Faro
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Faro clear sky chart from 2005-11-06 to 2026-04-02.

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	7% 11% 5% 12% 8% 7% 7% 6% 7% 9% 23%	134622
	Night	4 11% 5% 13% 9% 8% 8% 6% 8% 9% 20%	135798
Transparency	Day	52% 9% 17% 15% 5%	134265
	Night	53% 9% 16% 16% 6% 1	135736
Seeing	Day	14% 9% 21% 28% 21% 7%	18932
	Night	10% 7% 19% 33% 25% 6%	25413

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% 14% 11% 11% 9% 6% 10% 8% 16%	2331
	11th to 20th	Day	1 8% 5% 12% 8% 9% 8% 7% 8% 10% 23%	2746
	21st to end	Day	2 10% 4 14% 8% 7% 9% 6% 7% 11% 22%	3872
February	1st to 10th	Day	1 8% 6% 12% 7% 6% 9% 10% 6% 11% 22%	1365
	11th to 20th	Day	2 11% 6% 12% 12% 8% 9% 7% 7% 11% 18%	1640
	21st to end	Day	1 8% 5% 12% 14% 12% 11% 5% 6% 9% 17%	1440
March	1st to 10th	Day	7% 6% 13% 10% 9% 9% 6% 9% 9% 22%	4582
	11th to 20th	Day	11% 7% 15% 8% 9% 8% 6% 10% 10% 17%	4731
	21st to end	Day	5% 5% 14% 10% 7% 7% 6% 8% 9% 28%	6101
April	1st to 10th	Day	2 13% 7% 14% 8% 6% 7% 5% 6% 8% 23%	2485
	11th to 20th	Day	2 16% 6% 12% 8% 7% 7% 5% 7% 7% 25%	2762
	21st to end	Day	2 12% 4 10% 8% 7% 7% 5% 8% 9% 27%	2895
May	1st to 10th	Day	5% 14% 5% 11% 7% 7% 7% 5% 6% 7% 26%	7148
	11th to 20th	Day	4 12% 5% 12% 7% 7% 7% 6% 8% 9% 24%	7374
	21st to end	Day	6% 13% 5% 12% 8% 7% 7% 6% 7% 8% 21%	8720
June	1st to 10th	Day	12% 11% 4 9% 7% 7% 6% 6% 7% 8% 23%	3600
	11th to 20th	Day	13% 11% 3 10% 8% 8% 6% 5% 7% 8% 20%	3600
	21st to end	Day	13% 10% 4 11% 8% 7% 8% 6% 8% 8% 18%	3600
July	1st to 10th	Day	8% 10% 5% 12% 8% 7% 7% 6% 8% 9% 19%	8160
	11th to 20th	Day	13% 12% 4 11% 7% 7% 7% 6% 8% 8% 18%	8160
	21st to end	Day	12% 8% 4 9% 7% 7% 7% 5% 7% 10% 25%	8026
August	1st to 10th	Day	9% 8% 2 7% 5% 5% 5% 4 7% 7% 40%	5197
	11th to 20th	Day	15% 10% 3 9% 5% 5% 5% 3 6% 8% 31%	4652
	21st to end	Day	14% 13% 4 11% 7% 6% 6% 4 7% 9% 19%	5027
September	1st to 10th	Day	15% 13% 4 11% 7% 6% 6% 5% 6% 7% 21%	2511
	11th to 20th	Day	9% 11% 4 11% 6% 7% 6% 4 6% 9% 27%	2375
	21st to end	Day	12% 16% 5% 11% 6% 7% 6% 5% 6% 7% 19%	2161
October	1st to 10th	Day	6% 13% 4 11% 5% 5% 7% 5% 7% 9% 28%	3174
	11th to 20th	Day	5% 11% 4 10% 6% 8% 7% 5% 6% 8% 28%	2933
	21st to end	Day	4 17% 8% 14% 8% 7% 7% 5% 7% 8% 17%	2667
November	1st to 10th	Day	3 15% 7% 14% 9% 6% 8% 7% 9% 9% 14%	1435
