Kathryn Kane's excellent article about the eruption of Mount Tambora HERE also mentions an eruption of 1809 which made the mini ice age worse, but as you will see from the data below, even before that the weather was at times pretty extreme at times!
| 
1800 
An overall dry summer from March - September | |
| 
Jan  | 
Mild at beginning of month, then severe frost and heavy
  rains with thunder to end of month. | 
| 
Feb | 
Mostly dry.  | 
| 
Mar | 
Dry winds throughout month. W and SW winds prevailing. 
Beef up from 6d/lb to 8d; veal from 7d/lb to 8d; Mutton
  from 5d/lb to 6d | 
| 
Apr | 
Mild, some gentle seasonal rain, but heavy rain at end of
  month.  W and SW winds prevailing | 
| 
May | 
Sunday 4th May violent thunder storms at Windsor, Liverpool, Hull,
  Northampton, Kettering, Warwick, Lincolnshire etc.
  Floods after the storm as far S. as London.   
May not as warm as sometimes but generally dry after the
  storm. 
W and SW winds prevailing.  | 
| 
Jun | 
Rain off and on for first ten days of month 
Wednesday 4th, rain all morning in London and pouring from 9am -1pm on the Review in Hyde Park; but George III rode throughout it.  
Dry and fine after the 10th.  | 
| 
Jul | 
Dry; hot and windy. 
  Harvest able to commence by the close of the month.  Drought.  | 
| 
Aug | 
First half of month hot and sultry 
Tuesday 19th, uncommonly hot but with clouds
  coming in from NW followed by storm of hail, greatest in Norfolk and across to Baldock.  There was a powerful storm in London, and damage to
  buildings from lightning.  Storms were
  experienced from Dorset to East
    Anglia, some flash floods.  4 men died at Lyme.  The harvest in East Anglia was saved, but not
  all of it further south.  | 
| 
Sept | 
A dry beginning to the month, then showery.  | 
| 
Oct | 
Mild | 
| 
Nov | 
The first weeks of the month very adverse. 
Saturday 8th heavy rains Exeter
  and Bath; a chaise was washed away near Exeter. 
Sunday 9th November dreadful storm of wind in
  the morning, Stevenage.  The Chartist meeting on Kennington Common
  was dispersed as much by heavy rain as by the horse volunteers…  
Largely temperate for most of rest of month 
Frost at the end of November. | 
| 
Dec | 
Mild for most of month. 
Snow fell Saturday 29th which slowed down the
  Mails.  6’ deep in some places. | 
| 
1801 | |
| 
Jan  | 
Mild, very little frost, wet | 
| 
Feb | 
Mild but damp, a few frosty nights. 
Tuesday 17th inclement weather | 
| 
Mar | 
Strong winds, and frequent rain. 
Falmouth:
  Tuesday 17th,  wind S,
  blowing strongly. 
Plymouth: 
Wednesday 18th, wind SW moderate 
Thursday 19th, wind WSW and blowing hard [Falmouth wind NW] 
Friday 20th wind SW hurricane with rain until
  2pm, hail in the evening 
Saturday 21sr wind SW hurricane 
Sunday 22nd wind SW, blowing hard [Falmouth, wind W,
  squally] 
Monday 23rd wind SW, moderate 
Tuesday 24th, wind W, moderate | 
| 
Apr | 
Prevailing winds W and SW. Dry and cold in North,
  especially cold on Sunday 12th. 
Wet in West of England, with W and SW gales.  | 
| 
May | 
First week in May, charming weather, but no dust.  Mild, warm rains and sunny weather,
  excellent for the crops.  Prevailing
  winds W and SW.  | 
| 
Jun | 
Largely dry and warm. 
  Early thunderstorms, at Southwell a woman and child were killed by
  lightning.  
Tuesday 30th, Ball lightning in Hoxton.  Full moon, wind WNW, showers.  | 
| 
Jul | 
Wednesday 1st, thunderstorm, lightning strikes
  near Bath,
  woman killed in her own home by ball lightning down the chimney. 
In general there were heavy and frequent showers in the
  early and mid part of the month. 
Wind and weather shipping report: 
Wed. 1st Plymouth SW, rain: Falmouth NW, fair 
Thur. 2nd Plymouth WSW, rain: Falmouth s, heavy rain 
Fri. 3rd Plymouth SW, rain; Falmouth NW showers 
Sat. 4th Plymouth SW, rain: Falmouth NW fine,
  later heavy rain 
Sun. 5th  Plymouth SW, fair: Falmouth variable, fresh 
Mon. 6th Plymouth SE, rain: Falmouth 
  S, strong, rain 
Tues. 7th Plymouth SW cloudy 
Showery rain in this month filled the ears of corn in the
  south, but  in the north and east there
  was hail and rain that was less welcome.  
Wednesday 22nd thunderstorm Suffolk 
Thursday 23rd Various thunderstorm, Portsmouth, Richmond, Berkshire [where 2 are killed]. Heavy storm of hail in Reading, also in Elmsett in Suffolk, corn [wheat] beaten down.  | 
| 
Aug | 
Early rain, including some heavy showers clearing around
  Thursday 6th, wind veering to ENE. 
Saturday 2nd, near Stratford, violent thunderstorm and rain,
  killed 2 men and a cow.  The Royal
  Cornish Gazette ran an article on how to preserve life from lightning
  strikes.  
Fine and warm for rest of month, though cold at night with
  mists.  
Wed. 26th Plymouth
  variable, sultry: Falmouth,
  variable, fine 
Thur. 27th Plymouth SSE, sultry: Falmouth E 
Fri. 28th Plymouth SSW, sultry: Falmouth E 
Sat. 29th Plymouth ESE sultry: Falmouth N 
Sun. 30th Plymouth SW fair until evening, then
  storm, rain and lightning and hail until 7am next morning: Falmouth SW fine,
  then heavy rain.  
Mon 31st Plymouth SW showery: Falmouth, N fresh breeze | 
| 
Sept | 
Tuesday 1st, Plymouth, westerly wind, fair weather.  
Sunday 6th Wellington/Salop thunderstorm and
  rain lasting 2 hours; dreadful lighting at Coalbrookdale 
14th September Thunderstorm, lightning and rain
  for 2 hours Gloucester.
   
Generally a lot of rain in the north, but hard, drying
  winds | 
| 
Oct | 
A month of mainly fine weather. 
Tues.6th Falmouth N, fair 
Wed, 7th Plymouth SW rain: Falmouth SE 
Thur. 8th Plymouth SW, rain: Falmouth SW 
Fri. 9th Plymouth SE, hard rain, incessant
  rain: Falmouth SW, heavy rain 
Friday 9th near Salisbury, in a circle about 3
  miles across, clear skies darkened, scattered clouds gathered then ‘exploded’
  as a whirlwind arose causing damage to property.  Lightning and thunder lasted 20 minutes,
  then heavy rain with hail, some stones 3” across.   
Sat. 10th Plymouth SE, cloudy, heavy rain. 
Saturday 10th London, fine and warm day until evening
  when a thunderstorm started, lightning for an hour before any rain, very
  violent, lasted until 3am Sunday 11th. 
Sun. 11th Plymouth SE, fair; Falmouth S 
Mon. 12th Plymouth
  variable; Falmouth,
  variable 
Tues. 13th Plymouth, SW, fair, wheat down to
  18/6d from 27/6d a fortnight previously.  
Tuesday 20th [full moon 21st] Lunar
  rainbow observed at Edinburgh
  on the Western horizon; a full bow for half an hour. 
Thursday 22nd very inclement weather.  | 
| 
Nov | 
Tuesday 3rd heavy seas in the channel and
  storms 
Wednesday 25th tempestuous weather, much damage
  at Margate,
  destroying Hubbard’s bathing room, Tennant’s shop and Hughes’s hot baths, and
  broke down portions of the parade in front of Mitchener’s Hotel. 
Friday 27th tempestuous weather all around
  coasts at night, storm NNE to NE, hail and rain.  Ships sunk and driven inshore. 
Saturday 28th Shrewsbury, mail delayed by deep snow in
  parts of Worcestershire and Goucestershire, the coach having often to quit
  the roads and travel over fields.  Near
  Marlborough,
  drifts were several feet deep. 
Rain at end of month | 
| 
Dec | 
Intense frost throughout month | 
| 
1802 
A dry year especially in London and the South | |
| 
Jan  | 
Intense frost but dry throughout the month, but in the
  south west of the country cold wet weather at beginning of the month, killing
  newborn lambs. 
Wednesday 20th and Thursday 21st
  hurricanes across the land.  In Manchester several
  houses blown down and 4 people crushed to death. Several vessels on the
  Mersey foundered as well as houses blown down in Liverpool.  Ships sunk at anchor, much loss of
  life.  A servant girl near Doncaster killed by a falling chimney.  
This great gale extended in a lessened form to the 28th
  of the month off and on, becoming severe again on the 28th.  | 
| 
Feb | 
 Frost at beginning
  of month, rain in later part | 
| 
Mar | 
Prevailing winds W and SW; on the whole fine weather.  | 
| 
Apr | 
Prevailing winds W and SW, fine and warm weather, late
  showers | 
| 
May | 
Changeable temperature in the onset of the month with cold
  and severely frosty nights.  Very
  little rain throughout, and most of month cold.  Prevailing winds W and SW.  | 
| 
Jun | 
Light showers at first but general drought.  Cool weather. ? low 50s F | 
| 
Jul | 
Drought ends with cold rain throughout the month, cold
  weather and high winds.  Rain
  continuous and heavy in later part of month. 
   
Tuesday 20th Thunderstorms in Scotland, much rain in Aberdeen | 
| 
Aug | 
Remarkably fine and dry, bountiful harvest 
Wednesday 25th Newcastle, thunderstorm with heavy wind and
  hail. | 
| 
Sept | 
Fine and very dry on the whole. 
Wednesday 8th, sudden storm of wind and rain,
  then thunder and lightning for about an hour, fairly widespread. | 
| 
Oct | 
Fine and mild, dry at beginning of month, late mild rain. 
Friday 8th Brighton,
  storm from SW  
Wednesday 13th Swansea, a fine day for a balloon ascension | 
| 
Nov | 
Mild for time of year, some gentle rain | 
| 
Dec | 
Mild temperatures for time of year throughout | 
| 
1803 On the
  whole a dry year but seldom 10 days at a stretch without rain.  However it was dry enough for many springs
  to dry up, to be replenished after October. | |
| 
Jan  | 
Severe weather following the warmth of the previous month;
  severe frosts. 
Saturday 8th [day after full moon] about
  midnight a hurricane from ESE, rain and sleet, Newcastle bears the brunt.  | 
| 
Feb | 
Milder weather, frosts largely dissipate after the early
  part of the month. | 
| 
Mar | 
Early frosts, then finer weather.  In Midlands,
  the early frosts were severe. 
  Prevailing winds W and SW. | 
| 
Apr | 
Generally dry weather. 
  In Yorkshire and the north frosty
  nights between 18th and 20th, then several days stormy
  and tempestuous.  
Sunday 24th wind and light showers on the
  Sunday promenade in Hyde Park and Kensington
   Gardens, with a ‘blush
  of sunshine’.  Then sudden storms and a
  torrent of rain scattered the promenaders to Piccadilly to avoid the storm,
  doubtless with many a blush at suddenly transparent muslin.  Prevailing winds W and SW. | 
| 
May | 
Cold northerly winds at the beginning of the month, but
  warm gentle rains towards the close, by which time prevailing winds W and SW | 
| 
Jun | 
Frequent but not torrential rains, but insufficient to
  relieve drought especially in Scotland
  [the wrong type of rain, no doubt, as our modern water board would put
  it.]  Some burning hot days but cold
  nights.  
Monday 6th violent thunderstorm which did
  damage across the country.  | 
| 
Jul | 
Fine and warm 
[Saturday 9th July fire broke out in tower of Westminster Abbey; roof fell in before
  fire was under control but no damage to monuments. ] | 
| 
Aug | 
Continuous heat and sunshine.  Rain in North, uninterrupted fine weather
  in Midlands. 
  Too dry for some crops to survive. | 
| 
Sept | 
Unusually fine weather, with late, fine rains | 
| 
Oct | 
Largely fine, rain at beginning of month, but still
  something of a drought 
[Friday 2nd Astley’s Amphitheatre burned down
  as well as some houses behind it; Mrs Wood, mother of Mrs Astley, lost her
  life.] | 
| 
Nov | 
Wet, especially near start of month, but heavy rains
  throughout month.  
Sunday 13th a most beautiful vivid meteor seen
  in London and
  other parts of the country about 8pm. 
  Some reported a noise like thunder accompanying it. 
Tuesday 22nd South Coast,
  rain, hail, thunder and lightning, W and WSW winds; storms across South.  At Bromswell the storm was preceded by an
  explosion in the air, followed by a ‘heavy noise’ for a few seconds, striking
  those observing it temporarily blind from the flash, and a strong sulphurous
  smell, followed by torrents of rain.  | 
| 
Dec | 
Snow and frost at beginning of month, then thaw and much
  rain.  
Tempestuous weather in the last week of the month
  especially along the Sussex
  coast especially Sunday 25th and Monday 26th.  Average temperatures 40.4 ° Fahrenheit in London, and temperature
  on 24th was 2° higher than on June 24th 1802! | 
| 
1804 
Dry summer continuing well into September with pleasant weather | |
| 
Jan  | 
Much rain throughout the month, but mild
  temperatures.  Tempest of wind and rain
  towards the end of the month. 
