Cascade Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Cascade Mountain Observatory clear sky chart from 2005-11-06 to 2025-02-21.

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	13% 6% 2 6% 4 4 4 3 5% 7% 46%	108396
	Night	13% 6% 2 5% 4 3 3 3 5% 6% 50%	110338
Transparency	Day	37% 5% 10% 24% 20% 5%	112042
	Night	34% 4 9% 24% 22% 7%	114833
Seeing	Day	15% 10% 23% 26% 20% 7%	15700
	Night	15% 8% 25% 32% 17% 2	25658

By Month

Percentage Cloud Cover SE (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	18% 7% 2 5% 3 3 3 2 4 4 49%	4077
	11th to 20th	Day	11% 8% 2 5% 4 3 3 2 4 5% 52%	4080
	21st to end	Day	21% 7% 2 4 3 2 2 2 3 4 49%	4488
February	1st to 10th	Day	23% 8% 2 6% 4 3 3 3 2 5% 41%	1800
	11th to 20th	Day	18% 7% 3 7% 4 3 3 3 4 5% 42%	1897
	21st to end	Day	23% 8% 3 7% 3 3 4 3 4 5% 37%	1593
March	1st to 10th	Day	22% 9% 2 7% 4 3 3 3 3 4 38%	3436
	11th to 20th	Day	14% 6% 3 6% 4 4 3 3 5% 5% 47%	3480
	21st to end	Day	17% 8% 3 7% 4 4 3 3 4 4 43%	3840
April	1st to 10th	Day	14% 7% 2 5% 4 4 4 3 5% 7% 44%	2280
	11th to 20th	Day	16% 6% 2 6% 5% 4 4 4 4 6% 44%	2254
	21st to end	Day	16% 6% 2 8% 5% 4 5% 3 5% 6% 41%	2280
May	1st to 10th	Day	17% 7% 2 6% 5% 4 5% 3 5% 6% 38%	3765
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 6% 9% 41%	4014
	21st to end	Day	12% 6% 3 8% 6% 6% 5% 4 7% 9% 34%	4562
June	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 7% 9% 42%	2660
	11th to 20th	Day	5% 4 1 5% 4 6% 5% 4 7% 11% 47%	2660
	21st to end	Day	5% 3 1 4 5% 4 6% 5% 7% 9% 52%	2599
July	1st to 10th	Day	8% 5% 2 5% 5% 5% 5% 5% 7% 10% 42%	4120
	11th to 20th	Day	6% 4 1 5% 3 4 4 5% 5% 10% 54%	4120
	21st to end	Day	10% 9% 3 8% 6% 6% 6% 4 6% 7% 35%	4465
August	1st to 10th	Day	4 2 1 4 3 3 4 4 5% 8% 62%	4104
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 5% 7% 10% 42%	3734
	21st to end	Day	5% 4 1 5% 4 4 5% 4 7% 8% 53%	4180
September	1st to 10th	Day	11% 6% 2 4 4 4 5% 5% 5% 7% 47%	2280
	11th to 20th	Day	14% 5% 2 4 4 4 5% 4 6% 8% 45%	2280
	21st to end	Day	9% 5% 2 4 4 4 4 4 5% 7% 53%	1997
October	1st to 10th	Day	7% 4 2 5% 3 3 3 3 4 6% 60%	1900
	11th to 20th	Day	13% 5% 2 4 2 3 3 2 4 6% 57%	1900
	21st to end	Day	18% 4 2 5% 2 2 4 2 4 5% 50%	2090
November	1st to 10th	Day	14% 6% 2 4 2 3 2 2 3 6% 56%	1958
