A psychrometric diagram is a psychrometry tool used to understand the relationship between humidity and air temperature conditions. Through the use of the psychrometric diagram and appropriate calculations, it is possible to know the amount of heat or cooling needed to achieve the desired temperature and humidity.
The Clima psychrometric diagram shows dry bulb temperature on the abscissae, specific humidity on the ordinates, and relative humidity as parametric curves inside the graph.
Temperature line in the Psychrometric diagram
The humidity ratio is the water vapor's weight per unit weight of dry air, the so-called specific humidity. It is important not to confuse it with relative humidity. The same specific humidity value can have different relative humidity conditions by changing the air temperature.
Humidity ratio line in the Psychrometric diagram
All air conditions cannot go beyond the 100% saturation curve, which means that the air contains the maximum amount of water vapor, in certain conditions of temperature ad pressure.
The simplest transformation to be analyzed on the psychrometric diagram is the heating and cooling processes. The transition from the starting condition (1) to the final one (2) occurs horizontally, at constant humidity ratio values. The final condition (2) can be inspected as a function of the starting one.
A common application of the diagram is humidification and dehumidification of environments, where the saturation curve is used. The air is cooled to (2), and then heat continues to be removed, so condensing some of the vapor and decreasing the specific humidity until the desired value (2'). Therefore, a post-heating process could bring the air back to the starting relative humidity (3).
The main application of the psychrometric diagram is in the design of large all-air systems. The air we breathe is differentiated into dry air and humid air. Humid air, which is what is analyzed in the psychrometric diagram, is composed of dry air and water vapor. Thanks to the diagram, we can run the HVAC system on heating/cooling air and mixing humid and dry air, and know exactly what temperature and relative humidity conditions we will obtain. For instance, an adiabatic mixing of air in the two starting conditions (1-2) will obtain a new mixture (3).
The diagram is applied whenever the humidity of a particular environment needs to be studied, for reasons of thermal comfort or for the preservation of valuable objects, such as in museums.
The Climate Profiles graph gives the user the opportunity to observe at a glance the distribution of the data in the EPW file for four key variables and their variation between day and night.
The Climate Profiles graph are Violin Plots. They show the probability density of the data at different values, usually smoothed by a kernel density estimator. Wider sections of the violin plot represent a higher probability that members of the population will take on the given value; the skinnier sections represent a lower probability.
On mouse hover, they display various statistical properties of the data:
maximum value
minimum value
mean
median
1st quartile
3rd quartile
Clima Dataframe
Clima calculates new variables and creates a new dataframe containing the variables already inside the original EPW files and other we calculate. Users can overlay all the variables on the , on the , and on the customizable graphs in the .
All the variables in the new Clima dataframe are listed below.
Degree Days Explained
you might want to start understanding what degree days are here:
Degree days are calculated as the integral of the difference between the outside air temperature and a base temperature over time.
If you can define a base temperature (the outside temperature above which a building needs no heating or cooling) then you can use this to estimate degree days. The building requires heating if the outside air temperature falls below the heating base temperature, and the heating degree days accrue; if the outside air temperature rises above the cooling base temperature, the structure requires cooling, and the cooling degree days accumulate.
The base temperature does not necessarily correspond to the desired building internal temperature, but is the homeostatic temperature between the interior and exterior. Base temperature might depend, among other factors, upon the use of the building, the internal heat gains and the level of insulation.
Select Weather File
This page explains how a user can load an EPW file in the Clima tool
Users can either choose to analyse the climate of the locations displayed on the map or upload a custom EPW file. After loading an EPW file the user can then access the other tabs to generate dynamic visualisations of the data.
Video Tutorial
Learn more about how to analyse the climate of a specific location and uploading your custom EPW file by watching the following video.
Climate Summary
The top section of the page provides information about the selected location such as longitude, latitude, and climate zone. Via this page, the user can also download the EPW data and the used to generate the plots, as shown below.
The bottom section of the page comprises the heating and cooling degree day chart and four violin plots showing the distribution of the dry-bulb air temperature (Tdb), relative humidity (RH), Global Horizontal Irradiance (GHI), and wind speed (U).
