Snow in the Sahara Desert – Unbelievable But True!

11 01 2018

Dale C. S. Destin |

sahara-desert-map

A few days ago, snow fell over parts of the Sahara Desert. To many this was unbelievable, especially in this era of fake news and alternative truth. However, it really snowed in the Sahara Desert, and not for the first time. Precipitation (rain, snow, hail, etc.) in deserts are rare but certainly not unheard of.

Algeria - dark green

Location of Algeria – dark green

 

Snow covered dumes in the Sahara Desert. Photo by Geoff

Snow in the Sahara Desert near the town of Ain Sefra, Algeria – 07 Jan 2018. Photo by Geoff Robinson 

It is understandable the disbelieve one would have about the news of snow taking place in the Sahara Desert. After all, it is “only” the hottest desert in the world – consisting of some of the hottest places on Earth. The mean temperature is around 29 °C (84 °F) but can reach as high as 50 °C (122 °F).

What is not well-known is the fact that very cold temperatures do occur in the Sahara Desert. Temperatures, particularly during winter can fall below freezing (below 0 °C or below 32 °F). So, temperatures do become conducive for snow in the Sahara, at times. However, temperature is not sufficient for snow – moisture is needed.

Contrary to popular belief, precipitation does take place in deserts, including the Sahara. Not a lot takes place but they do get precipitation. There is no universal definition for a desert but a good working one is an area receiving less than 250 mm (10 in) of rainfall annually.

In the case of the Sahara, the annual rainfall is less than 100 mm (3.9 in). It rains from December to March and in August. The rains in August are said to be characterized by thunderstorms, which can produce flash floods. And of course, snow falls at times during the winter months, as was the case a few days ago.

It snowed in the same location last year; however, before then it had not snowed in 2012 and 1979 -nearly 40 years ago.

Sahara snow event of 2012 as reported by Algerian TV channel, Central TV

Weird precipitation events are rare for deserts but they do occur:

In January 2013, a NASA satellite observed the Taklimakan Desert, in western China, covered with snow. The Taklimakan Desert temperature range from – 20 to 38 °C (68 °F to 100 °F) and the annual average rainfall is less than 40 mm (1.60 in).

In August 2011, Stephane Guisard of the European Southern Observatory photographed snow in the Atacama Desert. The annual average rainfall is around 1 mm (0.04 in).

A dusting of snow in the Atacama Desert

A dusting of snow in the Atacama Desert. Photograph was taken by Stéphane Guisard on 1 August 2011. 

In March 2015, Flash floods took place in the Atacama Desert – the driest desert in the world and one of the driest places on Earth. The annual average rainfall is around 1 mm (0.04 in). The floods caused two deaths and left 24 persons missing.

Precipitation may be rare for the deserts of the world but far from unprecedented. No only do they get light precipitation but also solid, on occasions.

Advertisements




Windy Weather to Cause More Hazardous Seas and Economic Losses

8 01 2018

Dale C. S. Destin |

Hazardous seas being caused by frequently strong winds – gusting to near gale force at times, will continue to keep most mariners in or near port over the upcoming week – causing further significant economic losses for many.

Seas around Antigua and Barbuda have been rough for most of the year, so far, and are set to remain that way or even worsen over the next seven days, at least. As usual, this type of weather is very disruptive to marine activities and have a negative economic impact, particularly on fisherfolk, those alone the fisheries value-chain and those involve in offshore pleasure cruises and adventures.

Small craft warning remains in effect for hazardous seas around Antigua and Barbuda and will likely remain in place for the rest of the week. Hence, small craft operators, especially inexperienced ones, should avoid navigating in these conditions.

Warnings are also in effect for beach-goers as high surfs are affecting beaches, producing beach erosion and dangerous swimming conditions. Beach-goers should avoid the waters, especially those on the northern and eastern side of the islands. These high surfs are likely to subside to more manageable levels by Tuesday.

High surfs can also cause strong rip currents, which can carry even the strongest swimmers out to sea, and seawater splashing onto low-lying coastal roads, causing damage. Further, high surfs can also knock spectators off exposed rocks and jetties. Breaking waves may also occasionally impact harbours making navigating the harbour channel dangerous.

