Irma
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Irma clear sky chart from 2005-11-06 to 2025-10-04.

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	9% 9% 3 9% 6% 5% 5% 4 6% 8% 35%	130827
	Night	7% 9% 4 10% 6% 6% 5% 5% 6% 8% 34%	127277
Transparency	Day	45% 6% 16% 24% 8% 1	130523
	Night	47% 6% 16% 22% 9% 1	127161
Seeing	Day	12% 8% 27% 31% 17% 5%	16805
	Night	12% 6% 29% 36% 15% 2	25761

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	6% 11% 4 14% 9% 8% 7% 5% 6% 6% 24%	3179
	11th to 20th	Day	4 9% 4 8% 8% 8% 7% 7% 10% 8% 26%	3159
	21st to end	Day	7% 13% 4 10% 6% 5% 6% 4 6% 8% 31%	3608
February	1st to 10th	Day	8% 12% 5% 15% 8% 7% 6% 5% 5% 7% 22%	1651
	11th to 20th	Day	6% 13% 6% 12% 6% 5% 5% 5% 7% 8% 26%	1755
	21st to end	Day	5% 13% 4 12% 8% 7% 7% 4 7% 10% 22%	1485
March	1st to 10th	Day	4 10% 5% 11% 8% 6% 7% 5% 7% 9% 28%	4593
	11th to 20th	Day	5% 11% 6% 16% 7% 5% 4 3 5% 8% 29%	4999
	21st to end	Day	8% 14% 6% 16% 7% 5% 4 4 5% 6% 25%	5486
April	1st to 10th	Day	7% 13% 4 9% 6% 6% 5% 4 5% 8% 32%	2495
	11th to 20th	Day	11% 11% 4 9% 5% 4 5% 4 7% 7% 31%	2570
	21st to end	Day	13% 11% 4 6% 5% 5% 6% 5% 6% 8% 31%	2630
May	1st to 10th	Day	11% 9% 3 8% 6% 5% 4 3 5% 6% 39%	6426
	11th to 20th	Day	16% 11% 3 9% 6% 6% 5% 5% 6% 7% 26%	6734
	21st to end	Day	10% 7% 2 7% 5% 5% 6% 4 6% 7% 40%	7405
June	1st to 10th	Day	14% 8% 3 7% 5% 6% 6% 5% 7% 8% 33%	3000
	11th to 20th	Day	19% 9% 3 8% 6% 5% 6% 6% 6% 7% 25%	3000
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 5% 8% 9% 36%	3000
July	1st to 10th	Day	7% 5% 2 5% 4 4 5% 4 7% 9% 48%	6780
	11th to 20th	Day	12% 5% 2 6% 5% 5% 4 4 6% 8% 42%	6780
	21st to end	Day	8% 6% 3 7% 4 5% 5% 4 7% 8% 44%	7376
August	1st to 10th	Day	13% 5% 2 6% 4 5% 5% 4 6% 8% 43%	4758
	11th to 20th	Day	7% 5% 2 6% 4 5% 5% 4 7% 10% 45%	4158
	21st to end	Day	8% 6% 2 6% 4 4 5% 4 6% 9% 47%	4664
September	1st to 10th	Day	9% 6% 2 7% 5% 4 5% 4 5% 7% 45%	2507
	11th to 20th	Day	14% 8% 3 6% 4 3 3 3 4 8% 43%	2253
	21st to end	Day	9% 5% 2 6% 4 4 4 4 5% 9% 49%	2200
October	1st to 10th	Day	8% 9% 3 8% 5% 5% 5% 4 5% 7% 42%	3209
	11th to 20th	Day	8% 9% 3 7% 4 4 4 4 6% 11% 39%	3059
	21st to end	Day	7% 9% 3 7% 5% 4 5% 4 6% 8% 41%	2945
November	1st to 10th	Day	11% 15% 3 10% 7% 6% 5% 5% 5% 7% 25%	1568
	11th to 20th	Day	6% 14% 6% 15% 6% 4 4 4 6% 7% 27%	1565
	21st to end	Day	6% 12% 6% 12% 8% 6% 5% 4 7% 9% 27%	1400
December	1st to 10th	Day	9% 13% 5% 12% 6% 7% 7% 6% 6% 7% 23%	2290
	11th to 20th	Day	9% 14% 4 12% 8% 8% 7% 5% 7% 7% 19%	2642
	21st to end	Day	3 8% 4 13% 8% 5% 5% 3 6% 7% 38%	3498
January	1st to 10th	Night	3 8% 5% 14% 9% 9% 8% 6% 8% 8% 21%	6766
	11th to 20th	Night	3 9% 4 11% 8% 7% 6% 6% 6% 7% 33%	6739
	21st to end	Night	7% 12% 5% 11% 6% 5% 6% 3 6% 6% 33%	7394
February	1st to 10th	Night	6% 11% 4 13% 9% 7% 6% 6% 7% 7% 25%	2748
	11th to 20th	Night	5% 11% 4 11% 7% 7% 6% 6% 8% 7% 28%	2532
