Fifth Meridian Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fifth Meridian Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 3 7% 5% 5% 5% 4 6% 8% 41%	134833
	Night	5% 9% 3 8% 5% 5% 5% 4 6% 9% 41%	135070
Transparency	Day	38% 6% 15% 25% 13% 3	134538
	Night	39% 6% 15% 25% 13% 3	134944
Seeing	Day	13% 15% 32% 26% 12% 3	16942
	Night	11% 11% 35% 30% 11% 2	26236

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	1 8% 6% 12% 7% 5% 5% 4 8% 9% 35%	4125
	11th to 20th	Day	1 8% 3 9% 5% 6% 5% 4 7% 7% 45%	4316
	21st to end	Day	1 6% 3 10% 5% 5% 5% 4 6% 9% 45%	5186
February	1st to 10th	Day	3 9% 5% 11% 7% 5% 6% 5% 7% 9% 33%	1687
	11th to 20th	Day	3 10% 4 10% 8% 5% 6% 5% 8% 9% 32%	1845
	21st to end	Day	2 11% 4 10% 8% 5% 6% 5% 6% 9% 34%	1699
March	1st to 10th	Day	2 9% 4 9% 5% 5% 4 3 6% 11% 41%	4539
	11th to 20th	Day	4 9% 4 9% 5% 5% 5% 4 5% 8% 43%	4950
	21st to end	Day	7% 19% 6% 13% 5% 5% 4 4 6% 7% 24%	5563
April	1st to 10th	Day	8% 14% 4 10% 6% 5% 4 4 6% 8% 31%	2446
	11th to 20th	Day	10% 14% 4 8% 5% 5% 5% 3 6% 8% 32%	2570
	21st to end	Day	11% 12% 4 7% 5% 5% 4 4 6% 7% 35%	2600
May	1st to 10th	Day	15% 11% 3 6% 3 3 3 4 6% 6% 40%	6086
	11th to 20th	Day	12% 9% 3 9% 6% 6% 6% 5% 7% 9% 26%	6734
	21st to end	Day	12% 6% 2 7% 6% 6% 6% 4 6% 7% 38%	7405
June	1st to 10th	Day	16% 8% 3 6% 5% 6% 6% 5% 7% 8% 32%	3000
	11th to 20th	Day	20% 8% 3 7% 5% 4 5% 5% 7% 7% 29%	3000
	21st to end	Day	9% 5% 2 5% 5% 5% 6% 4 7% 9% 42%	3000
July	1st to 10th	Day	7% 4 2 5% 4 4 4 4 6% 9% 50%	6780
	11th to 20th	Day	10% 4 1 4 4 4 5% 4 7% 9% 49%	6780
	21st to end	Day	7% 5% 2 5% 3 3 4 3 5% 8% 54%	7113
August	1st to 10th	Day	9% 6% 2 5% 4 4 5% 5% 8% 10% 44%	4541
	11th to 20th	Day	7% 5% 2 5% 3 4 5% 5% 8% 10% 47%	4169
	21st to end	Day	8% 4 1 4 3 3 4 4 6% 8% 56%	4664
September	1st to 10th	Day	11% 5% 2 6% 4 4 3 3 5% 7% 51%	2465
	11th to 20th	Day	12% 7% 2 5% 3 4 4 4 5% 8% 47%	2200
	21st to end	Day	8% 8% 2 5% 4 4 4 4 5% 8% 50%	2200
October	1st to 10th	Day	6% 6% 2 6% 4 4 4 4 6% 10% 47%	3468
	11th to 20th	Day	4 7% 2 6% 4 4 4 4 8% 11% 46%	3014
	21st to end	Day	5% 8% 3 5% 4 5% 4 4 6% 8% 48%	3200
November	1st to 10th	Day	7% 13% 3 8% 6% 5% 4 4 5% 7% 37%	1744
	11th to 20th	Day	4 11% 3 7% 4 5% 5% 4 6% 8% 42%	1627
	21st to end	Day	3 12% 4 10% 5% 4 4 4 7% 9% 38%	1470
December	1st to 10th	Day	2 8% 5% 9% 6% 6% 6% 5% 7% 9% 39%	2360
	11th to 20th	Day	2 8% 4 11% 6% 5% 5% 5% 8% 8% 38%	2712
	21st to end	Day	3 9% 4 10% 4 5% 5% 4 6% 8% 43%	3575
January	1st to 10th	Night	2 10% 5% 11% 8% 6% 5% 4 7% 9% 32%	8768
	11th to 20th	Night	2 7% 4 11% 6% 5% 6% 5% 7% 9% 39%	8641
	21st to end	Night	2 7% 4 11% 6% 5% 5% 4 6% 8% 42%	8994
February	1st to 10th	Night	3 10% 5% 9% 6% 6% 7% 5% 7% 9% 34%	2932
	11th to 20th	Night	3 12% 5% 11% 6% 6% 5% 4 6% 8% 34%	2662
	21st to end	Night	3 9% 5% 10% 8% 6% 5% 4 6% 6% 38%	2105
March	1st to 10th	Night	2 9% 4 11% 7% 6% 6% 4 6% 10% 36%	5400
	11th to 20th	Night	2 8% 4 9% 6% 5% 5% 4 7% 9% 41%	5049
	21st to end	Night	9% 20% 6% 10% 4 3 4 3 7% 8% 25%	5409
April	1st to 10th	Night	8% 13% 5% 9% 5% 4 4 4 6% 8% 36%	1953
	11th to 20th	Night	9% 11% 5% 9% 5% 4 5% 4 5% 6% 35%	1781
	21st to end	Night	10% 11% 3 8% 6% 5% 6% 3 6% 7% 37%	1800
May	1st to 10th	Night	14% 11% 3 6% 4 4 4 3 4 7% 40%	3885
	11th to 20th	Night	12% 10% 3 9% 5% 5% 5% 4 7% 10% 31%	3159
	21st to end	Night	11% 7% 2 6% 5% 5% 5% 4 6% 7% 42%	3470
June	1st to 10th	Night	14% 8% 3 6% 5% 3 4 3 7% 7% 40%	1400
	11th to 20th	Night	19% 7% 2 6% 5% 5% 4 4 5% 8% 34%	1400
	21st to end	Night	8% 4 1 5% 4 4 5% 5% 6% 9% 48%	1400
July	1st to 10th	Night	6% 4 2 5% 4 5% 5% 4 6% 8% 52%	3180
	11th to 20th	Night	7% 4 2 4 3 3 5% 4 7% 9% 52%	3180
	21st to end	Night	10% 5% 2 4 3 3 3 3 5% 8% 54%	3726
August	1st to 10th	Night	8% 6% 2 5% 5% 4 5% 4 7% 9% 45%	2609
	11th to 20th	Night	6% 4 2 7% 5% 5% 5% 5% 6% 10% 45%	2888
	21st to end	Night	7% 3 1 4 2 3 4 4 6% 8% 58%	3245
September	1st to 10th	Night	11% 4 1 5% 3 3 4 3 6% 7% 54%	1933
	11th to 20th	Night	13% 7% 1 4 2 3 3 3 4 8% 53%	2200
	21st to end	Night	10% 6% 2 5% 2 4 4 3 6% 9% 50%	2200
October	1st to 10th	Night	7% 5% 2 5% 3 3 4 4 6% 8% 54%	3727
	11th to 20th	Night	5% 6% 2 5% 4 4 4 4 7% 9% 51%	4120
	21st to end	Night	5% 9% 3 6% 4 4 4 3 7% 8% 48%	4600
November	1st to 10th	Night	7% 12% 3 7% 5% 5% 5% 3 7% 8% 37%	2754
	11th to 20th	Night	3 9% 3 8% 6% 4 5% 4 7% 9% 41%	2987
	21st to end	Night	3 11% 4 9% 4 5% 5% 4 5% 9% 41%	3150
December	1st to 10th	Night	2 7% 3 8% 5% 5% 6% 5% 8% 11% 39%	5040
	11th to 20th	Night	2 9% 4 11% 7% 5% 6% 4 8% 10% 33%	5700
	21st to end	Night	3 11% 4 9% 5% 5% 4 4 6% 8% 41%	7623

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	42% 7% 12% 22% 13% 3	4125
	11th to 20th	Day	37% 4 12% 27% 16% 4	4337
	21st to end	Day	34% 5% 13% 31% 15% 1	5187
February	1st to 10th	Day	45% 7% 14% 20% 10% 3	1687
	11th to 20th	Day	44% 7% 14% 22% 10% 3	1854
	21st to end	Day	45% 5% 14% 20% 14% 1	1699
March	1st to 10th	Day	35% 8% 13% 24% 19% 1	4539
	11th to 20th	Day	38% 5% 12% 26% 18% 1	4951
	21st to end	Day	55% 7% 15% 16% 6% 1	5563
April	1st to 10th	Day	49% 6% 15% 20% 9% 1	2446
	11th to 20th	Day	50% 5% 14% 20% 10% 2	2570
	21st to end	Day	46% 5% 16% 22% 11% 1	2600
May	1st to 10th	Day	44% 4 9% 22% 17% 4	6086
	11th to 20th	Day	50% 7% 13% 22% 8%	6738
	21st to end	Day	40% 8% 16% 22% 13% 1	7406
June	1st to 10th	Day	46% 8% 15% 18% 11% 2	3000
	11th to 20th	Day	52% 6% 14% 18% 9% 2	3000
	21st to end	Day	34% 8% 19% 24% 12% 3	3000
July	1st to 10th	Day	29% 4 17% 27% 18% 5%	6706
	11th to 20th	Day	29% 5% 21% 29% 14% 2	6699
	21st to end	Day	28% 4 14% 27% 17% 10%	6913
August	1st to 10th	Day	32% 5% 18% 33% 10% 2	4524
	11th to 20th	Day	29% 6% 24% 30% 9% 2	4208
	21st to end	Day	25% 4 14% 38% 16% 2	4663
September	1st to 10th	Day	32% 4 13% 27% 18% 6%	2465
	11th to 20th	Day	34% 6% 14% 28% 15% 2	2200
	21st to end	Day	33% 6% 14% 30% 14% 3	2200
October	1st to 10th	Day	32% 5% 15% 36% 10% 1	3468
	11th to 20th	Day	30% 4 21% 27% 15% 3	3014
	21st to end	Day	34% 5% 11% 33% 13% 4	3200
November	1st to 10th	Day	47% 6% 12% 22% 12% 1	1744
	11th to 20th	Day	39% 6% 18% 24% 12% 1	1627
	21st to end	Day	41% 6% 18% 23% 9% 3	1470
December	1st to 10th	Day	41% 6% 15% 26% 9% 3	2360
	11th to 20th	Day	42% 5% 23% 21% 8% 1	2714
	21st to end	Day	38% 5% 10% 25% 18% 4	3575
January	1st to 10th	Night	47% 7% 12% 22% 11% 2	8769
	11th to 20th	Night	40% 5% 16% 22% 12% 5%	8682
	21st to end	Night	39% 5% 16% 26% 13% 1	8994
February	1st to 10th	Night	45% 7% 12% 20% 13% 3	2932
	11th to 20th	Night	48% 6% 12% 20% 12% 2	2663
	21st to end	Night	46% 5% 12% 23% 11% 3	2105
March	1st to 10th	Night	41% 7% 15% 22% 14% 2	5400
	11th to 20th	Night	37% 8% 14% 24% 16% 1	5049
	21st to end	Night	54% 7% 14% 15% 9% 1	5409
April	1st to 10th	Night	44% 7% 12% 23% 12% 1	1953
	11th to 20th	Night	48% 6% 12% 24% 9% 1	1781
	21st to end	Night	46% 6% 13% 24% 9% 2	1800
May	1st to 10th	Night	44% 4 11% 23% 15% 2	3882
	11th to 20th	Night	46% 6% 13% 28% 7% 1	3159
	21st to end	Night	38% 7% 16% 26% 11% 2	3470
June	1st to 10th	Night	42% 6% 15% 25% 12% 1	1400
	11th to 20th	Night	47% 5% 14% 22% 9% 2	1400
	21st to end	Night	30% 7% 20% 27% 14% 2	1400
July	1st to 10th	Night	29% 5% 18% 23% 21% 4	3145
	11th to 20th	Night	29% 5% 21% 33% 10% 3	3138
	21st to end	Night	30% 4 16% 26% 15% 9%	3621
August	1st to 10th	Night	34% 5% 17% 27% 14% 2	2601
	11th to 20th	Night	34% 5% 21% 29% 10% 2	2915
	21st to end	Night	24% 4 16% 37% 17% 3	3245
September	1st to 10th	Night	29% 4 16% 30% 16% 5%	1933
	11th to 20th	Night	31% 5% 14% 29% 16% 4	2200
	21st to end	Night	31% 5% 17% 28% 15% 4	2200
October	1st to 10th	Night	28% 5% 12% 31% 22% 2	3727
	11th to 20th	Night	29% 4 20% 25% 19% 3	4120
	21st to end	Night	34% 4 12% 34% 16% 1	4600
November	1st to 10th	Night	43% 6% 14% 22% 12% 2	2754
	11th to 20th	Night	37% 6% 17% 23% 16% 1	2987
	21st to end	Night	38% 5% 19% 22% 12% 4	3150
December	1st to 10th	Night	36% 7% 23% 21% 10% 2	5040
	11th to 20th	Night	44% 6% 23% 18% 8% 1	5697
	21st to end	Night	39% 4 11% 25% 15% 4	7623

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	5% 13% 40% 23% 12% 6%	220
	11th to 20th	Day	12% 13% 38% 25% 10% 3	220
	21st to end	Day	9% 13% 43% 24% 7% 3	242
February	1st to 10th	Day	12% 21% 37% 22% 6% 2	225
	11th to 20th	Day	6% 17% 36% 31% 8% 1	330
	21st to end	Day	14% 18% 35% 23% 9%	415
March	1st to 10th	Day	10% 24% 38% 17% 8% 3	509
	11th to 20th	Day	9% 20% 36% 25% 8% 3	500
	21st to end	Day	14% 23% 34% 19% 8% 2	550
April	1st to 10th	Day	17% 23% 33% 20% 6% 1	500
	11th to 20th	Day	20% 16% 34% 19% 10% 1	492
	21st to end	Day	19% 22% 28% 21% 8% 3	500
May	1st to 10th	Day	20% 13% 27% 28% 8% 4	518
	11th to 20th	Day	19% 11% 22% 27% 15% 4	725
	21st to end	Day	19% 10% 23% 30% 15% 3	789
June	1st to 10th	Day	16% 14% 33% 23% 11% 2	730
	11th to 20th	Day	20% 14% 29% 25% 10% 2	730
	21st to end	Day	11% 10% 30% 30% 15% 4	730
July	1st to 10th	Day	10% 8% 30% 32% 16% 3	721
	11th to 20th	Day	10% 9% 30% 33% 15% 3	718
	21st to end	Day	7% 12% 32% 29% 16% 3	684
August	1st to 10th	Day	10% 10% 31% 32% 13% 4	527
	11th to 20th	Day	7% 9% 32% 33% 14% 4	517
	21st to end	Day	9% 12% 34% 24% 15% 6%	583
September	1st to 10th	Day	12% 14% 29% 28% 12% 6%	530
	11th to 20th	Day	16% 11% 27% 26% 16% 4	530
	21st to end	Day	17% 18% 31% 24% 7% 2	530
October	1st to 10th	Day	14% 13% 30% 30% 11% 2	450
	11th to 20th	Day	13% 15% 35% 21% 12% 3	330
	21st to end	Day	17% 18% 34% 22% 7% 2	363
November	1st to 10th	Day	19% 18% 33% 20% 9% 2	330
	11th to 20th	Day	15% 19% 36% 19% 10% 2	302
	21st to end	Day	10% 16% 33% 25% 9% 7%	220
December	1st to 10th	Day	8% 17% 40% 24% 9% 2	220
	11th to 20th	Day	9% 19% 38% 21% 10% 3	220
	21st to end	Day	8% 13% 34% 28% 13% 5%	242
January	1st to 10th	Night	9% 17% 40% 25% 9% 1	1050
	11th to 20th	Night	8% 13% 44% 26% 7% 1	1003
	21st to end	Night	6% 15% 44% 27% 7% 1	924
February	1st to 10th	Night	8% 17% 36% 28% 10% 2	840
	11th to 20th	Night	9% 11% 40% 30% 10% 1	824
	21st to end	Night	9% 12% 38% 26% 13% 2	692
March	1st to 10th	Night	9% 11% 40% 26% 12% 2	808
	11th to 20th	Night	8% 14% 40% 28% 9% 2	800
	21st to end	Night	11% 11% 36% 30% 9% 3	880
April	1st to 10th	Night	13% 14% 34% 28% 10% 1	753
	11th to 20th	Night	16% 9% 36% 30% 8%	593
	21st to end	Night	14% 12% 33% 30% 10%	600
May	1st to 10th	Night	18% 11% 26% 32% 13%	530
	11th to 20th	Night	16% 8% 25% 30% 21% 1	394
	21st to end	Night	21% 7% 23% 32% 17% 1	433
June	1st to 10th	Night	18% 10% 28% 30% 13% 2	400
	11th to 20th	Night	21% 9% 29% 31% 12%	400
	21st to end	Night	11% 6% 23% 39% 21% 1	400
July	1st to 10th	Night	10% 4 20% 43% 23%	390
	11th to 20th	Night	9% 4 25% 43% 17% 2	388
	21st to end	Night	8% 5% 21% 42% 21% 2	438
August	1st to 10th	Night	11% 5% 15% 42% 26% 2	398
	11th to 20th	Night	9% 3 22% 44% 20% 2	581
	21st to end	Night	9% 6% 28% 39% 17% 1	660
September	1st to 10th	Night	11% 5% 26% 35% 20% 3	713
	11th to 20th	Night	17% 5% 24% 34% 19% 3	800
	21st to end	Night	14% 9% 34% 30% 11% 3	800
October	1st to 10th	Night	15% 7% 30% 32% 14% 3	800
	11th to 20th	Night	12% 10% 35% 28% 12% 2	800
	21st to end	Night	11% 12% 41% 27% 7% 1	880
November	1st to 10th	Night	14% 13% 41% 25% 7%	909
	11th to 20th	Night	11% 18% 42% 24% 4 1	1050
	21st to end	Night	9% 15% 40% 27% 8% 2	1050
December	1st to 10th	Night	8% 13% 41% 27% 9% 2	1050
	11th to 20th	Night	6% 14% 43% 29% 7% 1	1050
	21st to end	Night	11% 13% 39% 26% 9% 2	1155

Page generated at 2026-02-27 23:40:23. Version ac07

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