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 2026-07-31.

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	14% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	142583
	Night	16% 10% 4 9% 6% 5% 5% 4 5% 6% 31%	139102
Transparency	Day	48% 7% 16% 19% 8% 1	142297
	Night	53% 7% 14% 18% 7% 1	138970
Seeing	Day	19% 10% 21% 26% 18% 6%	17380
	Night	22% 10% 18% 29% 19% 3	25645

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	24% 15% 5% 13% 6% 6% 6% 4 5% 5% 11%	4125
	11th to 20th	Day	14% 10% 4 13% 8% 6% 6% 4 5% 7% 24%	4638
	21st to end	Day	21% 13% 5% 11% 6% 5% 4 3 4 5% 22%	5384
February	1st to 10th	Day	21% 12% 4 10% 9% 6% 6% 6% 7% 6% 14%	1890
	11th to 20th	Day	18% 15% 5% 11% 5% 5% 5% 4 6% 7% 20%	1845
	21st to end	Day	17% 13% 5% 10% 6% 5% 5% 4 6% 5% 24%	1615
March	1st to 10th	Day	14% 13% 4 12% 7% 7% 7% 5% 6% 7% 18%	4753
	11th to 20th	Day	15% 12% 4 10% 6% 5% 5% 4 5% 6% 27%	5110
	21st to end	Day	15% 11% 5% 11% 8% 8% 8% 5% 7% 7% 16%	5775
April	1st to 10th	Day	17% 12% 4 9% 7% 7% 5% 5% 7% 7% 20%	2520
	11th to 20th	Day	16% 12% 4 8% 6% 5% 5% 4 7% 7% 27%	2533
	21st to end	Day	15% 10% 4 10% 6% 6% 5% 5% 7% 6% 27%	2765
May	1st to 10th	Day	11% 7% 2 7% 5% 5% 5% 4 6% 7% 40%	8195
	11th to 20th	Day	16% 9% 4 9% 6% 5% 5% 5% 6% 8% 27%	8160
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 4 7% 9% 30%	8993
June	1st to 10th	Day	15% 9% 3 8% 6% 5% 5% 4 7% 7% 30%	3023
	11th to 20th	Day	16% 10% 4 10% 7% 7% 6% 4 6% 7% 24%	3150
	21st to end	Day	11% 8% 3 8% 5% 5% 4 4 6% 8% 37%	3150
July	1st to 10th	Day	8% 5% 2 5% 3 3 3 3 4 5% 58%	6930
	11th to 20th	Day	4 4 1 5% 3 3 3 2 4 5% 66%	6699
	21st to end	Day	7% 7% 2 6% 4 4 4 3 6% 8% 51%	7015
August	1st to 10th	Day	5% 4 2 5% 3 4 4 3 6% 8% 57%	4432
	11th to 20th	Day	8% 9% 4 7% 5% 4 5% 4 7% 8% 39%	4156
	21st to end	Day	13% 8% 3 9% 5% 4 4 3 5% 6% 41%	4486
September	1st to 10th	Day	8% 7% 3 6% 4 4 4 3 5% 7% 50%	2400
	11th to 20th	Day	10% 8% 3 9% 5% 5% 5% 4 5% 7% 40%	2400
	21st to end	Day	12% 9% 3 7% 5% 4 4 4 5% 7% 39%	2109
October	1st to 10th	Day	10% 7% 2 6% 5% 5% 5% 5% 8% 10% 36%	3280
	11th to 20th	Day	19% 11% 4 8% 6% 5% 5% 4 6% 7% 24%	3261
	21st to end	Day	18% 11% 4 12% 8% 6% 6% 4 6% 7% 18%	3373
November	1st to 10th	Day	23% 11% 3 11% 6% 5% 6% 4 6% 7% 18%	1678
	11th to 20th	Day	25% 11% 3 8% 7% 5% 5% 3 5% 5% 21%	1680
	21st to end	Day	24% 13% 4 10% 6% 5% 5% 3 6% 6% 19%	1680
December	1st to 10th	Day	17% 9% 3 13% 9% 8% 7% 5% 6% 7% 17%	2700
	11th to 20th	Day	27% 11% 5% 14% 7% 6% 5% 4 4 6% 12%	3105
	21st to end	Day	23% 13% 5% 14% 8% 6% 5% 4 5% 5% 12%	3575
January	1st to 10th	Night	25% 14% 6% 12% 7% 6% 5% 4 4 4 11%	8770
	11th to 20th	Night	15% 12% 4 10% 6% 5% 4 3 4 6% 32%	8242
	21st to end	Night	20% 12% 4 10% 6% 6% 5% 4 5% 6% 24%	8794
February	1st to 10th	Night	19% 13% 4 10% 6% 5% 5% 4 5% 7% 21%	2730
	11th to 20th	Night	17% 13% 3 9% 6% 5% 5% 4 5% 6% 26%	2662