UTCI explained
The UTCI tab allows users to analyze outdoor thermal comfort for a combination of different meteorological conditions based on the presence or absence of sun and wind.
Clima leverages the several models implemented in .
The "" calculates the solar gain to the human body, so the mean radiant temperature. To simulate a sunless situation, Clima considers the person surrounded by surfaces that shade him, all of which tend toward dry bulb temperature;
Wind data is obtained directly from the weather file. The windless situation sets the value at 0.5 m/s, which is the minimum value allowed by the UTCI model.
The UTCI can then be visualized for the entire year for the scenario chosen.
The values are then converted into a scale assessing thermal stress, either because of cold or heat. Therefore, a second chart maps if people will experience thermal stress for all the hours of the year for corresponding UTCI temperatures.
The UTCI is a useful tool to design the outdoor space, to maximize the number of comfortable hours. The designer can influence two factors out of the four driving outdoor comfort: radiant temperature (i.e. exposure to the sun) and wind speed (i.e. exposure to the wind).
UTCI perceived temperature annual heatmap in the four conditions for Rome, ITA
UTCI heat stress index heatmap in the four conditions for Rome, ITA
example deegree days for Palermo Boccadifalco Airport, ITALY
Relative humidity explained
Yearly chart
The annual graph allows the different months' relative humidity ranges to be evaluated. Moreover, overlaying the average value trend for each day helps visualize the differences between the minimum and maximum daily values for the investigated location.
The humidity comfort band is overlaid, considering 30-70% RH the comfortable range. With these trends, climates can be assessed, whether too dry or too humid, then evaluating design solutions that include humidification or dehumidification system.
Yearly relative humidity trend in four common climatic conditions: hot dry, tropical, temperate, and continental
Outdoor relative humidity varies greatly depending on the amount of rainfall. As expected, hot dry and tropical climates have diametrically opposite trends, always outside the comfort range, and some measures must be taken to recreate comfortable situations in buildings.
Daily chart
Monthly show all hourly relative humidity. The humidity excursion is much more evident than in the annual graphs. Daily medians, i.e., the most frequently occurring values, help evaluate the outliers.
Typical monthly days are useful to study relative humidity patterns. It is possible to read the variability of rainfall in a given location, from the constant high rates of tropical climate to the confusing and variable patterns of continental ones.
Heatmap
Heatmap is another very useful method for evaluating relative humidity excursion over a year (by evaluating the horizontal gradient) or over individual days (by evaluating the vertical gradient).
The four heatmaps give a clear idea of the comparison of the four climate types. Albeit little differences in the percentage scale, observations can be made about relative humidity trends, especially between day and night. Except on rainy days, we can see how daily sunshine decreases relative humidity, even in tropical climates.
Descriptive statistics
The last tool for relative humidity assessment is the statistics table. The earlier graphically made evaluations can be supported by the numbers. The following are listed, for each month:
the relative humidity means;
the ;
the minimum values;
Temperatures explained
Yearly chart
The annual graph allows the different months' ranges to be evaluated. Moreover, overlaying the average value trend for each day helps visualize the differences between the minimum and maximum daily values for the investigated location.
Comfortable temperature ranges for 80% and 90% of the population, calculated according to , are overlaid (see also the excellent ). For each location, it is therefore possible to assess the temperature difference between outdoors and comfort conditions, or to evaluate passive strategies using outside air in the summer season. For more information on .
Tabs Explained
The following sections will explain the tabs organizing the structure of CBE Clima
The Clima app is organized in a series of tabs that allow the exploration of various topics. All the tabs other than "Select Weather File" are active after a weather file has been selected.
Although there is a logical sequence in the organization of the tabs, thy can be accessed in any order.
The Followin section will explain the content and the usage of each tab.
Sun and Clouds
The Sun and Clouds tab presents an overview of various climatic factors that relate to sun, solar position, intensity, and cloud cover, in particular:
How to read a spherical sun path diagram
The spherical sun path is comprised of various graphical elements overlayed on one another.
We'll attempt to describe them individually below.