	11th to 20th	Day	3 13% 7% 15% 12% 10% 8% 6% 8% 8% 11%	1154
	21st to end	Day	1 12% 5% 13% 9% 9% 8% 8% 10% 12% 11%	1050
December	1st to 10th	Day	2 10% 5% 16% 8% 9% 10% 6% 9% 9% 15%	1382
	11th to 20th	Day	1 10% 5% 11% 9% 10% 9% 7% 10% 9% 19%	1542
	21st to end	Day	2 12% 6% 16% 9% 8% 7% 7% 8% 8% 17%	2024
January	1st to 10th	Night	1 8% 5% 16% 11% 11% 10% 6% 9% 8% 15%	10562
	11th to 20th	Night	2 9% 5% 13% 9% 9% 7% 5% 8% 11% 22%	10197
	21st to end	Night	2 11% 6% 16% 8% 7% 8% 6% 8% 11% 16%	10308
February	1st to 10th	Night	1 11% 6% 14% 8% 7% 7% 6% 8% 11% 21%	3252
	11th to 20th	Night	2 11% 7% 14% 9% 7% 8% 5% 9% 9% 19%	2867
	21st to end	Night	7% 7% 14% 9% 9% 7% 6% 10% 10% 21%	2364
March	1st to 10th	Night	7% 4 11% 9% 10% 9% 9% 9% 9% 23%	5512
	11th to 20th	Night	1 10% 6% 15% 8% 8% 7% 5% 8% 10% 22%	5451
	21st to end	Night	1 6% 5% 14% 10% 8% 9% 7% 9% 9% 22%	5094
April	1st to 10th	Night	2 12% 7% 14% 8% 7% 5% 4 5% 8% 27%	1938
	11th to 20th	Night	1 11% 5% 11% 7% 7% 8% 5% 7% 9% 29%	1584
	21st to end	Night	2 12% 4 10% 7% 6% 6% 5% 8% 8% 31%	1505
May	1st to 10th	Night	4 12% 4 11% 6% 6% 6% 6% 7% 8% 29%	2796
	11th to 20th	Night	5% 10% 4 13% 9% 7% 7% 6% 7% 8% 24%	2521
	21st to end	Night	6% 9% 4 10% 7% 6% 8% 5% 7% 9% 29%	2153
June	1st to 10th	Night	9% 9% 2 7% 6% 6% 6% 6% 9% 10% 30%	800
	11th to 20th	Night	12% 8% 4 9% 6% 5% 4 5% 10% 10% 27%	800
	21st to end	Night	12% 10% 4 8% 6% 6% 7% 5% 9% 11% 23%	800
July	1st to 10th	Night	9% 10% 4 14% 8% 8% 7% 4 8% 8% 22%	1800
	11th to 20th	Night	12% 9% 3 9% 6% 6% 6% 6% 8% 10% 24%	1800
	21st to end	Night	12% 9% 3 8% 6% 5% 6% 5% 8% 10% 27%	2800
August	1st to 10th	Night	8% 7% 3 7% 5% 5% 4 4 5% 8% 44%	1953
	11th to 20th	Night	14% 10% 2 8% 5% 5% 5% 4 6% 7% 33%	2402
	21st to end	Night	11% 12% 5% 11% 8% 7% 7% 5% 6% 7% 20%	2882
September	1st to 10th	Night	12% 11% 4 9% 7% 6% 6% 5% 7% 8% 25%	1886
	11th to 20th	Night	9% 11% 3 9% 7% 5% 5% 4 7% 7% 34%	2025
	21st to end	Night	12% 16% 4 11% 7% 7% 5% 4 5% 6% 22%	2236
October	1st to 10th	Night	6% 12% 5% 10% 5% 6% 7% 4 6% 8% 30%	4016
	11th to 20th	Night	5% 15% 6% 13% 8% 7% 6% 4 6% 8% 20%	4215
	21st to end	Night	4 17% 7% 13% 8% 9% 8% 5% 7% 8% 14%	5124
November	1st to 10th	Night	3 14% 7% 14% 9% 7% 8% 6% 8% 10% 14%	3081
	11th to 20th	Night	2 15% 8% 16% 12% 8% 8% 6% 7% 9% 9%	3444
	21st to end	Night	3 13% 6% 17% 10% 8% 7% 6% 10% 9% 12%	3570