The wind speed will range between 14 and 22 knots (16 to 25 mph) and at times gusting to near 30 knots (35 mph). The winds will be strongest on Tuesday and Friday and will blow from near east for most of the week.

Wind speed – valid at 6 am, Tue, Jan 9, 2018

Seas will remain hazardous with steep waves ranging between 2 and 3 metres (7 and 10 ft), occasionally reaching near 4 metres (13 ft), mainly in open waters on the eastern or windward side of the islands.

Significant wave height (ft) – valid at 6 am, Tue, Jan 9, 2018

These strong winds not only cause hazardous seas but also cause certain onshore activities to be uncomfortable, if not dangerous. Hence, certain outdoor work will be hampered, if not halted, at times; thus, reducing productivity in other sectors.

The windy conditions and hazardous seas will also be experienced by all the islands of the Eastern Caribbean along with Hispaniola and the Bahamas, particularly the Atlantic coastal waters or the eastern and northern coastal waters.

The strong winds are not due to any storm system but rather because of steep pressure gradients across the area. Recall that winds blow due to pressure differences or pressure gradients, and the greater the gradients the stronger the winds and vice versa.

Also, the strength and position of the ever-present subtropical/Atlantic high-pressure system modulate the steepness of the pressure gradient. The closer and or stronger the subtropical high, the steeper the gradient, the stronger the winds and vice versa.

The subtropical high-pressure system will be stronger than normal for much of the next week; hence, the forecast continuation of strong winds, hazardous seas, disrupted marine activities and economic losses.

Although windy, the weather will be mostly dry with only occasional brief showers likely.

Follow us for all you need to know about this windy weather and all things weather and climate. We can be followed on twitterfacebookinstagramtumblrflickrgoogle+, and youtube.





The 2017 Atlantic Hurricane Season Summary

1 12 2017

Dale C. S. Destin|

The 2017 Atlantic hurricane season came to an end yesterday but will long be remembered for the costliest and one of the most destructive on record. It was also one of the most active hurricane seasons on record, breaking or equalling several records.

Hurricanes Katia (left), Irma (middle) and Jose (right)

Hurricanes Katia (left), Irma (middle) and Jose (right)

The hyperactive season produced 17 named storms, 10 hurricanes, 6 major hurricanes and 223 Accumulated Cyclone Energy (ACE). Overall, it is the seventh most active season on record (based on ACE) dating back to 1851 and the most active since 2005.

AtlanticHUrricaneSeasonSummary

Relative to the normal season of 12 named storms, 6 hurricanes, 3 major hurricanes and 106 ACE, this season had 42% more named storms than normal, 67% more hurricanes than usual, twice the normal number of major hurricanes and over twice the normal amount of ACE.

2017HurricaneSeasonSummaryGraphic

The furious 2017 Atlantic hurricane season left in its wake cataclysmic damage and is now the costliest hurricane season on record with cost amounting to over US$367 billion. The Caribbean contribution to this total is about US$120 billion.

Relative to Antigua and Barbuda

It was the most active hurricane season for Antigua and Barbuda, based on the number and strength of hurricanes passing within 105 nm of the islands. This season marks the first time on record that three major hurricanes passed in such proximity of the country. There have been other times with three or more tropical cyclones but none with three major hurricanes.

For the first three weeks of September, we had a ominous procession of major hurricanes – Irma, Jose and Maria. Irma was the strongest, in terms of sustained winds. The current hurricane scale goes from 1 to 5; however; if there were a 6, Irma would have been a Category 6 – it was super strong with peak sustained winds of 298 km/h (185 mph).

Super-Category 5 Hurricane Irma virtually wiped out Barbuda – damaging or destroying around 90% of buildings. Meanwhile, Antigua got away almost “scot-free” with only storm-force winds causing minimal damage.

Damaged and destroyed properties in Barbuda in the wake of Irma – Sep 6, 2017

Both Antigua and Barbuda dodge the bullets from Jose and Maria. Maria also produced storm-force winds; however, Jose passed without causing any notable winds. Notwithstanding, with Jose passing three days after Irma destroying Barbuda – the whole island had to be evacuated.

The expense to Antigua and Barbuda according to the National Office of Disaster Service (NODS): about US$140 million in damage; around US$20 million in losses and a recovery cost of about US$220 million. Most of the damage took place on Barbuda, where one person died during the passage of Irma.