The sunpath can be read as a compass, with the radial lines indicating the different directions.
The concentric circles can be understood as sections of the immaginary sky dome. Each ring represents an increment of 10 degreees from the horizon. As such, they help visualize the solar altitude for each sun position.
Daily relative humidity trend in four common climatic conditions: hot dry, tropical, temperate, and continental
Annual relative humidity heatmap for four common climatic conditions: hot dry, tropical, temperate, and continental
<figure><img src="../../../.gitbook/assets/Desc stat RH.png" alt=""><figcaption><p>Descriptive statistics of relative humidity trend in a temperate climate, Berkeley (USA)</p></figcaption></figure>
Annual dry bulb temperatures trend in four common climatic conditions: hot dry, tropical, temperate, and continental
Building design must inevitably consider local climatic trends. Comparing the different annual trends in the previous image, keeping the comfort zone as a fixed reference point, it is very clear that we expect to find four aesthetically and functionally dramatically different buildings.
Daily chart
Monthly scatterplots show all hourly temperatures. The temperature excursion is much more evident than in the annual graphs. Daily medians, i.e., the most frequently occurring values, help evaluate the outliers.
Daily dry bulb temperatures trend in four common climatic conditions: hot dry, tropical, temperate, and continental
Typical monthly days show what buildings must provide to create comfortable environments. Below is an example of a comparison between four drastically opposite climates
Overall, the first observation is the climatic variability of desert and continental climates, while the other two are mostly stable at constant levels.
More in detail, imaging that we might design a building, we start from the factual need to reduce incoming heat into the environment with solar shading or other passive solutions in desert climates, to the constant need to create air movement that gives a sense of coolness in the constant tropical climates; we get to the steady temperate climates where it is easier to recreate comfortable conditions, to the continental climates, which are highly variable from summer to winter and certainly more challenging to handle.
Heatmap
Heatmap is another useful method for evaluating thermal excursion over a year (by evaluating the horizontal gradient) or over individual days (by evaluating the vertical gradient).
Daily dry bulb temperatures heatmap for four common climatic conditions: hot dry, tropical, temperate, and continental
Albeit with different scales, the four heatmaps give a clear idea of the comparison of the four given climate types, especially for the temperature excursion between day and night.
the desert climate, despite having a scale with a large delta T, shows clear contrasting vertical gradients, thus between day and night temperatures;
the tropical climate has no variability throughout the year, with a nearly constant pattern. The same observation can be made for daily and nighttime temperatures;
the temperate climate shows a certain constancy between day and night, but especially a slight increase over the summer month temperatures;
the continental climate has a highly variable temperature scale, but despite this, there is a clear gradient between the winter and summer months. The difference between day and night is not clear but, as already evident in the daily graphs, the patterns are not regular and show a very variable climate.
Descriptive statistics
The last tool for temperature assessment is the statistics table. The earlier graphically made evaluations can be supported by the numbers. The following are listed, for each month:
The daily path of the sun on a given day can be traced by following a sun path spline from the east (sunrise) to the west (sunset).
Characterization of the maximum, mean, and minimum paths line (altitudes) of the sun in the spherical sun path
The upper spline represents the sun's path during the summer solstice, i.e. the maximum height of the sun above the horizon for the examined location. Meanwhile, the lower spline is the sun’s path during the winter solstice, when the sun reaches the lowest height above the horizon. The spline in the middle is the sun's path during the Equinoxes.
Spline created by the sun's positions depending on daily hours, in the spherical sun path
Gathering the sun's positions for each time of day, during all days of the year, creates splines called Analemmas.
Analemma of the sun over a whole year
Source: National Geographic
Overlapping photos taken at the same time of day over the course of an entire year results in an analemma like the one in the picture.
Sun paths change shape depending on the latitude of the selected location
Spherical sun path
Geographical coordinates in the spherical sun path
Sun altitudes for each circle of the spherical sun path
Wind speed can be estimated with several scales. One of the first was created by Britain's Admiral Sir Francis Beaufort (1805). The Beaufort scale is an empirical scale that relates wind speed to observed conditions at sea or on land. The original scale goes from 0 to 12, but the Clima Tool will show the results from 1 to 9 since it is not common to construct buildings in places with recurrent winds over 100 km/h.