	21st to end	Night	4 10% 4 10% 8% 6% 6% 6% 7% 9% 31%	2145
March	1st to 10th	Night	5% 10% 4 12% 7% 5% 6% 5% 8% 11% 28%	5299
	11th to 20th	Night	4 9% 5% 12% 7% 7% 5% 6% 7% 9% 29%	5000
	21st to end	Night	10% 12% 5% 15% 7% 6% 5% 4 5% 7% 24%	5489
April	1st to 10th	Night	6% 10% 3 8% 6% 5% 5% 4 7% 9% 36%	1902
	11th to 20th	Night	9% 10% 4 11% 7% 6% 7% 5% 7% 7% 28%	1781
	21st to end	Night	12% 11% 3 8% 5% 4 4 4 7% 7% 35%	1770
May	1st to 10th	Night	15% 9% 3 8% 4 4 4 4 6% 7% 37%	3548
	11th to 20th	Night	13% 10% 4 9% 5% 5% 5% 5% 8% 8% 28%	3159
	21st to end	Night	13% 7% 3 7% 5% 5% 6% 4 7% 8% 34%	3470
June	1st to 10th	Night	11% 10% 3 8% 6% 6% 5% 6% 7% 7% 32%	1400
	11th to 20th	Night	17% 9% 3 8% 8% 5% 5% 6% 7% 7% 26%	1400
	21st to end	Night	9% 4 2 6% 5% 6% 6% 6% 7% 9% 39%	1400
July	1st to 10th	Night	6% 5% 2 5% 4 4 6% 6% 7% 9% 48%	3180
	11th to 20th	Night	10% 4 2 5% 4 4 5% 4 7% 9% 45%	3180
	21st to end	Night	8% 4 2 6% 5% 5% 5% 4 7% 9% 47%	3467
August	1st to 10th	Night	12% 6% 2 5% 4 4 5% 4 6% 8% 44%	2390
	11th to 20th	Night	8% 5% 3 6% 5% 5% 5% 4 6% 9% 45%	2901
	21st to end	Night	9% 6% 2 7% 4 4 4 4 6% 8% 46%	3245
September	1st to 10th	Night	9% 4 2 6% 4 5% 5% 3 5% 8% 48%	1892
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 5% 7% 47%	2145
	21st to end	Night	7% 6% 2 6% 4 4 4 4 7% 7% 48%	2200
October	1st to 10th	Night	8% 7% 3 7% 5% 5% 5% 4 6% 9% 42%	3826
	11th to 20th	Night	7% 7% 3 6% 4 3 4 4 6% 9% 46%	3866
	21st to end	Night	7% 10% 4 9% 4 5% 5% 4 6% 8% 39%	4581
November	1st to 10th	Night	12% 14% 5% 9% 5% 4 5% 4 6% 6% 30%	2730
	11th to 20th	Night	7% 12% 5% 13% 8% 6% 5% 4 6% 8% 26%	2834
	21st to end	Night	5% 12% 5% 12% 8% 5% 5% 4 6% 7% 30%	3000
December	1st to 10th	Night	10% 16% 6% 14% 7% 6% 5% 5% 5% 7% 19%	4890
	11th to 20th	Night	7% 15% 5% 15% 8% 7% 6% 5% 6% 7% 19%	5550
	21st to end	Night	3 6% 3 10% 8% 6% 5% 4 6% 7% 42%	7458

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	59% 4 11% 20% 6%	3179
	11th to 20th	Day	50% 7% 14% 14% 14%	3180
	21st to end	Day	51% 5% 19% 19% 5%	3608
February	1st to 10th	Day	61% 5% 11% 16% 8%	1651
	11th to 20th	Day	54% 4 14% 18% 9% 1	1764
	21st to end	Day	55% 5% 13% 19% 7%	1485
March	1st to 10th	Day	50% 5% 20% 19% 6%	4593
	11th to 20th	Day	53% 5% 17% 17% 7%	5000
	21st to end	Day	59% 5% 10% 20% 6%	5486
April	1st to 10th	Day	49% 6% 14% 21% 10% 1	2495
	11th to 20th	Day	50% 5% 14% 22% 9% 1	2570
	21st to end	Day	48% 6% 14% 21% 10% 1	2630
May	1st to 10th	Day	46% 4 9% 24% 14% 3	6426
	11th to 20th	Day	54% 5% 10% 21% 8%	6738
	21st to end	Day	41% 6% 16% 23% 12% 2	7406
June	1st to 10th	Day	46% 7% 14% 22% 10% 1	3000
	11th to 20th	Day	55% 8% 15% 17% 4 1	3000
	21st to end	Day	41% 8% 23% 20% 8% 1	3000
July	1st to 10th	Day	31% 7% 23% 29% 10% 1	6706
	11th to 20th	Day	39% 6% 16% 30% 10%	6699
	21st to end	Day	34% 8% 21% 30% 7%	7169