December	1st to 10th	Night	3 14% 6% 15% 9% 7% 7% 6% 9% 10% 15%	6016
	11th to 20th	Night	1 9% 5% 13% 10% 10% 10% 9% 11% 9% 14%	6870
	21st to end	Night	1 12% 6% 16% 10% 9% 8% 6% 9% 10% 14%	9174

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	54% 5% 11% 13% 16% 1	2331
	11th to 20th	Day	49% 6% 17% 16% 10% 2	2760
	21st to end	Day	46% 11% 15% 18% 10%	3873
February	1st to 10th	Day	45% 9% 15% 20% 10% 1	1365
	11th to 20th	Day	53% 9% 16% 13% 7% 1	1648
	21st to end	Day	57% 9% 16% 11% 6% 1	1440
March	1st to 10th	Day	51% 8% 11% 19% 11% 1	4582
	11th to 20th	Day	50% 9% 14% 16% 10% 1	4731
	21st to end	Day	44% 13% 18% 19% 7%	6101
April	1st to 10th	Day	56% 13% 17% 10% 4	2485
	11th to 20th	Day	54% 13% 14% 12% 6% 1	2762
	21st to end	Day	47% 16% 20% 14% 3	2895
May	1st to 10th	Day	49% 8% 14% 21% 8%	7148
	11th to 20th	Day	46% 11% 18% 19% 5%	7378
	21st to end	Day	55% 8% 13% 15% 8%	8721
June	1st to 10th	Day	54% 11% 14% 15% 5%	3600
	11th to 20th	Day	58% 12% 14% 15% 2	3600
	21st to end	Day	58% 15% 14% 11% 1	3600
July	1st to 10th	Day	54% 7% 22% 14% 3	8071
	11th to 20th	Day	60% 8% 22% 10% 1	8061
	21st to end	Day	52% 8% 29% 11% 1	7801
August	1st to 10th	Day	41% 8% 21% 17% 12%	5178
	11th to 20th	Day	50% 8% 23% 17% 1	4698
	21st to end	Day	60% 8% 18% 14%	5026
September	1st to 10th	Day	60% 12% 16% 11% 1	2511
	11th to 20th	Day	50% 13% 20% 16% 1	2375
	21st to end	Day	60% 11% 13% 13% 2	2161
October	1st to 10th	Day	48% 8% 18% 24% 2	3174
	11th to 20th	Day	52% 6% 13% 26% 3	2933
	21st to end	Day	62% 8% 17% 13%	2667
November	1st to 10th	Day	60% 10% 15% 11% 3	1435
	11th to 20th	Day	63% 9% 13% 12% 3	1154
	21st to end	Day	56% 12% 15% 13% 3	1050
December	1st to 10th	Day	58% 9% 18% 10% 4	1382
	11th to 20th	Day	53% 6% 17% 13% 11%	1544
	21st to end	Day	61% 8% 14% 8% 7% 2	2024
January	1st to 10th	Night	56% 7% 13% 11% 12% 1	10563
	11th to 20th	Night	49% 6% 14% 16% 10% 4	10245
	21st to end	Night	56% 9% 12% 16% 7% 1	10308
February	1st to 10th	Night	50% 7% 12% 19% 12% 1	3252
	11th to 20th	Night	52% 9% 15% 16% 6% 2	2870
	21st to end	Night	49% 9% 17% 16% 7% 1	2364
March	1st to 10th	Night	49% 7% 14% 22% 8% 1	5512
	11th to 20th	Night	49% 9% 12% 18% 11% 1	5453
	21st to end	Night	44% 16% 16% 13% 9%	5094
April	1st to 10th	Night	51% 10% 20% 15% 3	1938