On average, Antigua and Barbuda gets one named storm passing within 105 nm every other year, one hurricane every three years and a major hurricane every seven years. This is the first year on record we have been affected by Category 5 hurricanes – Irma and Maria.

AntiguaHurricaneSeasonSummary

Why was the season so active?

The season was hyperactive because of warmer than normal sea surface temperatures and very friendly atmospheric conditions – deep moisture levels and below normal vertical wind shear. This was especially so from around mid-August to early September.

The absence of an El Nino and conditions trending toward a La Nina also allowed for a more active season than normal.

Did climate change play a part?

There is no scientific evidence to support the notion that climate change had anything to do with the hyperactive 2017 Atlantic hurricane season or any previous hurricane season.

This is not to say that climate will not eventually have an impact on the hurricane season in the future. According to the Intergovernmental Panel on Climate Change (IPCC), climate change will, in the future, cause an overall decrease in the number of tropical cyclones but an increase in the number of major hurricanes.

Other notable records

Irma is the strongest hurricane on record to occur in the Atlantic Ocean – outside the Caribbean Sea and the Gulf of Mexico.

Maintaining peak intensity for 37 consecutive hours, Irma is the only tropical cyclone on record worldwide to have had winds that strong for so long.

Irma tied with 1935 US Labour Day hurricane for the strongest hurricane to ever make landfall in the Atlantic Basin.

September 2017 is the most active month on record for the Atlantic

Ten hurricanes in a row form during the 2017 Atlantic hurricane season – the most consecutive hurricanes on record.

Click here for more records.

Keep following for more on the just ended hurricane season, tropical cyclones and climate change and all things weather and climate. The next hurricane season starts June 1, 2018 – six months from now, let us all be prepared. Our first forecast for the next season will be issued around April 10.





Tropical Cyclone Size and Climate Change

21 11 2017

Dale C. S. Destin |

This is a continuation of our series – Tropical Cyclones and Climate Change – TCs (tropical depressions, tropical storms, hurricanes). In our previous blog in this series, we looked at Tropical Cyclone Rainfall Rates and Climate Change. In this blog, we will look at whether climate change is having an impact on TC size.

The soon ended 2017 Atlantic hurricane season has produced 17 named storms – five more than the average or 42% more than usual. This is still the most since 2012, when 19 formed. Meanwhile, there have been 10 hurricanes – four more than the average or 67% more than usual. This season remains tied with 2010 for the most hurricanes since 2012. The number of major hurricanes remains unchanged at six – three more than average or twice the usual amount. This is the most major hurricanes since 2005.

Have TCs size increased?

The Intergovernmental Panel on Climate Change (IPCC) is silent on this question. Further, the research on TC size has been quite few and the datasets very limited; however, the current answer is likely no based on available data.

Studies have shown that the size of TCs varies within and across ocean basins. Generally, the size of TCs increases with the increase in latitude or as you move poleward. For the Atlantic, this means that they generally increase in size as you go northward.

Hurricanes Katia (left), Irma (middle) and Jose (right)

Hurricanes Katia (left), Irma (middle) and Jose (right). All three were small hurricanes in terms of the extent of the hurricane and storm winds from their centres.

The initial size of a TC is also indicative of its eventual size when it reaches the maximum intensity. The rate of growth is largest at intensification.

There appears to be a trend toward smaller TCs in most ocean basins that they do form over; however, this trend is not deemed to be significant globally or regionally.

While there appears to be a negative trend in most ocean basins, no significant trend in size have been detected globally or regionally. However, the confidence level of this finding is considered relatively low since the dataset used is relatively short and somewhat subjectively obtained.

Will TCs increase in size in the future?

Although some studies have indicated a decrease in size, it is not clear how size will change in the future with a warming climate. The increase in intensity could be theorized to have both a positive or negative trend on size.

More intense TCs could mean higher pressure gradients due to a tightly ‘rap’ system and no change in central pressure, in that case, the trend in size could be toward smaller systems. On the other hand, if cyclones exhibit lower central pressure, then the trend in size could be toward larger systems. Much more research is needed in this area.

