The City is England's smallest ceremonial county by area and population, and the fourth most densely populated. Of the 326 English districts, it is the second smallest by population, after the Isles of Scilly, and the smallest by area. It is also the smallest English city by population (and in Britain, only two cities in Wales are smaller), and the smallest in the UK by area.

The elevation of the City ranges from sea level at the Thames to 21.6 metres (71 ft) at the junction of High Holborn and Chancery Lane. Two small but notable hills are within the historic core, Ludgate Hill to the west and Cornhill to the east. Between them ran the Walbrook, one of the many "lost" rivers or streams of London (another is the Fleet).

Boundary

Beginning in the west, where the City borders Westminster, the boundary crosses the Victoria Embankment from the Thames, passes to the west of Middle Temple, then turns for a short distance along Strand and then north up Chancery Lane, where it borders Camden. It turns east along Holborn to Holborn Circus, and then goes north east to Charterhouse Street. As it crosses Farringdon Road it becomes the boundary with Islington. It continues to Aldersgate, goes north, and turns east into some back streets soon after Aldersgate becomes Goswell Road, since 1994 embracing all of the Corporation's Golden Lane Estate. Here, at Baltic Street West, is the most northerly extent. The boundary includes all of the Barbican Estate and continues east along Ropemaker Street and its continuation on the other side of Moorgate, becomes South Place. It goes north, reaching the border with Hackney, then east, north, east on back streets, with Worship Street forming a northern boundary, so as to include the Broadgate estate. The boundary then turns south at Norton Folgate and becomes the border with Tower Hamlets. It continues south into Bishopsgate, and takes some backstreets to Middlesex Street (Petticoat Lane) where it continues south-east then south. It then turns south-west, crossing the Minories so as to exclude the Tower of London, and then reaches the river. It then runs up the centre of the Thames, with the exception that Blackfriars Bridge falls within the City; the City controls London Bridge (as part of Bridge ward) but only half of the river underneath it,^{[citation needed]} a feature which is unique in British local administration.^{[dubious – discuss]}

The boundaries are marked by black bollards bearing the City's emblem, and by dragon boundary marks at major entrances, such as Holborn. A more substantial monument marks the boundary at Temple Bar on Fleet Street.

In some places the financial district extends slightly beyond the boundaries, notably to the north and east, into the London boroughs of Tower Hamlets, Hackney and Islington, and informally these locations are seen as part of the "Square Mile". Since the 1990s the eastern fringe, extending into Hackney and Tower Hamlets, has increasingly been a focus for large office developments due to the availability of large sites compared to within the City.

Gardens and public art

The City has no sizeable parks within its boundary, but does have a network of a large number of gardens and small open spaces, many of them maintained by the Corporation. These range from formal gardens such as the one in Finsbury Circus, containing a bowling green and bandstand, to churchyards such as St Olave Hart Street, to water features and artwork in courtyards and pedestrianised lanes.

Gardens include:

• Barber-Surgeon's Hall Garden, London Wall
• Cleary Garden, Queen Victoria Street
• Finsbury Circus, Blomfield Street/London Wall/Moorgate
• Jubilee Garden, Houndsditch
• Portsoken Street Garden, Portsoken Street/Goodman's Yard
• Postman's Park, Little Britain
• Seething Lane Garden, Seething Lane
• St Dunstan-in-the-East, St Dunstan's Hill
• St Mary Aldermanbury, Aldermanbury
• St Olave Hart Street churchyard, Seething Lane
• St Paul's churchyard, St Paul's Cathedral
• West Smithfield Garden, West Smithfield
• Whittington Gardens, College Street

There are a number of private gardens and open spaces, often within courtyards of the larger commercial developments. Two of the largest are those of the Inner Temple and Middle Temple Inns of Court, in the far southwest.

The Thames and its riverside walks are increasingly being valued as open space and in recent years efforts have been made to increase the ability for pedestrians to access and walk along the river.

Climate

The nearest weather station has historically been the London Weather Centre at Kingsway/ Holborn, although observations ceased in 2010. Now St. James Park provides the nearest official readings.

The City has an oceanic climate (Köppen "Cfb") modified by the Urban Heat Island in the centre of London. This generally causes higher night-time minima than outlying areas. For example, the August mean minimum of 14.7 °C (58.5 °F) compares to a figure of 13.3 °C (55.9 °F) for Greenwich and Heathrow whereas is 11.6 °C (52.9 °F) at Wisley in the middle of several square miles of Metropolitan Green Belt. All figures refer to the observation period 1971–2000.

Accordingly, the weather station holds the record for the UK's warmest overnight minimum temperature, 24.0 °C (75.2 °F), recorded on 4 August 1990. The maximum is 37.6 °C (99.7 °F), set on 10 August 2003. The absolute minimum for the weather station is a mere −8.2 °C (17.2 °F), compared to readings around −15.0 °C (5.0 °F) towards the edges of London. Unusually, this temperature was during a windy and snowy cold spell (mid-January 1987), rather than a cold clear night—cold air drainage is arrested due to the vast urban area surrounding the city.

The station holds the record for the highest British mean monthly temperature, 24.5 °C (76.1 °F) (mean maximum 29.2 °C (84.6 °F), mean minimum 19.7 °C (67.5 °F) during July 2006). However, in terms of daytime maximum temperatures, Cambridge NIAB and Botanical Gardens with a mean maximum of 29.1 °C (84.4 °F), and Heathrow with 29.0 °C (84.2 °F) all exceeded this.