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 2025-12-18.

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	6% 9% 3 8% 6% 6% 6% 5% 6% 8% 37%	130770
	Night	6% 10% 4 9% 6% 6% 6% 5% 7% 8% 35%	128951
Transparency	Day	42% 8% 16% 21% 10% 2	130484
	Night	45% 9% 18% 19% 8% 1	128815
Seeing	Day	16% 15% 26% 23% 15% 5%	16842
	Night	13% 9% 25% 33% 18% 2	25254

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% 5% 15% 7% 7% 6% 4 7% 7% 14%	3179
	11th to 20th	Day	4 11% 4 9% 7% 7% 6% 5% 8% 11% 28%	3384
	21st to end	Day	7% 13% 6% 13% 8% 7% 7% 5% 5% 6% 25%	4004
February	1st to 10th	Day	6% 15% 6% 11% 8% 7% 7% 4 7% 8% 22%	1689
	11th to 20th	Day	3 13% 5% 10% 7% 6% 6% 5% 8% 8% 29%	1755
	21st to end	Day	4 15% 4 14% 9% 6% 8% 5% 7% 8% 21%	1576
March	1st to 10th	Day	3 9% 4 12% 7% 8% 8% 6% 8% 8% 27%	4518
	11th to 20th	Day	3 10% 4 9% 6% 7% 8% 7% 8% 9% 31%	4900
	21st to end	Day	8% 11% 4 11% 7% 6% 7% 5% 7% 9% 24%	5824
April	1st to 10th	Day	8% 11% 5% 9% 7% 7% 7% 7% 8% 8% 25%	2400
	11th to 20th	Day	7% 11% 4 9% 7% 6% 7% 5% 7% 8% 29%	2381
	21st to end	Day	7% 11% 4 9% 6% 6% 5% 4 7% 9% 31%	2775
May	1st to 10th	Day	6% 9% 3 8% 5% 5% 5% 4 6% 7% 44%	6320
	11th to 20th	Day	6% 9% 4 10% 6% 7% 6% 5% 8% 11% 28%	6277
	21st to end	Day	9% 10% 4 8% 5% 5% 5% 4 7% 9% 34%	6903
June	1st to 10th	Day	11% 8% 3 7% 6% 6% 6% 5% 7% 7% 34%	3058
	11th to 20th	Day	11% 9% 3 8% 6% 5% 6% 5% 7% 8% 32%	3200
	21st to end	Day	7% 6% 2 7% 6% 6% 6% 5% 6% 8% 43%	3127
July	1st to 10th	Day	4 4 1 4 3 4 3 3 5% 7% 62%	6780
	11th to 20th	Day	4 3 1 3 3 3 3 3 5% 7% 67%	6692
	21st to end	Day	4 3 2 4 3 3 3 3 5% 8% 62%	6874
August	1st to 10th	Day	2 2 1 3 3 3 4 4 6% 8% 62%	4541
	11th to 20th	Day	3 4 1 6% 4 5% 5% 4 7% 9% 51%	4355
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 7% 9% 47%	4445
September	1st to 10th	Day	7% 5% 2 6% 4 4 4 4 5% 7% 52%	2400
	11th to 20th	Day	9% 6% 3 6% 5% 5% 5% 3 6% 7% 44%	2400
	21st to end	Day	11% 8% 3 7% 5% 5% 5% 5% 7% 8% 35%	2139
October	1st to 10th	Day	10% 8% 3 7% 5% 6% 5% 4 5% 7% 40%	3280
	11th to 20th	Day	16% 10% 3 8% 6% 6% 5% 4 5% 7% 32%	3234
	21st to end	Day	9% 12% 4 11% 7% 6% 5% 5% 7% 9% 27%	3200
November	1st to 10th	Day	12% 13% 5% 10% 7% 5% 6% 5% 6% 7% 24%	1688
	11th to 20th	Day	8% 13% 4 11% 8% 7% 7% 4 6% 7% 25%	1680
	21st to end	Day	8% 14% 4 11% 9% 6% 7% 5% 7% 8% 22%	1655
December	1st to 10th	Day	5% 11% 4 12% 10% 9% 7% 6% 7% 7% 22%	2360
	11th to 20th	Day	6% 15% 5% 14% 9% 7% 8% 6% 6% 7% 16%	2440
	21st to end	Day	6% 13% 6% 15% 10% 9% 9% 5% 6% 7% 13%	3337
January	1st to 10th	Night	7% 18% 7% 15% 10% 7% 7% 5% 7% 6% 11%	6766
	11th to 20th	Night	4 11% 5% 11% 6% 5% 6% 5% 6% 10% 30%	6511
	21st to end	Night	8% 13% 6% 12% 8% 7% 6% 5% 5% 7% 24%	6952
February	1st to 10th	Night	7% 17% 7% 12% 7% 5% 6% 4 6% 7% 22%	2710
	11th to 20th	Night	3 12% 5% 11% 7% 5% 6% 5% 7% 7% 32%	2532
	21st to end	Night	4 12% 6% 13% 8% 6% 5% 5% 7% 8% 25%	2053