	11th to 20th	Day	11% 6% 2 5% 3 3 3 3 4 5% 55%	1880
	21st to end	Day	19% 7% 2 6% 3 2 2 2 4 5% 46%	1765
December	1st to 10th	Day	17% 7% 3 6% 3 3 3 2 3 3 49%	2610
	11th to 20th	Day	17% 8% 3 7% 4 4 3 3 3 4 44%	3008
	21st to end	Day	20% 8% 2 8% 4 3 3 3 4 5% 39%	4240
January	1st to 10th	Night	16% 6% 2 5% 3 3 3 2 4 5% 50%	5888
	11th to 20th	Night	12% 4 1 4 3 2 3 2 4 4 59%	5900
	21st to end	Night	22% 8% 2 5% 3 3 3 2 3 4 47%	6490
February	1st to 10th	Night	21% 8% 3 6% 4 2 3 2 4 6% 41%	2600
	11th to 20th	Night	19% 8% 3 6% 3 3 2 3 3 5% 44%	2388
	21st to end	Night	18% 7% 2 5% 3 4 4 2 3 5% 46%	1879
March	1st to 10th	Night	20% 8% 2 6% 3 3 3 3 4 5% 42%	3419
	11th to 20th	Night	15% 7% 2 5% 4 3 3 3 4 5% 50%	3480
	21st to end	Night	12% 6% 2 7% 5% 4 4 3 5% 7% 45%	3791
April	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 6% 6% 51%	1900
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 6% 6% 50%	1875
	21st to end	Night	14% 5% 2 5% 4 4 3 3 6% 7% 47%	1900
May	1st to 10th	Night	19% 7% 2 6% 4 4 4 3 4 7% 40%	3130
	11th to 20th	Night	15% 6% 2 6% 3 3 3 3 5% 7% 46%	2872
	21st to end	Night	14% 6% 2 7% 4 5% 5% 4 6% 7% 40%	2995
June	1st to 10th	Night	6% 4 1 4 3 4 4 4 7% 11% 52%	1520
	11th to 20th	Night	3 3 1 5% 4 3 3 4 5% 9% 60%	1520
	21st to end	Night	3 3 1 3 3 4 3 4 6% 9% 60%	1573
July	1st to 10th	Night	7% 4 2 6% 4 4 5% 4 7% 10% 49%	2850
	11th to 20th	Night	4 3 1 4 3 4 4 4 6% 10% 58%	2850
	21st to end	Night	8% 8% 3 8% 5% 4 5% 4 6% 8% 42%	3098
August	1st to 10th	Night	2 1 1 4 3 3 3 3 6% 7% 67%	2836
	11th to 20th	Night	4 2 1 5% 4 3 4 4 7% 10% 54%	3095
	21st to end	Night	3 3 1 5% 4 4 5% 4 6% 9% 56%	3476
September	1st to 10th	Night	7% 4 2 6% 4 4 4 3 4 8% 55%	1900
	11th to 20th	Night	10% 4 2 5% 4 4 4 3 4 7% 53%	1900
	21st to end	Night	8% 5% 1 4 3 3 3 3 4 7% 59%	2159
October	1st to 10th	Night	8% 3 2 5% 3 3 3 3 5% 6% 62%	2280
	11th to 20th	Night	11% 3 1 4 3 2 3 3 3 5% 62%	2280
	21st to end	Night	17% 5% 2 4 3 3 3 2 3 4 54%	2508
November	1st to 10th	Night	11% 6% 2 5% 4 3 3 3 4 5% 56%	2340
	11th to 20th	Night	12% 4 2 4 3 2 3 3 4 6% 58%	2517
	21st to end	Night	15% 6% 2 5% 3 3 3 3 5% 5% 50%	2630
December	1st to 10th	Night	16% 6% 2 5% 3 3 3 3 4 5% 51%	4580
	11th to 20th	Night	19% 8% 3 6% 4 3 3 3 3 4 46%	5199
	21st to end	Night	20% 7% 3 6% 4 4 4 3 4 5% 41%	6720

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	40% 2 8% 28% 19% 2	4078
	11th to 20th	Day	36% 3 7% 24% 26% 5%	4080
	21st to end	Day	41% 2 5% 17% 27% 8%	4488
February	1st to 10th	Day	47% 4 9% 21% 15% 3	1800
	11th to 20th	Day	45% 4 7% 20% 18% 5%	1907
	21st to end	Day	49% 4 8% 15% 16% 8%	1593
March	1st to 10th	Day	51% 3 6% 13% 17% 10%	3436
	11th to 20th	Day	40% 4 9% 24% 19% 4	3480
	21st to end	Day	45% 3 8% 22% 20% 1	3840
April	1st to 10th	Day	39% 5% 9% 22% 22% 4	2280
	11th to 20th	Day	42% 5% 8% 22% 17% 6%	2254
	21st to end	Day	43% 5% 9% 23% 18% 2	2280
May	1st to 10th	Day	45% 6% 12% 21% 15% 2	3800
	11th to 20th	Day	38% 6% 14% 26% 14% 1	4018
	21st to end	Day	44% 7% 13% 23% 12% 2	4575
June	1st to 10th	Day	35% 7% 13% 26% 17% 1	2660
	11th to 20th	Day	29% 7% 15% 29% 18% 2	2660
	21st to end	Day	27% 7% 13% 28% 20% 4	2599
July	1st to 10th	Day	34% 7% 16% 30% 12% 1	4056
	11th to 20th	Day	25% 6% 10% 35% 22% 2	4051
	21st to end	Day	47% 5% 14% 25% 8%	4325
August	1st to 10th	Day	21% 4 16% 31% 22% 6%	4090
	11th to 20th	Day	37% 6% 13% 31% 12% 1	3770
	21st to end	Day	28% 6% 11% 32% 20% 3	4179
September	1st to 10th	Day	36% 7% 11% 21% 18% 8%	2280
	11th to 20th	Day	36% 6% 10% 18% 24% 5%	2280
	21st to end	Day	32% 5% 9% 21% 27% 7%	1997
October	1st to 10th	Day	16% 3 5% 14% 37% 26%	3160
	11th to 20th	Day	19% 2 3 20% 39% 17%	3140
	21st to end	Day	31% 3 5% 28% 26% 6%	3428
November	1st to 10th	Day	32% 3 9% 25% 24% 8%	1958
	11th to 20th	Day	31% 3 7% 22% 27% 10%	1880
	21st to end	Day	42% 3 6% 20% 23% 6%	1765
December	1st to 10th	Day	42% 2 6% 27% 20% 3	2610
	11th to 20th	Day	45% 3 5% 22% 20% 4	3005
	21st to end	Day	47% 4 7% 24% 14% 4	4240
January	1st to 10th	Night	38% 3 8% 27% 22% 2	5888
	11th to 20th	Night	30% 3 8% 22% 31% 6%	5900
	21st to end	Night	43% 3 7% 18% 25% 4	6490
February	1st to 10th	Night	46% 3 8% 21% 18% 5%	2600
	11th to 20th	Night	44% 3 7% 22% 19% 6%	2389
	21st to end	Night	42% 3 7% 19% 20% 9%	1879
March	1st to 10th	Night	45% 4 8% 14% 18% 11%	3419
	11th to 20th	Night	37% 5% 8% 26% 19% 6%	3480
	21st to end	Night	39% 4 9% 26% 20% 2	3791
April	1st to 10th	Night	33% 4 10% 25% 20% 8%	1900
	11th to 20th	Night	33% 5% 8% 23% 21% 10%	1875
	21st to end	Night	36% 4 11% 24% 22% 3	1900
May	1st to 10th	Night	45% 5% 10% 23% 14% 3	3160
	11th to 20th	Night	39% 5% 15% 25% 16% 1	2873
	21st to end	Night	42% 6% 14% 23% 14% 2	3003
June	1st to 10th	Night	26% 6% 14% 30% 21% 2	1520
	11th to 20th	Night	22% 4 12% 32% 27% 3	1520
	21st to end	Night	21% 5% 14% 29% 24% 7%	1573
July	1st to 10th	Night	30% 6% 17% 29% 18% 1	2805
	11th to 20th	Night	21% 6% 12% 34% 23% 4	2796
	21st to end	Night	41% 5% 13% 26% 14%	2999
August	1st to 10th	Night	15% 4 16% 37% 23% 5%	2826
	11th to 20th	Night	23% 5% 19% 35% 16% 2	3125
	21st to end	Night	24% 5% 12% 28% 27% 3	3476
September	1st to 10th	Night	28% 6% 10% 25% 21% 9%	1900
	11th to 20th	Night	33% 5% 9% 21% 26% 6%	1900
	21st to end	Night	27% 5% 9% 22% 29% 9%	2159
October	1st to 10th	Night	15% 3 6% 15% 30% 31%	3800
	11th to 20th	Night	17% 2 3 16% 39% 24%	3779
	21st to end	Night	30% 2 5% 22% 29% 12%	4120
November	1st to 10th	Night	31% 4 7% 23% 26% 10%	2340
	11th to 20th	Night	28% 3 6% 25% 27% 11%	2517
	21st to end	Night	37% 3 7% 21% 27% 6%	2630
December	1st to 10th	Night	37% 3 6% 32% 19% 4	4580
	11th to 20th	Night	44% 3 4 21% 21% 6%	5201
	21st to end	Night	46% 3 8% 24% 15% 3	6720

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	27% 18% 32% 18% 4 1	300
	11th to 20th	Day	24% 20% 42% 12% 1 1	300
	21st to end	Day	21% 22% 36% 17% 4	330
February	1st to 10th	Day	29% 19% 33% 14% 4 1	300
	11th to 20th	Day	25% 28% 32% 12% 2	300
	21st to end	Day	35% 22% 31% 10% 2	225
March	1st to 10th	Day	25% 19% 35% 17% 4	270
	11th to 20th	Day	27% 25% 30% 17% 1	270
	21st to end	Day	25% 22% 37% 12% 4	311
April	1st to 10th	Day	20% 18% 32% 22% 7% 1	460
	11th to 20th	Day	23% 15% 33% 20% 9% 1	452
	21st to end	Day	18% 16% 32% 22% 10% 2	460
May	1st to 10th	Day	19% 13% 29% 28% 10% 1	463
	11th to 20th	Day	16% 7% 26% 28% 18% 5%	633
	21st to end	Day	9% 7% 24% 34% 20% 6%	734
June	1st to 10th	Day	10% 5% 17% 30% 28% 10%	680
	11th to 20th	Day	5% 2 15% 38% 30% 10%	680
	21st to end	Day	5% 3 17% 35% 28% 12%	680
July	1st to 10th	Day	6% 2 10% 31% 37% 14%	671
	11th to 20th	Day	6% 2 9% 31% 34% 17%	668
	21st to end	Day	7% 1 11% 30% 35% 15%	739
August	1st to 10th	Day	7% 2 10% 36% 35% 11%	676
	11th to 20th	Day	15% 3 8% 29% 34% 11%	484
	21st to end	Day	11% 1 7% 24% 37% 20%	539
September	1st to 10th	Day	10% 2 12% 26% 36% 14%	490
	11th to 20th	Day	12% 2 13% 23% 38% 11%	490
	21st to end	Day	13% 6% 16% 34% 24% 8%	335
October	1st to 10th	Day	8% 4 28% 32% 23% 5%	300
	11th to 20th	Day	12% 13% 33% 25% 14% 3	300
	21st to end	Day	25% 13% 30% 21% 9% 1	330
November	1st to 10th	Day	19% 13% 32% 22% 14% 1	300
	11th to 20th	Day	11% 15% 38% 27% 8% 1	300
	21st to end	Day	26% 17% 27% 19% 10% 1	300
December	1st to 10th	Day	22% 21% 29% 20% 7% 2	300
	11th to 20th	Day	25% 18% 33% 16% 8%	300
	21st to end	Day	26% 20% 34% 17% 2 1	330
January	1st to 10th	Night	24% 10% 35% 26% 5%	1000
	11th to 20th	Night	21% 13% 36% 26% 3 1	1000
	21st to end	Night	20% 12% 38% 27% 3	1100
February	1st to 10th	Night	25% 13% 31% 27% 4	1000
	11th to 20th	Night	23% 15% 37% 24% 2	833
	21st to end	Night	23% 15% 39% 20% 3	623
March	1st to 10th	Night	20% 11% 35% 30% 4	570
	11th to 20th	Night	19% 13% 36% 27% 4 1	570
	21st to end	Night	17% 12% 37% 27% 8%	627
April	1st to 10th	Night	15% 13% 36% 28% 8%	570
	11th to 20th	Night	15% 11% 35% 27% 11% 1	563
	21st to end	Night	16% 9% 35% 29% 10%	570
May	1st to 10th	Night	16% 5% 24% 42% 12%	570
	11th to 20th	Night	14% 7% 29% 30% 18% 3	561
	21st to end	Night	8% 5% 21% 39% 23% 5%	617
June	1st to 10th	Night	6% 4 13% 36% 36% 5%	570
	11th to 20th	Night	3 1 8% 48% 36% 3	570
	21st to end	Night	5% 1 11% 40% 38% 6%	570
July	1st to 10th	Night	5% 1 7% 36% 42% 9%	555
	11th to 20th	Night	4 2 7% 33% 45% 9%	552
	21st to end	Night	6% 1 7% 36% 43% 7%	624
August	1st to 10th	Night	2 1 6% 40% 41% 9%	567
	11th to 20th	Night	8% 1 8% 39% 38% 6%	564
	21st to end	Night	6% 1 10% 32% 44% 7%	627
September	1st to 10th	Night	8% 2 6% 40% 38% 6%	570
	11th to 20th	Night	9% 2 12% 35% 37% 4	570
	21st to end	Night	9% 5% 14% 36% 33% 3	679
October	1st to 10th	Night	10% 2 18% 35% 31% 4	760
	11th to 20th	Night	14% 5% 27% 35% 18% 1	760
	21st to end	Night	18% 7% 26% 32% 14% 1	836
November	1st to 10th	Night	15% 6% 26% 37% 14% 1	780
	11th to 20th	Night	14% 9% 31% 35% 12%	800
	21st to end	Night	17% 10% 31% 29% 12%	830
December	1st to 10th	Night	22% 8% 33% 30% 6%	1000
	11th to 20th	Night	23% 10% 28% 33% 6%	1000
	21st to end	Night	23% 13% 31% 27% 5%	1100

Page generated at 2025-02-18 02:26:29. Version ac07

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