Merritt
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Merritt clear sky chart from 2005-11-06 to 2024-01-13.

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	7% 8% 4 8% 7% 4 6% 6% 6% 8% 35%	74598
	Night	7% 9% 4 8% 7% 4 6% 6% 7% 8% 34%	75121
Transparency	Day	45% 10% 16% 19% 9% 2	74289
	Night	46% 11% 18% 17% 7% 1	74949
Seeing	Day	16% 15% 26% 23% 15% 5%	14281
	Night	13% 8% 25% 33% 18% 2	22847

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	9% 14% 5% 12% 10% 7% 8% 7% 6% 8% 15%	1329
	11th to 20th	Day	6% 10% 5% 11% 13% 6% 7% 7% 6% 9% 20%	1361
	21st to end	Day	7% 11% 5% 10% 10% 5% 8% 7% 8% 7% 22%	1584
February	1st to 10th	Day	7% 15% 6% 9% 9% 5% 7% 5% 6% 8% 22%	1520
	11th to 20th	Day	4 13% 5% 9% 8% 4 5% 7% 8% 9% 28%	1575
	21st to end	Day	4 13% 5% 12% 10% 5% 7% 8% 7% 8% 22%	1413
March	1st to 10th	Day	6% 10% 5% 10% 9% 4 7% 7% 7% 8% 27%	1800
	11th to 20th	Day	5% 11% 5% 8% 7% 5% 8% 8% 7% 9% 27%	1932
	21st to end	Day	6% 10% 6% 10% 8% 5% 7% 7% 8% 11% 22%	2309
April	1st to 10th	Day	8% 10% 5% 8% 8% 5% 7% 8% 7% 8% 25%	2160
	11th to 20th	Day	8% 11% 5% 9% 8% 4 7% 7% 7% 9% 27%	2139
	21st to end	Day	7% 10% 4 8% 7% 4 6% 5% 7% 10% 30%	2497
May	1st to 10th	Day	8% 9% 4 7% 7% 4 6% 6% 6% 9% 34%	2520
	11th to 20th	Day	7% 8% 3 8% 8% 4 7% 6% 7% 10% 33%	2477
	21st to end	Day	13% 8% 4 8% 7% 4 6% 5% 6% 8% 33%	2723
June	1st to 10th	Day	12% 8% 3 7% 7% 4 6% 5% 6% 7% 33%	2751
	11th to 20th	Day	12% 8% 3 7% 8% 4 6% 6% 7% 8% 32%	2880
	21st to end	Day	7% 6% 2 6% 7% 4 6% 6% 5% 8% 44%	2815
July	1st to 10th	Day	5% 4 2 5% 5% 4 5% 5% 5% 8% 52%	2700
	11th to 20th	Day	4 3 1 3 4 3 5% 5% 6% 8% 56%	2668
	21st to end	Day	3 2 1 3 3 3 3 4 5% 8% 64%	2741
August	1st to 10th	Day	4 3 1 4 4 3 3 5% 6% 7% 59%	2507
	11th to 20th	Day	4 3 1 4 4 3 4 4 5% 8% 59%	2416
	21st to end	Day	6% 5% 2 5% 5% 3 6% 6% 8% 9% 45%	2453
September	1st to 10th	Day	8% 5% 3 5% 6% 3 4 6% 5% 7% 50%	2160
	11th to 20th	Day	10% 6% 3 6% 6% 4 5% 5% 6% 8% 42%	2160
	21st to end	Day	11% 8% 3 6% 6% 4 5% 6% 6% 8% 36%	1926
October	1st to 10th	Day	11% 9% 4 7% 7% 4 5% 5% 4 6% 38%	1800
	11th to 20th	Day	11% 9% 3 8% 8% 6% 7% 6% 5% 8% 29%	1787
	21st to end	Day	10% 11% 4 9% 7% 5% 7% 7% 6% 9% 27%	1782
November	1st to 10th	Day	13% 11% 6% 9% 8% 4 6% 6% 5% 7% 25%	1524
	11th to 20th	Day	8% 12% 4 10% 8% 5% 7% 6% 6% 7% 27%	1520
	21st to end	Day	8% 12% 6% 8% 10% 5% 7% 6% 8% 8% 22%	1498
December	1st to 10th	Day	6% 12% 5% 11% 12% 7% 8% 7% 6% 7% 20%	1330
	11th to 20th	Day	7% 11% 5% 12% 11% 6% 9% 7% 6% 8% 18%	1518
	21st to end	Day	6% 10% 6% 12% 12% 7% 10% 7% 7% 8% 15%	2323
January	1st to 10th	Night	10% 15% 7% 11% 9% 5% 9% 7% 7% 6% 13%	2836
	11th to 20th	Night	7% 12% 7% 15% 11% 5% 7% 6% 6% 7% 16%	2652
	21st to end	Night	6% 13% 6% 10% 10% 6% 7% 7% 6% 8% 21%	2772
February	1st to 10th	Night	8% 16% 8% 11% 7% 4 5% 5% 6% 8% 23%	2439
	11th to 20th	Night	3 11% 5% 10% 8% 4 7% 6% 7% 7% 31%	2272