	11th to 20th	Night	48% 14% 15% 17% 6% 1	1584
	21st to end	Night	44% 13% 19% 19% 4	1505
May	1st to 10th	Night	42% 9% 18% 25% 5%	2793
	11th to 20th	Night	49% 9% 15% 19% 7%	2521
	21st to end	Night	46% 9% 12% 23% 10%	2153
June	1st to 10th	Night	43% 13% 18% 22% 5%	800
	11th to 20th	Night	49% 11% 22% 16% 3 1	800
	21st to end	Night	52% 13% 20% 12% 3	800
July	1st to 10th	Night	57% 6% 19% 15% 2	1780
	11th to 20th	Night	51% 7% 30% 11% 1	1776
	21st to end	Night	48% 7% 32% 12% 1	2720
August	1st to 10th	Night	36% 7% 27% 18% 12%	1947
	11th to 20th	Night	47% 6% 22% 22% 3	2422
	21st to end	Night	59% 7% 17% 15% 1	2882
September	1st to 10th	Night	52% 14% 18% 15% 2	1886
	11th to 20th	Night	45% 11% 23% 19% 2	2025
	21st to end	Night	60% 9% 13% 15% 2	2236
October	1st to 10th	Night	46% 7% 15% 27% 4	4016
	11th to 20th	Night	59% 6% 11% 21% 2	4215
	21st to end	Night	64% 7% 15% 14% 1	5124
November	1st to 10th	Night	59% 11% 16% 11% 3	3081
	11th to 20th	Night	65% 9% 12% 11% 3	3444
	21st to end	Night	60% 9% 15% 12% 4	3570
December	1st to 10th	Night	59% 10% 17% 11% 3	6016
	11th to 20th	Night	58% 8% 16% 13% 6%	6867
	21st to end	Night	59% 8% 14% 11% 5% 2	9174

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% 5% 11% 36% 31% 13%	110
	11th to 20th	Day	5% 5% 30% 31% 20% 11%	132
	21st to end	Day	5% 11% 21% 29% 24% 9%	242
February	1st to 10th	Day	7% 7% 16% 36% 27% 8%	318
	11th to 20th	Day	5% 9% 23% 32% 22% 10%	425
	21st to end	Day	4 8% 25% 32% 23% 9%	355
March	1st to 10th	Day	2 7% 24% 29% 26% 13%	430
	11th to 20th	Day	5% 8% 27% 31% 23% 6%	441
	21st to end	Day	5% 7% 24% 30% 24% 8%	594
April	1st to 10th	Day	8% 9% 28% 30% 19% 6%	505
	11th to 20th	Day	6% 9% 27% 30% 22% 5%	686
	21st to end	Day	9% 10% 25% 29% 20% 6%	700
May	1st to 10th	Day	13% 13% 25% 26% 17% 6%	703
	11th to 20th	Day	10% 8% 20% 30% 24% 7%	802
	21st to end	Day	11% 10% 20% 27% 23% 8%	1006
June	1st to 10th	Day	16% 8% 20% 29% 19% 7%	930
	11th to 20th	Day	17% 6% 16% 29% 26% 6%	930
	21st to end	Day	19% 6% 17% 28% 23% 8%	930
July	1st to 10th	Day	19% 6% 20% 27% 22% 6%	916
	11th to 20th	Day	21% 9% 16% 25% 22% 8%	912
	21st to end	Day	20% 6% 14% 28% 23% 9%	828
August	1st to 10th	Day	16% 7% 19% 28% 21% 8%	726
	11th to 20th	Day	25% 8% 23% 25% 13% 5%	715
	21st to end	Day	25% 8% 20% 28% 17% 3	803
September	1st to 10th	Day	23% 12% 22% 24% 14% 6%	540