August	1st to 10th	Day	38% 8% 18% 34% 3	4740
	11th to 20th	Day	31% 7% 24% 30% 7%	4197
	21st to end	Day	33% 6% 24% 30% 7% 1	4663
September	1st to 10th	Day	37% 5% 19% 27% 10% 3	2507
	11th to 20th	Day	41% 4 17% 29% 8%	2253
	21st to end	Day	33% 7% 18% 30% 11% 2	2200
October	1st to 10th	Day	41% 5% 23% 26% 6%	3209
	11th to 20th	Day	40% 5% 16% 31% 9%	3059
	21st to end	Day	40% 5% 22% 28% 5%	2945
November	1st to 10th	Day	58% 5% 13% 19% 5% 1	1568
	11th to 20th	Day	56% 5% 16% 18% 4	1565
	21st to end	Day	53% 6% 18% 18% 5% 1	1400
December	1st to 10th	Day	58% 9% 18% 12% 4	2290
	11th to 20th	Day	61% 7% 16% 14% 3	2644
	21st to end	Day	46% 3 10% 27% 13% 1	3498
January	1st to 10th	Night	57% 6% 14% 15% 8%	6766
	11th to 20th	Night	50% 5% 13% 16% 15% 1	6780
	21st to end	Night	51% 4 18% 22% 5%	7394
February	1st to 10th	Night	55% 5% 13% 17% 9% 1	2748
	11th to 20th	Night	50% 6% 13% 19% 11%	2533
	21st to end	Night	47% 6% 14% 23% 9% 1	2145
March	1st to 10th	Night	48% 6% 19% 20% 7%	5299
	11th to 20th	Night	49% 6% 17% 21% 7%	5000
	21st to end	Night	57% 7% 9% 18% 8%	5489
April	1st to 10th	Night	42% 6% 15% 22% 14% 1	1902
	11th to 20th	Night	52% 6% 11% 18% 12% 2	1781
	21st to end	Night	45% 5% 13% 23% 11% 2	1770
May	1st to 10th	Night	46% 4 7% 25% 16% 2	3548
	11th to 20th	Night	49% 7% 13% 21% 9%	3159
	21st to end	Night	45% 7% 15% 23% 9% 1	3470
June	1st to 10th	Night	47% 5% 14% 24% 9% 1	1400
	11th to 20th	Night	51% 10% 15% 17% 7%	1400
	21st to end	Night	38% 9% 23% 23% 7% 1	1400
July	1st to 10th	Night	29% 8% 24% 26% 13%	3145
	11th to 20th	Night	33% 7% 16% 31% 12%	3138
	21st to end	Night	31% 8% 22% 28% 10% 1	3369
August	1st to 10th	Night	36% 8% 18% 35% 4	2383
	11th to 20th	Night	34% 7% 21% 30% 7%	2928
	21st to end	Night	33% 6% 19% 32% 9%	3245
September	1st to 10th	Night	33% 6% 19% 26% 12% 4	1892
	11th to 20th	Night	36% 5% 16% 31% 10% 2	2145
	21st to end	Night	33% 7% 14% 32% 13% 2	2200
October	1st to 10th	Night	38% 6% 19% 29% 7% 1	3826
	11th to 20th	Night	34% 4 16% 36% 9%	3866
	21st to end	Night	42% 6% 20% 28% 5%	4581
November	1st to 10th	Night	54% 5% 14% 18% 8% 1	2730
	11th to 20th	Night	56% 6% 15% 16% 6% 1	2834
	21st to end	Night	52% 6% 19% 17% 5%	3000
December	1st to 10th	Night	64% 6% 13% 12% 5% 1	4890
	11th to 20th	Night	63% 7% 16% 10% 4	5547
	21st to end	Night	42% 4 13% 23% 16% 2	7458

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	10% 3 34% 34% 17% 4	200
	11th to 20th	Day	8% 8% 33% 32% 19% 2	200
	21st to end	Day	14% 7% 36% 26% 14% 3	220
February	1st to 10th	Day	11% 8% 36% 32% 10% 4	226
	11th to 20th	Day	11% 5% 30% 35% 16% 3	300
	21st to end	Day	8% 7% 30% 35% 14% 6%	249
March	1st to 10th	Day	11% 9% 30% 31% 17% 1	470
	11th to 20th	Day	9% 7% 36% 29% 14% 5%	500
	21st to end	Day	12% 10% 28% 30% 16% 3	550
April	1st to 10th	Day	10% 12% 35% 31% 9% 3	500
	11th to 20th	Day	14% 12% 30% 29% 12% 3	492
