The weather: its measurement and data representation Building Physics, Lo-Lo CDT Thursday 6 th October 2011

Lecture summary 1.The sun and its movement 2.Climatic regions 3.Measuring the weather 4.Hourly weather data 5.Heat Islands 6.Balance pint temperature and degree days 7.Synoptic weather data

The sun’s position The sun ‘drives’ the earth’s weather system. The location of the sun in the sky is important for calculating the radiation incident on surfaces of different orientation. The position can be precisely calculated (e.g. in dynamic thermal models of buildings). The position can be represented usefully by various charts. The position is expressed by azimuth 1 and altitude. 1 Degrees from true north

Revolution of Earth round the sun Summer solstice Spring Equinox Vernal equinox Winter solstice

Sunpath: cylindrical projection Alt = 90-Lat.+23 Alt = 90-Lat.-23 deg.

Middleberg, Netherlands: latitude: 51.5 / longitude: 3.6: source Photograph of Solar Path: 51.5 o N

Stereographic sunpath diagram for Trondheim Latitude: 63° 36'

The Koeppen classification of climate

Weather data and some uses for it -1 Ambient temperature ( o C or o F) –To figure out if natural ventilation and heating alone can maintain internal temperatures or if mechanical cooling is needed. –For calculating the heat loss through the building fabric by conduction and by ventilation and so the heating energy demand and perhaps the the cooling energy demands. –to work out how big a heating system must be –To work out how much energy is needed to cool air using a mechanically ventilation system. Solar radiation (and sun position) – W/m 2 (energy) lux = lm/m 2 (light) –To calculate the natural light available to illuminate a building interior. –To calculate the solar energy (heat) coming through windows. –To calculate the yield form a solar thermal or solar PV system.

Moisture content of air –To determine if air-conditioning (to dehumidify ventilation air) is needed. –To calculate the energy needed by a cooling machine to ‘dry’ the ventilation air. –To estimate if condensation is likely to be a problem. Wind speed and direction –To help calculate the wind pressures on buildings and so estimate the possible ventilation rate –To estimate the additional fabric heat losses. Weather data and some uses for it -2

Typical research weather station

Components of a weather station

Dynamic thermal models use the hourly data in simulations Typical Meteorological Year (TMY) -A whole year chosen to be representative of the local weather. Test Reference Year (TRY) -A year of data built up of 12 typical months, taken from different years. Design Summer Year (DSY) -A year of data chosen to show a particular characteristic, such as a particularly hot year.

London TRY – temperature and solar radiation

London TRY – hourly summer-time values

London TRY – hourly winter-time values

Summer-time comfort In the UK an overheating criterion is used to assess overheating risk in dynamic, hourly simulation. Operative temperature should not exceed 28°C for more than 1% of the occupied hours per year (CIBSE, Guide A, 2006) Use a near-extreme design weather year for simulation – the Design Summer Year (DSY).

Selection of a DSY This is the DSY

Cumulative frequency of occurrence of Summer-time Temperatures: London DSY Source: CIBSE Guide J For a building occupied for 10 hrs per day, 5 days per week, for 50 weeks per year, 1% of occupied hours is 250hrs.

But micro-climate can be important Urban heat islands Altitude Local water features – coastal locations. etc

Urban Heat Island Intensity, London Heathrow airport SSEES building Source: Watkins et al Temperatures (K) relative to rural reference on 02/08/1999 at 02:00 hours Source: Watkins et al British Museum

London’s Heat Island Intensity 31 May to 31 August 1999 (all data, mean +/- standard deviation)

The idea of balance point temperature Balance point temp. Internal desired temp. Internal gain Qi Solar gain Qs Heat loss by ventilation and through fabric Source – Vital Signs LBL Berkeley. Qv + Qf

Building balance point and the base temperature The Balance Point is the outdoor air temperature (Tb) causing building heat gains to be dissipated at a rate that creates a desired indoor air temperature (Ts). It is determined by design. (LBL Vital signs). Tb = Ts - (Qs+Qi) /(HLC) HLC = ∑ UA V.N Each degree deviation from that balance point temperature results in heating (if the temperature is below the balance point) or cooling (if the temperature is above the balance point). The Base Temperature is the outside temperature above which a building needs no heating (general UK definition) The two are functionally the same for heated buildings

Base temperatures In the UK the base temperature has, by convention, been taken as 15.5 o C and 18.5 o C (for use in e.g. hospital design because the internal temperature is higher). But the base temperature for any particular building depends on –the temperature that the building is heated to, –the nature of the building (including the heat-generating occupants and equipment within it), and –the fabric air tightness and insulation levels. So as buildings become better insulated these ‘conventional’ values are no longer appropriate – base temperature needs to be much lower. Question – what is the problem with very low base temperatures?

Heating degree-days (HDD) Conventionally, one degree day is counted for every degree that the ambient temperature is below the base temperature. With hourly temperature records it is easier and more accurate to calculate it from hourly temperature differences. Hourly (or daily mean) temperatures above the base temp. values do not contribute to the totals.

Some daily HDDs calculated from hourly data. Up to date hourly, monthly and annual HDD data are available from the Oxford, Environmental Change Institute’s web site at:

Monthly Degree Days: 2005 year-monthbase_15.5base_16base_16.5base_17base_17.5base_18base_

Standard UK Annual average degree days for SAP calculations

Energy use and HDD data Total heating energy use (E T ) can be estimated form the known degree days for the location: E T = (HLC × 24 × HDD)/1000 kWh Where: DD are the heating degree days ( o C.days) HLC is the buildings' heat loss coefficient (W/K). For a given building (HLC) the differences in the HDD from one place to another are a direct indication of the likely differences in heating energy use. Note, changes in building insulation levels affect both the HLC and the HDD values.

HDDs are useful for energy management. Deviations from an established regression line can indicate ‘problems’. Regression analysis is relatively straightforward - a

Simpler models might use synoptic weather data BREDEM12 v2001 estimates heating degree days from the mean monthly temperatures

Synoptic solar radiation data BREDEM12 v2001 uses average monthly horiz. solar flux to calculate solar heat gain

ASHRAE and CIBSE - sources of synoptic, hourly and future weather data

Some synoptic data provided by ASHRAE - heating system design

Some synoptic data provided by ASHRAE - cooling system design StationLatLongElevDatesWind DataCooling 0.4% MWS 0.4% PWD 0.4% DB NameDeg mYearm/sDeg°C ASHRAE Table 1AASHRAE Table 1B 1a1c1d1e1g5c5d2a Riyadh24.72N46.72E London (Heathrow) 51.48N0.45W Chicago (O ’ Hare) 41.98N87.9W Macau22.2N113.5E MWS: Mean wind speed PWD: Prevailing wind direction

Summary Weather data can easily be measured hourly. It is available world wide for most cities in this format. Dynamic thermal models of buildings use hourly weather data. Simpler models use reduced weather data such as degree-days. Such data assists energy managers. Synoptic data can assist strategic design decisions and plant sizing calculations. Weather data is increasingly easy to measure at low cost.