Courtenay
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Courtenay clear sky chart from 2005-11-06 to 2024-02-05.

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% 10% 4 7% 6% 4 5% 5% 5% 6% 30%	77694
	Night	20% 11% 4 7% 6% 4 5% 5% 5% 6% 29%	81118
Transparency	Day	53% 5% 16% 18% 7% 1	77396
	Night	57% 4 14% 17% 7% 1	80996
Seeing	Day	20% 7% 21% 26% 19% 8%	14613
	Night	25% 4 14% 29% 22% 6%	22796

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	26% 21% 5% 7% 7% 4 4 4 4 5% 14%	2219
	11th to 20th	Day	21% 12% 5% 8% 7% 4 6% 4 4 8% 22%	2199
	21st to end	Day	39% 18% 4 6% 5% 3 3 3 3 4 10%	2666
February	1st to 10th	Day	21% 16% 6% 8% 7% 3 6% 6% 5% 6% 17%	1657
	11th to 20th	Day	17% 14% 5% 8% 7% 4 5% 5% 4 6% 26%	1575
	21st to end	Day	17% 14% 4 7% 8% 5% 7% 6% 6% 6% 21%	1332
March	1st to 10th	Day	19% 13% 4 7% 7% 4 6% 7% 5% 6% 21%	1882
	11th to 20th	Day	19% 13% 4 8% 7% 5% 6% 5% 6% 7% 20%	1980
	21st to end	Day	18% 12% 5% 10% 8% 5% 7% 6% 4 5% 21%	2178
April	1st to 10th	Day	19% 13% 5% 8% 8% 5% 5% 5% 5% 6% 20%	2163
	11th to 20th	Day	16% 12% 4 9% 7% 5% 7% 6% 6% 6% 22%	2308
	21st to end	Day	18% 11% 5% 9% 6% 4 5% 5% 5% 6% 26%	2340
May	1st to 10th	Day	14% 7% 3 8% 8% 4 6% 6% 6% 7% 31%	2502
	11th to 20th	Day	13% 9% 4 7% 6% 4 6% 6% 6% 8% 29%	2479
	21st to end	Day	16% 7% 4 7% 7% 4 7% 6% 6% 7% 29%	2917
June	1st to 10th	Day	17% 9% 4 8% 9% 4 6% 6% 5% 7% 25%	2700
	11th to 20th	Day	14% 9% 4 9% 9% 6% 7% 6% 8% 7% 22%	2700
	21st to end	Day	14% 6% 4 8% 6% 3 6% 5% 6% 6% 35%	2700
July	1st to 10th	Day	10% 6% 3 7% 7% 3 5% 5% 6% 7% 42%	2700
	11th to 20th	Day	7% 4 2 5% 5% 4 6% 5% 6% 8% 48%	2700
	21st to end	Day	6% 3 2 4 5% 3 5% 5% 6% 7% 55%	2925
August	1st to 10th	Day	9% 5% 2 4 4 3 5% 5% 5% 7% 52%	2404
	11th to 20th	Day	5% 4 2 3 4 3 5% 5% 7% 9% 53%	2298
	21st to end	Day	9% 5% 2 6% 6% 4 5% 5% 7% 9% 42%	2574
September	1st to 10th	Day	9% 4 2 5% 5% 3 4 5% 6% 7% 50%	2179
	11th to 20th	Day	16% 6% 2 6% 6% 3 4 4 5% 6% 40%	2062
	21st to end	Day	16% 7% 3 6% 6% 3 5% 5% 5% 8% 36%	1959
October	1st to 10th	Day	21% 7% 3 6% 5% 3 5% 5% 5% 5% 35%	1800
	11th to 20th	Day	24% 9% 4 7% 6% 3 5% 5% 4 7% 26%	1800
	21st to end	Day	25% 10% 3 7% 6% 3 6% 4 5% 5% 27%	1802
November	1st to 10th	Day	27% 11% 3 7% 7% 5% 7% 5% 5% 6% 19%	1488
	11th to 20th	Day	21% 14% 4 8% 6% 3 5% 6% 5% 5% 23%	1520
	21st to end	Day	24% 11% 4 8% 7% 3 5% 5% 5% 6% 21%	1520
December	1st to 10th	Day	19% 13% 5% 6% 6% 4 6% 5% 5% 6% 24%	1339
	11th to 20th	Day	27% 17% 4 8% 6% 3 5% 4 4 4 16%	1685
	21st to end	Day	32% 16% 6% 8% 7% 3 5% 4 4 3 12%	2442
January	1st to 10th	Night	25% 17% 4 8% 6% 4 5% 4 4 5% 17%	4726
	11th to 20th	Night	21% 14% 4 7% 7% 3 5% 4 4 6% 24%	4699
	21st to end	Night	37% 17% 4 7% 6% 3 4 4 4 4 12%	4989
February	1st to 10th	Night	19% 15% 4 8% 7% 3 6% 6% 4 6% 20%	2397
	11th to 20th	Night	17% 12% 3 6% 6% 3 5% 5% 5% 7% 30%	2272