	21st to end	Night	18% 13% 4 10% 6% 5% 5% 3 6% 6% 24%	2191
March	1st to 10th	Night	13% 12% 4 10% 7% 7% 7% 5% 7% 8% 20%	5345
	11th to 20th	Night	16% 11% 4 8% 5% 4 5% 4 6% 7% 31%	5109
	21st to end	Night	19% 14% 4 10% 7% 6% 6% 4 7% 6% 17%	5442
April	1st to 10th	Night	17% 9% 3 9% 6% 5% 6% 5% 6% 7% 26%	2100
	11th to 20th	Night	16% 8% 3 8% 6% 5% 5% 4 5% 7% 32%	2033
	21st to end	Night	15% 10% 3 7% 5% 6% 5% 4 7% 7% 32%	1853
May	1st to 10th	Night	10% 7% 2 6% 5% 4 5% 3 4 6% 48%	4724
	11th to 20th	Night	13% 9% 4 10% 6% 5% 5% 4 5% 7% 32%	4700
	21st to end	Night	11% 8% 3 9% 7% 6% 6% 5% 8% 8% 30%	5178
June	1st to 10th	Night	14% 8% 3 7% 6% 5% 5% 4 6% 7% 36%	1592
	11th to 20th	Night	16% 9% 2 8% 7% 6% 5% 4 5% 7% 31%	1470
	21st to end	Night	8% 6% 2 7% 6% 5% 4 4 6% 8% 45%	1470
July	1st to 10th	Night	9% 6% 2 6% 4 3 3 2 3 4 58%	3250
	11th to 20th	Night	3 2 1 3 2 2 3 2 3 4 75%	3480
	21st to end	Night	5% 5% 2 6% 3 3 3 4 6% 7% 57%	4049
August	1st to 10th	Night	3 3 1 4 4 3 3 2 3 6% 69%	2706
	11th to 20th	Night	9% 6% 3 7% 4 3 4 3 4 7% 51%	2902
	21st to end	Night	13% 5% 3 6% 4 3 4 3 5% 6% 48%	3428
September	1st to 10th	Night	6% 4 1 5% 4 4 4 3 5% 6% 57%	2000
	11th to 20th	Night	11% 7% 4 6% 5% 4 4 4 5% 6% 43%	2000
	21st to end	Night	13% 8% 3 7% 5% 5% 4 4 5% 6% 41%	2288
October	1st to 10th	Night	11% 7% 3 8% 5% 5% 4 4 7% 8% 39%	3920
	11th to 20th	Night	18% 9% 3 8% 5% 5% 5% 4 6% 7% 31%	3897
	21st to end	Night	18% 10% 3 10% 6% 6% 5% 4 6% 7% 24%	4432
November	1st to 10th	Night	24% 10% 4 9% 6% 6% 4 4 5% 5% 24%	2840
	11th to 20th	Night	23% 12% 3 10% 6% 4 4 4 5% 5% 23%	2940
	21st to end	Night	21% 11% 5% 10% 6% 6% 4 4 6% 7% 20%	2940
December	1st to 10th	Night	15% 12% 4 14% 7% 6% 5% 4 5% 6% 23%	4700
	11th to 20th	Night	25% 15% 6% 12% 7% 5% 5% 3 5% 4 12%	5320
	21st to end	Night	23% 14% 5% 13% 7% 6% 6% 3 5% 5% 12%	7605

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% 7% 8% 8% 2	4125
	11th to 20th	Day	58% 7% 9% 17% 8%	4662
	21st to end	Day	63% 6% 10% 14% 6%	5385
February	1st to 10th	Day	67% 9% 11% 8% 4 1	1890
	11th to 20th	Day	65% 8% 10% 10% 6%	1854
	21st to end	Day	62% 7% 13% 13% 5%	1615
March	1st to 10th	Day	61% 6% 16% 13% 4	4753
	11th to 20th	Day	56% 6% 12% 20% 4	5110
	21st to end	Day	64% 8% 14% 12% 1	5775
April	1st to 10th	Day	60% 8% 18% 13% 2	2520
	11th to 20th	Day	52% 7% 15% 17% 7% 2	2533
	21st to end	Day	54% 7% 16% 16% 6% 1	2765
May	1st to 10th	Day	39% 6% 18% 27% 9% 1	8195
	11th to 20th	Day	48% 9% 20% 19% 4	8163
	21st to end	Day	45% 8% 26% 18% 3	8994
June	1st to 10th	Day	47% 6% 22% 17% 6% 1	3023
	11th to 20th	Day	52% 8% 19% 16% 5%	3150
	21st to end	Day	41% 7% 24% 19% 7% 1	3150
July	1st to 10th	Day	25% 5% 18% 23% 19% 10%	6856
	11th to 20th	Day	18% 5% 14% 31% 28% 4	6618
	21st to end	Day	26% 7% 22% 32% 11% 1	6822
August	1st to 10th	Day	22% 5% 16% 37% 19% 2	4415
	11th to 20th	Day	37% 9% 20% 23% 8% 2	4195