	21st to end	Night	3 10% 7% 12% 9% 5% 6% 6% 7% 8% 27%	1842
March	1st to 10th	Night	4 10% 5% 9% 7% 4 6% 6% 6% 9% 32%	2160
	11th to 20th	Night	5% 8% 5% 8% 7% 5% 6% 6% 7% 8% 35%	2025
	21st to end	Night	4 10% 5% 8% 7% 5% 6% 8% 8% 9% 30%	2046
April	1st to 10th	Night	5% 9% 4 8% 8% 4 7% 7% 6% 8% 35%	1800
	11th to 20th	Night	5% 8% 4 8% 9% 4 7% 6% 8% 9% 33%	1771
	21st to end	Night	4 9% 3 6% 6% 3 6% 5% 7% 10% 41%	1462
May	1st to 10th	Night	6% 9% 3 6% 5% 4 5% 5% 7% 7% 43%	1440
	11th to 20th	Night	6% 5% 3 4 6% 3 6% 7% 7% 10% 43%	1416
	21st to end	Night	9% 7% 3 8% 6% 3 5% 5% 6% 7% 42%	1552
June	1st to 10th	Night	9% 6% 3 6% 6% 5% 6% 6% 8% 11% 35%	1202
	11th to 20th	Night	8% 4 2 6% 6% 4 6% 8% 7% 10% 39%	1080
	21st to end	Night	4 4 2 4 5% 3 5% 5% 8% 9% 52%	1139
July	1st to 10th	Night	4 5% 2 4 4 3 5% 5% 6% 8% 55%	1260
	11th to 20th	Night	4 2 2 3 3 2 3 5% 7% 8% 60%	1291
	21st to end	Night	3 1 1 2 3 2 3 4 5% 7% 70%	1576
August	1st to 10th	Night	3 2 2 2 3 1 4 4 5% 7% 67%	1432
	11th to 20th	Night	3 2 1 2 4 3 4 4 5% 6% 65%	1481
	21st to end	Night	4 3 1 4 4 3 6% 6% 8% 9% 50%	1902
September	1st to 10th	Night	5% 3 2 4 4 3 4 5% 5% 6% 58%	1800
	11th to 20th	Night	8% 4 1 5% 5% 4 5% 5% 6% 9% 49%	1800
	21st to end	Night	10% 7% 2 5% 5% 4 4 4 5% 9% 43%	2030
October	1st to 10th	Night	8% 6% 3 6% 6% 5% 6% 6% 6% 6% 43%	2160
	11th to 20th	Night	11% 9% 3 7% 7% 3 5% 5% 6% 7% 36%	2173
	21st to end	Night	9% 10% 3 8% 7% 4 5% 6% 6% 7% 34%	2574
November	1st to 10th	Night	13% 12% 5% 9% 7% 4 6% 6% 7% 7% 24%	2552
	11th to 20th	Night	8% 12% 6% 9% 8% 4 7% 5% 6% 7% 26%	2660
	21st to end	Night	9% 13% 6% 10% 8% 5% 7% 7% 7% 7% 22%	2660
December	1st to 10th	Night	7% 12% 6% 11% 9% 5% 6% 6% 7% 7% 24%	2850
	11th to 20th	Night	8% 17% 7% 14% 10% 5% 6% 6% 6% 5% 15%	3668
	21st to end	Night	6% 13% 6% 12% 11% 7% 9% 7% 7% 6% 16%	5346

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	63% 13% 14% 7% 2	1329
	11th to 20th	Day	57% 14% 16% 10% 2 1	1361
	21st to end	Day	55% 12% 14% 15% 3	1584
February	1st to 10th	Day	59% 12% 12% 11% 6% 1	1520
	11th to 20th	Day	49% 13% 17% 14% 6%	1584
	21st to end	Day	57% 11% 14% 14% 5%	1413
March	1st to 10th	Day	53% 9% 16% 15% 8% 1	1800
	11th to 20th	Day	53% 10% 16% 15% 6%	1932
	21st to end	Day	53% 11% 19% 14% 3	2309
April	1st to 10th	Day	53% 11% 17% 16% 3	2160
	11th to 20th	Day	51% 11% 14% 15% 8% 1	2139
	21st to end	Day	50% 8% 18% 15% 8%	2497
May	1st to 10th	Day	47% 9% 17% 18% 8% 1	2520
	11th to 20th	Day	43% 11% 17% 20% 8% 1	2480
	21st to end	Day	49% 9% 15% 20% 6% 1	2724
June	1st to 10th	Day	49% 8% 14% 17% 8% 4	2751
	11th to 20th	Day	47% 10% 17% 19% 6%	2880
	21st to end	Day	37% 9% 17% 21% 13% 2	2815
July	1st to 10th	Day	30% 7% 15% 25% 18% 4	2626
	11th to 20th	Day	25% 8% 17% 28% 18% 4	2587
	21st to end	Day	18% 5% 14% 31% 24% 7%	2589
August	1st to 10th	Day	22% 8% 18% 25% 21% 5%	2490
