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 2024-12-02.

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	8% 8% 3 7% 5% 5% 5% 4 6% 8% 41%	105773
	Night	6% 9% 3 8% 5% 5% 5% 4 6% 8% 42%	102445
Transparency	Day	38% 6% 15% 25% 12% 3	105520
	Night	38% 6% 15% 25% 13% 3	102344
Seeing	Day	14% 15% 32% 26% 11% 3	15414
	Night	11% 11% 35% 30% 12% 2	24543

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	2 9% 7% 11% 6% 5% 5% 3 7% 9% 37%	2219
	11th to 20th	Day	2 8% 3 10% 6% 7% 6% 5% 8% 8% 37%	2298
	21st to end	Day	2 8% 3 9% 5% 6% 5% 5% 7% 9% 42%	2794
February	1st to 10th	Day	3 10% 5% 11% 6% 5% 6% 4 7% 9% 32%	1525
	11th to 20th	Day	3 11% 5% 10% 8% 5% 6% 5% 8% 9% 31%	1665
	21st to end	Day	2 12% 4 11% 8% 5% 6% 5% 6% 9% 33%	1541
March	1st to 10th	Day	1 9% 4 11% 7% 7% 5% 4 6% 8% 37%	3138
	11th to 20th	Day	6% 9% 3 6% 4 4 5% 3 5% 8% 48%	3453
	21st to end	Day	8% 19% 6% 13% 6% 5% 4 4 5% 6% 25%	3876
April	1st to 10th	Day	9% 14% 5% 10% 6% 5% 4 4 7% 8% 30%	2323
	11th to 20th	Day	10% 14% 4 8% 5% 5% 5% 3 6% 8% 32%	2440
	21st to end	Day	12% 12% 4 7% 5% 5% 4 4 6% 7% 34%	2470
May	1st to 10th	Day	19% 14% 3 7% 4 4 4 4 5% 6% 29%	4250
	11th to 20th	Day	11% 7% 3 8% 6% 6% 6% 5% 7% 9% 32%	4694
	21st to end	Day	16% 9% 3 8% 7% 6% 6% 5% 6% 7% 27%	5161
June	1st to 10th	Day	16% 8% 3 7% 5% 6% 6% 5% 7% 8% 30%	2850
	11th to 20th	Day	21% 8% 3 7% 5% 4 5% 5% 6% 7% 29%	2850
	21st to end	Day	9% 5% 2 6% 4 4 6% 4 7% 9% 43%	2850
July	1st to 10th	Day	7% 4 2 5% 4 4 5% 5% 6% 8% 50%	4740
	11th to 20th	Day	6% 3 1 4 3 3 4 4 7% 10% 55%	4740
	21st to end	Day	3 3 1 3 3 3 4 3 6% 8% 62%	4959
August	1st to 10th	Day	9% 6% 2 5% 3 4 5% 5% 8% 10% 44%	4401
	11th to 20th	Day	7% 5% 2 5% 3 4 5% 5% 8% 10% 47%	4039
	21st to end	Day	8% 4 1 4 3 3 4 4 6% 8% 56%	4521
September	1st to 10th	Day	11% 5% 2 6% 4 4 3 3 5% 7% 51%	2342
	11th to 20th	Day	12% 7% 2 5% 3 4 4 4 5% 7% 46%	2090
	21st to end	Day	8% 8% 2 5% 4 4 3 4 5% 8% 49%	2090
October	1st to 10th	Day	6% 6% 2 6% 4 5% 4 4 6% 10% 46%	3362
	11th to 20th	Day	4 7% 2 6% 4 4 4 4 8% 12% 45%	2921
	21st to end	Day	5% 8% 3 5% 4 5% 4 4 6% 8% 47%	3101
November	1st to 10th	Day	7% 14% 3 8% 7% 5% 4 4 5% 7% 36%	1659
	11th to 20th	Day	4 10% 3 8% 4 5% 5% 4 6% 8% 42%	1550
	21st to end	Day	3 12% 4 10% 5% 4 4 4 7% 10% 38%	1400
December	1st to 10th	Day	3 9% 6% 10% 6% 5% 6% 5% 6% 8% 38%	1337
	11th to 20th	Day	3 10% 5% 13% 6% 6% 5% 5% 7% 7% 34%	1682
	21st to end	Day	3 8% 3 7% 4 5% 5% 4 6% 8% 47%	2442
January	1st to 10th	Night	2 10% 5% 11% 7% 6% 6% 5% 7% 9% 31%	4726
	11th to 20th	Night	3 7% 4 14% 6% 5% 6% 6% 7% 8% 35%	4631
	21st to end	Night	3 11% 4 10% 6% 5% 5% 4 6% 8% 40%	4862
February	1st to 10th	Night	4 10% 5% 9% 5% 5% 6% 5% 7% 8% 35%	2654
	11th to 20th	Night	4 13% 5% 11% 5% 5% 5% 4 6% 8% 34%	2402
	21st to end	Night	3 9% 5% 11% 9% 6% 5% 4 6% 6% 37%	1911