Graphical illustration of the Beaufort scale. The colors shown for each scale value are those used in the tool's charts. Source:
Each circle segment shows the winds according to the cardinal direction along which they blow from.
in the wind rose
The length of each radius around the circle shows how often the wind blew from that direction. A click of the mouse over each slice of the rose shows therefore the recurrence frequency in which the wind of such intensity is repeated over the analyzed period.
As most graphs in Clima Tool, the wind rose is strongly interactive. Clicking on the legend will hide or highlight the selected category. As such, it is easy to go from a wind rose showing all the wind directions and frequency to one that highlights only the selected speed range. This can be particularly useful to identify low-frequency, high-speed wind patterns.
In the seasonal graphs section, Clima shows 4 wind roses for the periods of:
December - February;
March - May;
June - August;
Personal viewing periods are available using the last portion of the Wind section, where a wind rose of the desired interval can be generated.
In building design, the wind is addressed in several forms:
assessing pedestrian and outdoor comfort;
design for wind-driven natural ventilation;
understanding pollution dispersal and outdoor air quality;
Clima allows the user to visualize the annual data trend through a customizable chart.
Example: annual dry bulb temperatures trend for Paris, FRA
Example: annual relative humidity trend for Paris, FRA
Daily scatter plot shows all hourly data on all days of the month and the typical monthly trend.
Example: daily dry bulb temperatures trend for Paris, FRA
Example: daily relative humidity trend for Paris, FRA
allow the intensity of values to be perceived through color palettes throughout the year.
Video Tutorial
Learn more about the Temperature and Relative Humidity tab by watching the following video.
How to read a cartesian sun path diagram
In the cartesian coordinates, the solar elevation is plotted on the y-axis and the azimuth is plotted on the x-axis. It shows the path we would see the sun follow if we took a video with the camera in the right direction of the horizon (as notable comparing the following two figures).
Long exposure makeshift pinhole camera, rising and falling of the sun over eight years
Source: National Geographic
A photographic paper left inside a cider exposed through a pinhole aperture captured 8 years of sun full cycles. The photographic paper immortalized a real cartesian sun path.
Cartesian sun path
The cartesian sun path is comprised of various graphical elements overlayed on one another.
We'll attempt to describe them individually below.
Cylindrical Miller projection of the globe
Source: GISGeography
The cartesian sun path can be read as a of the spherical sun path diagram.
The map projection encompasses a wide range of transformations used to represent the curved two-dimensional surface of a globe on a plane. The cylindrical projection is obtained by unraveling the globe inside a cylinder. Among the different distortion versions of the globe in a flat map, the is a compromise that does not sacrifice either area or map shapes excessively at the extremes.
The planes parallel to the x-axis can be understood as sections of the imaginary sky dome. Each plane represents an increment of 10 degrees from the horizon. Thereby, it is possible to read the height of the sun in each of its positions.
The geographical coordinates are plotted on the x-axis in degrees, going from left to right: North (0°), East (90°), south (180°), and West (270°).
The upper spline represents the sun's path during the summer solstice, i.e. the maximum height of the sun above the horizon for the examined location. Meanwhile, the lower spline is the sun’s path during the winter solstice, when the sun reaches the lowest height above the horizon. The spline in the middle is the sun's path during the Equinoxes.
Wind
The Wind tab presents an overview of how to visualize the intensity, frequency, and direction of the wind.
Clima allows the user to visualize the annual, seasonal, daily and customizable period wind data in a .
Moreover, Clima shows wind intensity and direction using .
Frequency of wind intensity recurrence in the wind rose
Left: wind rose showing all velocity ranges
Right: the same data can be easily filtered (by clicking on the legend) to show only direction and frequency of wind speeds above 10.7m/s
Video Tutorial
Learn more about the Wind tab by watching the following video.