	21st to end	Day	43% 6% 13% 27% 11% 1	4485
September	1st to 10th	Day	33% 6% 19% 25% 13% 5%	2400
	11th to 20th	Day	43% 7% 15% 18% 15% 2	2400
	21st to end	Day	44% 7% 18% 19% 9% 2	2109
October	1st to 10th	Day	39% 9% 19% 27% 6%	3280
	11th to 20th	Day	58% 8% 14% 14% 6%	3261
	21st to end	Day	63% 6% 12% 11% 7% 1	3373
November	1st to 10th	Day	65% 6% 13% 11% 4	1678
	11th to 20th	Day	65% 7% 10% 12% 5% 1	1680
	21st to end	Day	66% 7% 9% 12% 6%	1680
December	1st to 10th	Day	62% 9% 10% 13% 6%	2700
	11th to 20th	Day	72% 8% 10% 9% 1	3108
	21st to end	Day	71% 8% 11% 8% 2	3575
January	1st to 10th	Night	76% 6% 8% 8% 2	8770
	11th to 20th	Night	51% 9% 8% 22% 10% 1	8280
	21st to end	Night	61% 6% 10% 17% 5%	8794
February	1st to 10th	Night	63% 7% 12% 11% 6%	2730
	11th to 20th	Night	59% 7% 13% 13% 7% 1	2664
	21st to end	Night	60% 8% 13% 14% 5%	2191
March	1st to 10th	Night	58% 8% 14% 15% 5%	5345
	11th to 20th	Night	51% 7% 16% 23% 3	5110
	21st to end	Night	65% 8% 14% 11% 1	5442
April	1st to 10th	Night	56% 9% 17% 16% 3	2100
	11th to 20th	Night	51% 8% 16% 14% 9% 1	2033
	21st to end	Night	52% 8% 16% 17% 7% 1	1853
May	1st to 10th	Night	39% 4 13% 33% 11%	4721
	11th to 20th	Night	48% 9% 17% 20% 5%	4700
	21st to end	Night	48% 12% 21% 17% 2	5182
June	1st to 10th	Night	48% 5% 20% 21% 6% 1	1592
	11th to 20th	Night	50% 7% 19% 19% 4	1470
	21st to end	Night	39% 6% 24% 21% 8% 3	1470
July	1st to 10th	Night	29% 5% 17% 21% 17% 11%	3215
	11th to 20th	Night	14% 4 16% 29% 34% 3	3438
	21st to end	Night	24% 7% 21% 31% 14% 3	3937
August	1st to 10th	Night	19% 4 13% 38% 23% 2	2698
	11th to 20th	Night	32% 8% 25% 22% 11% 2	2929
	21st to end	Night	37% 7% 15% 28% 12% 1	3428
September	1st to 10th	Night	27% 6% 19% 27% 15% 5%	2000
	11th to 20th	Night	42% 7% 13% 19% 16% 4	2000
	21st to end	Night	43% 6% 14% 24% 10% 1	2288
October	1st to 10th	Night	40% 8% 19% 28% 5%	3920
	11th to 20th	Night	52% 6% 15% 21% 6%	3897
	21st to end	Night	57% 9% 12% 15% 7% 1	4432
November	1st to 10th	Night	62% 8% 13% 13% 4 1	2840
	11th to 20th	Night	64% 6% 12% 10% 5% 2	2940
	21st to end	Night	63% 7% 12% 11% 7% 1	2940
December	1st to 10th	Night	62% 8% 9% 15% 6%	4700
	11th to 20th	Night	74% 7% 11% 6% 2	5316
	21st to end	Night	72% 7% 11% 9% 2	7605

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% 20% 15% 11% 4	220
	11th to 20th	Day	28% 13% 21% 17% 15% 6%	220
	21st to end	Day	30% 15% 22% 17% 13% 3	310
February	1st to 10th	Day	31% 14% 20% 22% 9% 3	430
	11th to 20th	Day	30% 17% 24% 19% 8% 2	425
	21st to end	Day	26% 17% 27% 17% 10% 2	386
March	1st to 10th	Day	26% 16% 26% 20% 10% 2	540
	11th to 20th	Day	31% 13% 20% 20% 11% 4	540
	21st to end	Day	25% 14% 29% 23% 8% 1	594
April	1st to 10th	Day	24% 14% 22% 24% 12% 4	540
	11th to 20th	Day	19% 14% 29% 22% 11% 5%	532
	21st to end	Day	19% 11% 27% 26% 14% 3	540
May	1st to 10th	Day	11% 9% 25% 35% 15% 4	543
