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Comparing climate-related design conditions for major U.S. cities requires the same weather variable, annual percentile, observation station, data edition and unit system for every city. ASHRAE climate zones provide useful thermal and moisture context, but they are not substitutes for station-specific heating and cooling design data. A defensible comparison therefore reports both the representative climate zone and the exact ASHRAE design-condition record used for HVAC decisions.
At a glance
| Name | Best for | Key spec | Checked |
|---|---|---|---|
| Miami | Hot, humid cooling context | Representative zone 1A | November 2026 |
| Houston | Hot, humid cooling context | Representative zone 2A | November 2026 |
| Phoenix | Hot, dry cooling context | Representative zone 2B | November 2026 |
| Atlanta | Mixed-humid conditions | Representative zone 3A | November 2026 |
| Los Angeles | Warm, dry coastal context | 3B in DOE examples; 3B-Coast in an ASHRAE addendum example | November 2026 |
| Seattle | Marine, mild-temperature context | Representative zone 4C | November 2026 |
| Chicago | Cold, mixed-humid heating and cooling | Representative zone 5A | November 2026 |
| Denver | Cold, dry high-elevation context | Representative zone 5B | November 2026 |
| Minneapolis | Very cold winter design context | Representative zone 6A | November 2026 |
The table is a climate-context map, not a list of design temperatures. Exact values change with station, percentile, edition and variable, so they must be taken from the applicable ASHRAE station record.
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Climate-related design conditions are statistical weather values used to select and size building systems. The data support heating, cooling, ventilation, energy analysis, equipment selection and related design work. ASHRAE’s Chapter 14, Climatic Design Information, includes dry-bulb temperature, wet-bulb temperature, dew point, humidity ratio, wind and precipitation information.
A design condition is not the city’s average temperature and not an absolute record high or low. It is a value associated with a stated probability of occurrence. That distinction keeps a system from being sized for a once-in-a-century record while still providing a common, documented basis for expected extreme operating conditions.
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How does ASHRAE’s percentile method work?
ASHRAE uses annual percentiles so that cities can be compared on the same probability basis. Warm-season temperature and humidity conditions are commonly reported at the 0.4%, 1.0% and 2.0% annual percentiles; cold-season conditions use the 99.6% and 99.0% annual percentiles.
For a cooling design value, the lower warm-season percentile represents a more demanding, less frequently exceeded condition than a higher percentile. For heating, the higher cold-season percentile represents a colder, less frequently undercut condition than the 99.0% value. The appropriate choice depends on the building, risk tolerance, operating schedule, local code and the engineer’s design criteria.
“The use of annual percentiles to define design conditions ensures that they represent the same probability of occurrence in any climate, regardless of the seasonal distribution of extreme temperature and humidity.” — ASHRAE Handbook, Chapter 14, Climatic Design Information
Percentiles should always be labeled with the variable and season. A cooling dry-bulb percentile is not interchangeable with a coincident wet-bulb or dew-point condition, and a heating percentile cannot be compared directly with a cooling percentile.
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What is the difference between a climate zone and a design temperature?
A climate zone is a broad classification of thermal and moisture conditions; a design temperature is a station-specific statistical value. Climate zones are useful for code compliance, reference-building assumptions and early feasibility decisions, while design temperatures are needed when an engineer calculates loads or selects equipment.
For example, the U.S. Department of Energy’s commercial reference-building table pairs Miami with 1A, Houston with 2A, Phoenix with 2B, Atlanta with 3A, Los Angeles with 3B, Seattle with 4C, Chicago with 5A, Boulder with 5B and Minneapolis with 6A. An ASHRAE standards addendum gives representative examples including Los Angeles as 3B-Coast and Denver as 5B.
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Those labels do not tell you the heating dry-bulb value for a particular airport, the cooling wet-bulb at that station or the humidity ratio at a selected percentile. Zone boundaries, code editions and city-versus-station definitions can also differ. Treat the label as context, then obtain the record for the actual project location.
How do the major-city examples differ?