March	1st to 10th	Night	2 9% 4 9% 6% 6% 6% 6% 8% 13% 31%	5376
	11th to 20th	Night	3 7% 3 9% 6% 6% 6% 5% 8% 10% 37%	5097
	21st to end	Night	8% 10% 4 9% 6% 7% 7% 6% 8% 7% 26%	5150
April	1st to 10th	Night	5% 10% 4 9% 7% 5% 6% 5% 6% 8% 34%	2000
	11th to 20th	Night	4 9% 4 8% 8% 5% 7% 5% 8% 8% 34%	1969
	21st to end	Night	4 10% 3 7% 5% 5% 6% 4 7% 10% 40%	1624
May	1st to 10th	Night	5% 8% 3 8% 5% 5% 5% 3 5% 6% 48%	3640
	11th to 20th	Night	4 8% 4 7% 5% 5% 6% 6% 8% 10% 37%	3616
	21st to end	Night	5% 5% 3 9% 5% 6% 5% 5% 6% 8% 43%	3972
June	1st to 10th	Night	9% 6% 3 7% 5% 6% 5% 5% 8% 10% 36%	1334
	11th to 20th	Night	7% 6% 2 7% 5% 5% 6% 6% 7% 9% 39%	1200
	21st to end	Night	5% 5% 1 5% 5% 3 6% 4 7% 9% 50%	1267
July	1st to 10th	Night	4 3 1 3 3 3 4 3 6% 7% 64%	3180
	11th to 20th	Night	3 2 1 3 2 2 2 3 5% 6% 71%	3267
	21st to end	Night	3 2 1 3 3 3 4 3 6% 7% 64%	3969
August	1st to 10th	Night	2 2 2 3 3 3 5% 4 7% 8% 60%	2609
	11th to 20th	Night	3 3 1 4 4 5% 6% 5% 7% 10% 53%	2693
	21st to end	Night	6% 4 1 6% 4 4 5% 5% 7% 9% 50%	3470
September	1st to 10th	Night	5% 3 1 4 3 4 4 4 6% 6% 60%	2000
	11th to 20th	Night	7% 5% 1 6% 4 5% 4 5% 6% 8% 50%	2000
	21st to end	Night	10% 7% 2 7% 4 5% 4 3 5% 9% 43%	2256
October	1st to 10th	Night	8% 7% 3 7% 5% 6% 5% 4 6% 7% 44%	3920
	11th to 20th	Night	14% 8% 3 7% 5% 5% 4 5% 7% 8% 34%	3923
	21st to end	Night	8% 10% 3 10% 6% 6% 5% 4 6% 7% 35%	4600
November	1st to 10th	Night	12% 12% 4 10% 6% 6% 6% 5% 7% 7% 25%	2828
	11th to 20th	Night	8% 14% 5% 11% 7% 6% 6% 4 6% 7% 25%	2940
	21st to end	Night	8% 15% 5% 11% 7% 6% 7% 6% 6% 7% 22%	2940
December	1st to 10th	Night	7% 12% 3 9% 7% 7% 6% 5% 8% 8% 27%	5040
	11th to 20th	Night	7% 17% 6% 15% 9% 7% 6% 4 6% 6% 16%	5901
	21st to end	Night	6% 14% 5% 14% 9% 9% 8% 5% 7% 6% 16%	7646

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% 11% 14% 10% 2	3179
	11th to 20th	Day	47% 9% 21% 17% 4 3	3407
	21st to end	Day	57% 11% 14% 14% 4	4004
February	1st to 10th	Day	60% 11% 11% 11% 7% 1	1689
	11th to 20th	Day	48% 12% 17% 15% 7%	1764
	21st to end	Day	57% 11% 13% 13% 5%	1576
March	1st to 10th	Day	49% 8% 15% 16% 11%	4518
	11th to 20th	Day	43% 10% 15% 23% 8%	4900
	21st to end	Day	53% 9% 15% 15% 8%	5824
April	1st to 10th	Day	52% 12% 17% 17% 3	2400
	11th to 20th	Day	49% 11% 14% 16% 9% 2	2381
	21st to end	Day	48% 8% 18% 16% 9% 1	2775
May	1st to 10th	Day	36% 9% 11% 30% 13% 1	6320
	11th to 20th	Day	43% 11% 21% 20% 5%	6280
	21st to end	Day	43% 10% 18% 23% 4 1	6904
June	1st to 10th	Day	46% 8% 14% 18% 9% 4	3058
	11th to 20th	Day	47% 9% 16% 20% 8% 1	3200
	21st to end	Day	38% 8% 17% 22% 13% 2	3127
July	1st to 10th	Day	23% 4 14% 23% 24% 13%	6706
	11th to 20th	Day	18% 4 16% 26% 26% 10%	6611
	21st to end	Day	22% 3 19% 31% 19% 6%	6681
August	1st to 10th	Day	19% 6% 13% 28% 29% 5%	4524
	11th to 20th	Day	30% 5% 24% 28% 11% 2	4397