Thursday 19th tremendous gale and storm from
  SW  Gloucester and other parts of the
  county; struck in Plymouth
  on Friday 20th.  Gales along
  the coast for several days. Damage done to the shipping at Falmouth on the 25th; reckoned a
  worse storm than the one the previous month.  | 
| 
Feb | 
Dry; frosts and thaws | 
| 
Mar | 
Largely fine in South with late sharp strong winds and
  frosty.  Weather in North more severe.
  Prevailing winds W and SW | 
| 
Apr | 
Strong frosty winds to open the month, and cold  rains prevailing to near the end of the
  month.  Prevailing winds W and SW. 
Monday 30th Brighton,
  2 hours before daybreak a tremendous thunderstorm with hail and rain, thunder
  and sheet lightning, so loud that many supposed the French had landed and the
  noise was artillery flashes and reports.  | 
| 
May | 
Very fine weather. 
  Prevailing winds W and SW 
Friday 4th Cheshire
  and Somerset  storm of rain described as tropical in its
  intensity.  Part of Berchen cliff fell.
   
Monday 7th Lewes tempest and heavy rains
  commencing after midnight and finishing between 6am and 7am. 
Tuesday 8th foggy at sea; Manchester,
  violent storm; tornado at Bolton and ball of
  lightning.  | 
| 
Jun | 
Dry and warm. 
Sunday 24th June, Brighton,
  it was sultry all day; the Steyne had a military parade, and shortly after it
  was over, around 8pm, a storm with rain and hail until 11pm.  At 10-30 ball lightning came down a chimney
  but no-one was harmed. It passed out through the window and exploded in a
  sulphurous reek.  The houses were
  shaken by thunder.   In Woolwich, the
  same storm’s lightning set light to the rigging of a sheer hulk.  It struck again and blinded two
  sailors.  The storm was experienced as
  far north as Bucks.  In London a woman was
  killed by lightning in Grosvenor
    Square.  | 
| 
Jul | 
Dry and warm in South, heavy rains in North. 
Wednesday 11th thunderstorm about 3pm of great
  severity in the region of Glasgow;
  a girl killed by lightning. Ball lightning and rain.  
Sunday 15th violent storm in Norfolk,
  Suffolk and eastern Essex,
  ricks set on fire, and two men, a dog and a cow struck dead by lightning,
  others singed or burned by it.  
Tuesday 31st storm on coast of France. | 
| 
Aug | 
Wednesday 1st heavy gales in the West;  ship driven from mooring  
Thursday 2nd thunderstorms in Ireland 
 Friday 3rd
  tempests in East; Thunderstorms Western England and ball lightning at
  Brailsford.  Vivid lightning Shropshire, and rain heavy enough to damage crops.  2” rain in Hampshire.  
Fine dry weather with some rain, but moderate, and mostly
  in North for rest of month.  | 
| 
Sept | 
Fine weather, still dry on the whole 
Friday 14th, Plymouth SSW, cloudy 
Saturday 15th Plymouth, sultry until noon then storm of
  thunder and lightning, lasting about an hour.  
Sunday 16th Plymouth, SW, fair.  
Monday 17th Plymouth, SW, fair and sultry.  St
    Lambert’s day celebrated 
Tuesday 18th, Plymouth ESE, cloudy and sultry 
Before Tuesday 18th: [reported on that date as
  ‘lately’] Small tornado out of sultry weather at Whitby. [probably the 15th?] | 
| 
Oct | 
Rain, but mild 
Friday 26th Plymouth SSE, fair 
Saturday 27th Plymouth SSW hurricane force,
  blowing up overnight and continuing into the day:  Falmouth,
  wind S.  Bristol also has hurricane.  
Sunday 28th Plymouth SW blow hard and rain: Falmouth wind S 
Monday 29th Plymouth SW, rain: Falmouth SE 
Tuesday 30th Plymouth SW, cloudy: Falmouth S 
Wednesday 31st Plymouth,
  SW: Falmouth
  SW 
Wednesday 31st Cornwall, storm of thunder and lightning
  and sudden gusts of wind.  Hereford  dreadful storm of thunder and lightning in
  the evening, and at around 10pm a tornado that levelled 130 apple and pear
  trees in a swathe 200 yards in length and 40 yards in breadth. Those to
  either side were uninjured.  Wind SE
  and NW changing momentarily.  10 or 12
  oaks destroyed in wood at end of the orchards. Before and after tornado,
  night very dark and calm. Heavy rain fell at intervals and frequent flashes
  of lightning. | 
| 
Nov | 
Rain for much of month. 
Thursday 1st Falmouth SW 
Friday 2nd Falmouth, N 
The astronomer La Lande calculated that more rain had
  fallen in the last 9 months than had fallen in the preceding 27 months.  