There are lots of talk in the press about TCs becoming larger due to climate change; however, there is no clear evidence to support this claim. There are two main scientific papers that address this matter – one found that the size is decreasing and the other said that the size will increase. However, both papers agree that no change in size has yet occurred due to climate change.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone duration.





Tropical Cyclone Rainfall Rates and Climate Change

3 11 2017

Dale C. S. Destin |

This is a continuation of our series – Tropical Cyclones and Climate Change – TCs (tropical depressions, tropical storms, hurricanes). In our previous blog in this series, we looked at Tropical Cyclone Frequency and Climate Change. In this blog, we will look at whether climate change is having an impact on TC rainfall rates.

To date, the 2017 Atlantic hurricane season has produced 16 named storms – four more than the average or 33% more than usual. This is the most since 2012, when 19 formed. Meanwhile, there have been 10 hurricanes – four more than the average or 67% more than usual. This tied with 2010 for the most hurricanes since 2012. Further, there have been six major hurricanes – three more than average or 100% more than usual. This is the most major hurricanes since 2005.

Have TC rainfall rates increased?

According to the Intergovernmental Panel on Climate Change (IPCC), the answer to the above question is no. Recall that the IPCC is the United Nations international body assigned with the task of assessing climate change.

Rainfall totals from Hurricane Maria based on (IMERG) or satellite data

Rainfall totals from Hurricane Maria based on (IMERG) or satellite data

Here is what the IPCC Assessment Report Five (AR5) actually says: “…no broad-scale, detectable long-term changes in tropical cyclone rainfall rates have been reported…”; hence, there can be no credible claim of  TCs producing increased rainfall.

This conclusion by the IPCC is supported by many TC researchers, including Kevin J.S. Walsh et al. and Thomas R. Knutson et al. They all indicate that although the moisture content of the Atmosphere has increased and will continue to do so, as the earth warms, there has been no detectable change in tropical cyclone rainfall rates.

Will TC rainfall rates increase?

To this question, the IPCC says yes! Here are the exact words from the IPCC AR5, “…the increase in rainfall rates associated with tropical cyclones is a consistent feature of the numerical models under greenhouse warming as atmospheric moisture content in the tropics and tropical cyclone moisture convergence is projected to increase.”

According to the IPCC, rainfall rates within 200 km of the centre of TCs are likely to increase by 5 to 20% by the year 2100, due to climate change. This conclusion was also arrived at by many TC researchers including Kevin J.S. Walsh et al. and Thomas R. Knutson et al.

HurricaneIrma_6Sep17

Hurricane Irma rainfall rates – Sep 5-6, 2017. The highest rates are near the centre

The science behind the IPCC conclusion is quite robust. The Clausius-Clapeyron equation indicates that the water holding capacity of the atmosphere increases by around 7% for every degree °C rise in temperature; hence, one can expect increasing water vapour levels in the atmosphere as the earth continues to warm. There is evidence to show that this has happened in terms of rising specific humidity levels; however, studies are yet to detect increased rainfall rates from TCs.

Going forward, TCs will likely take advantage of the increasing moisture levels, in the Atmosphere, to produce higher rainfall rates. However, this is not to say that overall rainfall will increase from these systems as decreasing frequency of TCs could counteract increases. Also, rainfall rates only speak to the amount of rainfall per unit time and not total overall rainfall, which could remain unchanged.

Based on the IPCC and virtually all TC researchers, climate change has not changed TC rainfall rates; the rates remain unchanged. However, they are projected to increase by the year 2100, as TCs will likely extract higher rainfall rates from a moistening Atmosphere, being caused by climate change.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone size.





The Wettest September for Antigua in Over Two Decades

31 10 2017

Dale C. S. Destin|

September 2017 was the wettest for Antigua in over two decades. The month yielded 270.3 mm (10.64 in). This is almost doubled the monthly of 144.0 mm (5.67 in). The last time September was wetter was in 1995, when 373.1 mm (14.69 in) of rain fell.

September 2017 now ranks sixth wettest of all Septembers on record, dating back to 1928. Only 5 other Septembers have seen more rainfall.

SeptemberRainfallTotals_RankedSeptember 2017 is also the wettest month for Antigua in nearly five years. No month has been wetter since October 2012.

The probability of such a high rainfall total for September is 8.6%. This means that this total occurs once in every 11 to 12 years, on average.