	11th to 20th	Day	15% 14% 22% 27% 16% 5%	530
	21st to end	Day	25% 14% 23% 22% 13% 4	508
October	1st to 10th	Day	22% 14% 29% 20% 10% 4	420
	11th to 20th	Day	19% 13% 22% 25% 16% 4	420
	21st to end	Day	13% 6% 20% 30% 23% 8%	360
November	1st to 10th	Day	13% 6% 21% 35% 17% 9%	220
	11th to 20th	Day	8% 6% 12% 33% 21% 20%	220
	21st to end	Day	5% 9% 21% 31% 23% 10%	220
December	1st to 10th	Day	10% 8% 19% 30% 24% 9%	124
	11th to 20th	Day	7% 3 21% 35% 19% 15%	110
	21st to end	Day	5% 11% 21% 22% 29% 12%	121
January	1st to 10th	Night	7% 5% 19% 34% 28% 7%	1160
	11th to 20th	Night	10% 8% 24% 33% 19% 6%	1074
	21st to end	Night	7% 9% 25% 34% 21% 5%	1045
February	1st to 10th	Night	6% 6% 19% 40% 23% 6%	950
	11th to 20th	Night	6% 6% 23% 38% 22% 5%	934
	21st to end	Night	3 4 18% 44% 25% 6%	783
March	1st to 10th	Night	2 6% 20% 37% 29% 6%	846
	11th to 20th	Night	4 4 21% 33% 32% 6%	816
	21st to end	Night	3 5% 16% 34% 32% 9%	693
April	1st to 10th	Night	5% 7% 17% 33% 29% 8%	603
	11th to 20th	Night	5% 5% 18% 36% 28% 8%	593
	21st to end	Night	9% 6% 21% 32% 29% 4	505
May	1st to 10th	Night	10% 8% 17% 33% 25% 7%	400
	11th to 20th	Night	11% 5% 20% 32% 28% 4	394
	21st to end	Night	8% 5% 13% 32% 37% 5%	302
June	1st to 10th	Night	15% 3 10% 33% 35% 5%	200
	11th to 20th	Night	15% 4 12% 35% 33% 3	200
	21st to end	Night	15% 5% 15% 29% 31% 6%	200
July	1st to 10th	Night	17% 3 9% 32% 35% 4	195
	11th to 20th	Night	18% 4 11% 22% 39% 7%	194
	21st to end	Night	14% 5% 12% 23% 37% 9%	399
August	1st to 10th	Night	16% 3 17% 27% 32% 5%	398
	11th to 20th	Night	23% 6% 15% 28% 24% 5%	526
	21st to end	Night	20% 5% 12% 30% 27% 6%	660
September	1st to 10th	Night	19% 9% 15% 28% 25% 4	600
	11th to 20th	Night	15% 8% 16% 27% 29% 5%	625
	21st to end	Night	23% 11% 19% 25% 18% 4	800
October	1st to 10th	Night	22% 9% 21% 29% 16% 3	800
	11th to 20th	Night	19% 10% 22% 25% 20% 4	800
	21st to end	Night	14% 7% 16% 35% 22% 5%	998
November	1st to 10th	Night	13% 5% 18% 33% 24% 6%	930
	11th to 20th	Night	9% 6% 18% 28% 25% 14%	1034
	21st to end	Night	9% 8% 19% 33% 20% 10%	1160
December	1st to 10th	Night	9% 8% 20% 36% 22% 5%	1160
	11th to 20th	Night	7% 7% 19% 35% 25% 7%	1160
	21st to end	Night	6% 6% 21% 35% 24% 8%	1276

Page generated at 2026-03-29 22:18:39. Version ac07

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