drop table if exists TT;
create table TT like Temp;
ALTER TABLE `TT`
	ADD COLUMN `AI` INT NOT NULL AUTO_INCREMENT FIRST,
	ADD PRIMARY KEY (`AI`),
	ADD UNIQUE INDEX `UniqueKey` (`City`, `MeasurmentDate`);

insert into TT
(City,MeasurmentDate,Temp)
select * from Temp
order by City,MeasurmentDate;

set @T=0;
select max(ContinousColdDays) into @MaxContinousColdDays
from (
	select AI,City,MeasurmentDate,Temp
	,if(Temp>=0,@T:=0,@T:=@T + if(Temp<0,1,0)) as ContinousColdDays
	from TT
	order by AI
) t;

select MeasurmentDate as FistColdDay
,date_sub(MeasurmentDate, INTERVAL @MaxContinousColdDays day) as LastColdDay
,@MaxContinousColdDays as MaxContinousColdDays
from (
	select AI,City,MeasurmentDate,Temp
	,if(Temp>=0,@T:=0,@T:=@T + if(Temp<0,1,0)) as ContinousColdDays
	from TT
	order by AI
) t
where ContinousColdDays=@MaxContinousColdDays;