The cities span the principal U.S. climate patterns, so they illustrate why one generic “U.S. design temperature” is meaningless. The differences below describe the design questions each location raises; they do not establish a universal hottest or coldest ranking.
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- Miami, zone 1A: Cooling loads are strongly affected by high outdoor moisture as well as temperature. Latent capacity, ventilation air treatment and coincident humidity data deserve as much attention as sensible dry-bulb load.
- Houston, zone 2A: This is another hot, humid context. A comparison with Miami should use the same cooling percentile and moisture variable rather than assuming the two stations have identical latent design conditions.
- Phoenix, zone 2B: The hot-dry classification shifts the emphasis toward high sensible cooling and lower outdoor moisture than humid-zone examples. Evaporative processes, ventilation assumptions and indoor humidity targets can change the design outcome.
- Atlanta, zone 3A: Mixed-humid conditions require both summer latent analysis and meaningful winter heating analysis. A dry-bulb-only comparison can hide the ventilation and dehumidification implications.
- Los Angeles, zone 3B or 3B-Coast: The coastal qualification matters. A coastal station and an inland station in the broader metropolitan area can produce different temperature and moisture records, so the station name must be recorded.
- Seattle, zone 4C: Marine conditions generally call for attention to moderate temperatures, long shoulder seasons and moisture. The relevant design challenge may be different from a short, intense summer peak even when the nominal zone number is close to another city’s.
- Chicago, zone 5A: Heating and cooling are both material. Winter design temperature, wind exposure and infiltration assumptions can drive heating capacity, while summer humidity still matters for cooling and ventilation.
- Denver, zone 5B: The dry, high-elevation context affects moisture, air density and equipment calculations. Altitude and the exact observation station should be documented rather than inferred from the zone code alone.
- Minneapolis, zone 6A: Winter design conditions are a major sizing concern. A heating percentile and station record should be stated explicitly, along with assumptions for envelope performance, infiltration and ventilation.
These examples demonstrate why a city can appear extreme on one axis and ordinary on another. “Hottest” might mean the highest cooling dry-bulb value, the greatest humid-air enthalpy or the largest latent ventilation load. “Coldest” might refer to heating dry bulb, wind-adjusted infiltration exposure or a particular percentile. Without those qualifiers, the ranking is incomplete.
Which fields should a city-to-city comparison include?
A useful comparison table identifies every choice that affects interpretation. At minimum, include the following fields:
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|---|---|
| City and weather station | A metropolitan area may contain airport, downtown, coastal, inland or elevated stations with different records. |
| Climate zone | Provides broad thermal and moisture context, not an equipment-sizing value. |
| Heating dry-bulb and percentile | Defines the cold-weather outdoor condition used for heating analysis. |
| Cooling dry-bulb and percentile | Defines the warm-weather sensible condition used for cooling analysis. |
| Coincident wet-bulb, dew point or humidity ratio | Shows the moisture condition that occurs with the selected temperature and affects latent load. |
| Source edition and observation period | Allows an engineer to reproduce the comparison and identify updates. |
| Units | Prevents errors when converting between IP and SI values. |
The research basis for this article does not provide one consistent, current numerical station table for all nine cities. It would be misleading to fill the missing values from unrelated summaries or to present a classroom reference table as a sizing standard. Use the current ASHRAE station data and record the station, variable, percentile, edition and units beside every number.
How do you build a defensible comparison?
Start with the project decision, then standardize the data before comparing cities.
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- Define the design task. Decide whether the comparison is for heating capacity, sensible cooling, total cooling, ventilation dehumidification, energy modeling or resilience. The task determines which weather variables are required.
- Choose the project station. Select the ASHRAE station that best represents the site. Document whether it is an airport or another observation location, and note elevation or coastal exposure when relevant.
- Select the percentile convention. Use the same warm-season percentile for every city and the same cold-season percentile for every city. Do not compare a 0.4% cooling value in one city with a 2.0% value in another.
- Retrieve coincident moisture data. For humidification, dehumidification and ventilation decisions, pair dry-bulb temperature with the corresponding wet-bulb, dew-point or humidity-ratio condition rather than adding an unrelated moisture statistic.