March	1st to 10th	Night	1 9% 4 13% 8% 8% 7% 4 6% 7% 33%	3736
	11th to 20th	Night	3 8% 3 6% 5% 4 5% 4 6% 8% 47%	3526
	21st to end	Night	10% 21% 6% 11% 5% 3 4 3 6% 6% 24%	3774
April	1st to 10th	Night	8% 13% 5% 9% 5% 4 4 4 6% 8% 35%	1856
	11th to 20th	Night	10% 11% 5% 10% 5% 4 5% 4 5% 6% 34%	1691
	21st to end	Night	10% 11% 3 9% 5% 4 6% 3 6% 7% 36%	1710
May	1st to 10th	Night	19% 13% 3 7% 5% 4 5% 4 4 6% 31%	2715
	11th to 20th	Night	10% 8% 3 7% 5% 6% 4 4 7% 10% 36%	2199
	21st to end	Night	15% 8% 3 7% 6% 6% 5% 5% 7% 8% 31%	2414
June	1st to 10th	Night	14% 8% 3 6% 5% 4 4 3 7% 7% 39%	1330
	11th to 20th	Night	20% 7% 2 5% 5% 5% 4 5% 5% 8% 33%	1330
	21st to end	Night	8% 4 1 5% 4 4 5% 5% 6% 9% 48%	1330
July	1st to 10th	Night	5% 3 2 6% 5% 5% 5% 4 6% 7% 51%	2220
	11th to 20th	Night	5% 3 1 3 3 3 5% 4 7% 9% 56%	2220
	21st to end	Night	4 3 1 3 3 3 3 3 5% 8% 64%	2589
August	1st to 10th	Night	9% 5% 2 5% 5% 4 5% 4 7% 8% 45%	2529
	11th to 20th	Night	5% 4 2 7% 5% 5% 5% 5% 7% 10% 45%	2798
	21st to end	Night	7% 3 1 4 2 3 4 3 6% 8% 57%	3146
September	1st to 10th	Night	11% 4 1 5% 3 3 3 3 6% 6% 54%	1836
	11th to 20th	Night	14% 7% 1 4 2 3 3 3 4 7% 52%	2090
	21st to end	Night	10% 6% 2 5% 2 4 4 3 6% 9% 50%	2090
October	1st to 10th	Night	7% 5% 2 5% 3 3 4 4 6% 7% 54%	3613
	11th to 20th	Night	5% 6% 1 5% 4 4 4 4 7% 10% 51%	3993
	21st to end	Night	5% 9% 3 6% 4 4 4 3 7% 8% 48%	4457
November	1st to 10th	Night	7% 12% 4 7% 5% 5% 5% 3 7% 8% 37%	2620
	11th to 20th	Night	3 9% 3 8% 6% 4 5% 4 7% 9% 42%	2845
	21st to end	Night	3 11% 3 9% 5% 5% 5% 4 6% 9% 40%	3000
December	1st to 10th	Night	3 9% 4 9% 6% 5% 6% 5% 7% 9% 37%	2878
	11th to 20th	Night	1 8% 4 12% 7% 5% 6% 4 7% 9% 36%	3510
	21st to end	Night	2 10% 3 8% 5% 5% 4 4 6% 7% 45%	5214

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% 7% 14% 25% 11% 3	2219
	11th to 20th	Day	43% 6% 10% 20% 18% 4	2319
	21st to end	Day	36% 7% 16% 30% 9% 1	2794
February	1st to 10th	Day	46% 8% 15% 20% 9% 3	1525
	11th to 20th	Day	45% 8% 14% 21% 9% 3	1674
	21st to end	Day	48% 5% 14% 19% 13% 1	1541
March	1st to 10th	Day	39% 11% 13% 20% 15% 2	3138
	11th to 20th	Day	34% 5% 16% 29% 15% 1	3454
	21st to end	Day	56% 7% 13% 18% 5%	3876
April	1st to 10th	Day	50% 6% 16% 20% 8% 1	2323
	11th to 20th	Day	50% 5% 14% 20% 10% 2	2440
	21st to end	Day	46% 5% 16% 21% 10% 1	2470
May	1st to 10th	Day	56% 4 10% 23% 6%	4250
	11th to 20th	Day	44% 7% 14% 24% 10% 1	4698
	21st to end	Day	50% 11% 15% 14% 9% 1	5162
June	1st to 10th	Day	48% 8% 16% 17% 10% 1	2850
	11th to 20th	Day	53% 6% 14% 17% 9% 2	2850
	21st to end	Day	34% 8% 19% 24% 12% 3	2850
July	1st to 10th	Day	29% 5% 15% 29% 17% 6%	4666
	11th to 20th	Day	24% 5% 18% 33% 18% 3	4659
	21st to end	Day	19% 4 10% 28% 24% 15%	4802
August	1st to 10th	Day	32% 6% 18% 34% 10% 2	4384
	11th to 20th	Day	29% 6% 24% 30% 9% 2	4078
	21st to end	Day	26% 4 14% 38% 15% 2	4520