	21st to end	Night	15% 15% 5% 7% 6% 4 5% 5% 5% 5% 28%	1924
March	1st to 10th	Night	17% 12% 3 7% 8% 4 6% 6% 6% 7% 25%	2077
	11th to 20th	Night	17% 10% 4 7% 7% 4 6% 5% 7% 7% 26%	1979
	21st to end	Night	16% 11% 5% 8% 8% 5% 7% 6% 6% 6% 23%	2178
April	1st to 10th	Night	17% 11% 3 7% 6% 4 6% 5% 7% 6% 26%	1792
	11th to 20th	Night	15% 11% 4 6% 8% 5% 6% 5% 6% 8% 27%	1601
	21st to end	Night	16% 8% 4 7% 7% 4 6% 4 6% 7% 31%	1620
May	1st to 10th	Night	14% 8% 4 5% 5% 4 6% 6% 5% 6% 38%	1457
	11th to 20th	Night	12% 8% 4 7% 8% 5% 6% 5% 7% 9% 30%	1412
	21st to end	Night	13% 9% 4 7% 7% 4 6% 6% 6% 7% 33%	1358
June	1st to 10th	Night	15% 8% 3 7% 7% 4 6% 5% 7% 8% 30%	1260
	11th to 20th	Night	14% 7% 3 9% 7% 5% 7% 5% 5% 8% 29%	1260
	21st to end	Night	13% 6% 3 6% 6% 3 5% 5% 6% 8% 39%	1260
July	1st to 10th	Night	8% 5% 2 6% 5% 4 5% 5% 5% 7% 47%	1260
	11th to 20th	Night	6% 4 2 4 3 2 4 6% 6% 8% 54%	1260
	21st to end	Night	6% 3 2 5% 3 2 5% 5% 4 6% 59%	1391
August	1st to 10th	Night	10% 5% 2 5% 5% 2 3 4 4 5% 55%	1532
	11th to 20th	Night	5% 3 1 4 3 3 4 4 6% 8% 57%	1602
	21st to end	Night	8% 6% 2 5% 6% 3 5% 6% 6% 8% 46%	1782
September	1st to 10th	Night	9% 5% 2 4 5% 3 4 4 4 8% 53%	1780
	11th to 20th	Night	14% 8% 3 5% 6% 3 4 5% 5% 6% 41%	1895
	21st to end	Night	15% 7% 2 5% 6% 3 4 5% 4 7% 41%	2000
October	1st to 10th	Night	16% 6% 2 5% 6% 3 4 4 6% 6% 41%	2160
	11th to 20th	Night	22% 9% 3 6% 6% 3 5% 5% 5% 6% 31%	2160
	21st to end	Night	23% 9% 3 7% 5% 3 4 4 4 6% 31%	2551
November	1st to 10th	Night	27% 13% 4 6% 6% 4 5% 5% 5% 6% 20%	2590
	11th to 20th	Night	21% 11% 4 7% 7% 4 6% 5% 5% 6% 24%	2660
	21st to end	Night	24% 10% 4 6% 7% 4 6% 5% 5% 5% 25%	2660
December	1st to 10th	Night	20% 13% 5% 7% 6% 4 5% 5% 5% 6% 24%	2840
	11th to 20th	Night	28% 15% 5% 9% 7% 4 5% 4 4 4 14%	3520
	21st to end	Night	34% 15% 4 7% 5% 3 4 4 4 4 16%	5214

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	76% 2 5% 13% 3 1	2219
	11th to 20th	Day	66% 3 12% 11% 6% 1	2220
	21st to end	Day	82% 2 7% 7% 2	2666
February	1st to 10th	Day	67% 5% 10% 11% 6% 1	1657
	11th to 20th	Day	62% 3 14% 16% 5%	1584
	21st to end	Day	61% 6% 13% 12% 7% 2	1332
March	1st to 10th	Day	61% 5% 13% 16% 6% 1	1882
	11th to 20th	Day	63% 4 14% 14% 4 1	1980
	21st to end	Day	62% 4 14% 16% 3	2178
April	1st to 10th	Day	60% 6% 14% 16% 4	2163
	11th to 20th	Day	57% 5% 12% 18% 8% 1	2308
	21st to end	Day	57% 4 12% 18% 9% 1	2340
May	1st to 10th	Day	48% 6% 19% 21% 6%	2502
	11th to 20th	Day	48% 5% 19% 21% 6% 1	2482
	21st to end	Day	47% 6% 20% 22% 5%	2918
June	1st to 10th	Day	53% 6% 16% 18% 6% 2	2700
	11th to 20th	Day	53% 8% 18% 16% 4 1	2700
	21st to end	Day	45% 5% 21% 21% 7% 1	2700
July	1st to 10th	Day	38% 5% 22% 23% 11% 1	2626
	11th to 20th	Day	29% 6% 26% 32% 7%	2619
	21st to end	Day	23% 4 24% 34% 14% 2	2762
August	1st to 10th	Day	29% 4 24% 31% 11% 1	2387
