Ritchie Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ritchie Observatory clear sky chart from 2005-11-06 to 2025-03-27.

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	15% 9% 3 9% 6% 5% 5% 4 6% 7% 31%	112939
	Night	17% 10% 4 9% 6% 5% 5% 4 5% 6% 30%	115020
Transparency	Day	50% 7% 15% 18% 8% 2	112694
	Night	54% 7% 13% 18% 7% 1	114911
Seeing	Day	19% 10% 21% 26% 18% 6%	15549
	Night	22% 9% 18% 29% 19% 3	24088

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% 13% 5% 12% 6% 5% 6% 4 5% 5% 12%	3179
	11th to 20th	Day	16% 9% 5% 13% 8% 6% 5% 4 6% 7% 21%	3552
	21st to end	Day	22% 14% 4 10% 6% 5% 4 3 5% 5% 22%	4160
February	1st to 10th	Day	21% 12% 4 9% 9% 6% 6% 6% 6% 6% 15%	1800
	11th to 20th	Day	19% 15% 5% 10% 5% 5% 5% 4 5% 7% 20%	1755
	21st to end	Day	16% 13% 5% 10% 6% 5% 5% 4 6% 5% 23%	1540
March	1st to 10th	Day	13% 12% 4 11% 7% 8% 7% 5% 7% 7% 19%	3334
	11th to 20th	Day	14% 9% 3 8% 5% 5% 5% 4 6% 6% 35%	3600
	21st to end	Day	18% 11% 4 9% 7% 7% 7% 4 6% 7% 19%	4006
April	1st to 10th	Day	18% 13% 4 9% 6% 7% 5% 5% 6% 7% 19%	2280
	11th to 20th	Day	16% 12% 4 8% 7% 5% 5% 4 6% 7% 27%	2289
	21st to end	Day	16% 10% 4 10% 6% 6% 5% 6% 6% 6% 25%	2501
May	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 5% 6% 37%	4420
	11th to 20th	Day	16% 7% 3 6% 5% 4 5% 4 6% 8% 36%	4377
	21st to end	Day	22% 12% 4 11% 6% 6% 5% 4 5% 6% 19%	4813
June	1st to 10th	Day	16% 9% 3 8% 6% 5% 5% 4 6% 7% 28%	2735
	11th to 20th	Day	17% 11% 4 10% 7% 7% 6% 4 6% 7% 21%	2850
	21st to end	Day	11% 7% 3 8% 5% 5% 4 4 6% 8% 38%	2850
July	1st to 10th	Day	7% 5% 2 6% 3 3 3 3 4 5% 60%	4740
	11th to 20th	Day	4 3 1 4 3 3 3 2 4 6% 65%	4584
	21st to end	Day	8% 6% 2 6% 3 4 4 3 5% 7% 50%	4790
August	1st to 10th	Day	5% 4 1 5% 3 4 4 3 6% 8% 58%	4295
	11th to 20th	Day	8% 9% 3 7% 5% 4 5% 4 7% 8% 39%	4026
	21st to end	Day	13% 8% 3 9% 5% 4 4 3 5% 6% 41%	4349
September	1st to 10th	Day	9% 7% 3 6% 4 4 3 3 5% 7% 51%	2280
	11th to 20th	Day	10% 8% 3 10% 5% 5% 5% 3 5% 7% 39%	2280
	21st to end	Day	12% 9% 3 7% 5% 4 4 4 5% 7% 39%	2004
October	1st to 10th	Day	11% 7% 2 6% 5% 5% 5% 5% 8% 10% 36%	3180
	11th to 20th	Day	19% 11% 4 9% 6% 5% 5% 4 6% 7% 23%	3161
	21st to end	Day	18% 11% 4 12% 8% 6% 6% 4 6% 7% 18%	3269
November	1st to 10th	Day	24% 11% 3 11% 6% 5% 6% 4 5% 7% 18%	1597
	11th to 20th	Day	25% 11% 3 9% 7% 6% 5% 3 5% 5% 22%	1600
	21st to end	Day	23% 13% 3 10% 7% 5% 5% 3 6% 6% 19%	1600
December	1st to 10th	Day	16% 9% 3 13% 9% 8% 7% 5% 6% 7% 17%	2620
	11th to 20th	Day	27% 11% 5% 14% 7% 5% 5% 4 4 6% 12%	3025
	21st to end	Day	23% 13% 5% 14% 8% 6% 5% 4 5% 4 12%	3498
January	1st to 10th	Night	26% 14% 6% 12% 8% 6% 5% 4 4 4 11%	6766
	11th to 20th	Night	16% 9% 4 11% 6% 5% 4 3 5% 7% 28%	6328
	21st to end	Night	22% 13% 4 9% 6% 5% 4 4 5% 6% 24%	6803
February	1st to 10th	Night	20% 14% 4 10% 6% 5% 5% 4 4 7% 21%	2600
	11th to 20th	Night	18% 13% 3 9% 6% 5% 5% 4 5% 6% 26%	2532
	21st to end	Night	18% 13% 4 10% 6% 5% 5% 3 6% 6% 24%	2090
March	1st to 10th	Night	14% 13% 5% 10% 7% 7% 7% 5% 6% 7% 18%	3744
	11th to 20th	Night	13% 8% 3 6% 4 4 4 4 6% 8% 40%	3599
	21st to end	Night	20% 15% 4 10% 6% 6% 5% 3 5% 5% 20%	3785
April	1st to 10th	Night	17% 10% 3 9% 7% 6% 6% 5% 6% 7% 25%	1900
	11th to 20th	Night	16% 9% 3 8% 6% 5% 5% 4 5% 7% 32%	1837
	21st to end	Night	15% 10% 4 8% 6% 6% 5% 4 7% 6% 29%	1677
May	1st to 10th	Night	15% 9% 3 6% 5% 4 4 3 4 5% 42%	2540
	11th to 20th	Night	12% 7% 3 8% 5% 5% 4 3 5% 7% 40%	2516
	21st to end	Night	13% 10% 4 10% 7% 6% 7% 4 6% 7% 26%	2762
June	1st to 10th	Night	15% 8% 3 8% 6% 5% 5% 4 6% 7% 35%	1440
	11th to 20th	Night	17% 9% 3 8% 7% 6% 6% 3 5% 7% 29%	1330
	21st to end	Night	8% 6% 2 7% 6% 5% 4 4 6% 7% 45%	1330
July	1st to 10th	Night	7% 6% 2 5% 3 3 3 2 3 3 62%	2220
	11th to 20th	Night	3 2 1 2 2 2 3 2 4 4 74%	2375
	21st to end	Night	6% 6% 2 6% 4 3 3 4 5% 5% 55%	2762
August	1st to 10th	Night	3 3 1 4 4 3 3 2 3 6% 70%	2623
	11th to 20th	Night	8% 6% 3 7% 4 4 4 3 4 7% 51%	2812
	21st to end	Night	14% 5% 3 6% 4 3 3 3 4 6% 48%	3324
September	1st to 10th	Night	6% 4 1 5% 3 4 4 3 5% 6% 58%	1900
	11th to 20th	Night	12% 8% 4 7% 5% 5% 4 4 5% 6% 42%	1900
	21st to end	Night	13% 8% 2 7% 5% 5% 4 4 5% 6% 41%	2173
October	1st to 10th	Night	11% 7% 2 8% 5% 5% 4 4 7% 8% 39%	3800
	11th to 20th	Night	18% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	3777
	21st to end	Night	18% 11% 3 10% 6% 6% 5% 4 6% 7% 24%	4294
November	1st to 10th	Night	24% 10% 4 9% 6% 5% 4 3 5% 5% 24%	2701
	11th to 20th	Night	23% 12% 3 9% 6% 4 4 4 5% 5% 24%	2800
	21st to end	Night	20% 11% 5% 10% 7% 6% 5% 4 6% 7% 20%	2800
December	1st to 10th	Night	15% 11% 4 14% 7% 6% 5% 4 5% 6% 23%	4560
	11th to 20th	Night	25% 15% 6% 12% 7% 5% 5% 3 5% 4 12%	5180
	21st to end	Night	24% 14% 5% 13% 7% 6% 6% 3 5% 5% 12%	7440

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	75% 6% 8% 9% 2	3179
	11th to 20th	Day	61% 8% 11% 17% 4 1	3576
	21st to end	Day	63% 6% 11% 13% 6%	4160
February	1st to 10th	Day	67% 9% 11% 8% 4 1	1800
	11th to 20th	Day	65% 8% 10% 10% 6%	1764
	21st to end	Day	62% 7% 12% 13% 6%	1540
March	1st to 10th	Day	60% 6% 17% 13% 4	3334
	11th to 20th	Day	48% 6% 15% 26% 4 1	3600
	21st to end	Day	63% 8% 14% 14% 1	4006
April	1st to 10th	Day	61% 8% 17% 12% 2	2280
	11th to 20th	Day	53% 7% 14% 16% 7% 2	2289
	21st to end	Day	56% 7% 16% 15% 5% 1	2501
May	1st to 10th	Day	44% 5% 15% 24% 9% 2	4420
	11th to 20th	Day	42% 6% 25% 21% 6%	4380
	21st to end	Day	61% 7% 20% 11% 2	4814
June	1st to 10th	Day	49% 7% 23% 15% 6% 1	2735
	11th to 20th	Day	55% 8% 19% 14% 4	2850
	21st to end	Day	40% 7% 25% 20% 7% 2	2850
July	1st to 10th	Day	24% 5% 11% 22% 24% 14%	4666
	11th to 20th	Day	18% 4 11% 29% 32% 4	4503
	21st to end	Day	28% 6% 16% 33% 15% 2	4639
August	1st to 10th	Day	22% 5% 15% 37% 19% 2	4278
	11th to 20th	Day	37% 9% 21% 24% 8% 2	4065
	21st to end	Day	43% 6% 13% 27% 11% 1	4348
September	1st to 10th	Day	33% 6% 18% 25% 14% 5%	2280
	11th to 20th	Day	44% 7% 15% 17% 15% 2	2280
	21st to end	Day	45% 7% 18% 19% 10% 2	2004
October	1st to 10th	Day	39% 9% 19% 27% 6%	3180
	11th to 20th	Day	58% 8% 14% 14% 5%	3161
	21st to end	Day	63% 6% 12% 11% 7% 1	3269
November	1st to 10th	Day	66% 7% 12% 11% 4	1597
	11th to 20th	Day	65% 7% 9% 12% 5% 2	1600
	21st to end	Day	66% 7% 9% 12% 7%	1600
December	1st to 10th	Day	62% 9% 10% 13% 6%	2620
	11th to 20th	Day	72% 8% 10% 8% 1	3028
	21st to end	Day	72% 8% 11% 7% 2	3498
January	1st to 10th	Night	77% 6% 7% 8% 2	6766
	11th to 20th	Night	52% 10% 10% 23% 4 1	6366
	21st to end	Night	62% 6% 11% 16% 5%	6803
February	1st to 10th	Night	64% 7% 12% 11% 6%	2600
	11th to 20th	Night	59% 7% 13% 13% 7% 1	2534
	21st to end	Night	60% 7% 13% 14% 5%	2090
March	1st to 10th	Night	64% 7% 12% 14% 3	3744
	11th to 20th	Night	42% 7% 19% 29% 3 1	3600
	21st to end	Night	67% 7% 14% 11% 1	3785
April	1st to 10th	Night	56% 9% 17% 15% 3	1900
	11th to 20th	Night	52% 8% 15% 15% 9% 1	1837
	21st to end	Night	55% 7% 14% 16% 7% 1	1677
May	1st to 10th	Night	46% 4 12% 26% 11%	2540
	11th to 20th	Night	41% 8% 22% 19% 9% 1	2516
	21st to end	Night	56% 13% 16% 12% 2	2762
June	1st to 10th	Night	49% 6% 19% 20% 5% 1	1440
	11th to 20th	Night	53% 8% 20% 16% 3	1330
	21st to end	Night	40% 5% 24% 20% 8% 3	1330
July	1st to 10th	Night	27% 4 11% 22% 20% 17%	2185
	11th to 20th	Night	15% 4 10% 30% 38% 2	2333
	21st to end	Night	28% 7% 12% 30% 19% 4	2674
August	1st to 10th	Night	19% 4 12% 39% 24% 2	2615
	11th to 20th	Night	31% 8% 25% 22% 11% 2	2839
	21st to end	Night	37% 7% 15% 29% 12% 1	3324
September	1st to 10th	Night	27% 6% 18% 28% 16% 5%	1900
	11th to 20th	Night	43% 7% 13% 18% 15% 4	1900
	21st to end	Night	43% 6% 14% 24% 11% 1	2173
October	1st to 10th	Night	39% 8% 19% 28% 5%	3800
	11th to 20th	Night	52% 6% 15% 21% 6%	3777
	21st to end	Night	57% 9% 12% 14% 7% 1	4294
November	1st to 10th	Night	62% 7% 13% 13% 4 1	2701
	11th to 20th	Night	63% 6% 12% 11% 6% 2	2800
	21st to end	Night	63% 7% 12% 11% 7% 1	2800
December	1st to 10th	Night	62% 8% 9% 15% 6%	4560
	11th to 20th	Night	74% 7% 11% 6% 2	5176
	21st to end	Night	72% 6% 10% 9% 2	7440

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	36% 14% 18% 17% 12% 4	200
	11th to 20th	Day	29% 14% 20% 18% 14% 6%	200
	21st to end	Day	30% 15% 23% 18% 12% 3	285
February	1st to 10th	Day	33% 15% 20% 22% 9% 3	400
	11th to 20th	Day	31% 16% 24% 18% 9% 2	395
	21st to end	Day	26% 17% 26% 18% 11% 2	359
March	1st to 10th	Day	24% 17% 27% 21% 10% 2	500
	11th to 20th	Day	28% 14% 21% 21% 12% 5%	500
	21st to end	Day	26% 14% 27% 23% 9% 2	531
April	1st to 10th	Day	26% 16% 22% 21% 11% 4	460
	11th to 20th	Day	19% 15% 30% 20% 10% 6%	452
	21st to end	Day	21% 12% 28% 25% 13% 2	460
May	1st to 10th	Day	13% 10% 24% 35% 14% 4	463
	11th to 20th	Day	18% 6% 19% 29% 19% 9%	488
	21st to end	Day	19% 7% 19% 29% 18% 8%	529
June	1st to 10th	Day	19% 9% 21% 26% 19% 6%	565
	11th to 20th	Day	19% 9% 20% 30% 19% 3	680
	21st to end	Day	12% 5% 23% 32% 22% 6%	680
July	1st to 10th	Day	9% 5% 16% 34% 26% 9%	671
	11th to 20th	Day	6% 3 14% 33% 33% 10%	577
	21st to end	Day	7% 3 9% 31% 38% 12%	531
August	1st to 10th	Day	6% 3 11% 31% 36% 13%	487
	11th to 20th	Day	6% 4 11% 29% 35% 15%	477
	21st to end	Day	14% 6% 19% 31% 23% 8%	539
September	1st to 10th	Day	11% 6% 21% 36% 22% 4	490
	11th to 20th	Day	14% 8% 19% 30% 23% 6%	490
	21st to end	Day	16% 11% 22% 28% 18% 4	490
October	1st to 10th	Day	19% 10% 22% 24% 18% 7%	490
	11th to 20th	Day	24% 9% 20% 27% 16% 4	490
	21st to end	Day	27% 16% 20% 21% 13% 3	435
November	1st to 10th	Day	29% 14% 25% 20% 9% 3	215
	11th to 20th	Day	32% 17% 19% 18% 9% 6%	200
	21st to end	Day	30% 12% 22% 16% 13% 8%	200
December	1st to 10th	Day	28% 14% 20% 16% 17% 7%	200
	11th to 20th	Day	33% 18% 27% 13% 9% 1	200
	21st to end	Day	32% 15% 20% 21% 9% 1	220
January	1st to 10th	Night	36% 13% 17% 18% 13% 2	900
	11th to 20th	Night	30% 10% 16% 25% 17% 2	810
	21st to end	Night	28% 8% 19% 27% 15% 3	880
February	1st to 10th	Night	28% 15% 20% 24% 11% 2	800
	11th to 20th	Night	25% 10% 21% 28% 14% 2	784
	21st to end	Night	27% 14% 19% 28% 11% 2	660
March	1st to 10th	Night	23% 13% 21% 29% 13% 2	800
	11th to 20th	Night	25% 12% 22% 25% 14% 2	800
	21st to end	Night	25% 12% 24% 28% 9% 2	862
April	1st to 10th	Night	22% 10% 23% 28% 15% 2	760
	11th to 20th	Night	19% 11% 23% 29% 15% 3	709
	21st to end	Night	22% 9% 23% 32% 13% 2	537
May	1st to 10th	Night	15% 11% 21% 34% 17% 2	380
	11th to 20th	Night	13% 6% 17% 35% 25% 4	374
	21st to end	Night	16% 6% 14% 37% 23% 4	411
June	1st to 10th	Night	18% 7% 19% 33% 20% 3	380
	11th to 20th	Night	16% 11% 21% 33% 17% 2	380
	21st to end	Night	11% 5% 17% 39% 24% 3	380
July	1st to 10th	Night	8% 3 13% 36% 36% 4	370
	11th to 20th	Night	7% 2 12% 40% 33% 5%	368
	21st to end	Night	6% 3 8% 38% 41% 5%	416
August	1st to 10th	Night	7% 2 8% 33% 39% 11%	433
	11th to 20th	Night	7% 1 9% 31% 44% 9%	564
	21st to end	Night	11% 3 13% 34% 34% 4	727
September	1st to 10th	Night	9% 3 16% 38% 31% 5%	760
	11th to 20th	Night	14% 7% 12% 28% 33% 6%	760
	21st to end	Night	18% 8% 15% 34% 22% 4	760
October	1st to 10th	Night	15% 5% 20% 31% 24% 4	760
	11th to 20th	Night	22% 10% 16% 29% 21% 3	760
	21st to end	Night	26% 10% 20% 27% 16% 1	836
November	1st to 10th	Night	30% 11% 20% 28% 10%	780
	11th to 20th	Night	30% 12% 24% 21% 14% 1	800
	21st to end	Night	28% 11% 20% 24% 15% 3	800
December	1st to 10th	Night	27% 12% 14% 29% 16% 2	800
	11th to 20th	Night	30% 15% 20% 23% 10% 2	800
	21st to end	Night	35% 14% 21% 21% 8% 1	987

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