Height above the horizon measured in degrees, in the cartesian sun path
Geographical coordinates in the cartesian sun path
Characterization of maximum, mean, and minimum splines in the cartesian sun path
Charts Tips and tricks
Global, Diffuse and Normal solar radiation explained
Example of meterelogical station able to measure Global Horizontal, Diffuse Horizontal, and Direct Normal Solar Radiation. Source: CSIRO
Global Horizontal Irradiance (GHI) is the total irradiance from the sun on a horizontal surface. It is the sum of the Diffuse Horizontal Irradiance and the Direct Normal Irradiance, projected onto the horizontal plane using the solar zenith angle (z).
GHI=DHI+DNI∗cos(z)
Diffuse Horizontal Irradiance (DHI) is the radiation that arrives from the entirety of the sky dome, except the solar disc. Is the radiation that has been scattered by molecules and particles in the atmosphere. It is measured on a horizontal surface
Direct Normal Irradiance (DNI) is the measurement of the intensity of sunlight on a surface perpendicular (normal) to the sun, as such, in very clear sky conditions and low solar altitudes, the Direct Normal Irradiance can be higher than the Global Horizontal Irradiance.
Customizable daily and hourly maps
Clima allows the user to visualize daily charts and heatmaps of all the variables listed in ''.
The chart above shows the of all hourly data on all days of the month and the typical monthly trend.
allow the intensity of values to be perceived through color palettes. These graphs are very helpful in seeing how magnitudes vary throughout the year.
Psychrometric Chart
Clima allows the user to visualize all annual weather conditions on a
The default diagram allows the users to overlay the frequency with which weather conditions recur throughout the year.
With the first choice in the drop-down list, "None", it is possible to view temperature conditions in the psychrometric diagram over the entire year. The visualized dots have the same gradient with a transparency rate, they are not colored according to a legend. Multiplying them when overlaid provides a visualization of their frequency, so the most common conditions.
Then, users can overlay another variable on the graphs, choosing from .
Data Explorer
The last section of Clima allows for a deeper analysis of all the data in the .
The tab is divided into three sections:
Single-variable analysis
Single-variable + filter analysis
Global and Diffuse Horizontal Solar Radiation
Clima displays hourly Horizontal values for a typical day for each month.
Typical daily graphs showing the amount of energy gained from the sun have many uses, such as:
design the building with a passive approach, to control solar gains and reduce energy consumption (allow solar gain during months of heating demand and block them during periods of cooling demand, see for reference the );
Natural Ventilation Potential
Clima allows an estimation of the hours when indoor environments can be ventilated with outside air depending on the external temperature.
is the process of introducing outdoor air into a building from the outside to improve air quality, provide fresh air and/or comfort cooling without actively conditioning the incoming airstream. Hourly air changes are required by most building regulations to ensure a healthy and smell-free environment, usually set at a minimum of one volume exchanged per hour.
In favorable climates and building types, natural ventilation can be used in combination with or as an alternative to air conditioning systems.
Clima Natural Ventilation Potential assessment is based primarily on outdoor temperatures. Natural ventilation is assumed to be possible when the outdoor air temperature is below the indoor comfort threshold and above a minimum to avoid drafts.
The maximum temperature threshold can be estimated depending on the internal loads, how many people are in the room, and how many electrical appliances. Considering the desired indoor air setpoint temperature, it can be dropped by 2 or 3 °C based on how much heat needs to be removed from the interior.
Outdoor Comfort
The Outdoor Comfort tab shows an overview of the perceived environmental condition based on the UTCI model.
The (UTCI), introduced in 1994, aims to be the measure of human physiological reaction to the atmospheric environment.
It considers:
dry bulb temperature
Cloud coverage
The cloud coverage diagram reports, for every month of the year the frequency of "clear", "cloudy" or "intermediate" conditions.
As the Cloud cover is reported in tenths of coverage (i.e. 0 is 0/10 covered. 10 is total coverage) for the purpose of this graph we have simplified the scale as per the table below.