The vast majority – over 75% of the rain was due to Hurricanes Irma, Jose and Maria. Maria was the single greatest rain maker with over 30% of the rainfall total for the month.

Interestingly, the last time September was wetter, we had two hurricanes – Luis and Marilyn of 1995. This time around we had three hurricanes.

September is the third wettest month on average behind October and November.

The rainfall total for the year, thus far, is running on the high side of the near normal range, notwithstanding the well above normal rainfall for September.

The forecast for October and the rest of the year is for near to above normal rainfall. Meanwhile, it is likely (55% chance) that the year (2017) will end with above normal rainfall – 1329.0 mm (52.3 in) with a 70 confidence of it being in the range of 1022.3 to 1690.6 mm (40.2 to 66.6 in). This would be the highest total since 2011.

Follow us for updated info on our local, regional and international weather and climate.





Tropical Cyclone Frequency and Climate Change

17 10 2017

Dale C. S. Destin |

This is a continuation of our series – Tropical Cyclones and Climate Change – (TCs; tropical depressions, tropical storms, hurricanes). In our previous blog in this series, we looked at Tropical Cyclone Intensity and Climate Change. In this blog, we will look at whether climate change is having an impact on TC frequency.

Hurricanes Katia (left), Irma (middle) and Jose (right)

Hurricanes Katia (left), Irma (middle) and Jose (right) – Sep 8, 2017

To date, the 2017 Atlantic hurricane season has produced 15 named storms – three more than the average or 25% more than usual. This has equalled the number of storms for last year. Meanwhile, there have been 10 hurricanes – four more than the average or 67% more than usual. This tied with 2010 for the most hurricanes since 2012. Further, there have been six major hurricanes – three more than average or 100% more than usual, the most since 2005.

Have TCs become more frequent?

According to the Intergovernmental Panel on Climate Change (IPCC), it is unlikely that TCs have become more frequent. Recall that the IPCC is the United Nations’ body task with assessing climate change.

Here is what the IPCC Assessment Report Five (AR5) actually says: “…recent assessments indicate that it is unlikely that annual numbers of tropical storms, hurricanes and major hurricanes counts have increased over the past 100 years in the North Atlantic basin…”. “No robust trends” exist.

So, having examined hundreds of peer-reviewed scientific research papers on the subject, the IPCC concluded that there has been change in the frequency of tropical cyclones.

One of the many papers cited by the IPCC, explains not only that there has been no change in the annual number of TCs but that there have been previous active Atlantic TC eras similar to the present. These active eras alternate with inactive eras; hence, in the not too distant future, the Atlantic will return to an inactive phase, perhaps similar to the 1970-1994 interval, when fewer storms occurred.

Further, although there has been no change in the number of  TCs over the past 100 years, research suggest that relative to about a thousand years ago, the annual number of TCs have decreased significantly. This is supported by Michael E. Mann et al., Michael J. Burn and Suzanne E. Palmer and others.

The increased greenhouse gases, the cause of climate change, substantially explain the observed SST increases over such places as the Atlantic and North West Pacific during the last 50 years. Added to this, are the apparent significant increases in TCs over this period. However, when the raw dataset is adjusted for short-lived TCs i.e. TCs lasting less than two days, a different picture emerges.

NA_TCs_Raw&Adjusted

Green-shaded curves depict global mean temperature (HadCRUT3 data set) and August–October main development region (MDR; 10° N–20° N, 80° W–20° W) SST anomalies (HadISST data set). Blue-shaded curves represent unadjusted tropical storm counts. Red-shaded curves include time-dependent adjustments for missing storms based on ship-track density. The curve labelled ‘>2-day’ depicts storms with a duration greater than 2.0 days. Orange-shaded curves depict US landfalling tropical storms and hurricanes (no adjustments). Solid black lines are five-year means (1878–2008); dashed black lines are linear trends. Vertical axis ticks represent one standard deviation. Series normalized to unit standard deviation. Only the top three series have significant linear trends (p = 0.05). Source: Knutson et al. 2010

Before the satellite era – pre-1966, a number of storms went undetected due to the fact that they never made landfall and occurred in unfrequented parts of the Atlantic Ocean by ships – the only source of TC detection over open waters before there were satellites. Thus, when the raw data is adjusted for these missing cyclones, no long-term trend is detected.