	21st to end	Day	33% 6% 20% 28% 11% 2	4444
September	1st to 10th	Day	31% 7% 16% 27% 15% 3	2400
	11th to 20th	Day	38% 7% 14% 23% 14% 3	2400
	21st to end	Day	43% 10% 20% 18% 7% 2	2139
October	1st to 10th	Day	44% 5% 18% 26% 6%	3280
	11th to 20th	Day	51% 7% 13% 24% 5%	3234
	21st to end	Day	52% 9% 17% 19% 3	3200
November	1st to 10th	Day	58% 10% 15% 14% 3 1	1688
	11th to 20th	Day	55% 10% 16% 12% 5% 1	1680
	21st to end	Day	56% 13% 14% 13% 4	1655
December	1st to 10th	Day	58% 9% 19% 12% 2	2360
	11th to 20th	Day	62% 12% 14% 10% 3	2442
	21st to end	Day	62% 13% 15% 9% 1	3337
January	1st to 10th	Night	68% 11% 12% 8% 2	6766
	11th to 20th	Night	49% 8% 21% 16% 4 3	6550
	21st to end	Night	55% 12% 15% 15% 3	6952
February	1st to 10th	Night	60% 9% 13% 12% 5% 1	2710
	11th to 20th	Night	49% 9% 18% 16% 7%	2534
	21st to end	Night	54% 9% 15% 16% 6%	2053
March	1st to 10th	Night	39% 12% 19% 23% 8%	5376
	11th to 20th	Night	35% 10% 19% 25% 11%	5098
	21st to end	Night	51% 10% 16% 15% 8%	5150
April	1st to 10th	Night	45% 12% 23% 17% 4	2000
	11th to 20th	Night	42% 11% 19% 18% 9% 1	1969
	21st to end	Night	38% 11% 23% 20% 8%	1624
May	1st to 10th	Night	35% 7% 16% 27% 15%	3637
	11th to 20th	Night	35% 10% 28% 23% 4	3616
	21st to end	Night	35% 11% 23% 25% 5%	3972
June	1st to 10th	Night	40% 9% 18% 20% 9% 3	1334
	11th to 20th	Night	39% 8% 24% 20% 9%	1200
	21st to end	Night	29% 7% 27% 23% 12% 2	1267
July	1st to 10th	Night	20% 5% 19% 25% 20% 10%	3145
	11th to 20th	Night	15% 3 22% 30% 25% 5%	3225
	21st to end	Night	20% 4 22% 31% 14% 9%	3857
August	1st to 10th	Night	20% 5% 15% 27% 29% 3	2601
	11th to 20th	Night	26% 5% 29% 26% 11% 3	2717
	21st to end	Night	30% 6% 24% 26% 13% 2	3470
September	1st to 10th	Night	24% 7% 19% 30% 16% 3	2000
	11th to 20th	Night	31% 8% 18% 24% 15% 4	2000
	21st to end	Night	38% 9% 22% 19% 10% 2	2256
October	1st to 10th	Night	39% 6% 22% 26% 6%	3920
	11th to 20th	Night	47% 7% 16% 25% 6%	3923
	21st to end	Night	47% 7% 18% 23% 4	4600
November	1st to 10th	Night	56% 10% 16% 12% 5%	2828
	11th to 20th	Night	55% 12% 15% 13% 5% 1	2940
	21st to end	Night	56% 12% 16% 11% 4	2940
December	1st to 10th	Night	53% 9% 20% 14% 4	5040
	11th to 20th	Night	67% 8% 14% 9% 2	5899
	21st to end	Night	59% 12% 14% 12% 2	7646

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% 10% 26% 26% 13% 8%	200
	11th to 20th	Day	13% 4 24% 36% 21% 3	200
	21st to end	Day	18% 7% 24% 32% 16% 4	220
February	1st to 10th	Day	18% 14% 28% 27% 9% 3	289
	11th to 20th	Day	14% 15% 27% 28% 12% 3	395
	21st to end	Day	18% 17% 29% 22% 12% 3	378
March	1st to 10th	Day	19% 19% 25% 25% 11% 2	500
	11th to 20th	Day	18% 19% 24% 18% 15% 6%	500
	21st to end	Day	20% 21% 28% 17% 11% 3	550
April	1st to 10th	Day	23% 22% 24% 20% 8% 2	500
	11th to 20th	Day	18% 24% 25% 18% 10% 5%	492
	21st to end	Day	19% 21% 29% 14% 13% 4	500
May	1st to 10th	Day	16% 19% 28% 20% 13% 3	503
	11th to 20th	Day	18% 13% 25% 24% 14% 6%	528
	21st to end	Day	15% 15% 31% 23% 12% 4	573
June	1st to 10th	Day	16% 21% 29% 19% 12% 2	788
	11th to 20th	Day	18% 18% 27% 22% 14% 3	930
	21st to end	Day	13% 15% 28% 23% 17% 4	857
July	1st to 10th	Day	9% 15% 33% 25% 13% 5%	721
	11th to 20th	Day	8% 16% 31% 26% 14% 5%	682
	21st to end	Day	6% 10% 26% 27% 21% 9%	575
August	1st to 10th	Day	6% 7% 25% 28% 24% 10%	527
	11th to 20th	Day	8% 10% 25% 26% 20% 10%	517
	21st to end	Day	11% 11% 28% 23% 17% 10%	583
September	1st to 10th	Day	12% 12% 21% 24% 20% 11%	530
	11th to 20th	Day	13% 15% 21% 25% 20% 6%	530
	21st to end	Day	22% 14% 25% 21% 14% 4	530
October	1st to 10th	Day	22% 11% 22% 20% 14% 10%	530
	11th to 20th	Day	25% 16% 18% 19% 17% 5%	515
	21st to end	Day	25% 13% 23% 25% 11% 3	363
November	1st to 10th	Day	24% 8% 28% 27% 11% 2	242
	11th to 20th	Day	23% 12% 24% 24% 13% 5%	220
	21st to end	Day	20% 10% 27% 25% 14% 5%	220
December	1st to 10th	Day	18% 5% 19% 31% 20% 7%	220
	11th to 20th	Day	21% 7% 27% 30% 12% 4	214
	21st to end	Day	16% 8% 21% 38% 14% 3	220
January	1st to 10th	Night	21% 9% 26% 32% 11% 1	900
	11th to 20th	Night	15% 6% 28% 34% 15% 1	850
	21st to end	Night	14% 7% 29% 34% 14% 2	880
February	1st to 10th	Night	18% 8% 29% 32% 11% 2	800
	11th to 20th	Night	11% 8% 25% 40% 16% 1	784
	21st to end	Night	10% 10% 30% 31% 17% 2	660
March	1st to 10th	Night	14% 10% 26% 29% 19% 2	800
	11th to 20th	Night	11% 15% 27% 29% 16% 2	800
	21st to end	Night	11% 15% 27% 28% 16% 3	880
April	1st to 10th	Night	14% 12% 22% 35% 16% 2	800
	11th to 20th	Night	12% 13% 27% 30% 16% 2	781
	21st to end	Night	10% 11% 24% 35% 17% 3	424
May	1st to 10th	Night	11% 8% 26% 35% 19% 2	400
	11th to 20th	Night	10% 5% 22% 35% 26% 2	394
	21st to end	Night	14% 4 24% 36% 21% 1	433
June	1st to 10th	Night	11% 10% 28% 37% 14% 1	400
	11th to 20th	Night	10% 6% 31% 33% 19% 1	400
	21st to end	Night	6% 7% 26% 37% 22% 3	400
July	1st to 10th	Night	7% 5% 22% 41% 23% 2	390
	11th to 20th	Night	5% 6% 25% 39% 23% 2	388
	21st to end	Night	4 3 18% 44% 28% 4	438
August	1st to 10th	Night	5% 3 14% 38% 36% 4	398
	11th to 20th	Night	6% 3 15% 36% 36% 4	460
	21st to end	Night	7% 5% 19% 36% 29% 3	794
September	1st to 10th	Night	7% 6% 19% 34% 30% 4	800
	11th to 20th	Night	9% 7% 17% 29% 32% 7%	800
	21st to end	Night	16% 10% 21% 29% 20% 4	800
October	1st to 10th	Night	13% 7% 20% 35% 23% 4	800
	11th to 20th	Night	20% 9% 21% 30% 18% 2	800
	21st to end	Night	17% 11% 27% 29% 12% 2	880
November	1st to 10th	Night	22% 10% 26% 34% 7% 1	820
	11th to 20th	Night	19% 11% 29% 28% 12% 2	840
	21st to end	Night	19% 9% 27% 32% 13% 2	840
December	1st to 10th	Night	14% 7% 25% 36% 15% 2	1050
	11th to 20th	Night	17% 10% 29% 33% 10% 1	1096
	21st to end	Night	15% 8% 29% 37% 10% 1	1074

Page generated at 2025-12-14 22:13:50. Version ac07

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