How much water vapor is in the air, and when does it condense into clouds?
Explain humidity, dewpoint and relative humidity, use the Reference Tables dewpoint and relative humidity charts from dry-bulb and wet-bulb readings, and relate cooling to condensation, cloud and precipitation formation.
A Regents answer on atmospheric moisture: the difference between dewpoint and relative humidity, how to read the Reference Tables dewpoint and relative humidity charts from the dry-bulb temperature and the wet-bulb depression, why air cooled to its dewpoint condenses, how clouds and precipitation form on condensation nuclei, and the saturation idea, with worked exam questions.
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What this topic is asking
The Regents wants you to explain humidity, dewpoint and relative humidity, to read the Reference Tables dewpoint and relative humidity charts from a dry-bulb temperature and a wet-bulb depression, and to relate cooling to condensation and cloud formation. This is a reliable source of Part B-2 chart marks.
Three ways to describe moisture
A key relationship: warmer air can hold more water vapor. So if the moisture stays the same and the air warms, relative humidity falls; if the air cools, relative humidity rises until, at the dewpoint, the air is saturated.
Reading the Reference Tables charts
Both charts are read the same way, using two measurements from a sling psychrometer (two thermometers, one with a wet wick):
- Find the wet-bulb depression = dry-bulb temperature - wet-bulb temperature.
- On the Dewpoint Temperature chart, read across from the dry-bulb temperature (left column) and down from the depression (top row) to the dewpoint.
- On the Relative Humidity chart, do the same to read the relative humidity percentage.
From cooling to clouds and precipitation
When moist air rises (over mountains, along fronts, or by convection), it expands and cools. Once it cools to its dewpoint, the relative humidity reaches 100 percent and the vapor condenses onto microscopic particles called condensation nuclei (dust, salt, pollution), forming cloud droplets. If the droplets grow large and heavy enough, they fall as precipitation (rain, snow, sleet or hail, depending on the temperature profile). So clouds form where rising air is cooled to saturation.
Try this
Q1. Define relative humidity. [1 point]
- Cue. The ratio of the water vapor in the air to the maximum the air could hold at that temperature, as a percentage.
Q2. Explain what happens to relative humidity as air is cooled toward its dewpoint. [2 points]
- Cue. It rises, because cooler air can hold less vapor, reaching 100 percent (saturation) at the dewpoint.
Exam-style practice questions
Practice questions written in the style of NYSED exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Regents (style)2 marksPart B-2. A sling psychrometer reads a dry-bulb temperature of 14 degrees Celsius and a wet-bulb temperature of 10 degrees Celsius. Using the Reference Tables, determine (a) the dewpoint and (b) the relative humidity. Explain how you used the tables.Show worked answer →
A 2-point Reference Tables question.
1 point for the dewpoint, 1 point for the relative humidity.
First find the wet-bulb depression: dry-bulb minus wet-bulb = 14 - 10 = 4 degrees Celsius.
(a) On the Dewpoint Temperature chart, read across from a dry-bulb of 14 and down from a depression of 4: the dewpoint is about 7 degrees Celsius.
(b) On the Relative Humidity chart, read across from a dry-bulb of 14 and down from a depression of 4: the relative humidity is about 61 percent (accept the NYSED value).
Markers reward computing the depression first, then reading each chart with the dry-bulb row and the depression column.
Regents (style)3 marksPart C. (a) Define relative humidity. (b) Explain what happens to the relative humidity as unsaturated air is cooled toward its dewpoint. (c) Explain why clouds form when moist air rises and cools.Show worked answer →
A 3-point extended-response question.
(a) 1 point: relative humidity is the ratio of the water vapor actually in the air to the maximum amount the air could hold at that temperature, expressed as a percentage.
(b) 1 point: as the air cools toward its dewpoint, the relative humidity increases (the air can hold less vapor as it cools), reaching 100 percent at the dewpoint.
(c) 1 point: rising moist air expands and cools to its dewpoint; the vapor then condenses onto condensation nuclei to form cloud droplets, so clouds form.
Markers reward the ratio definition, rising relative humidity as temperature falls, and the cool-to-dewpoint-then-condense mechanism.
Related dot points
- Describe the layered structure and composition of the atmosphere and explain how energy is transferred by radiation, conduction and convection, including how surfaces absorb and reflect insolation.
A Regents answer on the atmosphere and energy transfer: the layered structure (troposphere to thermosphere) and temperature profile on the Reference Tables, the composition (nitrogen, oxygen, trace gases), the three modes of heat transfer (radiation, conduction, convection), and how surface color and texture affect the absorption and reflection of insolation, with worked exam questions.
- Classify air masses, describe the weather at warm and cold fronts and around high- and low-pressure systems, and interpret weather maps and the Reference Tables station model.
A Regents answer on weather systems: how air masses are classified (maritime/continental, tropical/polar), the weather at cold and warm fronts, high-pressure (clear, sinking, diverging) versus low-pressure (cloudy, rising, converging) systems, the typical west-to-east movement across New York, and how to decode the Reference Tables weather station model, with worked exam questions.
- Describe the water cycle and its processes, and explain the factors that control infiltration, runoff and groundwater storage (porosity, permeability, slope, saturation and the water table).
A Regents answer on the water cycle and groundwater: evaporation, transpiration, condensation, precipitation and runoff, the factors that control infiltration versus runoff (porosity, permeability, particle size, slope, saturation, vegetation), the water table and zones of saturation and aeration, and the energy that drives the cycle, with worked exam questions.
- Explain the factors that control climate (latitude, elevation, proximity to water, ocean currents, mountain barriers and prevailing winds) and distinguish climate from weather.
A Regents answer on climate controls: the difference between weather and climate, how latitude, elevation, proximity to large bodies of water, ocean currents, mountain barriers (orographic effect and rain shadows) and prevailing winds set a region's temperature and precipitation, and why coastal and inland climates differ, with worked exam questions.
- Explain how ocean surface currents form (winds, the Coriolis effect) and how they redistribute heat, moderate coastal climates and connect to density-driven deep circulation.
A Regents answer on the oceans: how prevailing winds and the Coriolis effect drive surface currents into gyres, how warm and cold currents redistribute heat and moderate coastal climates (for example the Gulf Stream), the difference between surface and density-driven deep circulation, and the link to the water specific heat on the Reference Tables, with worked exam questions.
Sources & how we know this
- Reference Tables for Physical Setting/Earth Science (2011 edition) — New York State Education Department (2011)
- Regents Examination in Physical Setting/Earth Science — New York State Education Department (2026)