	11th to 20th	Day	25% 7% 28% 26% 13% 2	2298
	21st to end	Day	36% 5% 26% 26% 7% 1	2574
September	1st to 10th	Day	31% 6% 24% 27% 10% 2	2179
	11th to 20th	Day	43% 4 16% 21% 14% 1	2062
	21st to end	Day	46% 5% 18% 21% 9% 1	1959
October	1st to 10th	Day	51% 5% 18% 17% 9% 1	1800
	11th to 20th	Day	59% 5% 17% 16% 3	1800
	21st to end	Day	61% 3 11% 16% 8%	1802
November	1st to 10th	Day	67% 4 11% 12% 6% 1	1488
	11th to 20th	Day	65% 5% 9% 14% 6% 1	1520
	21st to end	Day	66% 4 12% 11% 6% 1	1520
December	1st to 10th	Day	63% 4 15% 12% 6% 1	1339
	11th to 20th	Day	75% 4 9% 8% 4 1	1688
	21st to end	Day	79% 2 8% 8% 3 1	2442
January	1st to 10th	Night	73% 2 8% 12% 5%	4726
	11th to 20th	Night	64% 3 13% 10% 8% 2	4740
	21st to end	Night	78% 2 7% 10% 2	4989
February	1st to 10th	Night	65% 4 11% 12% 7% 1	2397
	11th to 20th	Night	56% 4 14% 18% 7% 1	2274
	21st to end	Night	59% 5% 10% 16% 8% 2	1924
March	1st to 10th	Night	57% 4 14% 17% 8%	2077
	11th to 20th	Night	55% 4 16% 17% 7% 1	1980
	21st to end	Night	61% 4 14% 16% 5%	2178
April	1st to 10th	Night	54% 5% 15% 19% 6%	1792
	11th to 20th	Night	54% 4 12% 20% 9% 1	1601
	21st to end	Night	52% 3 13% 23% 9% 1	1620
May	1st to 10th	Night	46% 4 15% 25% 9%	1457
	11th to 20th	Night	51% 4 19% 19% 7% 1	1412
	21st to end	Night	48% 5% 18% 22% 6%	1358
June	1st to 10th	Night	51% 5% 19% 18% 5% 2	1260
	11th to 20th	Night	53% 4 20% 17% 6%	1260
	21st to end	Night	42% 5% 22% 22% 7% 2	1260
July	1st to 10th	Night	36% 4 22% 26% 11% 2	1225
	11th to 20th	Night	26% 5% 30% 27% 10% 1	1218
	21st to end	Night	23% 3 20% 36% 14% 3	1314
August	1st to 10th	Night	31% 5% 16% 34% 11% 2	1524
	11th to 20th	Night	24% 5% 26% 27% 15% 3	1602
	21st to end	Night	35% 5% 24% 30% 6% 1	1782
September	1st to 10th	Night	30% 4 19% 32% 13% 2	1780
	11th to 20th	Night	44% 4 14% 19% 16% 2	1895
	21st to end	Night	44% 3 19% 21% 10% 2	2000
October	1st to 10th	Night	43% 4 17% 23% 12% 1	2160
	11th to 20th	Night	53% 5% 17% 20% 6%	2160
	21st to end	Night	55% 4 15% 18% 8%	2551
November	1st to 10th	Night	66% 3 12% 12% 5% 1	2590
	11th to 20th	Night	62% 5% 14% 12% 6% 1	2660
	21st to end	Night	63% 4 12% 14% 6% 1	2660
December	1st to 10th	Night	64% 4 13% 14% 5%	2840
	11th to 20th	Night	75% 3 9% 8% 3 1	3516
	21st to end	Night	74% 2 7% 12% 4 1	5214

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	38% 8% 21% 19% 7% 7%	180
	11th to 20th	Day	39% 6% 19% 18% 15% 2	180
	21st to end	Day	32% 7% 22% 20% 16% 3	293
February	1st to 10th	Day	31% 8% 27% 22% 10% 2	353
	11th to 20th	Day	25% 9% 27% 24% 13% 2	335
	21st to end	Day	30% 9% 26% 20% 11% 5%	280
March	1st to 10th	Day	24% 12% 23% 23% 14% 4	394
	11th to 20th	Day	29% 8% 25% 20% 14% 3	420
	21st to end	Day	26% 11% 24% 22% 13% 4	462
April	1st to 10th	Day	23% 13% 29% 20% 11% 5%	420
	11th to 20th	Day	22% 11% 23% 23% 15% 5%	412
	21st to end	Day	24% 11% 24% 23% 12% 6%	420
May	1st to 10th	Day	17% 8% 23% 28% 19% 5%	423
	11th to 20th	Day	18% 5% 18% 28% 22% 9%	450
	21st to end	Day	15% 4 19% 33% 23% 6%	680
June	1st to 10th	Day	20% 8% 21% 22% 24% 5%	630
	11th to 20th	Day	17% 7% 25% 27% 20% 5%	630
	21st to end	Day	15% 6% 18% 29% 25% 7%	630
July	1st to 10th	Day	11% 3 18% 31% 28% 9%	621
	11th to 20th	Day	7% 4 21% 32% 25% 10%	618
	21st to end	Day	6% 4 15% 29% 31% 15%	671
August	1st to 10th	Day	12% 1 13% 28% 27% 19%	447
	11th to 20th	Day	7% 3 14% 32% 29% 15%	437
	21st to end	Day	11% 4 21% 34% 21% 10%	495
September	1st to 10th	Day	12% 5% 20% 31% 22% 9%	450
	11th to 20th	Day	20% 6% 18% 24% 21% 10%	450
	21st to end	Day	20% 8% 21% 23% 21% 7%	450
October	1st to 10th	Day	25% 7% 17% 22% 20% 9%	450
	11th to 20th	Day	28% 5% 22% 19% 18% 8%	450
	21st to end	Day	29% 10% 20% 24% 11% 5%	384
November	1st to 10th	Day	28% 9% 22% 23% 15% 3	180
	11th to 20th	Day	30% 7% 15% 18% 18% 12%	180
	21st to end	Day	36% 4 21% 17% 11% 11%	180
December	1st to 10th	Day	33% 3 17% 21% 14% 12%	180
	11th to 20th	Day	31% 4 14% 28% 17% 6%	180
	21st to end	Day	37% 3 20% 25% 13% 2	198
January	1st to 10th	Night	40% 7% 18% 21% 10% 4	850
	11th to 20th	Night	36% 6% 11% 28% 16% 3	850
	21st to end	Night	36% 3 15% 24% 17% 5%	915
February	1st to 10th	Night	29% 5% 17% 32% 14% 2	738
	11th to 20th	Night	27% 4 17% 33% 17% 2	704
	21st to end	Night	26% 6% 20% 29% 16% 4	592
March	1st to 10th	Night	24% 5% 22% 26% 17% 5%	720
	11th to 20th	Night	25% 7% 21% 28% 16% 4	720
	21st to end	Night	24% 6% 17% 27% 20% 6%	792
April	1st to 10th	Night	23% 7% 14% 32% 20% 5%	702
	11th to 20th	Night	20% 5% 18% 36% 19% 2	533
	21st to end	Night	22% 4 18% 33% 20% 4	540
May	1st to 10th	Night	19% 3 12% 36% 24% 5%	377
	11th to 20th	Night	16% 3 12% 33% 27% 10%	354
	21st to end	Night	19% 3 13% 30% 30% 5%	389
June	1st to 10th	Night	17% 3 13% 37% 26% 4	360
	11th to 20th	Night	19% 3 15% 33% 24% 6%	360
	21st to end	Night	18% 4 10% 34% 28% 7%	360
July	1st to 10th	Night	9% 2 8% 29% 42% 10%	350
	11th to 20th	Night	9% 2 10% 32% 37% 9%	348
	21st to end	Night	7% 1 4 30% 43% 14%	394
August	1st to 10th	Night	12% 2 4 24% 42% 17%	458
	11th to 20th	Night	6% 1 5% 28% 51% 10%	534
	21st to end	Night	11% 3 10% 30% 39% 7%	594
September	1st to 10th	Night	12% 2 7% 35% 34% 10%	700
	11th to 20th	Night	17% 3 11% 21% 31% 17%	720
	21st to end	Night	22% 5% 12% 28% 25% 8%	720
October	1st to 10th	Night	21% 3 11% 30% 29% 6%	720
	11th to 20th	Night	27% 3 13% 28% 22% 8%	720
	21st to end	Night	29% 5% 18% 28% 17% 4	792
November	1st to 10th	Night	36% 4 16% 29% 13% 1	740
	11th to 20th	Night	30% 4 18% 30% 15% 3	760
	21st to end	Night	31% 4 18% 26% 16% 5%	760
December	1st to 10th	Night	31% 6% 13% 27% 19% 4	845
	11th to 20th	Night	39% 3 16% 27% 12% 3	850
	21st to end	Night	38% 5% 20% 26% 11% 1	935

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