	21st to end	Day	16% 12% 27% 30% 12% 3	530
May	1st to 10th	Day	13% 10% 26% 29% 17% 5%	703
	11th to 20th	Day	11% 9% 24% 32% 18% 6%	725
	21st to end	Day	15% 8% 22% 29% 19% 8%	789
June	1st to 10th	Day	15% 9% 27% 33% 13% 3	730
	11th to 20th	Day	19% 9% 25% 28% 15% 5%	730
	21st to end	Day	12% 7% 22% 30% 20% 9%	730
July	1st to 10th	Day	10% 6% 18% 32% 25% 9%	721
	11th to 20th	Day	12% 6% 22% 31% 20% 8%	718
	21st to end	Day	9% 5% 20% 30% 28% 8%	794
August	1st to 10th	Day	11% 5% 19% 30% 25% 10%	687
	11th to 20th	Day	9% 5% 25% 33% 21% 6%	517
	21st to end	Day	11% 6% 22% 30% 21% 9%	583
September	1st to 10th	Day	12% 8% 31% 30% 15% 5%	530
	11th to 20th	Day	18% 8% 23% 28% 17% 5%	530
	21st to end	Day	14% 12% 28% 30% 13% 3	530
October	1st to 10th	Day	14% 8% 27% 32% 15% 4	318
	11th to 20th	Day	15% 8% 33% 29% 13% 1	300
	21st to end	Day	12% 11% 38% 26% 10% 3	330
November	1st to 10th	Day	16% 6% 35% 30% 11% 2	300
	11th to 20th	Day	10% 11% 40% 30% 8%	283
	21st to end	Day	9% 8% 30% 38% 11% 5%	200
December	1st to 10th	Day	12% 8% 30% 32% 14% 5%	200
	11th to 20th	Day	11% 11% 29% 30% 20% 1	200
	21st to end	Day	9% 5% 26% 36% 19% 5%	220
January	1st to 10th	Night	10% 7% 31% 38% 12% 2	1000
	11th to 20th	Night	9% 9% 33% 35% 13% 2	1000
	21st to end	Night	13% 11% 35% 31% 9% 1	1075
February	1st to 10th	Night	9% 8% 26% 39% 16% 3	800
	11th to 20th	Night	9% 5% 32% 36% 16% 2	784
	21st to end	Night	7% 4 25% 39% 22% 3	660
March	1st to 10th	Night	8% 6% 28% 42% 15% 2	800
	11th to 20th	Night	10% 5% 32% 36% 16% 2	800
	21st to end	Night	12% 7% 29% 34% 15% 3	880
April	1st to 10th	Night	9% 8% 29% 35% 16% 2	630
	11th to 20th	Night	12% 8% 31% 36% 11% 2	593
	21st to end	Night	17% 8% 24% 38% 14%	600
May	1st to 10th	Night	16% 6% 25% 36% 16% 2	560
	11th to 20th	Night	13% 6% 25% 38% 18% 1	394
	21st to end	Night	19% 6% 22% 30% 21% 2	433
June	1st to 10th	Night	16% 8% 26% 36% 13% 2	400
	11th to 20th	Night	22% 5% 20% 33% 18% 3	400
	21st to end	Night	11% 4 23% 36% 24% 3	400
July	1st to 10th	Night	9% 5% 19% 42% 25%	390
	11th to 20th	Night	9% 4 21% 44% 21% 2	388
	21st to end	Night	8% 5% 18% 40% 25% 4	438
August	1st to 10th	Night	9% 5% 17% 38% 26% 6%	419
	11th to 20th	Night	11% 5% 19% 42% 22% 2	594
	21st to end	Night	8% 4 24% 39% 23% 1	660
September	1st to 10th	Night	11% 4 25% 39% 21% 1	600
	11th to 20th	Night	15% 5% 18% 43% 18% 1	745
	21st to end	Night	10% 5% 33% 38% 14% 1	800
October	1st to 10th	Night	12% 5% 30% 35% 16% 2	772
	11th to 20th	Night	13% 4 32% 36% 13% 2	760
	21st to end	Night	14% 8% 33% 35% 10% 1	911
November	1st to 10th	Night	18% 6% 32% 32% 10% 2	975
	11th to 20th	Night	13% 7% 35% 32% 12% 2	1000
	21st to end	Night	8% 7% 36% 38% 10% 1	1000
December	1st to 10th	Night	14% 7% 31% 29% 16% 4	1000
	11th to 20th	Night	11% 10% 34% 32% 12% 1	1000
	21st to end	Night	8% 4 33% 36% 17% 3	1100

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