Sunday 4th, Glasgow heavy wind and swell in harbour
  such that a ship parted its cables and blew away.  
Sunday 18th conjunction of Venus and Saturn.   Gales in the Channel. | 
| 
Dec | 
Some rain and mild in first half of month.  Dry frosty weather towards close of
  month.  Average temperature 37° F in London. Heavy snow in
  middle of month in Scotland,
  which delayed the London
  mails on the 20th.  | 
| 
1805 
Wet summer in London | |
| 
Jan  | 
Sharp frosts through most of month and continuous rain in
  the East 
Sunday 6th and Monday 7th heavy rain
  around Chester.   In the south east there were also severe
  storms and heavy rain and strong wind, and during the Sunday night rain, snow
  and frost in turn. 
Monday 21st The Serpentine being frozen,
  hundreds of skaters repaired there, but decided it was unsafe.  About 3 pm some score decided to test it,
  and they skated for a while without mishap encouraging others.  About 50 were on the ice when it gave way
  simultaneously in 2 places.  4 men fell
  through, one in attempting to rescue another, and all but 1 were saved using
  ladders.  | 
| 
Feb | 
Mild | 
| 
Mar | 
Cold, dry weather 
  through much of month; winds as much from E and NE as from W and SW | 
| 
Apr | 
Fine, some light seasonal rains at start of month, sharp
  frosty night at end of month, generally dry. 
8th Apr fine bright warm day.  | 
| 
May | 
Sharp cold winds from NE; cold, especially at night, often
  frost in the mornings, wet weather. 
  Wind veered W at end of month. 
  As many winds during month from E and NE as from W and SW.  | 
| 
Jun | 
Seasonal rains; heavy showers in Home Counties.  North Easterly winds over much of month
  caused damage to exposed fruit trees. 
Friday 14th Plymouth wind NNW, cold rain:  
Saturday 15th Plymouth wind NNW, cold. Falmouth, wind  N 
Sunday 16th Plymouth
  wind variable, some light showers, cold evening: Falmouth wind W 
.  
Monday 17th Plymouth
  wind W, cloudy and cold in the evening: Falmouth
  wind W  
Monday 17th Whitehaven, a torrent of rain 
Tuesday 18th Plymouth wind NW, cloudy and cold. Falmouth, wind NNW.
  Storm in the Yarmouth Roads [shipping lanes near Isle of
   Wight].  
Wednesday 19th Plymouth
  wind NNW: Falmouth,
  wind N 
Thursday 20th Falmouth wind N 
Friday 21st Falmouth wind variable 
Friday 28th London about noon, tremendous storm or
  rain, hail, lightning and thunder, lasting 20 minutes.  Many windows were broken and the streets
  deluged rising so high in Clement’s Lane, some 2-3’ deep, that a stall was
  carried away.  The hailstones were the
  size of marbles. Kingston-upon-Thames,
  a dreadful storm with torrents of hail and rain. Rain ran off hills, and
  lightning caused much damage. Floods in low lying areas. | 
| 
Jul | 
Fine weather, after cold winds and dull weather at start
  of month,  Average temperature 61.5° F 
Monday 22nd Manchester, violent thunderstorm. 
Tuesday 23rd Leeds
  thunderstorm, and what appeared to be two sheets of fire descended on the
  roof of a factory, but no harm was done. 
  In Craven a small boy was struck dead by lightning, and fires were
  started by the lightning in various places. 
  Cheshire
  in general suffered storms, also Staffordshire. 
Monday 22nd to Thursday 25th
  Carlisle and other parts of Cumbria
  much rain fell, rendering the streets almost impassable.  Thunder and lightning off and on.  
This summer reckoned to have experienced more thunder than
  in living memory.  | 
| 
Aug | 
Fine weather on the whole 
Rain and thunderstorms in middle of month improved the
  hops. 
Saturday 10th, full moon such as to cause very
  high spring tides, and for the next three days | 
| 
Sept | 
Fine on the whole 
Saturday 21st Margate, a torrent of rain about noon,
  spoiling the public breakfast for the fashionable at Dandelion. It brightened
  for them however by 3pm.  | 
| 
Oct | 
Started fine and dry, but wet at end 
Friday 25th hurricane followed by gusty weather
  Devon.  
Tuesday 29th, 
  weather deteriorates generally | 
| 
Nov | 
Relatively mild and moist | 
| 
Dec | 
Severe weather to open, with snow; generally cold wet
  weather.  Lowest temperature  Tuesday 17th, 20° F, wind SW
  [Polar maritime returning if anyone cares]. 
  Average temperature 38.3° F | 
 