September	1st to 10th	Day	33% 4 13% 26% 18% 6%	2342
	11th to 20th	Day	35% 6% 15% 27% 15% 2	2090
	21st to end	Day	34% 6% 13% 30% 14% 3	2090
October	1st to 10th	Day	32% 5% 15% 36% 10% 1	3362
	11th to 20th	Day	31% 4 21% 27% 15% 3	2921
	21st to end	Day	35% 5% 11% 33% 13% 4	3101
November	1st to 10th	Day	47% 6% 13% 22% 12% 1	1659
	11th to 20th	Day	39% 6% 18% 23% 13% 1	1550
	21st to end	Day	40% 7% 18% 23% 10% 3	1400
December	1st to 10th	Day	43% 7% 14% 20% 12% 4	1337
	11th to 20th	Day	46% 6% 18% 20% 8% 1	1684
	21st to end	Day	33% 5% 12% 25% 19% 6%	2442
January	1st to 10th	Night	45% 8% 14% 22% 8% 2	4726
	11th to 20th	Night	44% 6% 12% 17% 15% 5%	4672
	21st to end	Night	39% 6% 16% 24% 12% 1	4862
February	1st to 10th	Night	44% 8% 12% 21% 12% 3	2654
	11th to 20th	Night	47% 6% 13% 20% 11% 2	2403
	21st to end	Night	47% 6% 10% 22% 12% 3	1911
March	1st to 10th	Night	46% 8% 11% 22% 12% 1	3736
	11th to 20th	Night	31% 10% 19% 24% 16% 1	3526
	21st to end	Night	57% 9% 11% 17% 5% 1	3774
April	1st to 10th	Night	45% 7% 13% 22% 12% 1	1856
	11th to 20th	Night	49% 6% 12% 23% 9% 1	1691
	21st to end	Night	46% 6% 13% 24% 9% 2	1710
May	1st to 10th	Night	53% 5% 12% 24% 6%	2715
	11th to 20th	Night	39% 7% 13% 31% 9% 1	2199
	21st to end	Night	47% 8% 16% 18% 9% 1	2414
June	1st to 10th	Night	43% 5% 15% 24% 11% 1	1330
	11th to 20th	Night	47% 5% 14% 22% 9% 2	1330
	21st to end	Night	30% 7% 20% 27% 15% 2	1330
July	1st to 10th	Night	30% 6% 17% 24% 19% 4	2185
	11th to 20th	Night	23% 5% 20% 36% 13% 4	2178
	21st to end	Night	19% 4 10% 32% 22% 13%	2507
August	1st to 10th	Night	34% 6% 16% 27% 15% 2	2521
	11th to 20th	Night	33% 5% 21% 29% 10% 2	2825
	21st to end	Night	24% 4 16% 37% 16% 3	3146
September	1st to 10th	Night	29% 4 16% 29% 17% 5%	1836
	11th to 20th	Night	32% 5% 15% 29% 16% 4	2090
	21st to end	Night	32% 5% 17% 28% 15% 4	2090
October	1st to 10th	Night	28% 5% 12% 30% 23% 2	3613
	11th to 20th	Night	28% 4 20% 26% 18% 3	3993
	21st to end	Night	34% 5% 12% 33% 16% 1	4457
November	1st to 10th	Night	44% 7% 14% 22% 12% 2	2620
	11th to 20th	Night	36% 6% 17% 23% 16% 1	2845
	21st to end	Night	39% 5% 19% 22% 12% 4	3000
December	1st to 10th	Night	41% 7% 16% 21% 11% 4	2878
	11th to 20th	Night	41% 7% 21% 22% 7% 1	3507
	21st to end	Night	36% 5% 13% 26% 14% 6%	5214

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% 12% 39% 24% 13% 7%	180
	11th to 20th	Day	11% 13% 36% 27% 11% 4	180
	21st to end	Day	9% 13% 43% 25% 8% 3	198
February	1st to 10th	Day	14% 21% 38% 20% 5% 2	183
	11th to 20th	Day	7% 20% 36% 27% 10% 1	270
	21st to end	Day	12% 16% 37% 25% 9%	353
March	1st to 10th	Day	9% 25% 39% 17% 7% 2	460
	11th to 20th	Day	9% 20% 34% 25% 8% 3	460
	21st to end	Day	13% 24% 33% 20% 8% 2	506
April	1st to 10th	Day	17% 24% 33% 20% 5%	460
	11th to 20th	Day	19% 15% 34% 20% 10% 1	452
	21st to end	Day	19% 23% 28% 21% 7% 2	460