- Record the edition and period. Climate data are revised. Cite the ASHRAE edition or dataset release and the observation period so a later design review can identify exactly what was used.
- Normalize units and definitions. Keep IP or SI units consistent, distinguish dry bulb from wet bulb, and avoid silently converting a design value into an average or record extreme.
- Interpret by load axis. Compare heating severity, sensible cooling severity, moisture load and wind or precipitation exposure separately. Only then draw a project-specific conclusion.
- Check local requirements. Codes, utility programs and the authority having jurisdiction may prescribe a particular edition, weather file or safety margin. The governing requirement controls the final design.
Why can two sources show different conditions for the same city?
Different results usually reflect different stations, editions, periods, variables or percentile choices rather than an error. “Los Angeles” may refer to a coastal station in one source and an inland station in another; “cooling design temperature” may mean dry bulb in one table and a combined temperature-moisture condition in another.
Reference-building studies can introduce another layer of difference. A paper on ventilation and filtration modeling, available through PubMed Central, states that its design-day conditions were reference values and were not used for HVAC system sizing. Such values can support a classroom or simulation scenario, but they should not replace current station-specific design data for a construction project.
What are the most common comparison mistakes?
- Using the climate-zone code as a temperature: “5A” does not specify a heating or cooling degree value.
- Mixing percentiles: A 0.4% cooling condition and a 2.0% cooling condition represent different exceedance probabilities.
- Ignoring moisture: Dry-bulb temperature alone can understate latent and ventilation loads in humid cities.
- Comparing different stations: Airport and downtown records may differ, especially near coasts, mountains and urban heat islands.
- Using record extremes: A historical maximum or minimum is not automatically an ASHRAE design condition.
- Leaving the edition unstated: Readers cannot reproduce or audit an unlabeled number.
- Ranking cities without a variable: A city can lead on dry-bulb heat but not on humid-air load, or be colder at one percentile but not another.
- Applying a reference table to equipment sizing: A model input or educational design day may be intentionally simplified.
How should the results be reported in construction documents?
Put the weather basis in the mechanical design narrative and schedules, not only in a spreadsheet. For every design condition, identify the city, station, climate-zone context, variable, percentile, source edition, observation period, unit system and whether the moisture value is coincident with the stated dry bulb.
A concise schedule might read: “Project station: [station name]; heating outdoor dry bulb: [value], [percentile]; cooling outdoor dry bulb: [value], [percentile]; coincident moisture: [wet bulb/dew point/humidity ratio]; source: ASHRAE [edition]; units: [IP or SI].” This format makes substitutions and value-engineering reviews safer because a reviewer can see whether a proposed weather file is genuinely equivalent.
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For early planning, the representative zone table is enough to signal broad differences between Miami, Houston, Phoenix, Atlanta, Los Angeles, Seattle, Chicago, Denver and Minneapolis. Before final load calculations, replace that context with the current station record required by the project and jurisdiction.
Frequently Asked Questions
Can I size HVAC equipment from an ASHRAE climate-zone label?
No. A label such as 2A or 5B describes a broad climate category but contains no station-specific dry-bulb, moisture or percentile value. Use the label for early context and the current ASHRAE station data for load calculations.
Why do humid cities need more than a cooling dry-bulb comparison?
Outdoor moisture determines the latent portion of ventilation and space loads. Two cities with similar dry-bulb design temperatures can require different coil selection and dehumidification strategies when their coincident dew point, wet bulb or humidity ratio differs.
Which weather station should a project in a large metro area use?
Use the station that best represents the site’s exposure and elevation, then document the choice. Coastal, inland, airport and mountain stations within one metropolitan region can produce materially different design conditions.
Are design-day values in energy-modeling papers suitable for construction sizing?
Not automatically. The cited ventilation and filtration modeling paper identifies its design-day values as reference conditions rather than HVAC sizing inputs, so construction documents should use the governing ASHRAE station data and local requirements.