Example: Global Horizontal radiation value for San Francisco, USA
Example: Heat map of the hourly Global Horizontal radiation on all days of the year for San Francisco, USA
Examples of daily graphs with different variables (from top left to bottom right): global horizontal radiation, global horizontal illuminance, zenith luminance, opaque sky cover
manage the direct solar gain through the glass, to evaluate solar shading devices (most useful in locations with high temperatures and a strong direct component);
manage the indirect solar gain transfer into the building with a time shift, exploiting the thermal mass, heating thick walls or concrete floors, or designing special rooms adjacent to the main spaces that rely on convection to transfer the heat, such as sunroom or Trombe wall;
evaluating sustainable renewable energy solutions such as solar thermal or photovoltaic panels.
The integral of the curves in the graphs is the total energy (in Wh/m²), supplied by the sun. Be careful in considering the different types of solar radiation.
Example: Solar radiation graphs on typical monthly days for San Francisco, USA
Conceptual representation of Global Horizontal, Diffuse Horizontal and Direct Normal Solar Radiation
Measurement of Direct Irradiation on a horizontal and a normal plane
Example: Psychrometric charts with different variables (from top left to bottom right): dry bulb temperature, relative humidity, global horizontal radiation, and wind speed
Moreover, data can be filtered by date, time, or one of the Clima dataframe variables.
Psychrometric charts filters
Video Tutorial
Learn more about the Psychrometric tab by watching the following video.
Example: Frequency of climatic condition in the Psychrometric chart of New York, USA
Example: Psychrometric chart with climatic conditions of New York, USA
Triple-variable + filter analysis
The single-variable analysis allows data to be displayed in 4 outputs: a yearly chart, monthly daily charts, an annual heatmap chart, and a descriptive statistics table.
The single-variable + filter analysis allows data to be displayed in a customizable heatmap. The chart can be created starting from one variable inside the Clima dataframe and, eventually, filtered by another one.
The triple-variable + filter analysis allows data to be displayed in two composite charts, a scatterplot, and a heat map. The baselines of the two graphs are driven by the choice of one variable to be placed on the x-axis and one on the y-axis. Then, data can be colored according to a third variable and filtered according to a fourth.
Video Tutorial
Learn more about the Data Explorer tab by watching the following video.
The minimum temperature threshold is typically dictated by local discomfort near the fresh air inlet source. The minimum limit to be considered may be as low as 10 °C, but a typically used value to minimize the risk of draft is 15 °C.
Clima shows by default a heatmap with all hours when the temperature is between 10 and 24 °C and a bar chart with the number of hours in which the filter was successful. The bar chart can be normalized, then displayed as a percentage of total monthly hours, or simply plotted with the total sum of the filtered hours.
Example: annual dry bulb temperature heatmap between 10 and 24 °C for Barcelona, SPA
Example: count of annual dry bulb temperature between 10 and 24 °C for Barcelona, SPA
Clima provides various options to customize the natural ventilation potential calculations for individual projects:
setting the minimum and maximum outdoor air temperatures to be considered;
selecting the time of the day or months to be analyzed;
Natural ventilation can be used in combination with and in aid of radiant cooling systems. The most common risk is that condensation will form on cold surfaces, creating slippery floors or potential mold. Clima allows this control to be performed with the "surface temperature" filter, which is a function of the dew temperature.
Video Tutorial
Learn more about the Natural Ventilation tab by watching the following video.
to calculate a reference environmental temperature causing strain when compared to an individual's response to the real environment. It is based on Fiala et al.'s multi-node model of thermo-regulation.
The UTCI equivalent temperature is a function of the above parameters, which are combined in a multinode thermo-physiological model that takes into account clothing insulation and metabolic rate. From this aUniversal Thermal Climate Index (UTCI) of perceived thermal stress is derived.
Clima allows the user to visualize the annual UTCI equivalent temperature as heatmap under various environmental conditions.
Example: UTCI perceived temperature annual heatmap in 'Sun and Wind' conditions for Rome, ITA
The UTCI temperature can be converted in a scale assessing thermal stress, displayed by Clima in a heatmap graph.
Example: UTCI heat stress index heatmap in 'Sun and Wind' conditions for Rome, ITA
Video Tutorial
Learn more about the Outdoor Comfort tab by watching the following video.