May	1st to 10th	Day	21% 12% 27% 27% 9% 4	477
	11th to 20th	Day	19% 11% 23% 28% 15% 4	675
	21st to end	Day	20% 11% 23% 29% 15% 3	734
June	1st to 10th	Day	17% 14% 32% 23% 11% 3	680
	11th to 20th	Day	21% 15% 29% 23% 10% 2	680
	21st to end	Day	11% 11% 31% 29% 15% 4	680
July	1st to 10th	Day	10% 8% 30% 32% 17% 3	671
	11th to 20th	Day	9% 10% 30% 33% 15% 3	668
	21st to end	Day	6% 12% 33% 29% 16% 3	635
August	1st to 10th	Day	10% 10% 32% 32% 13% 3	487
	11th to 20th	Day	7% 9% 31% 34% 14% 4	477
	21st to end	Day	10% 13% 36% 23% 14% 5%	539
September	1st to 10th	Day	12% 15% 30% 28% 10% 5%	490
	11th to 20th	Day	17% 12% 28% 26% 13% 3	490
	21st to end	Day	19% 19% 29% 24% 7% 2	490
October	1st to 10th	Day	14% 14% 29% 29% 11% 2	414
	11th to 20th	Day	14% 15% 35% 21% 12% 4	300
	21st to end	Day	17% 18% 34% 22% 7% 2	330
November	1st to 10th	Day	18% 17% 33% 21% 10% 2	300
	11th to 20th	Day	15% 19% 37% 19% 9% 1	275
	21st to end	Day	9% 17% 34% 24% 9% 8%	200
December	1st to 10th	Day	7% 16% 38% 26% 10% 2	182
	11th to 20th	Day	8% 19% 39% 22% 9% 3	180
	21st to end	Day	9% 11% 32% 29% 14% 5%	198
January	1st to 10th	Night	9% 17% 39% 25% 9% 1	950
	11th to 20th	Night	8% 12% 44% 27% 8% 1	908
	21st to end	Night	7% 15% 42% 28% 8% 1	836
February	1st to 10th	Night	8% 17% 36% 27% 10% 3	760
	11th to 20th	Night	9% 12% 39% 29% 10% 1	744
	21st to end	Night	8% 11% 36% 28% 14% 2	628
March	1st to 10th	Night	9% 12% 40% 26% 12% 2	760
	11th to 20th	Night	8% 15% 40% 26% 9% 2	760
	21st to end	Night	11% 11% 35% 31% 9% 3	836
April	1st to 10th	Night	14% 14% 34% 28% 9% 1	716
	11th to 20th	Night	16% 9% 35% 30% 9%	563
	21st to end	Night	14% 13% 34% 30% 10%	570
May	1st to 10th	Night	18% 11% 27% 31% 13%	504
	11th to 20th	Night	15% 8% 25% 31% 21% 1	374
	21st to end	Night	22% 7% 23% 32% 16% 1	411
June	1st to 10th	Night	18% 10% 28% 29% 13% 2	380
	11th to 20th	Night	22% 9% 29% 28% 12%	380
	21st to end	Night	10% 6% 23% 38% 21% 1	380
July	1st to 10th	Night	10% 4 20% 43% 24%	370
	11th to 20th	Night	9% 4 26% 42% 17% 2	368
	21st to end	Night	6% 5% 22% 44% 22% 2	416
August	1st to 10th	Night	10% 5% 15% 43% 26% 2	378
	11th to 20th	Night	8% 3 22% 44% 21% 2	551
	21st to end	Night	10% 6% 28% 39% 16% 1	627
September	1st to 10th	Night	12% 5% 27% 35% 18% 3	677
	11th to 20th	Night	18% 4 24% 34% 17% 3	760
	21st to end	Night	15% 9% 32% 29% 11% 3	760
October	1st to 10th	Night	16% 7% 29% 33% 13% 3	760
	11th to 20th	Night	12% 10% 35% 28% 13% 2	760
	21st to end	Night	11% 11% 41% 28% 7% 1	836
November	1st to 10th	Night	14% 13% 40% 25% 7%	865
	11th to 20th	Night	12% 18% 42% 23% 4 1	1000
	21st to end	Night	9% 14% 40% 27% 8% 2	1000
December	1st to 10th	Night	8% 13% 39% 28% 9% 2	960
	11th to 20th	Night	6% 15% 42% 30% 7% 1	950
	21st to end	Night	11% 13% 37% 28% 10% 2	1045

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