	11th to 20th	Day	24% 7% 18% 28% 19% 4	2416
	21st to end	Day	32% 9% 17% 29% 11% 2	2453
September	1st to 10th	Day	33% 7% 16% 26% 14% 3	2160
	11th to 20th	Day	40% 8% 14% 21% 15% 3	2160
	21st to end	Day	43% 11% 19% 18% 8% 2	1926
October	1st to 10th	Day	46% 7% 16% 24% 7%	1800
	11th to 20th	Day	50% 11% 15% 19% 4	1787
	21st to end	Day	50% 13% 16% 17% 4	1782
November	1st to 10th	Day	58% 10% 14% 14% 3 1	1524
	11th to 20th	Day	54% 11% 17% 13% 4 1	1520
	21st to end	Day	55% 13% 14% 13% 5%	1498
December	1st to 10th	Day	59% 13% 14% 12% 3	1330
	11th to 20th	Day	57% 17% 14% 8% 4	1520
	21st to end	Day	59% 18% 14% 9% 1	2323
January	1st to 10th	Night	64% 13% 13% 8% 2	2836
	11th to 20th	Night	64% 12% 14% 9% 1 1	2652
	21st to end	Night	57% 13% 13% 13% 3	2772
February	1st to 10th	Night	59% 9% 14% 12% 5%	2439
	11th to 20th	Night	49% 10% 18% 16% 6%	2274
	21st to end	Night	53% 10% 15% 16% 6%	1842
March	1st to 10th	Night	47% 12% 19% 14% 7% 1	2160
	11th to 20th	Night	43% 11% 20% 20% 5%	2026
	21st to end	Night	48% 11% 22% 15% 4	2046
April	1st to 10th	Night	44% 12% 22% 17% 5%	1800
	11th to 20th	Night	43% 12% 20% 17% 8%	1771
	21st to end	Night	38% 11% 22% 21% 8%	1462
May	1st to 10th	Night	39% 10% 23% 19% 9% 1	1440
	11th to 20th	Night	34% 12% 25% 21% 9% 1	1416
	21st to end	Night	41% 9% 20% 22% 8%	1552
June	1st to 10th	Night	41% 9% 19% 21% 7% 3	1202
	11th to 20th	Night	39% 9% 26% 20% 7%	1080
	21st to end	Night	28% 7% 27% 22% 13% 2	1139
July	1st to 10th	Night	26% 8% 17% 29% 16% 4	1225
	11th to 20th	Night	20% 7% 21% 32% 18% 2	1249
	21st to end	Night	13% 6% 19% 30% 25% 8%	1488
August	1st to 10th	Night	17% 7% 21% 28% 22% 5%	1424
	11th to 20th	Night	19% 7% 22% 28% 19% 5%	1481
	21st to end	Night	25% 10% 21% 27% 14% 2	1902
September	1st to 10th	Night	26% 8% 20% 30% 14% 3	1800
	11th to 20th	Night	31% 9% 17% 21% 16% 5%	1800
	21st to end	Night	38% 9% 21% 19% 10% 2	2030
October	1st to 10th	Night	39% 10% 18% 24% 9%	2160
	11th to 20th	Night	47% 9% 16% 21% 8%	2173
	21st to end	Night	47% 9% 18% 20% 6%	2574
November	1st to 10th	Night	56% 11% 15% 12% 5%	2552
	11th to 20th	Night	54% 13% 15% 12% 5% 1	2660
	21st to end	Night	56% 13% 16% 11% 5%	2660
December	1st to 10th	Night	56% 10% 15% 14% 4	2850
	11th to 20th	Night	67% 11% 12% 8% 2	3666
	21st to end	Night	58% 16% 13% 11% 2	5346

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	18% 11% 24% 26% 13% 9%	180
	11th to 20th	Day	15% 5% 24% 33% 20% 4	164
	21st to end	Day	20% 6% 25% 31% 15% 3	176
February	1st to 10th	Day	20% 16% 30% 26% 8%	240
	11th to 20th	Day	17% 17% 28% 26% 10% 2	335
	21st to end	Day	14% 19% 29% 23% 12% 3	317
March	1st to 10th	Day	20% 20% 26% 23% 10% 2	420
	11th to 20th	Day	19% 19% 25% 18% 14% 6%	420
	21st to end	Day	19% 24% 28% 15% 11% 3	462
April	1st to 10th	Day	24% 22% 24% 20% 8% 3	420
	11th to 20th	Day	18% 25% 25% 18% 10% 5%	412
	21st to end	Day	20% 22% 29% 14% 12% 3	420
