17% 8% 19% 27% 25% 5%	792
April	1st to 10th	Night	15% 7% 19% 32% 23% 4	663
	11th to 20th	Night	15% 7% 20% 29% 25% 4	533
	21st to end	Night	16% 7% 26% 28% 22% 1	410
May	1st to 10th	Night	19% 7% 23% 34% 15% 3	360
	11th to 20th	Night	16% 6% 21% 32% 24% 1	354
	21st to end	Night	22% 5% 15% 29% 26% 3	389
June	1st to 10th	Night	16% 6% 23% 34% 18% 2	360
	11th to 20th	Night	18% 4 25% 29% 21% 4	360
	21st to end	Night	13% 4 22% 36% 22% 3	360
July	1st to 10th	Night	18% 3 19% 29% 28% 3	350
	11th to 20th	Night	16% 3 21% 29% 27% 3	348
	21st to end	Night	12% 3 23% 34% 25% 4	394
August	1st to 10th	Night	10% 4 13% 33% 32% 8%	358
	11th to 20th	Night	12% 4 19% 33% 26% 6%	436
	21st to end	Night	18% 3 17% 33% 26% 4	594
September	1st to 10th	Night	15% 4 19% 30% 29% 2	666
	11th to 20th	Night	13% 4 15% 31% 31% 6%	720
	21st to end	Night	23% 7% 21% 32% 15% 3	720
October	1st to 10th	Night	21% 6% 19% 30% 19% 4	720
	11th to 20th	Night	24% 6% 21% 28% 19% 2	720
	21st to end	Night	25% 9% 21% 31% 14% 1	792
November	1st to 10th	Night	24% 9% 22% 27% 16% 2	740
	11th to 20th	Night	21% 10% 24% 32% 12% 1	770
	21st to end	Night	21% 8% 24% 30% 14% 3	977
December	1st to 10th	Night	19% 6% 19% 37% 17% 3	1040
	11th to 20th	Night	20% 8% 21% 34% 15% 2	1040
	21st to end	Night	18% 6% 24% 34% 15% 3	1144

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