Essex resides in the latitude of predominately South-Westerly winds where depressions and their associated bands of cloud and rain move Eastwards, often bringing unsettled weather - particularly during the autumn.
The air streams associated with the depressions often originate in very warm or very cold regions, but by the time they reach the United Kingdom their extreme temperatures have been modified by the seas and oceans over which they have travelled. Consequently, summers in Essex are cooler than continental Europe, but the winters are usually much milder.
Sometimes large, stationary anticyclones become established near the United Kingdom and they block the depressions. These anticyclones are most common in spring, but can occur in all seasons and they can persist for several weeks and completely change the character of the weather.
In summer months, the blocking anticyclones often bring a prolonged spell of very warm or hot weather to Essex. In winter, dry weather prevails in anticyclones, but during cloudless nights the temperature falls away and does not recover the following day because of weak sunshine or dense patches of fog. If a winter anticyclone settles to the North or North-East of the United Kingdom, then Easterly winds can draw bitterly cold air from the continent into Essex for a long period of time.
Falls of snow in Essex are confined to the period from November to April, though falls of sleet and wet snow occasionally occur in late October. Snow rarely lies before December or after March.
The number of days with snow lying is less than the number of days with snow falling because in many cases when snow is falling, the temperature of the ground remains above freezing point with the result that the snow does not lie for long.
Thunderstorms can occur at any time of the year, but the maximum frequency of occurrence is between May and September. During the summer months, bands of thunderstorms often push up from Northern France during the evening hours which bring spectacular lightning displays to many parts of Essex. One such thunderstorm delivered 115 mm of rainfall in just two hours on August, 1st 1888 in Romford.
Because of the variation in length of day from winter to summer, the duration of sunshine shows a marked seasonal variation. As a consequence, December is on average the month with least sunshine and June is the sunniest. Rettendon notched up 1850 hours of sunshine in 1990 – making it one of the sunniest places in the whole of the United Kingdom that year.
The strongest winds that occur in Essex are associated with the passage of depressions across or close to England and Wales. Shoeburyness recorded a maximum wind gust of 101 mph during the Great Storm in October 1987.
Air temperature varies on a daily, seasonal and geographical basis. On a daily basis, the temperature is usually lower at night than by day, with the minimum temperature normally occurring shortly after dawn and the maximum temperature two or three hours after midday. February is on average the coldest month and August the warmest.
In February, the coldest districts are Uttlesford and Braintree. The main factor determining the distribution of temperature in Essex is distance from the North Sea. The coldest night on record occurred in South Ockendon on February, 7th 1879 when -21.4 degrees Celsius was observed.
In August, the warmest districts are those bordering London – Brentwood, Thurrock and Epping Forest. Districts near the coast are less warm than inland, the opposite to what happens in February and the temperature decreases from South to North Essex. The Met Office station at Andrewsfield recorded 35.9 degrees Celsius on August, 10th 2003 - the highest official temperature recorded in Essex. An unofficial 37.9 degrees Celsius was recorded in Epping on the same day.