	11th to 20th	Day	19% 9% 18% 29% 17% 8%	568
	21st to end	Day	18% 7% 19% 28% 19% 8%	617
June	1st to 10th	Day	18% 10% 20% 28% 19% 6%	653
	11th to 20th	Day	17% 9% 21% 31% 19% 3	780
	21st to end	Day	12% 6% 23% 32% 21% 7%	780
July	1st to 10th	Day	9% 5% 16% 34% 27% 9%	771
	11th to 20th	Day	6% 4 15% 33% 32% 9%	667
	21st to end	Day	7% 3 10% 31% 36% 13%	616
August	1st to 10th	Day	6% 4 11% 31% 36% 13%	527
	11th to 20th	Day	7% 4 13% 30% 33% 14%	517
	21st to end	Day	13% 5% 19% 32% 23% 8%	583
September	1st to 10th	Day	11% 5% 20% 35% 23% 6%	530
	11th to 20th	Day	14% 8% 19% 30% 24% 6%	530
	21st to end	Day	16% 11% 23% 28% 18% 4	530
October	1st to 10th	Day	19% 9% 22% 25% 18% 7%	530
	11th to 20th	Day	24% 10% 20% 26% 15% 4	530
	21st to end	Day	27% 16% 22% 20% 12% 3	473
November	1st to 10th	Day	29% 14% 25% 20% 9% 3	236
	11th to 20th	Day	33% 16% 18% 19% 9% 5%	220
	21st to end	Day	30% 13% 22% 16% 12% 7%	220
December	1st to 10th	Day	31% 14% 19% 15% 15% 6%	220
	11th to 20th	Day	35% 19% 25% 12% 8% 1	220
	21st to end	Day	31% 16% 19% 21% 11% 2	242
January	1st to 10th	Night	36% 13% 18% 18% 13% 2	950
	11th to 20th	Night	30% 10% 16% 24% 18% 2	851
	21st to end	Night	27% 8% 19% 27% 15% 3	924
February	1st to 10th	Night	27% 15% 20% 25% 11% 2	840
	11th to 20th	Night	25% 11% 22% 27% 14% 2	824
	21st to end	Night	26% 14% 20% 28% 11% 1	692
March	1st to 10th	Night	23% 13% 21% 28% 12% 2	840
	11th to 20th	Night	28% 12% 21% 25% 13% 2	840
	21st to end	Night	25% 12% 24% 29% 9% 2	924
April	1st to 10th	Night	22% 9% 22% 29% 16% 2	840
	11th to 20th	Night	19% 11% 23% 30% 15% 2	785
	21st to end	Night	21% 8% 22% 33% 13% 2	593
May	1st to 10th	Night	14% 10% 21% 35% 18% 2	420
	11th to 20th	Night	14% 8% 18% 33% 24% 3	414
	21st to end	Night	16% 6% 14% 37% 23% 4	455
June	1st to 10th	Night	18% 7% 18% 32% 22% 3	420
	11th to 20th	Night	15% 10% 20% 35% 18% 2	420
	21st to end	Night	10% 5% 17% 41% 25% 2	420
July	1st to 10th	Night	8% 3 13% 36% 37% 4	410
	11th to 20th	Night	7% 2 12% 42% 33% 4	408
	21st to end	Night	6% 3 9% 36% 40% 5%	458
August	1st to 10th	Night	8% 2 7% 33% 40% 11%	456
	11th to 20th	Night	8% 1 9% 30% 43% 9%	594
	21st to end	Night	11% 3 12% 35% 34% 5%	765
September	1st to 10th	Night	8% 2 16% 37% 31% 5%	800
	11th to 20th	Night	14% 7% 12% 29% 32% 6%	800
	21st to end	Night	17% 8% 15% 35% 22% 4	800
October	1st to 10th	Night	15% 6% 21% 31% 24% 4	800
	11th to 20th	Night	22% 10% 16% 29% 20% 3	800
	21st to end	Night	25% 11% 21% 27% 15% 1	880
November	1st to 10th	Night	30% 12% 19% 28% 10%	820
	11th to 20th	Night	30% 12% 24% 21% 13% 1	840
	21st to end	Night	28% 11% 20% 24% 14% 3	840
December	1st to 10th	Night	29% 12% 13% 29% 16% 2	840
	11th to 20th	Night	31% 15% 20% 22% 10% 2	840
	21st to end	Night	34% 15% 21% 21% 9% 1	1042

Page generated at 2026-07-28 00:25:25. Version ac07

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