May	1st to 10th	Day	16% 19% 29% 22% 11% 3	423
	11th to 20th	Day	17% 13% 23% 24% 16% 6%	448
	21st to end	Day	15% 14% 31% 23% 13% 5%	485
June	1st to 10th	Day	17% 22% 28% 19% 12% 2	681
	11th to 20th	Day	17% 18% 27% 22% 14% 2	810
	21st to end	Day	12% 15% 28% 22% 18% 5%	745
July	1st to 10th	Day	9% 15% 32% 27% 13% 4	621
	11th to 20th	Day	8% 17% 31% 26% 13% 5%	586
	21st to end	Day	6% 10% 25% 28% 22% 9%	487
August	1st to 10th	Day	6% 7% 25% 27% 23% 11%	447
	11th to 20th	Day	7% 11% 24% 26% 22% 10%	437
	21st to end	Day	11% 13% 30% 23% 16% 8%	495
September	1st to 10th	Day	14% 14% 23% 26% 18% 6%	450
	11th to 20th	Day	15% 16% 20% 24% 20% 5%	450
	21st to end	Day	20% 14% 25% 22% 15% 5%	450
October	1st to 10th	Day	20% 13% 21% 21% 14% 11%	450
	11th to 20th	Day	22% 17% 17% 20% 17% 6%	437
	21st to end	Day	24% 13% 21% 26% 12% 4	297
November	1st to 10th	Day	24% 7% 25% 29% 13% 2	198
	11th to 20th	Day	23% 13% 22% 22% 14% 6%	180
	21st to end	Day	18% 10% 27% 23% 15% 7%	180
December	1st to 10th	Day	17% 5% 17% 32% 22% 7%	180
	11th to 20th	Day	16% 6% 27% 32% 14% 5%	180
	21st to end	Day	15% 8% 22% 39% 14% 3	198
January	1st to 10th	Night	22% 9% 27% 31% 10% 1	850
	11th to 20th	Night	16% 7% 28% 33% 14% 2	774
	21st to end	Night	15% 7% 30% 33% 13% 2	792
February	1st to 10th	Night	19% 8% 30% 32% 10% 2	720
	11th to 20th	Night	11% 8% 26% 39% 15% 1	704
	21st to end	Night	8% 9% 30% 33% 18% 2	592
March	1st to 10th	Night	14% 10% 27% 29% 18% 2	720
	11th to 20th	Night	12% 16% 27% 28% 16% 2	720
	21st to end	Night	11% 16% 26% 28% 16% 3	792
April	1st to 10th	Night	12% 12% 21% 36% 17% 2	720
	11th to 20th	Night	12% 12% 26% 31% 16% 2	703
	21st to end	Night	10% 11% 24% 37% 16% 3	382
May	1st to 10th	Night	11% 8% 27% 35% 18% 2	360
	11th to 20th	Night	10% 4 21% 34% 28% 3	354
	21st to end	Night	15% 2 23% 36% 23% 2	389
June	1st to 10th	Night	11% 9% 28% 37% 14% 1	360
	11th to 20th	Night	10% 5% 29% 35% 19% 1	360
	21st to end	Night	5% 6% 26% 38% 23% 3	360
July	1st to 10th	Night	7% 4 22% 42% 23% 2	350
	11th to 20th	Night	6% 6% 24% 39% 23% 1	348
	21st to end	Night	3 3 16% 45% 28% 4	394
August	1st to 10th	Night	5% 3 12% 38% 38% 4	358
	11th to 20th	Night	6% 3 14% 37% 38% 4	413
	21st to end	Night	7% 4 21% 36% 29% 3	714
September	1st to 10th	Night	8% 6% 20% 35% 27% 4	720
	11th to 20th	Night	10% 7% 18% 27% 32% 7%	720
	21st to end	Night	16% 9% 21% 30% 21% 4	720
October	1st to 10th	Night	12% 7% 19% 33% 24% 4	720
	11th to 20th	Night	19% 10% 21% 30% 19% 2	720
	21st to end	Night	17% 11% 27% 29% 13% 2	792
November	1st to 10th	Night	22% 11% 26% 33% 8% 1	740
	11th to 20th	Night	18% 11% 30% 27% 12% 2	760
	21st to end	Night	19% 9% 27% 30% 13% 2	760
December	1st to 10th	Night	13% 7% 25% 37% 16% 2	950
	11th to 20th	Night	16% 9% 30% 34% 10% 1	1001
	21st to end	Night	15% 8% 28% 37% 10% 1	1015

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