The trend in major hurricanes is not (statistically) significance. Notwithstanding, the current trend is unreliable since studies have shown that the wind speeds for TCs, over the period 1851 to 1920, were systematically underestimated, a fact supported by paleoclimatology.

To a great extent, the numbers of TCs on record, pre-1966, are there only because they made landfall. Counting landfalling TCs is not a robust method for determining overall trend; nevertheless, the method shows no trend in various regions of the world.

Will TCs become more frequent in the future?

To this question, the IPCC says no! Here are the exact words from the IPCC AR5, “…it is likely that the global frequency…of tropical cyclones will either decrease or remain essentially unchanged…. However, “substantial increases in the frequency of the most intense cyclones and it is more likely than not that this increase will be larger than 10% in some basins”.

One of the main scientific papers used by the IPCC to come to this conclusion is one done by Knutson TR, McBride JL, Chan J, et al. They say that late in this century, there will be a 6 to 34% decrease in TC frequency. However, on the other hand, the same paper projects a 2 to 11% increase in TC winds; hence, by virtue of this, predicts an increased frequency of major hurricanes being more likely than not by year 2100. Notable TC researchers Kevin J.E. Walsh, John L. McBride, Philip J. Klotzbach et al. endorsed this conclusion.

TrendOfTCs

General consensus assessment of the numerical experiments described in Supplementary Material Tables 14.SM.1 to 14.SM.4. All values represent expected percent change in the average over period 2081–2100 relative to 2000–2019, under an A1B-like scenario, based on expert judgement after subjective normalization of the model projections. Four metrics were considered: the percent change in (I) the total annual frequency of tropical storms, (II) the annual frequency of Category 4 and 5 storms, (III) the mean Lifetime Maximum Intensity (LMI; the maximum intensity achieved during a storm’s lifetime) and (IV) the precipitation rate within 200 km of storm centre at the time of LMI. For each metric plotted, the solid blue line is the best guess of the expected percent change, and the coloured bar provides the 67% (likely) confidence interval for this value (note that this interval ranges across –100% to +200% for the annual frequency of Category 4 and 5 storms in the North Atlantic). Where a metric is not plotted, there are insufficient data (denoted ‘insf.d.’) available to complete an assessment. A randomly drawn (and coloured) selection of historical storm tracks are underlain to identify regions of tropical cyclone activity. Source: IPCC AR5

One of the fallacious arguments being advanced for the increase in the frequency of TCs is increasingly warmer SSTs being caused by climate change. But quite obviously, warm SSTs are not the only parameters required for increased frequency. SSTs are very necessary but far from being sufficient for causing increased TC frequency.

A recently issued paper by Jeffrey P. Donnelly & Jonathan D. Woodruff indicates that such high SSTs as at present are not even necessary to support periods of frequent major hurricane activity. There have been eras with similar or higher number of TCs with significantly lower SSTs.

The Kevin J.E. Walsh et al. paper indicates the models used to simulate TCs, produce an increased frequency of TCs as temperature decreases and decreased frequency when temperature increases. Thus, increased SSTs do not necessarily mean increased TC frequency, by consensus, the opposite seems to be true.

Also required for increased number of TCs are conducive atmospheric conditions – unstable Atmosphere, moist middle Atmosphere and the overturning of the tropical Atmosphere caused by the Hadley cell. All three are needed for the formation of tropical disturbances – the precursors of TCs. However, fortunate for us, climate change is causing these things to go in the negative direction; hence, the projected decrease in the frequency of TCs by the end of this century.

Based on the IPCC and the vast majority of the TC researchers, TC frequency has not changed over the last century. By the preponderance of research papers, climate change has NOT caused TCs to become more frequent. However, by late this century the frequency of TCs is projected to decrease or perhaps remain unchanged relative to present. However, the number of major hurricanes is more likely than not to increase late in the current century. This is the conclusion of the IPCC AR5 – we either accept the full report or none at all. And if we accept all of it, we would not be blaming current TC and hurricane frequencies on climate change, at this time.

Our next blog in this series will look at the impact, if any, of climate change on tropical cyclone rainfall.








%d bloggers like this: